The things you can tell from a pile of corpses…

I’m really late to this party, but I never claimed to be timely, and the thing about the reproductive habits of Fractofusus is too interesting not to cover.* Rangeomorphs  like Fractofusus are really odd creatures. They lived in that Ediacaran twilight zone between older Precambrian seas devoid of macroscopic animals and younger Cambrian seas teeming with recognisable members of modern groups. Rangeomorphs such as RangeaCharnia and Fractofusus itself have such a unique fractal body plan (Narbonne, 2004) that no one really knows what they are. Although they were probably not photosynthetic like plants or algae (they are abundant in deep sea sediments where there wouldn’t have been enough light), their odd body architectures are equally difficult to compare to any animal that we know.

Mitchell et al. (2015) don’t bring us any closer to the solution of that mystery; they do, however, use the ultimate power of Maths to deduce how the enigmatic creatures might have reproduced. Fractofusus is an oval-shaped thingy that could be anywhere from 1 cm to over 40 cm in length. Unlike some other rangeomorphs, it lay flat on the seafloor with no holdfasts or stalks to be seen. Fractofusus fossils are very common in the Ediacaran deposits of Newfoundland. Since there are so many of them, and there is no evidence that they were capable of movement in life, the researchers figured their spatial distribution might offer some clues as to their reproductive habits. A bit of seafloor covered in Fractofusus might look something like this (drawing from the paper):

clusters within clusters

(The lines between individuals don’t actually come from the fossils, they just represent the putative connection between a parent and its babies.)

Statistical models suggest that the fossils are not randomly distributed but clearly clustered: small specimens around medium-sized ones, which are in turn gathered around the big guys. Two out of three populations examined show these clusters-within-clusters; the third has only one layer of clustering, but it’s still far from random. As the authors note, the real populations they studied involve a lot more specimens than shown in the diagram, but they “rarefied” them a bit for clarity of illustration while keeping their general arrangement.

The study looked not only at the distances between small, medium and large specimens, but also directions – both of where the specimens were and which way they pointed. If young Fractofusus spread by floating on the waves, they’d be influenced by currents in the area. It seems the largest specimens were – they are unevenly distributed in different directions. In contrast, smaller individuals were clustered around the bigger ones without regard to direction. Small and large specimens alike pointed randomly every which way.

What does this tell us about reproduction? The authors conclude that the big specimens probably arrived on the current as waterborne youngsters, hence their arrangement along particular lines . However, once there, they must have colonised their new home in a way that doesn’t involve currents. Mitchell et al. think that way was probably stolons – tendrils that grew out from the parent and sprouted a new individual at the end. This idea is further strengthened by the fact that among thousands of specimens, not a single one shows evidence for other types of clonal reproduction – no fragments, and no budding individuals, are known. (Plus if a completely sessile organism fragments, surely the only way the pieces could spread anywhere would be by riding currents, and that would show up in their distribution.)

Naturally, none of this tells us whether Fractofusus was an animal, a fungus or something else entirely. Sending out runners is not a privilege of a particular group, and while there is evidence that the original founders of the studied populations came from far away on the waves, we have no idea what it was that floated in to take root in those pieces of ancient seafloor. Was it a larva? A spore? A small piece of adult tissue? Damned if we know. Despite what Wikipedia and news headlines would have you believe, there is nothing to suggest that sex was involved. It may have been, but the evidence is silent on that count. (Annoyingly, the news articles themselves acknowledge that. Fuck headlines is all I’m saying…)

While sometimes we gain insights into ancient reproductive habits via spectacular fossils like brooding dinosaurs or pregnant ichthyosaurs, this study is a nice reminder that in some cases, a lot can be deduced even in the absence of such blatant evidence. This was an interesting little piece of Precambrian ecology, and a few remarks in the paper suggest more to come: “Other taxa exhibit an intriguing range of non-random habits,” the penultimate paragraph says, “and our preliminary analyses indicate that Primocandelabrum and Charniodiscus may have also reproduced using stolons.”

An intriguing range of non-random habits? No citations? I wanna know what’s brewing!

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*Also, I’ve got to write something so I can pat myself on the back for actually achieving something beyond getting out of bed. Let’s just say Real Life sucks, depression sucks worse, and leave it at that.

***

References:

Mitchell EG et al. (2015) Reconstructing the reproductive mode of an Ediacaran macro-organism. Nature 524:343-346

Narbonne GM (2004) Modular construction of Early Ediacaran complex life forms. Science 305:1141-1144

The unexpected complexity of nothing

I don’t think I’ve covered anything theoretical in a while, so here’s an interesting modelling study that I’ve just come across in PNAS (Shah et al., 2015). It discusses a key point in evolutionary theory – that “mutations” and “fitness” don’t exist in a vacuum. More specifically, it investigates how mutations that have little effect on fitness at the time interact with other mutations in a protein under strong purifying selection. Lots of studies deal with the role of interactions (or epistasis) between mutations in adaptation and innovation, but apparently, the question is much less explored when selection is keeping things the way they are.

Shah et al.’s approach is a mixture of theory and empirical data. The protein they consider is perfectly real – it’s the amino acid-binding protein argT from the bacterium Salmonella typhimurium (yep, that salmonella), and it was chosen because its structure is well-known and relatively simple. In contrast, the mutations happen entirely on a computer, although the models used to calculate their fitness effects in a simulated population of bacteria were calibrated to match the real-world distribution of the effects of mutations under similar circumstances.

The most important of these circumstances is the fact that this protein is not actively adapting to anything. It is already well-adapted to its function, and there is nothing pushing it in a new direction. Nonetheless, mutations happen whether or not they are needed; what the authors wanted to know is whether the mutations that arise in this environment constrain the course of evolution.

The researchers took the real argT protein sequence and introduced changes. In each round, ten random mutations were proposed, only one of which made it into the next round. For each proposed mutation, they used a program designed to model protein structure to calculate the stability of the new protein. Proteins are long chains of amino acids twisted and folded into specific 3D shapes. The stability of these shapes is important because a protein that is too rigid or too floppy can’t bind the right molecules with the right strength (remember, the function of argT is to grab certain amino acids).

The simulations assumed that the real protein is pretty much optimally stable already, and either increased or decreased its stability would decrease its fitness*. Protein stability was converted to fitness in a way that a realistic percentage of mutations were neutral, kinda bad or plain lethal (you can’t have beneficial mutations here, since the original protein is assumed to be optimised for its function). Finally, the least bad mutation in each round was chosen to update the protein sequence. This procedure was repeated until the protein had accumulated 30 changes, and the whole process was replicated 100 times.

With a hundred new virtual proteins in hand, the really interesting part of the experiment could begin. The grand aim of this whole study was to examine mutations in their historical context. All mutations that were added to the original argT sequence were neutral or nearly neutral at the time of their introduction – but would they be neutral if they were introduced at an earlier point in the evolutionary sequence? And would they still be neutral, or rather, reversible, after a bunch of other mutations had accumulated on top of them?

As you may have guessed, the answer to both questions is no. Even though the final 100 proteins were pretty much as good as the original, and each of the mutations that made it through had close to zero effect on fitness at the time, taking mutations out of their context and sticking them into different backgrounds showed that their lack of effect was highly contingent on the history of that particular sequence.

The graph below summarises what happens if you take mutation 16 and either shift it to an earlier point, or take it out at a later point (a similar pattern holds no matter which mutation you start from). On the vertical axis is the fitness effect of the same mutation at different points relative to its effect at the time it actually occurred. The left side of the graph is consistently below zero – at any point before its “proper” time, mutation 16 would have been more deleterious. It only worked with all 15 previous mutations already in place.

Shah_etal2015-contingency.large

On the right side – mutation 16’s future – fitness effects rise rapidly. The more new mutations are added, the more “beneficial” (or more precisely, irreversible) mutation 16 becomes. Even though it didn’t do much at the time, as soon as other mutations come to rely on it, you can’t take it out without royally screwing up the whole protein. The mutation has become entrenched, to use the authors’ terminology. This figure is an average of all 100 simulations; the results are pretty consistent.

Of course, there are some caveats. One of the most important is that in real populations, mutations are not necessarily fixed one at a time, and the way multiple co-existing mutations interact could be quite different from the way individual mutations affect subsequent individual mutations. Another big if is the accuracy of the software that calculates protein stability – getting from protein sequence to structure and physical/chemical properties is still notoriously difficult. In this study, considering only the first few mutations in each series (i.e. before the virtual protein diverged too far from the original with known properties) doesn’t change the main results, so the authors don’t think this is a major problem for their conclusions. There is also the fact that global protein stability, the variable used here to estimate fitness, is not the same thing as function (in this case, binding specific amino acids). However, the latter depends only on a tiny proportion of the larger structure, so global stability is probably a reasonable proxy.

It occurs to me that what Shah et al.’s study simulated is basically the evolution of irreducibly complex nothing. Here we have a protein that does the exact same thing its ancestor did (with the above caveat) despite having a rather different sequence. This utter lack of change evolved one tiny step at a time; each step dispensable, each step insignificant. Yet try to take out any of the earlier steps from the final product, and the whole edifice collapses.

Call me strange, but I find this… amusing.

***

*They actually repeated the entire experiment with an alternative assumption that increases in stability are neutral rather than deleterious, but they got very similar results, largely due to the fact that very few mutations actually increased the stability of argT.

***

Reference:

Shah P et al. (2015) Contingency and entrenchment in protein evolution under purifying selection. PNAS 112:E3226–E3235

Msp130 adventures, or the Mammal does science

I’ve been writing this blog almost since I started my PhD, but the closest I actually got to writing about my own work was a long fangirl squee about fan worms. Most of my project involved describing some really basic things about a relatively unknown animal, and probably not terribly interesting unless you’re an expert in my field (also, my brain is convinced that nearly everything I do is shit, so I don’t particularly like talking about it…). However, I do have this cool little story I’ve been burning to tell the world, and couldn’t because we wanted it published… Now it is (Szabó and Ferrier [2015]; there goes my super-secret identity, I suppose ;) )

My story involves a family of proteins called msp130. I wish they had a more fun name than that, but they were named by sea urchin people, and unlike the fruit fly community, they don’t really seem to care about making their gene names fun. (Msp130 stands for “mesenchyme-specific protein, 130 kDa”, in case you wondered; kDa, kilodaltons, being units of molecular mass.)

It all started with a sea urchin

The original msp130 was discovered in sea urchin larvae. It is found in – or rather, on the surface of – primary mesenchyme cells (PMCs), a specialised population of cells that build the calcareous skeleton of the larva. Here’s a photo of a sea urchin embryo with PMCs stained blue, from Illies et al. (2002). At this stage, the embryo is basically a squashed ball with a hole through most of it; the hole is going to become the gut, and its opening is the future anus.

Illies_etal2002-msp130r2_gastrula

Here’s a polarised light photograph of an older larva of a sea biscuit. The skeleton is pretty much the only thing you can see, highlighted in stunning rainbow colours due to the birefringence of the mineral (Bruno Vellutini, flickr):

Msp130 turned out to be essential for skeleton formation – when researchers blocked its surface with antibodies, PMCs cultured in a dish couldn’t take up calcium and couldn’t make spicules (Carson et al., 1985; Anstrom et al., 1987). Not quite so long ago, Illies et al. (2002) found that S. purpuratus has at least three msp130 genes, and in the embryo/larva, the other two are also exclusively expressed in PMCs. This is what the first picture above shows: the blue stain appears in cells that express one of the msp130-related genes.

Anyway. A few years later, after the sequencing of the S. purpuratus genome, it turned out that there were at least seven such genes, residing in a couple of clusters in the genome (Livingston et al., 2006). However, until very recently, the msp130 family was only studied in echinoderms.

Horizons are expanded and weirdness is found

BUT, this being the genomic era, sea urchin guru Charles Ettensohn wanted to know more about these buggers – just how common are they? Where do they come from? Are they always lurking in genomes that have to produce calcified skeletons? What he found in sifting through the vast repository of sequence data that is Genbank was very interesting and somewhat puzzling: across the entire tree of life, msp130 genes only seemed to be present in echinoderms, acorn worms, lancelets, molluscs, a handful of algae… plus loads of bacteria and archaea (Ettensohn, 2014). There was no mistaking it: to someone accustomed to comparing protein sequences, the bacterial sequences very clearly were the same thing as the ones from animals and algae.

So, Ettensohn concluded, it looks like animals (and algae) probably didn’t inherit this thing directly from their common ancestor with other life forms. That would imply a lot of independent losses, and Occam’s razor dictates that we shouldn’t postulate so many hypothetical events without good reason (although, as Maeso et al. [2012] point out, animal genomes don’t seem to be quite as keen on Occam’s razor as scientists).

Instead, supposing that animals and algae repeatedly acquired these genes by horizontal gene transfer from bacteria (or each other?) seems like a simpler explanation. At least one loss probably did occur – among deuterostomes, vertebrates and sea squirts are the odd ones out in not having msp130 genes, and the most Occamific explanation of that pattern is that we just mislaid them somewhere along the line. Here’s a graphical representation of Ettensohn’s scenario from his paper – “HGT” stands for horizontal gene transfer events, and grey circles are meant to represent the extra msp130 genes that later evolved in each lineage by gene duplication:

Ettensohn2014-animalMsp130

However, Ettensohn also pointed out that whole genome-level information about most animal groups is still pretty thin on the ground (seriously, everyone, stop sequencing more stupid vertebrates. We’re all the same.) We don’t, for example, have published genomes from calcareous sponges, or from annelid worms who build calcareous tubes or have other calcareous hard parts. Like my wormies. And here’s where I come in – I happen to have a decent amount of transcriptome data (alas, no genome) from just the right kind of annelid. Better, my data are derived specifically from an organ with calcareous parts (the operculum – see my fanworm post).

Naturally, as soon as I read Ettensohn’s paper, the first thing I did was grab the sequence of the “original” msp130 protein and search my own data for a match. Ettensohn said that msp130 sequences were very easy to recognise… And yep, they are. With not much effort at all, I found a lovely, full-length msp130-like sequence in my big pile of data. Much as I hate doing molecular biology, I also managed to confirm the presence of the messenger RNA (or at least the presence of one end of it) in an actual test tube of actual RNA taken from the operculum. But that’s not really saying much re: the whole gene thievery issue – yeah, another animal fairly closely related to molluscs has an msp130 gene, and it’s active somewhere within a millimetre of a calcareous hard part. That, unfortunately, says precisely bugger all about their evolutionary origin.

But I had an idea, peeps. Introns!!!

Genes in pieces make answers come together

There is an important difference between the genes of prokaryotes like bacteria and eukaryotes like algae or animals. In the former, most genes are uninterrupted stretches of DNA. A bacterial gene is transcribed into messenger RNA, and everything in that mRNA that stands between the “start protein” and “end protein” signals is translated into a protein using the appropriate genetic code.

Most of the genes of eukaryotes, however, consist of chunks that encode parts of the protein product (exons) interrupted by chunks that get discarded during or after transcription (introns)*. So there’s a potentially easy way of telling whether a gene in two different animals came from their common ancestor or from some overly generous microbes. If they have introns in matching locations, that’s not a similarity they could have acquired just by getting the gene from the same bacterium!

I say potentially easy for at least two reasons. One, while some gene families keep their introns in the same places for a very long time, introns can come and go in evolution. They can even disappear completely under some circumstances, although something the size of msp130 does usually have at least a few. If msp130 genes have fast-evolving structures, we may not be able to tell whether molluscs and deuterostomes acquired them independently, or whether the positions of introns just changed too much since their common ancestor.

Two, introns can theoretically evolve twice in the same place – just as some parts of a genome can be hotspots for mutations, parts of a gene can be hotspots for new introns. Of course, the more similar the overall structure of two genes, the less likely “intron hotspots” become as an explanation.

I compared the exon-intron structures of all msp130 genes in a few representative species with sequenced genomes in which Ettensohn found such genes. Besides sea urchins (which are from one of the two main deuterostome lineages), I chose lancelets (from the other great branch of deuterostomes) and limpets (which are molluscs). Together, these three creatures represent all major animal lineages in which msp130 genes have been found. Alas, I couldn’t do it with my own animals, because I don’t have a genome to play with :( . I also checked all three algae – the two green algae on Ettensohn’s list are fairly closely related, but the third one is a brown alga separated by upwards of a billion years.

As I said, all of these species have fully sequenced genomes, but you really need two sources of data to do this kind of thing properly. A genome sequence includes the complete gene with all the introns – but without the corresponding mRNA sequences, we must use clever computer programs that search for characteristic DNA motifs and/or sequence similarity to other organisms to predict where introns begin and end. Aside from clever programs occasionally being remarkably stupid or getting confused by sequencing errors, you can hopefully see how relying on similarity doesn’t exactly provide unbiased evidence for my purposes.

Sequences derived from transcripts only contain exons, however, and not because a computer predicted them, but because they’re read from the fully edited mRNA. So aligning transcripts with genomes should tell you exactly where the introns are, although transcript data were incomplete or altogether missing for some of the genes I looked at. (I didn’t have that problem with sea urchins – Tu et al. [2012] helpfully sequenced transcripts of pretty much all urchin genes and uploaded the results to the genome browser.)

Nonetheless, the data that did exist told us enough to doubt Ettensohn’s idea. Importantly, I found enough to piece together the entire protein-coding portion of the mRNA for two of the limpet msp130 genes – in other words, the animals that Ettensohn thought likely to have acquired the family independently from sea urchins. In total, the animal species I investigated share not just one or two but seven intron locations (an msp130 gene has maybe a dozen introns altogether). One of those is also present in the algae, and the sequence next to it is almost identical across all of the genes. There’s really no mistaking that one! A few more introns are in generally similar locations, though they don’t line up perfectly in my best alignment**.

What can we conclude from this? I think we can probably say with reasonable certainty that deuterostomes and molluscs didn’t get msp130 genes from bacteria separately. Given the similarity with algae, they might not have got it from bacteria at all, although one similarly positioned intron is a lot easier to explain away as convergent evolution.

As I see it, either the last common ancestor of molluscs+annelids and deuterostomes had msp130 genes and only a few of its descendants kept them, or one of the two lineages snatched it from the other after those seven introns had originated. (Animals stealing genes from other animals is relatively uncommon, as far as I know.)

…some answers, anyway…

If you put the evidence for a single origin together with the incredibly gappy distribution of this gene family, the other side of the equation is a ridiculous number of losses. Why? And what’s the deal with msp130 and calcification? Is there a deal at all? Ettensohn speculated that acquiring msp130 might have had something to do with acquiring calcareous skeletons – did it?

IMO we really don’t have enough examples to properly assess this association, and my impression is that we actually know very little about the roles of these genes. Oh, we know that some of them are pretty specific to calcification in certain echinoderms, and they seem to be around in multiple organs in molluscs given that the hundreds of RNA sequences I found had been extracted from anything from gonads to mouthparts. And, of course, at least one of them is doing something in a partly calcified body part in my annelid, though we haven’t yet checked exactly where or what.

But calcification is pretty much the only context in which msp130s have been investigated; since everyone thought they were just echinoderm “calcification genes”, no one thought to look elsewhere. What do they do in, say, lancelets, which have six of the genes but not much of a calcified skeleton that we know of? Lancelets may well have something calcareous that isn’t a skeleton – other animals with no obvious calcareous skeletons, such as arachnids or earthworms, produce little calcareous granules that might work to store calcium or get rid of a surplus. Most of the limpet transcripts I found come from testicles or ovaries, which don’t tend to calcify, but gonads are a bit special and turn on lots of random genomic shit that may or may not actually have a function. AFAIK, none of the three algae from which msp130 genes are known has a calcareous skeleton, but many other algae do.

In summary, I did some detective work and discovered something and I feel rather clever about all of that, but in the process I learned just how much more we don’t know about this obscure but intriguing little gene family.

… actually, that sounds like a fairly typical summer in science. :)

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*Don’t ask me how that happened (it’s not even remotely my area), but now that the system exists, it does enable eukaryotes to make loads of different proteins from a single gene just by picking and choosing which exons to keep. See fruit fly Dscam, or the “brutally murdering the one gene, one protein hypothesis, forty thousand splice variants at a time” gene. Introns can also contain a variety of regulatory sequences that determine either the behaviour of their own gene or even that of a different gene, so introns are far from useless. They’re just a bit… counterintuitive.

**Aligning similar sequences is part science, part art. Often, there’s no single clear best way to align two or more genes or proteins; the various programs people have written for this job will all come up with slightly different answers, and an experienced pair of eyes will probably want to tweak all of them. Whether introns are really in the same place in two genes can therefore be a bit ambiguous, depending on the degree of sequence similarity.

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References:

Anstrom JA et al. (1987) Localization and expression of msp130, a primary mesenchyme lineage-specific cell surface protein in the sea urchin embryo. Development 101:255-265

Carson DD et al. (1985) A monoclonal antibody inhibits calcium accumulation and skeleton formation in cultured embryonic cells of the sea urchin. Cell 41:639-648

Ettensohn CA (2014) Horizontal transfer of the msp130 gene supported the evolution of metazoan biomineralization. Evolution & Development 16:139-148

Illies MR et al. (2002) Identification and developmental expression of new biomineralization proteins in the sea urchin Strongylocentrotus purpuratus. Development Genes and Evolution 212:419-431

Livingston BT et al. (2006) A genome-wide analysis of biomineralization-related proteins in the sea urchin Strongylocentrotus purpuratus. Developmental Biology 300:335-348

Maeso I et al. (2012) Widespread recurrent evolution of genomic features. Genome Biology and Evolution 4:486-500

Szabó R & Ferrier DEK (2015) Another biomineralising protostome with an msp130 gene and conservation of msp130 gene structure across Bilateria. Evolution & Development 17:195-197

Tu Q et al. (2012) Gene structure in the sea urchin Strongylocentrotus purpuratus based on transcriptome analysis. Genome Research 22:2079-2087

Putting the cart before the… snake?

Time to reexamine some assumptions (again)! And also, talk about Hox genes, because do I even need a reason?

Hox genes often come up when we look for explanations for various innovations in animal body plans – the digits of land vertebrates, the limbless abdomens of insects, the various feeding and walking and swimming appendages of crustaceans, the strongly differentiated vertebral columns of mammals, and so on.

Speaking of differentiated vertebral columns, here’s one group I’d always thought of as having pretty much the exact opposite of them: snakes. Vertebral columns are patterned, among other things, by Hox genes. Boundaries between different types of vertebrae such as cervical (neck) and thoracic (the ones bearing the ribcage) correspond to boundaries of Hox gene expression in the embryo – e.g. the thoracic region in mammals begins where HoxC6 starts being expressed.

In mammals like us, and also in archosaurs (dinosaurs/birds, crocodiles and extinct relatives thereof), these boundaries can be really obvious and sharply defined – here’s Wikipedia’s crocodile skeleton for an example:

In contrast, the spine of a snake (example from Wikipedia below) just looks like a very long ribcage with a wee tail:

Snakes, of course, are rather weird vertebrates, and weird things make us sciencey types dig for an explanation.

Since Hox genes appear to be responsible for the regionalisation of vertebral columns in mammals and archosaurs, it stands to reason that they’d also have something to do with the comparative lack of regionalisation (and the disappearance of limbs) seen in snakes and similar creatures. In a now classic paper, Cohn and Tickle (1999) observed that unlike in chicks, the Hox genes that normally define the neck and thoracic regions are kind of mashed together in embryonic pythons. Below is a simple schematic from the paper showing where three Hox genes are expressed along the body axis in these two animals. (Green is HoxB5, blue is C8, red is C6.)

Cohn_Tickle1999_hoxRegions

As more studies examined snake embryos, others came up with different ideas about the patterning of serpentine spines. Woltering et al. (2009) had a more in-depth look at Hox gene expression in both snakes and caecilians (limbless amphibians) and saw that there are in fact regions ruled by different Hoxes in these animals, if a little fuzzier than you’d expect in a mammal or bird – but they don’t appear to translate to different anatomical regions. Here’s their summary of their findings, showing the anteriormost limit of the activity of various Hox genes in a corn snake compared to a mouse:

Woltering_etal2009-mouse_vs_snake

Such differences aside, both of the above studies operated on the assumption that the vertebral column of snakes is “deregionalised” – i.e. that it evolved by losing well-defined anatomical regions present in its ancestors. But is that actually correct? Did snakes evolve from more regionalised ancestors, and did they then lose this regionalisation?

Head and Polly (2015) argue that the assumption of deregionalisation is a bit stinky. First, that super-long ribcage of snakes does in fact divide into several regions, and these regions respect the usual boundaries of Hox expression. Second, ordinary lizard-shaped lizards (from which snakes descended back in the days of the dinosaurs) are no more regionalised than snakes.

The study is mostly a statistical analysis of the shapes of vertebrae. Using an approach called geometric morphometrics, it turned these shapes from dozens of squamate (snake and lizard) species into sets of coordinates, which could then be compared to see how much they vary along the spine and whether the variation is smooth and continuous or clustered into different regions. The authors evaluated hypotheses regarding the number of distinct regions to see which one(s) best explained the observed variation. They also compared the squamates to alligators (representing archosaurs).

The results were partly what you’d expect. First, alligators showed much more overall variation in vertebral shape than squamates. Note that that’s all squamates – leggy lizards are nearly (though not quite) as uniform as their snake-like relatives. However, in all squamates, the best-fitting model of regionalisation was still one with either three or four distinct regions in front of the hips/cloaca, and in the majority, it was four, the same number as the alligator had.

Moreover, there appeared to be no strong support for an evolutionary pattern to the number of regions – specifically, none of the scenarios in which the origin of snake-like body plans involved the loss of one or more regions were particularly favoured by the data. There was also no systematic variation in the relative lengths of various regions; the idea that snakes in general have ridiculously long thoraxes is not supported by this analysis.

In summary, snakes might show a little less variation in vertebral shape than their closest relatives, but they certainly didn’t descend from alligator-style sharply regionalised ancestors, and they do still have regionalised spines.

Hox gene expression is not known for most of the creatures for which vertebral shapes were analysed, but such data do exist for mammals (mice, here), alligators, and corn snakes. What is known about different domains of Hox gene activation in these three animals turns out to match the anatomical boundaries defined by the models pretty well. In the mouse and alligator, Hox expression boundaries are sharp, and the borders of regions fall within one vertebra of them.

In the snake, the genetic and morphological boundaries are both gradual, but the boundaries estimated by the best model are always within the fuzzy boundary region of an appropriate Hox gene expression domain. Overall, the relationship between Hox genes and regions of the spine is pretty consistent in all three species.

To finish off, the authors make the important point that once you start turning to the fossil record and examining extinct relatives of mammals, or archosaurs, or squamates, or beasties close to the common ancestor of all three groups (collectively known as amniotes), you tend to find something less obviously regionalised than living mammals or archosaurs – check out this little figure from Head and Polly (2015) to see what they’re talking about:

Head_Polly2015-phylogeny_of_spines

(Moving across the tree, Seymouria is an early relative of amniotes but not quite an amniote; Captorhinus is similarly related to archosaurs and squamates, Uromastyx is the spiny-tailed lizard, Lichanura is a boa, Thrinaxodon is a close relative of mammals from the Triassic, and Mus, of course, is everyone’s favourite rodent. Note how alligators and mice really stand out with their ribless lower backs and suchlike.)

Although they don’t show stats for extinct creatures, Head and Polly argue that mammals and archosaurs, not snakes, are the weird ones when it comes to vertebral regionalisation. For most of amniote evolution, the norm was the more subtle version seen in living squamates. It was only during the origin of mammals and archosaurs that boundaries were sharpened and differences between regions magnified. Nice bit of convergent/parallel evolution there!

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References:

Cohn MJ & Tickle C (1999) Developmental basis of limblessness and axial patterning in snakes. Nature 399:474-479

Head JJ & Polly PD (2015) Evolution of the snake body form reveals homoplasy in amniote Hox gene function. Nature 520:86-89

Woltering JM et al. (2009) Axial patterning in snakes and caecilians: evidence for an alternative interpretation of the Hox code. Developmental Biology 332:82-89

Wherein scientists DON’T spill blood over a Precambrian animal

Having gone through much of my backlog, I was going to post about pretty blue limpet shells, then I saw that people have been arguing over Haootia. You remember Haootia? It’s that Precambrian fossil with probable muscle impressions that looks kind of like a modern-day staurozoan jellyfish (living staurozoan Haliclystus californiensis by Allen Collins, Encyclopedia of Life; Haootia quadriformis reconstruction from Liu et al., 2014):

Liu_etal2014-haootia_recon

It’s pretty much a law of Precambrian palaeontology that no interpretation of a fossil can ever remain uncontested, and Haootia is no exception. Nonetheless, this might be the tamest debate anyone ever had about a Precambrian fossil, and it gives me all kinds of warm feels.

Good news: Miranda et al. (2015) don’t dispute that the fossils show muscle impressions. They don’t even dispute that they belong to a cnidarian-grade creature. However, they question some of the details of the muscular arrangement, which could have implications for what this creature was and how it functioned.

They don’t have much of an issue with the muscles that run along the stalk and arms. The main point of contention, as far as I can tell, is that the muscles that run around the body (called coronal muscles in modern jellies) are not that big in living staurozoans. Those are the muscles that regular jellyfish use to contract their bells while swimming, but staurozoans don’t swim and therefore don’t need huge coronal muscles.

By Liu et al.‘s (2014) reconstruction (see above), Haootia has pretty massive coronal muscles. Miranda et al. (2015) wonder whether this was really the case, or the deformation of the fossils combined with the subconscious influence of regular jellyfish misled the original authors. They offer an alternative reconstruction, in which most of the body musculature runs up and down rather than around the body wall:

Miranda_etal2015-haootia_alt_recon

However, they also entertain the possibility that Liu et al.‘s reconstruction is correct – in which case, they note, Haootia must have done something with those muscles. Did jellyfish-like pulsations somehow form part of its feeding method? Could this even be a precursor to the jellyfish way of swimming? Who knows!

Liu et al. (2015) gave the most amazing response – much of their short reply to Miranda et al.‘s comments is basically thanking them for all the extra information and insight. They seem really pleased that biologists who study living cnidarians are taking an interest in their fossils, and enthusiastic about fruitful discussions in the future. (I concur. Biologists and palaeontologists need to talk to each other!)

They did take another, closer look at Haootia and maintain that they still see a large amount of musculature running around the body. So perhaps this peculiar Precambrian animal was doing something peculiarly Precambrian that has few or no parallels in modern seas. “We must keep in mind,” they write,  “that some, or maybe most, Ediacaran body plans and feeding strategies may have been specifically adapted to Ediacaran conditions.”

Either way, the whole exchange makes me very warm and fuzzy – I love to see scientists having constructive debates and learning from each other. (I also love that Miranda et al. thank Alex Liu in their acknowledgements; they were so obviously not out to tear one another down.) Plus both teams agree that we DO have a cnidarian-type creature from the Precambrian, and we DO have lovely lovely muscle impressions. Here’s to nice people, and to the slowly sizzling fuse of the Cambrian explosion! :)

***

References:

Liu AG et al. (2014) Haootia quadriformis n. gen., n. sp., interpreted as a muscular cnidarian impression from the Late Ediacaran period (approx. 560 Ma). Proceedings of the Royal Society B 281:20141202

Liu AG et al. (2015) The arrangement of possible muscle fibres in the Ediacaran taxon Haootia  quadriformis. Proceedings of the Royal Society B 282:20142949

Miranda LS et al. (2015) Is Haootia quadriformis related to extant Staurozoa (Cnidaria)? Evidence from the muscular system reconsidered. Proceedings of the Royal Society B 282:20142396

Finally, that sponge ParaHox gene

ParaHox genes are a bit like the underappreciated sidekicks of Hox genes. Or little sisters, as the case may be, since the two families are closely related. Hox genes are probably as famous as anything in evo-devo. Being among the first genes controlling embryonic development to be (a) discovered, (b) found to be conserved between very distantly related animals, they are symbolic of the late 20th century evo-devo revolution.

ParaHoxes get much less attention despite sharing some of the most exciting properties of Hox genes. Like those, they are involved in anteroposterior patterning – that is, partitioning an embryo along its head to tail axis. Also like Hox genes, they are often neatly clustered in the genome, and when they are, they tend to be expressed in the same order (both in space and time) in which they sit in the cluster*. Their main ancestral roles for bilaterian animals seem to be in patterning the gut and the central nervous system (Garstang and Ferrier, 2013).

There are three known types of ParaHox gene, which are generally thought to be homologous to specific Hox subsets of Hox genes – by the most accepted scheme, Gsx is the closest sister of Hox1 and Hox2, Xlox is closest to Hox3, and Cdx to Hox9 and above. It is abundantly clear that Hoxes and ParaHoxes are closely related, but there has been a bit of debate concerning the number of genes in the ancestral gene cluster that gave rise to both – usually called “ProtoHox” (Garcia-Fernàndez, 2005).

Another big question about these genes is precisely when they originated, and in this regard, ParaHox genes are proving much more interesting than Hoxes. You see, there are plenty of animals with both Hox and ParaHox genes, which is what you’d expect given the ProtoHox hypothesis, but there are also animals with only ParaHoxes. If there really was a ProtoHox gene/cluster that then duplicated to give rise to Hoxes and ParaHoxes, then lone ParaHoxes (or Hoxes for that matter) shouldn’t happen – unless the other cluster was lost along the way.

So a suspiciously Gsx-like gene in the weird little blob-creature Trichoplax, which has nothing remotely resembling a Hox gene, was a big clue that (a) Hox/ParaHox genes might go back further in animal evolution than we thought, (b) the loss of the entire Hox or ParaHox cluster is totally possible**, despite how fundamental these genes appear to be for correctly building an animal.

I wrote (at length) about a study by Mendivil Ramos et al. (2012), which revealed that while Trichoplax had no Hox genes and only one of the three types of ParaHox gene, it preserved the more or less intact genomic neighbourhoods in which Hox and ParaHox clusters are normally situated. One of the more interesting results of that paper was that the one sponge genome available at the time – that of Amphimedon queenslandica, which had no trace of either Hox or ParaHox genes – also contained statistically significant groupings of Hox and ParaHox neighbour genes, as if it had a Hox neighbourhood and a ParaHox neighbourhood, but the Hoxes and ParaHoxes themselves had moved out.

That study thus pointed towards an intriguing hypothesis, previously championed by Peterson and Sperling (2007) based solely on gene phylogenies: sponges once did have Hox and ParaHox genes/clusters, which at least some of them later lost. This would essentially mean that the two gene clusters go straight back to the origin of animals if not further***, and we may never find any surviving remnant of the ancestral ProtoHox cluster, since the closest living relatives of animals have neither the genes nor their neighbourhoods (that we know of).

Hypotheses are nice, but as we know, they do have a tendency to be tragically slain by ugly facts. Can we further test this particular hypothesis about sponges? Are there facts that could say yay or nay? (Of course there are. I wouldn’t be writing this otherwise ;) )

I keep saying that we should always be careful when generalising from one or a few model organisms, that we ignore diversity in the animal kingdom at our own peril, and that “distantly related to us” = “looks like our distant ancestors” is an extremely dodgy assumption. Well, here’s another lesson in that general vein: unlike Amphimedon, some sponges have not just the ghosts of vanished ParaHox clusters, but intact, honest to god ParaHox genes!

It’s calcareous sponges again. Sycon ciliatum and Leucosolenia complicata, two charming little calcisponges, recently had their genomes sequenced (alas, they weren’t yet public last time I checked), and since then, there’s been a steady stream of “cool stuff we found in calcisponge genomes” papers from Maja Adamska’s lab and their collaborators. I’ve discussed one of them (Robinson et al., 2013), in which the sponges revealed their rather unhelpful microRNAs, and back in October (when I was slowly self-destructing from thesis stress), another study announced a couple of delicious ParaHoxes (Fortunato et al., 2014).

(Exciting as it is, the paper starts by tickling my pet peeves right off the bat by calling sponges “strong candidates for being the earliest extant lineage(s) of animals”… I suppose nothing can be perfect… *sigh*)

The study actually covers more than just (Para)Hox genes; it looks at an entire gene class called Antennapedia (ANTP), which includes Hoxes and ParaHoxes plus a handful of related families I’m far less interested in. Sycon and Leucosolenia don’t have a lot of ANTP genes – only ten in the former and twelve in the latter, whereas a typical bilaterian like a fruit fly or a lancelet has several times that number – but from phylogenetic analyses, these appear to be a slightly different assortment of genes from those present in Amphimedon, the owner of the first sequenced sponge genome. This picture is most consistent with a scenario in which all of the ANTP genes in question were present in our common ancestor with sponges, and each sponge lineage lost some of them independently. (You may not realise this until you start delving into the history of various gene families, but genes come and go a LOT in evolution.)

Sadly, many of the branches on these gene trees are quite wonky, including the one linking a gene from each calcisponge to the ParaHox gene Cdx. However, somewhat fuzzy trees are not the only evidence the study presents. First, the putative sponge Cdxes possess a little motif in their protein sequences that is only present in a handful of gene families within the ANTP class. If you take only these families rather than everything ANTP and make trees with them, the two genes come out as Cdx in every single tree, and with more statistical support than the global ANTP trees gave them. Another motif they share with all Hoxes, ParaHoxes and a few of their closest relatives, but not with other ANTP class families.

Second, at least the gene in Sycon appears to have the right neighbours (Leucosolenia was not analysed for this). Since the Sycon genome sequence is currently in pieces much smaller than whole chromosomes, only four or so of the genes flanking ParaHox clusters in other animals are clearly linked to the putative Cdx in the sponge. However, when the researchers did the same sort of simulation Mendivil Ramos et al. (2012) did for Amphimedon, testing whether Hox neighbours and ParaHox neighbours found across all fragments of the genome are (a) close to other Hox/ParaHox neighbours or randomly scattered (b) mixed or segregated, they once again found cliques of genes with little overlap, indicating the once-existence of separate Hox and ParaHox clusters.

Fortunato et al. (2014) also examined the expression of their newfound Cdx gene, and found it no less intriguing than its sequence or location in the genome, although their description in the paper is very limited (no doubt because they’re trying to cram results on ten genes into a four-page Nature paper). The really interesting activity they mention and picture is in the inner cell mass of the young sponge in its post-larval stages – the bit that develops into the lining of its feeding chambers. Which, Adamska’s team contend, may well be homologous to our gut lining. In bilaterians, developing guts are one of the major domains of Cdx and ParaHox genes in general!

So at least three different lines of evidence – sequence, neighbours and expression – make this picture hang together quite prettily. It’s incredibly cool – the turning on their heads of long-held assumptions is definitely the most exciting part of science, I say! On the other hand, it’s also a little disheartening, because now that everyone in the animal kingdom except ctenophores has definitive ParaHox genes and at least the empty seats once occupied by Hox genes, are we ever going to find a ProtoHox thingy? May it be that it’ll turn up in one of the single-celled beasties people like Iñaki Ruiz-Trillo are sequencing? That would be cool and weird.

The coolest twist on this story, though, would be to discover traces of ProtoHoxes in a ctenophore, since solid evidence for ProtoHox-wielding ctenophores would (a) confirm the strange and frankly quite dubious-sounding idea that ctenophores, not sponges, are the animal lineage farthest removed from ourselves, (b) SHOW US A FREAKING PROTOHOX CLUSTER. (*bounces* >_> Umm, * cough* OK, maturity can suck it :D ) However, given how horribly scrambled at least one ctenophore genome is (Ryan et al., 2013), that’s probably a bit too much to ask…

***

Notes

*Weirdly, the order of expression in time is the opposite of that of the Hox cluster. In both clusters, the “anterior” gene(s), i.e. Hox1-2 or Gsx, are active nearest the front of the embryo, but while anterior Hox genes are also the earliest to turn on, in the ParaHox cluster the posterior gene (Cdx) wakes up first. /end random trivia

**Of course we’ve long known that losing a Hox cluster is not that big a deal, but previously, all confirmed losses occurred in animals with more than one Hox cluster to begin with – a fish has plenty of Hox genes left even after chucking an entire set of them.

***With the obligatory ctenophore caveat

***

References

Fortunato SAV et al. (2014) Calcisponges have a ParaHox gene and dynamic expression of dispersed NK homeobox genes. Nature 514:620-623

Garcia-Fernàndez J (2005) The genesis and evolution of homeobox gene clusters. Nature Reviews Genetics 6:881-892

Garstang M & Ferrier DEK (2013) Time is of the essence for ParaHox homeobox gene clustering. BMC Biology 11:72

Mendivil Ramos O et al. (2012) Ghost loci imply Hox and ParaHox existence in the last common ancestor of animals. Current Biology 22:1951-1956

Peterson KJ & Sperling EA (2007) Poriferan ANTP genes: primitively simple or secondarily reduced? Evolution and Development 9:405-408

Robinson JM et al. (2013) The identification of microRNAs in calcisponges: independent evolution of microRNAs in basal metazoans. Journal of Experimental Zoology B 320:84-93

Ryan JF et al. (2013) The genome of the ctenophore Mnemiopsis leidyi and its implications for cell type evolution. Science 342:1242592

Hi, real world, again!

The Mammal has emerged from a thesis-induced supermassive black hole and a Christmas-induced food coma, only to find that in the month or so that she spent barely functional and buried in chapters covered in the supervisor’s dreaded Red Pen, things actually happened in the world outside. This, naturally, manifested in thousands of items feeling thoroughly neglected in RSS readers and email inboxes. (Jesus. How many times have I vowed never to neglect my RSS feed again? Oh well, it’s not like unemployment is such a busy occupation that I can’t deal with a measly two and a half thousand articles :-P )

… earlier tonight, the paragraph here said I wasn’t doing a proper post yet, “just pointing out” a couple of the cooler things I’ve missed. Then somehow this thing morphed into a 1000+ word post that goes way beyond “pointing things out”. It’s almost like I’ve been itching to write something that isn’t my thesis. >_>

So the first cool thing I wanted to “point out” is the genome paper of the centipede Strigamia maritima, which is a rather nondescript little beast hiding under rocks on the coasts of Northwest Europe. This is the first sequenced genome of a myriapod – the last great class of arthropods to remain untouched by the genome sequencing craze after many genomes from insects, crustaceans and chelicerates (spiders, mites and co.).  The genome sequence itself has been available for years (yay!), but its “official” paper (Chipman et al., 2014) is just recently out.

Part of the appeal of Strigamia – and myriapods in general – is that they are considered evolutionarily conservative for an arthropod. In some respects, the genome analysis confirms this. Compared to its inferred common ancestor with us, Strigamia has lost fewer genes than insects, for example. Quite a lot of its genes are also linked together similarly to their equivalents in distantly related animals, indicating relatively little rearrangement in the last 600 million years or so. But this otherwise conservative genome also has at least one really unique feature.

Specifically, this centipede – which is blind – has not only lost every bit of DNA coding for known light-sensing proteins, but also all known genes specific to the circadian clock. In other animals, genes like clock and period mutually regulate one another in a way that makes the abundance of each gene product oscillate in a regular manner (this is about the simplest graphical representation I could find…). The clock runs on a roughly daily cycle all by itself, but it’s also connected to external light via the aforementioned light-sensing proteins, so we can constantly adjust our internal rhythms according to real day-night cycles.

There are many blind animals, and many that live underground or otherwise find day and night kind of irrelevant, but even these are often found to have a functioning circadian clock or keep some photoreceptor genes around. However, based on the genome data, our favourite centipede may be the first to have completely lost both. The authors of the genome paper hypothesise that this may be related to the length of evolutionary time the animals have spent without light. Things like mole rats are relatively recent “inventions”. However, the geophilomorph order of centipedes, to which Strigamia belongs, is quite old (its most likely sister group is known from the Carboniferous, so they’re probably at least that ancient). Living geophilomorphs are all blind, so chances are they’ve been that way for the last 300+ million years.

Nonetheless, the authors also note that geophilomorphs are still known to avoid light – the question now is how the hell they do it… And, of course, whether Strigamia has a clock is not known – only that it doesn’t have the clock we’re used to. We also have no idea at this point how old the gene losses actually are, since all the authors know is that one other centipede from a different group has perfectly good clock genes and opsins.

In comparison with fruit flies and other insects, the Strigamia genome also reveals some of the ways in which evolutionary cats can be skinned in multiple ways. There is an immune-related gene family we share with arthropods and other animals, called Dscam. The product of this gene is involved in pathogen recognition among other things, and in flies, Dscam genes are divided into roughly 100 chunks or exons, most of which are are found in clusters of variant copies. When the gene is transcribed, only one of these copies is used from each such cluster, so in practical terms the handful of fruit fly Dscam genes can encode tens of thousands of different proteins, enough to adapt to a lot of different pathogens.

A similar arrangement is seen in the closely related crustaceans, although with fewer potential alternative products. In other groups – the paper uses vertebrates, echinoderms, nematodes and molluscs for comparison – the Dscam family is pretty boring with at most one or two members and none of these duplicated exons and alternative splicing business. However, it looks like insects+crustaceans are not the only arthropods to come up with a lot of DSCAM proteins. Strigamia might also make lots of different ones (“only” hundreds in this case), but it achieved this by having dozens of copies of the whole gene instead of performing crazy editing feats on a small number of genes. Convergent evolution FTW!

Before I paraphrase the entire paper in my squeeful enthusiasm (no, seriously, I’ve not even mentioned the Hox genes, and the convergent evolution of chemoreceptors, and I think it’s best if I shut up now), let’s get to something else that I can’t not “point out” at length: a shiny new vetulicolian, and they say it’s related to sea squirts!

Vetulicolians really deserve a proper discussion, but in lieu of a spare week to read up on their messiness, for now, it’s enough to say that these early Cambrian animals have baffled palaeontologists since day one. Reconstructions of various types look like… a balloon with a fin? Inflated grubs without faces? I don’t know. Drawings below (Stanton F. Fink, Wikipedia) show an assortment of the beasts, plus Yunnanozoon, which may or may not have something to do with them. Here are some photos of their fossils, in case you wondered.

Vetulicolians from Wiki

They’re certainly difficult creatures to make sense of. Since their discovery, they’ve been called both arthropods and chordates, and you can’t get much farther than that with bilaterian animals (they’re kind of like the Nectocaris of old, come to think of it…).

The latest one was dug up from the Emu Bay Shale of Australia, the same place that yielded our first good look at anomalocaridid eyes. Its newest treasure has been named Nesonektris aldridgei by its taxonomic parents (García-Bellido et al., 2014), and it looks something like this (Diego García-Bellido’s reconstruction from the paper):

Garcia-Bellido_etal2014-nesonektris_recon

In other words, pretty typical vetulicolian “life but not as we know it”, at first glance. Its main interest lies in the bit labelled “nc” in the specimens shown below (from the same figure):

García-Bellido_etal2014-nesonektris_notochords

This chunky structure in the animal’s… tail or whatever is a notochord, the authors contend. Now, only one kind of animal has a notochord: a chordate. (Suspicious annelid muscle bundles notwithstanding. Oh yeah, I also wanted to post on Lauri et al. 2014. Oops?) So if this thing in the middle of Nesonektris’s tail is a notochord, then at the very least it is more closely related to chordates than anything else.

Why do they think it is one? Well, there are several long paragraphs devoted to just that, so here goes a summary:

1. It’s probably not the gut. A gut would be the other obvious ID, but it doesn’t fit very well in this case. Structures interpreted as guts in other vetulicolians – which sometimes contain stuff that may be half-digested food – (a) start in the front half of the body, where the mouth is, (b) constrict and expand and coil and generally look much floppier than this, (c) don’t look segmented, (d) sometimes occur alongside these tail rod-like thingies, so probably aren’t the same structure.

2. It positively resembles modern half-decayed notochords. The notochords of living chordates are long stacks of (muscular or fluid-filled) discs, which fall apart into big blocks as the animal decomposes after death. Here’s what remains of the notochord of a lamprey after two months for comparison (from Sansom et al. (2013)):

Sansom_etal2013-adult_lamprey_notochord_d63

This one isn’t as regular as the blockiness in the fossils, I think, but that could just be the vetulicolians not being quite as rotten.

There is, of course, a but(t). To be precise, there are also long paragraphs discussing why the structure might not be a notochord after all. It’s much thicker than anything currently interpreted as such in reasonably clear Cambrian chordates, for one thing. Moreover, it ends right where the animal does, in a little notch that looks like a good old-fashioned arsehole. By the way, the paper notes, vetulicolian tails in general don’t go beyond their anuses by any reasonable interpretation of the anus, and a tail behind the anus is kind of a defining feature of chordates, though this study cites a book from the 1970s claiming that sea squirt larvae have a vestigial bit of proto-gut going all the way to the tip of the tail. (I suspect that claim could use the application of some modern cell labelling techniques, but I’ve not actually seen the book…)

… and there is a phylogenetic analysis, in which, if you interpret vetulicolians as deuterostomes (which impacts how you score their various features), they come out specifically as squirt relatives whether or not you count the notochord. I’m never sure how much stock to put in a phylogenetic analysis based on a few bits of anatomy gleaned from highly contentious fossils, but at least we can say that there are other things – like a hefty cuticle – beyond that notochord-or-not linking vetulicolians to a specific group of chordates.

Having reached the end, I don’t feel like this paper solved anything. Nice fossils either way :)

And with that, I’m off. Maybe next time I’ll write something that manages to be about the same thing throughout. I’ve been thinking that I should try to do more posts about broader topics rather than one or two papers (like the ones I wrote about ocean acidification or homology versus developmental genetics), but I’ve yet to see whether I’ll have the willpower to handle the necessary reading. I’m remarkably lazy for someone who wants to know everything :D

(Aside: holy crap, did I ALSO miss a fucking Nature paper about calcisponges’ honest to god ParaHox genes? Oh my god, oh my GOD!!! *sigh* This is also a piece of incredibly exciting information I’ve known for years, and I miss it when it actually comes out in a journal bloody everyone reads. You can tell I’ve been off-planet!)

References:

Chipman AD et al. (2014) The first myriapod genome sequence reveals conservative arthropod gene content and genome organisation in the centipede Strigamia maritima. PLoS Biology 12:e1002005

García-Bellido DC et al. (2014) A new vetulicolian from Australia and its bearing on the chordate affinities of an enigmatic Cambrian group. BMC Evolutionary Biology 14:214

Lauri A et al. (2014) Development of the annelid axochord: insights into notochord evolution. Science 345:1365-1368

Sansom RS et al. (2013) Atlas of vertebrate decay: a visual and taphonomic guide to fossil interpretation. Palaeontology 56:457-474

Because I couldn’t not post about Dendrogramma

And the deep sea surprises us yet again (photos of the type specimen of Dendrogramma enigmatica from Just et al. [2014]).

I totally ignored the original hype about these beasties. I saw them pop up on I Fucking Love Science the other day, read the headline, decided it was probably another annoyingly sensationalised news story about a moderately strange new species and went on with my life. (The fact that they kinda look like weird flatworms didn’t help) Well, now that I’ve seen the paper, I… nah, I don’t regret the decision to ignore the news story, because hyperbole like that headline about rewriting the tree of life drives me up the wall, but I am glad that I finally checked what the hype was all about.

It’s really cool, after all these years of humanity cataloguing the living world, to find something so weird that basically all we can say about it is that it’s an animal. At this point it’s not clear to me how much of that is genuine weirdness and how much is simply down to the lack of data. The organisms were found in bulk seafloor samples brought up from depths of 400 and 1000 m somewhere off Tasmania nearly thirty years ago, and they are apparently quite poorly preserved. There’s no DNA, though commenters on the PLoS article seem to think it might be possible to get some out of the specimens. (That would be nice!)

According to the authors’ description, the general organisation of Dendrogramma species can be discerned and is much like a cnidarian or a ctenophore – two basic germ layers with thick jelly in between, and a blind gut – but they appear to lack anything that would clearly identify them as a member of either group, such as comb rows or stinging cells. Because they appear to have only two germ layers, the authors conclude they are probably not bilaterians, but because they don’t have diagnostic features of any other kind of animal, and because there’s so much more we don’t know about them, they don’t feel brave enough to place them beyond that.

The beasties are made of a stalk and a flat disc; the mouth opens at the tip of the stalk and the gut extends into the disc, where it bifurcates repeatedly to form dozens of branches. Two comments on the PLoS website point out that this arrangement is a bit like a flatworm – many of which have a long pharynx that they can poke out to feed, and a highly branched intestine occupying most of the body (a lovely diagram and photo can be found in the bottom half of this page).

Superficially at least, it sounds possible that Dendrogramma‘s “stalk” is an extended pharynx. However, flatworms are bilaterians, and between their skin and their gut wall they are full of the tissues of the mesoderm, the third germ layer – muscles, simple kidneys, reproductive organs and quite a lot of cell-rich connective tissue. Just et al.‘s description of Dendrogramma states that the equivalent space in these creatures is filled with mesogloea, i.e. jelly with few or no cells. If Dendrogramma really lacks mesodermal tissues, then it wouldn’t make a very good flatworm! (The paper itself doesn’t discuss the flatworm option at all, presumably for similar reasons.)

Of course, the thing that piqued my interest in Dendrogramma is its supposed resemblance to certain Ediacaran fossils, specifically these ones. It would be awesome if we could demonstrate that the living and the fossil weirdos are related, since then determining what Dendrogramma is would also classify the extinct forms, but I’m not holding my breath on this count. The branching… whatevers in the fossils in question may look vaguely like the branching gut of Dendrogramma, but, as discussed above, so do flatworm guts. The similarity to the fossils may well have nothing to do with actual phylogenetic relatedness, which the authors sound well aware of.

Nature, helpful as always. >_>

It seems all we can do for the moment is wait for more material to come along, hopefully in a good enough state to make detailed investigations including genetic studies. My inner developmental biologist is also praying for embryos, but the gods aren’t generally kind enough to grant me these sorts of wishes :-P

I do quite like the name, though. Mmmmm, Dendrogramma. :)

***

Reference:

Just J et al. (2014) Dendrogramma, new genus, with two new non-bilaterian species from the marine bathyal of southeastern Australia (Animalia, Metazoa incertae sedis) – with similarities to some medusoids from the Precambrian Ediacara. PLoS ONE 9:e102976

Precambrian muscles??? Oooooh!

Okay, consider this a cautious squee. I wish at least some of those Ediacaran fossils were a little more obvious. I mean, I might love fossils, but I’m trained to squirt nasty chemicals on bits of dead worm and play with protein sequences, not to look at faint impressions in rock and see an animal.

Most putative animals from the Ediacaran period, the “dark age” that preceded the Cambrian explosion, are confusing to the actual experts, not just to a lab/computer biologist with a fondness for long-dead things. The new paper by Liu et al. (2014) this post is about lists a “but see” for pretty much every interpretation they cite. The problem is twofold: one, as far as I can tell, most Ediacaran fossils don’t actually preserve that much interpretable detail. Two, Ediacaran organisms lived at a time when the kinds of animal body plans we’re familiar with today were just taking shape. The Ediacaran is the age of ancestors, and it would be more surprising to find a creature we can easily categorise (e.g. a snail) than a weird beastie that isn’t quite anything we know.

Having said that, Liu et al. think they are able to identify the new fossil they named Haootia quadriformis. Haootia comes from the well-known Fermeuse Formation of New Foundland, and is estimated to be about 560 million years old. The authors say its body plan – insofar as it can be made out on a flat image pressed into the rock – looks quite a lot like living staurozoan jellyfish, with a four-part symmetry and what appear to be branching arms or tentacles coming off the corners of its body. The most obvious difference is that Haootia seems to show the outline of a huge circular holdfast that’s much wider than usual for living staurozoans.

However, the most exciting thing about this fossil is not its shape, but the fact that most of it is made up of fine, highly organised parallelish lines – what the authors interpret as the impressions of muscle fibres. The fibres run in different directions according to their position in the body; for example, they seem to follow the long axes of the arms.

(Below: the type specimen of Haootia with some of the fibres visible, and various interpretive drawings of the same fossil. Liu et al. is a free paper, so anyone can go and look at the other pictures, which include close-ups of the fibres and an artistic reconstruction of the living animal.)

If the lines do indeed come from muscle fibres, then regardless of its precise affinities, Haootia is certainly an animal, and it is probably at least related to the group called eumetazoans, which includes cnidarians like jellyfish and bilaterians like ourselves (and maybe comb jellies, but let’s not open that can of jellies just now). Non-eumetazoans – sponges and Trichoplax – do not have muscles, and unless comb jellies really are what some people think they are, we can be almost certain that the earliest animals didn’t either.

Finding Ediacaran muscles is also interesting because it gives us further evidence that things capable of the kinds of movement attributed to some Ediacaran fossils really existed back then. Of course, it would have been nicer to find evidence of muscle and evidence of movement in the same fossils, but hey, this is the Precambrian. You take what you get.

(P.S.: Alex Liu is cool and I heart him. OK, I saw him give one short talk, interviewing for a job at my department that he didn’t get *sniffles*, so maybe I shouldn’t be pronouncing such fangirlish judgements, but that talk was awesome. As I’ve said before, my fangirlish affections are not very hard to win :) )

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Reference:

Liu AG et al. (2014) Haootia quadriformis n. gen., n. sp., interpreted as a muscular cnidarian impression from the Late Ediacaran period (approx. 560 Ma). Proceedings of the Royal Society B 281:20141202

A bit of Hox gene nostalgia

I had the most random epiphany over my morning tea today. I don’t even know what got me thinking about the Cambrian explosion (as if I needed a reason…). Might have been remembering something from the Euro Evo Devo conference I recently went to. (I kind of wanted to post about that, because I saw some awesome things, but too much effort. My brain isn’t very cooperative these days.)

Anyway.

I was thinking about explanations of the Cambrian explosion and remembering how the relevant chapter in The Book of Life (otherwise known as the book that made me an evolutionary biologist)  tried to make it all about Hox genes. It’s an incredibly simplistic idea, and almost certainly wrong given what we now know about the history of Hox genes (and animals)*. At the time, and for a long time afterwards, I really wanted it to be true because it appeals to my particular biases. But I digress.

Then it dawned on me just how new and shiny Hox genes were when this book was written. I thought, holy shit, TBoL is old. And how far evo-devo as a field has come since!

The Book of Life was first published in 1993. That is less than a decade after the discovery of the homeobox in fruit fly genes that controlled the identity of segments (McGinnis et al., 1984; Scott and Weiner, 1984), and the finding that homeoboxes were shared by very distantly related animals (Carrasco et al., 1984). It was only four years after the recognition that fly and vertebrate Hox genes are activated in the same order along the body axis (Graham et al., 1989; Duboule and Dollé, 1989).

This was a HUGE discovery. Nowadays, we’re used to the idea that many if not most of the genes and gene networks animals use to direct embryonic development are very ancient, but before the discovery of Hox genes and their clusters and their neatly ordered expression patterns, this was not at all obvious. What were the implications of these amazing, deep connections for the evolution of animal form? It’s not surprising that Hox genes would be co-opted to explain animal evolution’s greatest mysteries.

It also occurred to me that 1993 is the year of the zootype paper (Slack et al., 1993). Slack et al. reads like a first peek into a brave new world with limitless possibilities. They first note the similarity of Hox gene expression throughout much of the animal kingdom, then propose that this expression pattern (their “zootype”) should be the definition of an animal. After that, they speculate that just as the pattern of Hox genes could define animals, the patterns of genes controlled by Hoxes could define subgroups within animals. Imagine, they say, if we could solve all those tough questions in animal phylogeny by looking at gene expression.

As always, things turned out More Complicated, what with broken and lost Hox clusters and all the other weird shit developmental “master” genes get up to… but it was nice to look back at the bright and simple childhood of my field.

(And my bright and simple childhood. I read The Book of Life in 1998 or 1999, not entirely sure, and in between Backstreet Boys fandom, exchanging several bookfuls of letters with my BFF and making heart-shaped eyes at long-haired guitar-playing teenage boys, I somehow found true, eternal, nerdy love. *nostalgic sigh*)

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*Caveat: it’s been years since I last re-read the book, and my copy is currently about 2500 km from me, so the discussion of the Cambrian explosion might be more nuanced than I remember. Also, my copy is the second edition, so I’m only assuming that the Hox gene thing is there in the original.

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References:

Carrasco AE et al. (1984) Cloning of an X. laevis gene expressed during early embryogenesis coding for a peptide region homologous to Drosophila homeotic genes. Cell 37:409-414

Duboule D & Dollé P (1989) The structural and functional organization of the murine HOX gene family resembles that of Drosophila homeotic genes. The EMBO Journal 8:1497-1505

Graham A et al. (1989) The murine and Drosophila homeobox gene complexes have common features of organization and expression. Cell 57:367-378

McGinnis W et al. (1984) A conserved DNA sequence in homoeotic genes of the Drosophila Antennapedia and bithorax complexes. Nature 308:428-433

Scott MP & Weiner AJ (1984) Structural relationships among genes that control development: sequence homology between the Antennapedia, Ultrabithorax, and fushi tarazu loci of Drosophila. PNAS 81:4115-4119

Slack JMW et al. (1993) The zootype and the phylotypic stage. Nature 361:490-492