Heads are rolling in Nature

The Nature website has been overrun with headless flatworms, my RSS feed tells me! These adorable guys called planarians are known for their ability to regenerate from almost any little scrap of their squishy bodies. (And yes, I find lots of things adorable.) More than a century ago, TH Morgan, who invented like half of modern biology, observed that a whole planarian complete with a head, tail, eyes, brain, and digestive system can regrow itself from a fragment as tiny as 1/279th of the original animal*.

If you’re not familiar with these neat creatures, here’s one of regeneration expert Alejandro Sánchez Alvarado’s pet planarians from Wikipedia:

(OK, “pet” is probably the wrong word. Unless your concept of pets involves frequent mutilation and poisoning.)

Of course, once humanity discovered that planarians can do that, we just had to try and figure out how. Solana_etal2012-ctrlheadWho knows, one day the knowledge might even help us boost our own pathetic regeneration abilities. (Well, aside from the fact that planarian regeneration is pretty weird compared to what vertebrates do. But they are dead easy to keep and you can do lots of fancy things with them.) I felt like I should include some pictures of head regeneration in action, so here’s a few shots from Solana et al. (2012). Watch the eyes!

When you chop bits off a planarian, the remainder of the body has to know a couple of things to repair itself correctly. First, of course, it has to recognise that something is missing. This happens when tissues that don’t normally meet come into contact as the wound closes. The system can be fooled – if you cut off a piece of worm, turn it upside down, and stick it back on, things start growing out even though technically nothing is missing (Kato et al., 1999). The next step is to recognise precisely what needs to be replaced. An example of failing at this step is this two-headed flatworm obtained from a chemical treatment (from Nogi et al. [2012] via Wikipedia):

Making heads or tails

So how does a headless planarian know whether it needs a new head or tail? There’s a venerable theory (apparently also TH Morgan’s) according to which the body of the animal has a sort of built-in molecular coordinate system. Some molecules are more abundant at one end of the beast, while different molecules mark the other end. The anterior (head) and posterior (tail) signals would interact negatively, banishing each other from their respective head(or tail)quarters and resulting in opposing gradients. So any particular point along the head to tail axis would have a precise level of “headness” and “tailness”, and a wounded worm would “know” exactly where it was cut based on this information.

The tail end has long been known to be a seat of an ancient signalling pathway. Wnt (pronounced “wint”) genes are really, really important in a variety of developmental processes; in fact, it’s been proposed that they were involved in defining the head to tail axes of animals long before the more famous Hox genes (Guder et al., 2006). (The merits of that proposition are a discussion for another day. :)) Similar to their axis-defining developmental role, they – or rather, one of the several pathways they act through – also signal “tail” in adult planarians (Gurley et al., 2008).

Butts rule?

In one of the three new planarian studies, Umesono et al. (2013) set out mainly to figure out how the less well-understood head signal worked, but they managed to chuck in something vastly more interesting (to me, evolution nerd that I am) in an almost throwaway paragraph towards the end of the paper. Not all planarians have awesome regeneration superpowers. In particular, many species have difficulty regrowing heads while they can still regenerate tails just fine. Umesono et al. found out why.

Knowing how important Wnt signalling is in making tails, they wondered if an over-enthusiastic Wnt system might be behind some species’ head regeneration defects. They took members of such a species, demolished their Wnt pathway by hijacking their own gene regulatory mechanisms, and proceeded to hack off their heads. A couple of weeks later, shiny new heads started appearing!

It’s not just that one species, either. The other two new headless worm studies (Sikes and Newmark, 2013; Liu et al., 2013) basically did the exact same thing with two other kinds of regeneration-deficient planarian, and got the exact same result. So it looks like the same failure to overcome tailness underlies head regeneration failure in these three species.

The latter two papers examined worms from the same family, and the two animals proved to fail at regeneration in eerily similar ways. Everything up to a point goes correctly: the wound heals properly, stem cells across the body start dividing and gather at the amputation site… and then it stops. Wnts run rampant, heady genes remain silent, and nothing regenerates.

What I’d like to know is why it’s nothing rather than a second tail. After all, the molecular makeup of their wounded parts is screaming “tail!”, and they can regenerate missing tail ends. If you overactivate Wnt signalling in better regenerators among planarians, you still get something growing out at the front, it’s just not a very good head. (Umesono et al., in fact, did a couple of experiments like that in the process of figuring out heads.)

I thought the key was in the Umesono paper, because their prime suspect for “head stuff” as they call it is actually briefly needed in tail regeneration as well, so if it’s not activated at all due to too much Wntiness, then nothing will happen. But if you can turn that on in the middle of a tail in one species, why can’t the same thing happen in the others, which are also perfectly capable of regenerating their tails? Does tail regeneration work differently in them?

Did I mention that living things are complicated?

(And this is the part where I don’t start discussing the whole issue of how and why regeneration is lost in evolution. ‘Tis beautiful, but the night’s getting old ;))

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*I can’t for the life of me find the original source, even though I distinctly remember having read Morgan’s account. The 1/279th figure is cited, among others, by Newmark and Sánchez Alvarado (2001).

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References:

Guder C et al. (2006) The Wnt code: cnidarians signal the way. Oncogene 25:7450-7460

Gurley KA et al. (2008) β-catenin defines head versus tail identity during planarian regeneration and homeostasis. Science 319:323-327

Kato K et al. (1999) The role of dorsoventral interaction in the onset of planarian regeneration. Development 126:1031-1040

Liu SY et al. (2013) Reactivating head regrowth in a regeneration-deficient planarian species. Nature advance online publication 24/07/2013, doi: 10.1038/nature12414

Newmark PA and Sánchez Alvarado A (2001) Regeneration in Planaria. In: Encyclopedia of Life Sciences. John Wiley & Sons Ltd, Chichester. http://www.els.net; doi: 10.1038/npg.els.0001097

Nogi T et al. (2009) A novel biological activity of praziquantel requiring voltage-operated Ca2+ channel β subunits: subversion of flatworm regenerative polarity. PLoS Neglected Tropical Diseases 3:e464

Sikes JM & Newmark PA (2013) Restoration of anterior regeneration in a planarian with limited regenerative ability. Nature advance online publication, 24/07/2013, doi: 10.1038/nature12403

Solana J et al. (2012) Defining the molecular profile of planarian pluripotent stem cells using a combinatorial RNA-seq, RNA interference and irradiation approach. Genome Biology 13:R19

Umesono Y et al. (2013) The molecular logic for planarian regeneration along the anterior-posterior axis. Nature advance online publication, 24/07/2013, doi: 10.1038/nature12359

Ocean acidification is complicated, case in point

I once wrote about the complicated way in which ocean acidification is mostly really bad for marine creatures with calcium carbonate shells/skeletons. Well, today, while reading a book I thought had nothing to do with ocean acidification, I came across a report of one such creature for whom the change is apparently for the better. (I’d expected to find all kinds of interesting information in Embryos in Deep Time, but this was a surprise…)

Dupont et al. (2010) studied common sun stars (above; Bernard Picton, habitas.org.uk), following the larvae right up to metamorphosis under current CO2 and pH values of their home seas, and also under a near-future predicted scenario with higher CO2 concentration and lower sea pH. Surprisingly, the larvae in the “future” tanks survived just as well, grew better, and showed no obvious defects in development or calcification compared to the control group.

The authors speculate that this might be related to the reproductive strategy of these animals. While the larvae of many echinoderms have very little yolk in their eggs and have to feed the moment they look vaguely like an animal, sun star larvae are provided with a lot of yolk that can sustain them until they’re ready to metamorphose. So they don’t have to face the burdens of hunting for food; all their energy can go towards growing, which might make them more resilient to harmful environmental effects.

I’m not sure I buy such a simplistic explanation – first, other echinoderms with a similar developmental strategy suffer quite badly in similar conditions; and second, they only examined one species during the early stage of its life cycle. In fact, the authors point out these exact same caveats. (Plus the creatures not only resisted acidification, they thrived.)

Whatever the mechanism, though, Dupont et al.‘s data show that there is at least one animal for which ocean acidification may be a boon. Considering that this guy happens to be a top predator in its ecosystem, that could have major consequences for said ecosystem.

Also, they are incredibly pretty. Echinoderms rock.

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Reference

Dupont S et al. (2010) Near future ocean acidification increases growth rate of the lecithotrophic larvae and juveniles of the sea star Crossaster papposus. Journal of Experimental Zoology 314B:382–389

Take two

So, the post that WordPress ate earlier today was me squealing like a tween over some baby worms. Specifically, these ones (Gibson and Paterson, 2003):

GibsonPatterson2003-amphipolydoraBabies

Don’t you just want to cuddle them?

The adorable little slug-creatures with their cute little dot eyes are the larvae of a small polychaete worm called Amphipolydora vestalis. The adult worms build muddy tubes inside some poor unfortunate sponge in the waters of New Zealand (Paterson and Gibson, 2003). Females lay their eggs in an egg capsule within the tube, and add some extra eggs filled with yolk for the babies’ nourishment (Gibson and Paterson, 2003). The larvae in these pictures are about a week old, and they are bulging with all that yummy egg stuff they’ve been eating.

By the time they hatch from the capsule and leave to set up their own tube, they no longer look morbidly obese (or all that cute), and appear more like a weird alien species with four eyes in a row, hairy “legs” everywhere, and a pair of nice long tentacle things (technically “palps”) sprouting off their heads. These worms and others of the spionid family use the palps to collect tiny food particles (image from Gibson and Paterson [2003]):

GibsonPaterson2003-amphipolydoraHatchling

They eventually grow up into something like this (Hans Hillewaert, Wiki Commons):

(This is a related species; good photos of adult Amphipolydora are kind of non-existent.)

Did I mention I love polychaetes? (Not that I work on one or anything…)

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References:

Gibson GD & Paterson IG (2003) Morphogenesis during sexual and asexual reproduction in Amphipolydora vestalis (Polychaeta: Spionidae). New Zealand Journal of Marine and Freshwater Research 37:741-752

Paterson IG & Gibson GD (2003) A new species of Amphipolydora (Polychaeta: Spionidae) from New Zealand. New Zealand Journal of Marine and Freshwater Research 37:733-740

The post-eating monster

This is the second time I wrote an entry, pressed publish, and ended up publishing a completely empty post. Why is this thing eating my carefully written blathers?

At least last time I had a saved version I could start again from. This one just disappeared into the aether.

I am sad and pissed and all kinds of grumpy now. Probably not a good time to ask tech support about the problem.

I might be back once I stopped screaming obscenities inside my head.

I couldn’t resist

Damn, I said I wasn’t going to talk about the Moroccan helicoplacoid-on-stalk, but it’s just so. Bloody. Amazing.

Here it is in its full glory, from the supplementary figures of Smith and Zamora (2013). Left is a cast of a young specimen, right is the authors’ reconstruction of the adult creature:

helicocystis_casthelicocystis_recon

So… the thing is a transitional form all right. It’s got a little stalk and cup like eocrinoids, built with a rather irregular arrangement of mineralised plates. On top of that it has a spiral body like helicoplacoids. It has ambulacra, the “rays” with porous plates where the tube feet that characterise living echinoderms can come out. This photo of the underside of a starfish is a pretty nice illustration of ambulacra (the white regions with little holes) and tube feet:

Even more interestingly, the new beastie (christened Helicocystis moroccoensis by the authors) seems to have five of them, like modern echinoderms (and a lot of extinct types, including eocrinoids). Helicoplacoids do have ambulacra, but only three or a single Y-shaped one, depending on interpetation.

Again unlike (one interpretation of) helicoplacoids but like modern echinoderms, the mouth of Helicocystis is right at the stalkless end. It’s also surrounded by an arrangement of skeletal plates that resembles more “conventional” echinoderms and has no equivalent in helicoplacoids proper. It’s about as neat a transitional form as you could hope for.

The question is which way the transition goes. It could be that the familiar five-rayed echinoderms are derived from a helicoplacoid-like ancestor, going through something like this guy. Or it could be that helicoplacoids are actually weird even for echinoderms, and their ancestors were more conventional stalked, five-armed beasties that lost their proper echinoderm shapes via something like Helicocystis.

Smith and Zamora actually did a phylogenetic analysis, but it’s not that helpful IMO. The tree in the paper is very pretty, and it says Helicocystis is the next branch after helicoplacoids on the path leading to “proper” echinoderms. The tree in the supplementary figures actually has measures of statistical support on it – which pretty confidently put Helicoplacus, Helicocystis, and a bunch of less weird echinoderms, together.

However, the relationships within that group are, shall we say, a little bit fluid. Granted, I come from a more sequency background and don’t often have to deal with morphology-based trees or parsimony as the method of analysis – but I’d definitely view a 56% bootstrap support with a big dose of scepticism, and this is the number they got for the hypothesis that Helicocystis is more closely related to “proper” echinoderms than to Helicoplacus. The other measure they display doesn’t make me any more confident about the relationship.

(I find it kind of amazing they got any resolution at all in that tree – with only 17 characters, some of which aren’t applicable to all species, and only nine species to begin with… yeah. The whole phylogenetic analysis is far from ideal even if it’s the best they could think of.)

So, based on that tree, the phylogenetic hypothesis they present is, at this point, just a plausible hypothesis. That doesn’t lessen the value of Helicocystis, though. The creature is still a damn neat transitional form – we just can’t be terribly sure which way the transition went.

There’s some interesting speculation in the paper about developmental evolution (yay!). Smith and Zamora point out that the spirally bit in Helicocystis looks like a complete helicoplacoid; the stalk and cup are kind of tacked onto that. The tissues of most modern echinoderm adults come from two different places: regular old tissues of the larva, and a special set of cells set aside for adult-making purposes*. So Smith and Zamora hypothesise that the two-part body of Helicocystis marks the point where this dual origin appeared. (Or, if they’re wrong about the phylogeny, the point where proto-helicoplacoids lost it?)

There’s also another interesting bit of evo-devo speculation (mixed with a bit of “eco”) about the stalk. Full-grown Helicocystis have pretty small stalks compared both to their own young and more typical stalked echinoderms. The authors wonder if this is because stalks for attachment originally functioned to help young echinoderms settle in a comfortable place, and only later became important for adults. I’m not sure how much sense that actually makes, and of course we only have a single species of Helicocystis to go by, but hey, ideas are fun.

Helicocystis has a random weird quirk as well, in that its spirals curl the opposite way to every proper helicoplacoid. That sort of variation happens even within species (e.g. in snail shells), but isn’t it a weird coincidence that such a unique creature should also twist the wrong way?

One thing is for sure: this beast is made of pure, distilled awesome. I think we should make a new Archaeopteryx out of it. Invertebrates need their evolutionary icons, too!

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*And that’s a nice reminder for me, because I thought they basically threw away the larva. Apparently I need a refresher on echinoderm development. Or just a reminder that not all echinoderms are sea urchins. The funny thing is a couple of years ago I actually specifically read and puzzled over literature discussing what comes from where in various echinderms…

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Reference:

Smith AB, Zamora S (2013) Cambrian spiral-plated echinoderms from Gondwana reveal the earliest pentaradial body plan. Proceedings of the Royal Society B advance online publication 26/06/2013, doi:10.1098/rspb.2013.1197

Petrified strawberries and the cnidarian that isn’t

In the last few weeks, tons of squee-worthy stuff has accumulated on my backlog. The echinoderm transitional form from Cambrian Morocco I got so excited about is now officially described (Smith and Zamora, 2013), Dennis Duboule and his team put out some really cool findings about how vertebrate Hox clusters work that connect to my old fascination with limb evo-devo (Andrey et al., 2013), the developmental hourglass returned (Schep and Adryan, 2013)…

I kind of regret not gushing about all of them, but let’s face it, I’m not gonna ever do that. Looking over the things I bookmarked recently, I decided I’d rather not ignore Yasui et al. (2013), though. One, it’s about early animals, two, it exploits one of the greatest treasures the fossil record has to offer: the record of ancient development. I almost don’t care what the findings are, because the fact that we can follow a 530+-million-year-old creature from egg to adult is so staggeringly awesome in itself that everything else pales in comparison.

The paper looks at a tiny creature known from the earliest Cambrian of China. The beastie is called Punctatus and looks something like this (the authors’ interpretation of its development from fig. 4 of the paper):

The observations in this paper come from some 10 thousand specimens of various developmental stages from a couple of different Punctatus species. With such an abundance of fossils covering the animal’s life cycle, it is possible to connect the different stages and identify them as the same animal. So how did Punctatus develop and what kind of animal was it?

The earliest development took place inside a smooth egg membrane. Broken or CT-scanned embryos show that the creatures went through a nice blastula stage that looked like a simple, hollow ball of cells which were maybe a bit fatter on one side than the other (below, left). This type of blastula is quite common, found in animals as disparate as jellyfish (middle, from celldynamics.org) and sea urchins (right, from exploratorium). So the blastulae don’t tell you much about the affinities of the creature. In fact, while the authors use the leftmost embryo as a pretty illustration, they’re not even sure this particular specimen belongs to Punctatus. (Which is not really surprising.)

coeloblastulae

By the time young Punctatus hatch from their eggs, they are much more identifiable. The authors compare them to strawberries (awwww! ^.^). They are spiny all over, slightly pointy on one end and slightly flattened on the other, and the flattened end is divided into five parts by a star-like pattern of Y-shaped grooves. At the centre of the star, there is the blastopore, the opening of the embryonic gut, which seems to develop straight into the mouth in this creature. Punctatus never develops another gut opening. The simple blastopore = mouth equation again isn’t terribly informative, since a lot of animals follow it, and it’s the most straightforward way to make a mouth. The only thing the lack of a through gut tells us is something that was already fairly obvious – Punctatus is not a bilaterian.

The early stages also exclude another group from the list of possible identities, that is ctenophores. Early embryos of modern ctenophores (comb jellies/sea gooseberries) have very unequal-sized cells (see image at the top of this article), and no such embryos are known from the deposits preserving Punctatus specimens. (Although given what I recently learned about living ctenophores having a very recent common ancestor, I wouldn’t bet on what their Cambrian ancestors were up to…)

Thirdly, embryos that haven’t yet hatched also tell us something important about the adults. The prickly covering of these animals had apparently been interpreted as the remnants of a tube in which the animal proper lived – but this covering clearly appears before the baby even pops out of the egg, and the mouth forms right in the middle of it. All of that makes it more likely to be the animal’s skin. And that weakens a possible link to a group of extinct tube-dwelling animals that are much more plausibly related to jellyfish.

After hatching, development enters a new stage. In young and adult Punctatus specimens, the strawberry-like hatchling body remains in place, but a new body region appears at the mouth end, which has a ringed appearance and no spines. Presumably, individuals with more rings were older.

CT cross-sections of such specimens (C-E below) show a huge, empty body cavity, with a small sac-like gut attached to the mouth. There’s apparently no “stuff” between the gut and the body wall: no mesenteries anchoring the gut, no jelly or mass of cells filling in the body cavity, just big fat nothing. This is unlike not just bilaterians or ctenophores, but also cnidarians, in which the gut wall tends to be much closer to the body wall, and a jelly-like layer containing a varying amount of cells fills any gaps between the two.

From this point, the basic body plan doesn’t seem to change. Specimens with only a couple of rings and those with a dozen have the same small gut and large body cavity. There’s nothing we might call metamorphosis – unlike most cnidarians, Punctatus didn’t have a larval stage. (BTW, can someone tell me what the hell the lumpy bit on top of G above is? The paper doesn’t bother to explain as far as I could tell, and it bugs me.)

An intriguing (and rather pretty) part of the animal is the mouth end, what the authors call the “oral ruffle”. You’ll see why it’s called that if you look at figure 3:

This is an inferred developmental series of the mouth region. The five-pointed star of the hatchlings develops into ten finely striped folds emerging from the body surface, and as the animal grows, a new oral ruffle appears inside the previous one. The old ruffle then becomes part of the body wall, forming the next ring. Rinse and repeat. There are no tentacles at any point, although this might still turn out to be an artefact of preservation.

Tenfold symmetry, stacks old oral ruffles, no tentacles, building an adult body on top of an intact piece of embryo – the whole thing is quite unlike your typical cnidarian. Or, indeed, your typical anything else. The authors use the unusual developmental and body plan features of this creature to question its previous assignment to cnidarians, but beyond that, they are unsure what to make of it.

Well, this is the Early Cambrian, when a lot of now-extinct animal lineages were kicking around. Of course they would give us classification headaches! 😉 Which probably means that we know an awful lot about the development of a member of a long-extinct lineage. That’s a comparative embryology goldmine right there, folks!

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References:

Andrey G et al. (2013) A switch between topological domains underlies HoxD genes colinearity in mouse limbs. Science 340:1234167, doi:10.1126/science.1234167

Schep AN, Adryan B (2013) A comparative analysis of transcription factor expression during metazoan embryonic development. PLoS ONE 8:e66826

Smith AB, Zamora S (2013) Cambrian spiral-plated echinoderms from Gondwana reveal the earliest pentaradial body plan. Proceedings of the Royal Society B advance online publication 26/06/2013, doi:10.1098/rspb.2013.1197

Yasui K et al. (2013) A diploblastic radiate animal at the dawn of cambrian diversification with a simple body plan: distinct from Cnidaria? PLoS ONE 8: e65890