From the weeks during which I failed to check my RSS reader…
1. The coolest ribozyme ever. (In more than one sense.)
I’ve made no secret of my fandom of the RNA world hypothesis, according to which early life forms used RNA both as genetic material and as enzymes, before DNA took over the former role and proteins (mostly) took over the latter. RNA is truly an amazing molecule, capable of doing all kinds of stuff that we traditionally imagined as the job of proteins. However, coaxing it into carrying out the most important function of a primordial RNA genome – copying itself – has proven pretty difficult.
To my knowledge, the previous record holder in the field of RNA copying ribozymes (Wochner et al., 2011) ran out of steam after making RNA strands only half of its own length. (Which is still really impressive compared to its predecessors!) In a recent study, the same team turned to an alternative RNA world hypothesis for inspiration. According to the “icy RNA world” scenario, pockets of cold liquid in ice could have helped stabilise the otherwise pretty easily degraded RNA as well as concentrate and isolate it in a weird inorganic precursor to cells.
Using experimental evolution in an icy setting, they found a variation related to the aforementioned ribozyme that was much quicker and generally much better at copying RNA than its ancestors. Engineering a few previously known performance-enhancing mutations into this molecule finally gave a ribozyme that could copy an RNA molecule longer than itself! It still wouldn’t be able to self-replicate, since this particular guy can only copy sequences with certain properties it doesn’t have itself, but we’ve got the necessary endurance now. Only two words can properly describe how amazing that is. Holy. Shit. :-O
Attwater J et al. (2013) In-ice evolution of RNA polymerase ribozyme activity. Nature Chemistry, published online 20/10/2013, doi: 10.1038/nchem.1781
Wochner A et al. (2011) Ribozyme-catalyzed transcription of an active ribozyme. Science 332:209-212
2. Cambrian explosion: evolution on steroids.
This one’s for those people who say there is nothing special about evolution during the Cambrian – and also for those who say it was too special. (Creationists, I’m looking at you.) It is also very much for me, because Cambrian! (How did I not spot this paper before? Theoretically, it came out before I stopped checking RSS…)
Lee et al. (2013) used phylogenetic trees of living arthropods to estimate how fast they evolved at different points in their history. They looked at both morphology and genomes, because the two can behave very differently. It’s basically a molecular clock study, and I’m still not sure I trust molecular clocks, but let’s just see what it says and leave lengthy ruminations about its validity to my dark and lonely hours 🙂
They used living arthropods because, obviously, you can’t look at genome evolution in fossils, but the timing of branching events in the tree was calibrated with fossils. With several different methods, they inferred evolutionary trees telling them how much change probably happened during different periods in arthropod history. They tweaked things like the estimated time of origin of arthropods, or details of the phylogeny, but always got similar results.
On average, arthropod genomes, development and anatomy evolved several times faster during the Cambrian than at any later point in time. Including the aftermath of the biggest mass extinctions. Mind you, not faster than modern animals can evolve under strong selection – they just kept up those rates for longer, and everyone did it.
(I’m jumping up and down a little, and at the same time I feel like there must be something wrong with this study, the damned thing is too good to be true. And I’d still prefer to see evolutionary rates measured on actual fossils, but there’s no way on earth the fossil record of any animal group is going to be good enough for that sort of thing. Conflicted much?)
Lee MSY et al. (2013) Rates of phenotypic and genomic evolution during the Cambrian explosion. Current Biology 23:1889-1895
3. Chitons to sausages
Aplacophorans are probably not what you think of when someone mentions molluscs. They are worm-like and shell-less, although they do have tiny mineralised scales or spines. Although they look like one might imagine an ancestral mollusc before the invention of shells, transitional fossils and molecular phylogenies have linked them to chitons, which have a more conventional “sluggy” body plan with a wide foot suitable for crawling and an armoured back with seven shell plates.
Scherholz et al. (2013) compared the musculature of a living aplacophoran to that of a chiton and found it to support the idea that aplacophorans are simplified from a chiton-like ancestor rather than simple from the start. As adults, aplacophorans and chitons are very different – chitons have a much more complex set of muscles that includes muscles associated with their shell plates. However, the missing muscles appear to be present in baby aplacophorans, who only lose them when they metamorphose. (As a caveat, this study only focused on one group of aplacophorans, and it’s not entirely certain whether the two main groups of these creatures should even be together.)
Scherholz M et al. (2013) Aplacophoran molluscs evolved from ancestors with polyplacophoran-like features. Current Biology in press, available online 17/10/2013, doi: 10.1016/j.cub.2013.08.056
4. Does adaptation constrain mammalian spines?
Mammals are pretty rigid when it comes to the differentiation of the vertebral column. We nearly all have seven neck vertebrae, for example. This kind of conservatism is surprising when you look at other vertebrates – which include not only fairly moderate groups like birds with their variable necks, but also extremists like snakes with their lack of legs and practically body-long ribcages. Mammalian necks are evolutionarily constrained, and have been that way for a long time.
Emily Buchholz proposes an interesting explanation with links to previous hypotheses. Mammals not only differ from other vertebrates in the less variable numbers of vertebrae in various body regions; these regions are also more differentiated. For example, mammals are the only vertebrates that lack ribs in the lower back. In Buchholz’s view, this kind of increased differentiation contributes to adaptation but costs flexibility.
Her favourite example is the muscular diaphragm unique to mammals. This helps mammals breathe while they move, and also makes breathing more powerful, which is nice for active, warm-blooded creatures that use a lot of oxygen. However, it also puts constraints on further changes. Importantly, Buccholz argues that these constraints don’t all have to work in the same way.
For example, the constraint on the neck may arise because muscle cells in the diaphragm come from the same place as muscle cells associated with specific neck vertebrae. Moving the forelimbs relative to the spine, i.e. changing the number of neck vertebrae, would mess up their migration to the right place, and we’d end up with equally messed up diaphragms.
A second possible constraint has less to do with developmental mishaps and more to do with plain old functionality. If you moved the pelvis forward, you may not screw with the development of other bits, but you’d squeeze the space behind the diaphragm, which you kind of need for your guts, especially when you’re breathing in using your lovely diaphragm.
Buccholz E (2013) Crossing the frontier: a hypothesis for the origins of meristic constraint in mammalian axial patterning. Zoology in press, available online 28/10/2013, doi: 10.1016/j.zool.2013.09.001
And… I think that approximately covers today’s squee moments 🙂