News bites

Just quickly before I completely forget about these…

(1) Common ancestry of segmentation: back-and-forth-and-back-and-forth

Seaver EC et al.(2012) Expression of the pair-rule gene homologs runt, Pax3/7, even-skipped-1 and even-skipped-2 during larval and juvenile development of the polychaete annelid Capitella teleta does not support a role in segmentation. EvoDevo 3:8

I’ve made throwaway mentions of segmentation before. The conundrum about segmentation is whether (or rather, to what extent) it is homologous in the three “eusegmented” phyla, arthropods, annelids and chordates. It arises because all three phyla are separated from the others by many lineages that aren’t usually considered segmented – yet the three share some tantalising similarities. People have been trying to solve the question by comparing the genetic mechanisms generating the segments in each group, with mixed results. One of the papers in my previous news bite post was about the similarity of segmentation in arthropods and vertebrates. Now, here’s one for the differences between arthropods and the wormies. (You can’t say I’m not fair :-P) The genes listed in the study’s title were originally described in the fruit fly Drosophila melanogaster, one of the best studied animals in developmental biology (and, like, every other area of biology). There, they have an interesting role in that each of them helps define every other body segment. IIRC, Pax3/7 (known in flies as paired) and even-skipped are for even-numbered segments, runt is for the odd ones. Now, segmentation in Drosophila is (to put it mildly) fucking weird, but if memory serves, several of these pair-rule genes have been confirmed to play similar roles in less eccentric arthropods. Elaine Seaver and colleagues looked at their expression in their favourite worm (this guy. Seaver’s group obviously didn’t pick it for its beauty :-P), and they found that they were active in… nothing resembling a two-segment pattern. Or anything segment-related. The more genetic studies come out, the more complicated the whole segmentation issue is looking…

(2) Someone found the cause of the Cambrian explosion. (Again.)

Peters SE & Gaines RR (2012) Formation of the ‘Great Unconformity’ as a trigger for the Cambrian explosion. Nature 484:363-366

The Cambrian explosion is probably not what you think it is (no, all animal phyla didn’t just suddenly pop into existence fully formed ;)). Nevertheless, the (relatively) quick rise of animals – particularly animals with hard parts – beginning in the Early Cambrian is still odd enough to fascinate generations of palaeontologists, evolutionary biologists and geologists. The list of proposed causes is pretty long by this point (and believe me, I really really would like to go into them once… but, uh. Huge, dauntingly huge topic). Explanations range from denying the need for an explanation through pinning it on oxygen levels, ice ages, developmental genetics, predation, biomineralisation, even the evolution of eyes, and, working from memory, I probably left some more out of that list. Peters and Gaines’ preferred explanation seems to be geological and ecological: they suggest that a combination of lots of erosion/weathering on land, and a subsequent rise in sea levels, led to large new shallow seas that were chock full of dissolved minerals. New habitats to conquer + widely available minerals = an explosion of new animals with mineralised hard parts. This study is a nice two-in-one: it purports to explain not only the Cambrian explosion, but also the conspicuous gap in the geological record that separates Cambrian from Precambrian rocks in many places.


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