How was the vertebrate/arthropod LCA segmented?
Vertebrates are modified segmented worms; that is, their body plan is made up of sequentially repeated units, most apparent in skeletal structures like the vertebrae.
Arthropods are also modified segmented worms. Look at a larval fly, for instance, and you can see they are made up of rings stacked together.
So here’s a simple and obvious question: can we infer that the last common ancestor of vertebrates and arthropods was also a segmented worm? That is, is segmentation a common ancestral trait, or did arthropods and vertebrates invent it independently? At first thought, you might assume they are: it’s a complex trait shared by two taxa, so the simplest assumption is that both groups inherited it from their common ancestor (making it a synapomorphy), but there are also substantial differences in the mechanism of segmentation, so it’s possible that this trait wasn’t present in the common ancestor (making it a homoplasy).
Which is it? And the answer is…we don’t know! There’s a great deal of sympathy for synapomorphy, driven largely by a molecular bias — we see that a lot of the genes involved in the process in both vertebrates and arthropods are shared. There is a whole family of Notch-related cyclic genes, for instance, that turn out to be important in both, but the catch is that Notch is a gene that gets recruited in all kinds of processes — it’s part of a handy developmental module for defining borders. So its presence doesn’t automatically imply homology.
And then there’s the whole problem of segmentation looking so different in flies (weird, highly derived arthropods) and vertebrates.
Flies build their segments almost all at once. In the fly embryo, there is first a broad gradient of position information, then a set of genes called the gap genes are switched on to define broad zones in the animal, and then, finally, the segmentation genes read the pattern of the gap genes and interact with each other to partition the fly into segments. It means segmentation is fast: all 14 segments form in one short interval, nearly simultaneously.
There’s another complication. The segmentation genes in flies are numerous and elaborate, and in particular, they set up a peculiar pattern of alternating periodicity using genes called pair-rule genes. That means that there is one set of genes active in all of the odd-numbered segments (eve, or even-skipped, for instance), and a different set active in all of the even-numbered segments (ftz, or fushitarazu). In addition, there’s a regular platoon of other pair-rule genes like odd and prd and hairy that are expressed in alternating segmental domains.
What it means is that fly segments alternate in their molecular substrates, like the floral wallpaper to the right.* It seems excessively complicated. Furthermore, when we look at the molecular circuitry of segmentation, it’s surprising inelegant — each stripe is hard-coded into the regulatory controls of the genes involved. The ugliness of evolutionary contingency is on full display here.
When we look at vertebrates, on the other hand, we see something very different. Segments form sequentially, one at a time from front to back, rather than nearly simultaneously. What we see here is a clock-and-wavefront model in operation, where cyclic surges in the expression of two molecules, fgf8 and Wnt3a, trigger a small pool of cells at the front of an undifferentiated band to pinch off and differentiate into a segment. Then the wave recedes, the front of the undifferentiated band matures a little more, and then when the wave rises again, another set of cells are recruited to make the next segment. And so on and on, until the whole band is delineated into an array of segments.
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