Are vertebrates upside-down insects?

October 19, 2019 6:10 AM

What's the current consensus on the dorsoventral axis inversion hypothesis? I can see in preview that it is a long-debated idea in biology. Is it still debated, or has evidence from genetics and microbiology decided it conclusively one way or the other?
posted by clawsoon to Science & Nature (2 answers total)

Best answer: I'm not an expert in developmental biology, but as far as I understand the evidence is fairly strongly in favor of the idea that a dorsoventral inversion occurred early in chordate (a taxonomic group including vertebrates) evolution. There's a lot of molecular genetic evidence, in particular, to suggest that the central nervous systems in protostomes (a group including insects) and chordates share a common evolutionary origin, with the major genes responsible for patterning the development of the brain being largely homologous across as diverse a range of species as humans, mice, flies, annelid worms, and perhaps even cephalopods. However, the dorso-ventral patterning of gene expression is inverted between protostomes and chordates.

There's a fairly nice-looking 2013 review available in full text on PubMedCentral discussing the evidence for a common origin of the central nervous system in bilaterians (the clade of all bilaterally symmetric animals). The first figure summarizes four major hypotheses of what urbilaterian (the hypothesized last common ancestor of all bilaterians) would have been like, and what that would mean for the subsequent evolution of the nervous system. Two of the four possibilities involve a dorsoventral axis inversion in chordate evolution.

(In case you're wondering why the comparison is between protostomes and chordates rather than protostomes and deuterostomes, I believe it's because the other major taxon of deuterostomes is the echinoderms, such as starfish, which took a wild trajectory off onto doing something else entirely with their nervous systems. They're so strongly derived compared to chordates and most protostomes that it's probably very hard to know whether the D/V inversion happened before or after they split off from the chordates.)

I don't know if the evidence is conclusive or not, but the developmental neurobiologists I've known generally treat D/V inversion as a given.
posted by biogeo at 12:40 AM on October 20, 2019 [4 favorites]

Actually on continuing to skim the review I linked the authors go on to discuss that echinoderms are treated separately in questions about the evolution of the central nervous system in bilaterians exactly because they seem to have followed a very different path from all the other bilaterians, just as I guessed.
posted by biogeo at 12:48 AM on October 20, 2019