Ctenophore nervous systems redux

… and reasons I suddenly find myself liking Joseph Ryan.

Ryan was the first author on the first ctenophore genome paper, published last December, though I’d known his name long before that thanks to his developmental genetic work on jelly creatures of various kinds. As is clear from the genome study, he leans quite strongly towards the controversial idea that ctenophores represent the sister lineage to all other animals.

And here’s reason one that my eyes suddenly have little cartoon hearts pulsing in their irises upon reading his short perspective paper in Zoology (Ryan, 2014). Throughout the paper, not once does he refer to ctenophores as “the” basal animal lineage. Instead, he uses phrases like “most distant relative to all other animals” or “the sister group to the rest of the animals”.

In other words, he’s scrupulously avoiding my giantest pet peeve, and I’m sure he doesn’t do it to please an obscure blogger, but gods, that’s even better. I don’t want to be pleased, I want evolutionary biology to get rid of stupid anthropocentric ladder-thinking nonsense.

Anyway, the little paper isn’t actually about animal phylogeny, it’s about nervous systems.

Both ctenophore genome papers argued that the ancestors of these pretty beasties might have evolved nervous systems independently of ours. The second one seemed positively convinced of this, but, as Ryan’s review points out, there are other possibilities even assuming that the placement of ctenophores outside the rest of the animals is correct.

While it’s possible that nerve cells and nervous systems evolved twice among the animals – it is equally possible that they have been lost twice (i.e. in sponges and blobby little placozoans). Full-fledged nerve cells wouldn’t be the first things that sponges and blobs have lost.

And Ryan basically wrote this short piece just to point that out. The argument that ctenophore nervous systems are their own invention is based on the absence or strange behaviour of many “conserved” nervous system-related genes. Ctenophores appear to completely lack some common neurotransmitters such as dopamine, as well as a lot of genes/proteins that are necessary for nerve synapses to work in us. Other genes that are “neural” in other animals are present but not associated with the nervous system in ctenophores.

BUT, Ryan cautions, there are also commonalities that shouldn’t be dismissed. While ctenophores can’t make dopamine, they do possess several other messenger molecules common in animal nervous systems. Same goes for the proteins involved in making synapses. Likewise, while they completely lack some of the genes responsible for defining various types of nerve cells (see: Hox genes), other genes involved in the same kind of stuff are definitely there.

The key thing, he says, is to take a closer look at more of these genes and find out what they do by manipulating them. Since there are clearly both similarities and differences, we must assess their extent.

And that, my friends, is the question at the heart of every homology argument ever. How similar is similar enough? Greater minds than mine have struggled with the answer, and I imagine they’ll continue to struggle until we invent time machines or find fossils of every single stage in the evolution of everything.

Until then, I’ll leave you with the closing lines of Ryan’s paper. I may not agree that we’ve “revealed” the position of ctenophores, but I’m absolutely on board with the excitement :)

One thing is quite clear: something remarkable happened regarding the evolution of the nervous system very early in animal evolution. Either a nervous system existed in the ancestor and was lost in certain lineages, or ctenophores invented their own nervous system independently (Fig. 1). Either possibility is quite extraordinary. The revelation that ctenophores are the sister group to the rest of animals has sparked a truly exciting debate regarding the evolutionary origins of the nervous system, one that will continue as additional genomic and functional data come to the fore.

Reference:

Ryan JF (2014) Did the ctenophore nervous system evolve independently? Zoology in press, available online 11/06/2014, doi: 10.1016/j.zool.2014.06.001

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