Ctenophores, or comb jellies, are a phylum of animals whose relative position in the Great Tree of Life—along with the other metazoan (multicellular) animal phyla of Cnidaria (jellyfish, corals sea anemones), Porifera (sponges) Placozoa (a single species resembling a multicellular amoeba, which forms its own bizarre phylum), and Bilateria (all the other animals we know, from worms to clams to squirrels)—has been a mystery. It’s now being resolved, and a paper in the latest Science by Joseph Ryan et al. (see also the nice short summary by Antonis Rokis; references and links with free download at bottom) may have resolved at least these major groups.
But first, here’s a weird placozoan, the species Trichoplax adhaerens, which is the only monspecific phylum I know (there may be others). It’s a marine animal that eats algae:
There have been been lots of arguments over the years about how these phyla are related, and that’s important because some of them have common features (colenterates, ctenophores, and bilaterians, for example, have nervous systems; others don’t; while only bilaterians and ctenophores have “mesoderm”, a middle layer of tissue in the zygote that forms, among other things, bilaterian muscles), common features that imply common ancestry. Just those similarities I described would imply that our closest relatives—and by “our” I mean Bilateria—may be ctenophores, but their mesoderms are different from ours. And they strongly resemble jellyfish. When I was younger I learned that sponges, because of some peculiar cells with flagella they have, may be the outgroup for all animals (the sister group of all the other groups).
All in all, the grouping of metazoan phyla has been contentious and unresolved, but now the advent of whole genome-sequencing offers one way to sort it out.
The paper by Ryan et al is based on whole-genome sequencing of a single species of ctenophore, Mnemiopsis leidyi (one species from each group will do when you’re trying to resolve such anciently-diverged taxa, which diverged around the time of the Cambrian explosion, over 500 million years ago). Here’s M. leidyi:
But let’s look at some ctenophores first, as they’re among the most beautiful of animals, iridescent marine species with shimmering waves of light:
I don’t want to dwell on the paper too long, but several of the main findings come from comparing the genome of this species (both DNA sequences and which genes are present or absent in the groups) with those of representatives of the other four phyla as well as a definite “outgroup” (single-celled animals; they used a “choanoflagellate“: a one-celled animal with a flagellum surrounded by a collar). The phylogenies differed a bit depending on how they did the analysis, but the most definitive one, statistically more supportable than any other family tree, involved using the presence or absence of groups of genes as a way to judge relatedness. Here’s their phylogeny as redrawn in the summary of Rokas:
- It shows that ctenophores are the outgroup of all other metazoans. That is, their ancestors branched off before the ancestors of sponges, placozoans, cnidarians, and bilaterians. That’s a surprise because ctenophores look far more similar to jellyfish than to anything else. But that similarity is superficial, and belies the true genetic relationships. (Looks are deceiving; genomes less so.) There’s no doubt that this is correct, and that the sister group to ourselves (Bilateria) is cnidarians. That’s a surprise. We are in fact more closely related to sponges and those weird placozoans than to ctenophores.
- The only animals in this tree that have nervous systems are Bilateria, Cnidaria, and Ctenophores; placozoans and sponges don’t. And, as the new DNA sequencing study shows, those nervous systems rest on the expression of similar genes in the three groups, so they didn’t evolve independently.
- The finding above implies either that 1.) the ancestor of all multicellular animals had a nervous system, which was later lost in sponges and placozoans, or that 2.) the ancestor had the requisite genes for building nervous systems, but they were originally used for something else and later co-opted in Bilateria, Cnidaria, and Ctenophora to build neurons and other components of that system. Although the latter may seem less likely, it’s not unknown for the same genes to be co-opted in different lineages to build similar structures. (The eyes of humans and fruit flies evolved independently, for instance, but both involve the important involvement of a gene called Pax6.) The representation in Rokas’s figure implies possibility 1)—a full nervous system in the common ancestor—but we don’t know that yet.
- Finally, the genes that make the mesoderm of ctenophores—the middle layer of tissue—are different from those making the mesoderm of bilaterians, like the layer of tissue that builds our muscles and connective tissue. It’s thus pretty clear that the mesoderm evolved twice independently, and that depiction in Rokas’s diagram is accurate. The mesoderms of Bilateria and Ctenophora are analogous but not “homologous”, i.e., they are similar in structure but not evolutionary origin.
There’s other stuff in the paper of Ryan et al. as well, but this is what most of us need to know. The paper is free if you want to read more. What strikes me most strongly is that the similarity between comb jellies and jellyfish does not reflect close relationship, and probably evolved independently—unless the common ancestor of all metazoans was jellyfish-like (unlikely!). And the possibility that the common ancestor also had a nervous system is also intriguing. That won’t be resolved until we can figure out what those genes in sponges that make nervous systems in ctenophores and bilaterians (but not in sponges really do)—that is, we need a functional analysis of sponge “nervous-system-type” genes.
Ryan, J. F., K. Pang, C. E. Schnitzler, A.-D. Nguyen, R. T. Moreland, D. K. Simmons, B. J. Koch, W. R. Francis, P. Havlak, S. A. Smith, N. H. Putnam, S. H. D. Haddock, C. W. Dunn, T. G. Wolfsberg, J. C. Mullikin, M. Q. Martindale, and A. D. Baxevanis. 2013. The genome of the ctenophore Mnemiopsis leidyi and its implications for cell type evolution. Science 342:1336-1344.
Rokas, A. 2013. My oldest sister is a sea walnut? Science 342:1327-1329.