IN THE PAST few years there has been some acrimony regarding the division of the protostomes into Lophotrochozoa and Ecdysozoa. This decision split the annelids and the arthropods, and placed nematodes in with the arthropods. At its first proposal support for this tree was weak, but successive discoveries have strengthened it. Now the most detailed tree yet confirms this division, and clarifies other relationships.
The new tree is based upon sequence data from 77 taxa and 150 genes. The result is one of the most beautiful phylogenetic trees I’ve seen.
Reprinted by permission from Macmillan Publishers Ltd:
Nature advanced online publication, 5 March 2008.
The improved resolution of the metazoan tree has led to several important findings:
- Lophotrochozoa and Ecdysozoa receive additional support.
- Chaetognatha, an enigmatic protostome with deuterostome-like gastrulation, is placed as the sister group of Lophotrochozoa.
- Coelomata, a proposed group of organisms possessing a body cavity, is not supported. This means the coelom was independently gained and lost along various lineages.
- A variety of groups with unstable positioning recovered Platyzoa, a group of acoelomates clustering near Platyhelminthes. This is probably due to long-branch attraction, an artifact produced when highly divergent lineages’ DNA sequences have time to mutate multiple times and happen to hit upon a similar sequence. This produces the superficial appearance of close relationship. There are a variety of methods to detect long-branch attraction, and the authors suggest concentrating upon these unstable groups in future. Better sampling may help, because for many of these groups there are limited DNA sequences or few representatives tested.
- A novel sister clade to Annelida was recovered containing the Nemerteans (ribbon worms), a phoronid (another protostome with deuterostome-like gastrulation), and a brachiopod. This supports the suggestion that extinct coelosclerites, which appear to have produced chaetae and shells, are ancestral to Mollusca and its sister clade (Annelida plus the new nemertea/phoronidae/brachiopoda clade). Additionally, it suggests that phoronids and brachiopods lost spiral cleavage, a form of embryonic development, secondarily.
And the weirdest finding of all:
- Ctenophores, comb jellies, are placed as the sister group to all other metazoans. Previously ctenophores were thought to be more closely related to cnidaria, and definitely to have branched off more recently than the sponges. This new finding is odd, since it suggests that either ctenophores evolved their complexity on a different route than the rest of the metazoans or that the sponges are simplified from a more complicated ancestor. More studies will be required to determine if this placement is correct.
Questions still remain, but this represents a significant advance in metazoan phylogenetics.
Dunn, C. W.; Hejnol, A.; Matus, D. Q.; Pang, K.; Browne, W. E.; Smith, S. A.; Seaver, E.; Rouse, G. W.; Obst, M.; Edgecombe, G. D.; Sorensen, M. V.; Haddock, S. H. D.; Schmidt-Rhaesa, A.; Okusu, A.; Kristensen, R. M.; Wheeler, W. C.; Martindale, M. Q.; Giribet, G. “Broad phylogenetic sampling improves resolution of the animal tree of life.” Nature advance online publication, 5 March 2008 (DOI:10.1038/nature06614).
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March 6, 2008 at 8:42 pm
Christopher Taylor
I really think that caution is called for as regards the position of Ctenophora, though this is actually not the first time such a position has been suggested. It certainly looks increasingly likely that ctenophores are less closely related to bilaterians than cnidarians are, and if the interpretation of Stromatoveris is correct then the ctenophores could be derived from sessile Ediacaran ancestors. The current analysis, however, includes only two sponges, the demosponge Suberites and the homoscleromorph Oscarella, and only Oscarella in the Bayesian analysis. No Calcarea, no Hexactinellida. The position of the included sponges as sister to Cnidaria is also suspicious, though support for that was pretty much non-existent. More sponges! More sponges, I say!
The position of Acoela within the Platyzoa is also a little unexpected and needs a further look, though the rest of this clade is believable. Zrzavý et al. interpreted myzostomids as platyzoans in 2001, identifying a clade of animals that had sperm with flagella placed anteriorly rather than posteriorly as in all other groups. I feel the urge to point out, too, that support for their nemertean-brachiozoan clade is pretty low, though the clade is not impossible.
The position of the tardigrades is interesting – sister to nematoids in one analysis, but sister to onychophorans + arthropods in the other. That definitely needs a further look…
March 6, 2008 at 9:17 pm
Nimravid
I agree with you about the sponges. We tend to give them short shrift because they just sit there lacking organ systems and filter feeding. Perhaps I should do an entry on sponges.
Articles like this make my ears perk up (as best they can), but I’m glad I don’t have to do all of the work to construct these trees.
March 6, 2008 at 10:24 pm
Christopher Taylor
I tutored a lab on sponges last week – the first practical lab in a course on invertebrate biology. I introduced the lab by putting up a phylogenetic tree showing the paraphyly of sponges in relation to other animals, and informed the students that while in terms of workload it was going to be one of the shorter labs, in evolutionary terms they were going to be covering more ground than they would in all the other labs put together.
March 27, 2008 at 9:16 pm
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