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.

Metazoan tree of life
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).