The Latest Word on Metazoan Phylogenetics

The architect of the latest Metazoan phylogeny - Casey Dunn - was in Rochester this week talking about everything from siphonophore development to basal metazoan relations. Fans of his recent Nature paper on metazoan phylogenetics will be delighted to learn that Casey and his colleagues are dramatically expanding their EST data set. In addition to adding data from a few new taxa, the number of genes has increased by an order of magnitude to around 1,500. Their preliminary matrix is around 20% complete and they're now very actively re-evaluating the patterns recovered by the previously published dataset.

Although preliminary results largely agree with the previously published phylogeny, there is one interesting new pattern: acoel worms+xenoturbella as the basal bilaterians. Along with other organisms lacking a true coelem, acoel worms were traditionally united with platyhelminthes and placed at the base of the metazoan tree. Although rapid rates of evolution and long-branch attraction complicated molecular phylogenetic analyses these acoelemate forms, a string of studies -- including Dunn et al.'s previous analysis -- rejected the basal position of the platyhelminthes and acoels and suggested instead that they form a clade nested within the lophotrochozoa. Nevertheless, the phylogenetic distinctness of acoels and platyhelminthes, and a return of the acoels to the base of bilaterians recovered by the new dataset and preliminary analsyes, is not entirely unexpected; indeed, this conclusion brings the Dunn et al. tree in line with the conclusions of a carefully implemented 18S study. Xenoturbella (see image above), which has long been one of the greatest phylogenetic mysteries in the tree of life, is another story. As recently as the late 1990s, it was hypothesized that these almond-looking creatures were protostomes allied with molluscs. Although this story fell apart rather dramatically, I'm not aware of previous studies that have united these creatures with the aceols at the base of the bilaterians.

In addition to addressing specific phylogenetic hypotheses -- which, in Dunn's own words basically amounts to figuring out how all the various types of worms are related -- his group is also devoting attention to general systematic issues related to data-sampling and analysis efficiency. He makes a convincing case for the idea that a revolution in analytical techniques will be needed as we enter an era during which computational capabilities will be more limiting than data availability. Exciting times. If you want to get involved, you should drop Casey a line, I know he's looking for bright students and post-docs to populate his new lab.