Acoelomorpha, a collection of worms which comprises roughly 350 species, is part of a much larger group called bilateral animals, which are organisms that have symmetrical body forms and include humans, insects and worms. Apparently there has been a question about acoelomorpha, namely where do they fit in taxonomically?

Acoelomorpha has been a "rogue animal," says Casey Dunn, an evolutionary biologist at Brown University. "It has been wandering throughout the animal tree of life."

(?!?)

The worm wanders no more, he now says. Through genetic sequencing analysis, Dunn and collaborators claim to have settled the debate and determined that acoelomorpha belongs as a sister clade to other bilateral animals. The finding is significant, Dunn said, because it shows the worm is a product of the deepest split within the bilateral animals, the first evolutionary divergence within the group. Because of that, scientists will have gained a key insight into what should be the most recent common ancestor to bilaterians, a species that remains unknown.

The worm is "as distant as an animal can be in bilateria and still be a bilaterian," said Dunn, who is assistant professor of biology at Brown. "So, now we have two perspectives to (find out about) this common ancestor, the acoelomorphs and all the other bilateral animals."


A group of researchers now says that the flatworm Acoelomorpha belongs as a sister clade to other bilateral animals. They say this means the worm is a product of the deepest split within the bilateral animals, the first evolutionary divergence within the group.  Credit: Eric Rottinger/Kahikai.org

The team, composed of 17 scientists from the United States, France Germany, Sweden, Spain and the United Kingdom, claimed two more interesting findings:

1) They say the debate appears to be over for Xenoturbella, a type of marine worm whose ancestral affiliation had been tossed between worms and mollusks. The researchers reported their genetic analysis shows diminishing evidence for placing xenoturbella within Deuterostomia, one of the major groups within the animal kingdom. Coincidentally, it also shows that xenoturbella may be a close relative to acoelomorpha.

2) Cycliophora, a single species discovered in 1994 that lives on the bristles surrounding the mouth of the Norway lobster Nephrops norvegicus, has found a home with Entoprocta and Ectoprocta. The researchers base their findings on an analysis that reached further into the genetic makeup of cycliophora than previous studies had done.

The team used a genetic sequencing technique called expressed sequence tags to carry out the phylogenetic studies. The aim of this approach, discusssed in a study led by Dunn that appeared in Nature last year, is to analyze a large number of genes from a large number of animals. For this paper, the researchers looked at 1,487 genes, a 10-fold increase in the number of genes analyzed in previous studies. In all, the researchers logged 2.25 million processor hours on a supercomputer in California to obtain the results. Dunn called the effort the most computationally intensive phylogenetic analysis to date.

The results appear online this week in the Proceedings of the Royal Society B.