Scientists Feel Confident That They Have Finally determined that the flatworm Acoelomorpha Belongs as a Sister Clade to Other Bilateral Animals

An international research team led by Brown University has determined that the flatworm Acoelomorpha belongs as a sister clade to other bilateral animals. The finding 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

An article in PHYSORG.COM explains: "When it comes to understanding a critical junction in animal evolution, some short, simple flatworms have been a real thorn in scientists' sides. Specialists have jousted over the proper taxonomic placement of a group of worms called Acoelomorpha. This collection of worms, which comprises roughly 350 species, is part of a much larger group called bilateral animals, organisms that have symmetrical body forms, including humans, insects and worms. The question about acoelomorpha, was: Where do they fit in?"

Acoelomorpha, the most primitive bilateral animals, has held the fascination of researchers for a number of years. It as been referred to as an "enigmatic,... rogue animal,." As it has been studied: "It has been wandering throughout the animal tree of life," said Casey Dunn, an evolutionary biologist at Brown University. Acoelomorpha are "tiny, cryptic worms that many consider to be the most primitive of all bilaterally symmetrical animals."

  

Acoelomorpha, a tiny, primitive flatworm

The reason why Acoelomorpha are a disputed phylum of animals with planula-like features (free-swimming or crawling larval type common in many species of the phylum Cnidaria (e.g., jellyfish, corals, and sea anemones). The Acoelomorpha has recently been classified by Jaume Baguñà and Marta Riutort as a separate phylum, basal, forming a basis; fundamental; basic form among the Bilateria."There are two or more superphyla (main lineages) of Bilateria" including ... "the protostomes (which) include most of the rest, such as arthropods, annelids, mollusks, flatworms, and so forth.... There are a number of differences, most notably in how the embryo develops. In particular, the first opening of the embryo becomes the mouth in protostomes."

Acoelomorpha are very primitive flatworms

PHYSORG.COM explains: "The worm wanders no more. Through a laborious genetic sequencing analysis, Dunn and an international team of scientists have settled the long-standing 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 have gained a key insight into the most recent common ancestor to bilaterians, a species that remains unknown.

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

A team of 17 scientists from the United States, France Germany, Sweden, Spain and the United Kingdom, published the results of their research recently in the Proceedings of the Royal Society B, and produced two interesting findings that stood out:

"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.

"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, discussed 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."

A color enhanced photo of the primitive flatworm Acoelomorpha