Animais bilaterais: finalmente um lugar na extinta Árvore da Vida de Darwin

First Evolutionary Branching For Bilateral Animals Found

ScienceDaily (Sep. 25, 2009) — 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?

To scientists, acoelomorpha, has been enigmatic, a “rogue animal,” said Casey Dunn, an evolutionary biologist at Brown University. “It has been wandering throughout the animal tree of life.”

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

The results appear online this week in the Proceedings of the Royal Society B.
The team, composed of 17 scientists from the United States, France Germany, Sweden, Spain and the United Kingdom, had two more interesting findings:

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.

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Journal reference:

Andreas Hejnol, Matthias Obst, Alexandros Stamatakis, Michael Ott, Greg W. Rouse, Gregory D. Edgecombe, Pedro Martinez, Jaume Baguñà, Xavier Bailly, Ulf Jondelius, Matthias Wiens, Werner E. G. Müller, Elaine Seaver, Ward C. Wheeler, Mark Q. Martindale, Gonzalo Giribet, and Casey W. Dunn.

Assessing the root of bilaterian animals with scalable phylogenomic methods.

Proceedings of The Royal Society B Biological Sciences, 2009

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Assessing the root of bilaterian animals with scalable phylogenomic methods

Andreas Hejnol1,*, Matthias Obst2, Alexandros Stamatakis3, Michael Ott3, Greg W. Rouse4, Gregory D. Edgecombe5, Pedro Martinez6, Jaume Baguñà6, Xavier Bailly7, Ulf Jondelius8, Matthias Wiens9, Werner E. G. Müller9, Elaine Seaver1, Ward C. Wheeler10, Mark Q. Martindale1, Gonzalo Giribet11 and Casey W. Dunn12,*

+ Author Affiliations

1Kewalo Marine Laboratory, University of Hawaii, 41 Ahui Street, Honolulu 96813, HI, USA

2Sven Lovén Centre for Marine Sciences, Göteborg University, Kristineberg 566 45034, Fiskebäckskil, Sweden

3Department of Computer Science, Technical University of Munich, Boltzmannstr. 3, 85748 Garching b. Munich, Germany

4Scripps Institution of Oceanography, University of California San Diego, 9500 Gilman Drive, La Jolla, CA 92093, USA

5Department of Palaeontology, Natural History Museum, Cromwell Road, London SW7 5BD, UK

6Departament de Genètica, Facultat de Biologia, Universitat de Barcelona, Diagonal 645 08028, Barcelona, Spain

7UPMC, CNRS – Station Biologique de Roscoff, Place Georges Teissier, 29680 Roscoff, France

8Department of Invertebrate Zoology, Swedish Museum of Natural History, Box 50007, 10405 Stockholm, Sweden

9Department of Applied Molecular Biology, Johannes-Gutenberg-University Mainz, Duesbergweg 6, 55099 Mainz, Germany

10Division of Invertebrate Zoology, American Museum of Natural History, Central Park West at 79th Street, New York, NY 10024, USA

11Museum of Comparative Zoology and Department of Organismic and Evolutionary Biology, Harvard University, 26 Oxford Street, Cambridge, MA 02138, USA

12Department of Ecology and Evolutionary Biology, Brown University, 80 Waterman Street, Providence, RI 02912, USA

*Authors for correspondence (hejnol@hawaii.edu; casey_dunn@brown.edu).

Abstract

A clear picture of animal relationships is a prerequisite to understand how the morphological and ecological diversity of animals evolved over time. Among others, the placement of the acoelomorph flatworms, Acoela and Nemertodermatida, has fundamental implications for the origin and evolution of various animal organ systems. Their position, however, has been inconsistent in phylogenetic studies using one or several genes. Furthermore, Acoela has been among the least stable taxa in recent animal phylogenomic analyses, which simultaneously examine many genes from many species, while Nemertodermatida has not been sampled in any phylogenomic study. New sequence data are presented here from organisms targeted for their instability or lack of representation in prior analyses, and are analysed in combination with other publicly available data. We also designed new automated explicit methods for identifying and selecting common genes across different species, and developed highly optimized supercomputing tools to reconstruct relationships from gene sequences. The results of the work corroborate several recently established findings about animal relationships and provide new support for the placement of other groups. These new data and methods strongly uphold previous suggestions that Acoelomorpha is sister clade to all other bilaterian animals, find diminishing evidence for the placement of the enigmatic Xenoturbella within Deuterostomia, and place Cycliophora with Entoprocta and Ectoprocta. The work highlights the implications that these arrangements have for metazoan evolution and permits a clearer picture of ancestral morphologies and life histories in the deep past.

phylogenomics Acoelomorpha Nemertodermatida Cycliophora Xenoturbella Ctenophora

Footnotes

Received May 26, 2009.
Accepted August 21, 2009.