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MBE Advance Access published online on April 27, 2005
Molecular Biology and Evolution, doi:10.1093/molbev/msi150
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© The Author 2005. Published by Oxford University Press on behalf of the Society for Molecular Biology and Evolution. All rights reserved. For permissions, please e-mail: journals.permissions@oupjournals.org
Accepted April 3, 2005

Research Article

The Trichoplax PaxB Gene: a Putative Proto-PaxA/B/C Gene Predating the Origin of Nerve and Sensory Cells

Thorsten Hadrys 1*, Rob DeSalle 2, Sven Sagasser 3, Nina Fischer 3, and Bernd Schierwater 4

1 IBU, Neurogenetics, Carl v. Ossietzky University, D-26129 Oldenburg, Germany; Dept. of Invertebrate Zoology & Molecular Labs, American Museum of Natural History, Central Park West at 79th Street, New York, NY 10024, USA; Dept. of Physiology and Neuroscience, NYU Medical Center, New York, NY 10016, USA
2 Dept. of Invertebrate Zoology & Molecular Labs, American Museum of Natural History, Central Park West at 79th Street, New York, NY 10024, USA
3 ITZ, Ecology & Evolution, TiHo Hannover, Bünteweg 17d, D-30559 Hannover Germany
4 ITZ, Ecology & Evolution, TiHo Hannover, Bünteweg 17d, D-30559 Hannover Germany; Yale University, MCDB, 165 Prospect St., New Haven, CT 06520, USA

* To whom correspondence should be addressed.
Thorsten Hadrys, E-mail: thorsten.hadrys{at}uni-oldenburg.de


  Abstract

Pax genes play key regulatory roles in embryonic and sensory organ development in metazoans but their evolution and ancestral functions remain widely unresolved. We have isolated a Pax gene from Placozoa, beside porifera the only metazoan phylum that completely lacks nerve and sensory cells or organs. These simplest known metazoans also lack any kind of symmetry, organs, extracellular matrix, basal lamina, muscle cells and main body axis. The isolated Pax gene from Trichoplax adhaerens harbors a paired domain, an octapeptide, and a full-length homeodomain. It displays structural features not only of PaxB and Pax2/5/8 like genes, but also of PaxC and Pax6 genes. Conserved splice sites between Placozoa, Cnidaria and triploblasts, mark the ancient origin of intron structures. Phylogenetic analyses demonstrate that the Trichoplax PaxB gene, TriPaxB, is basal not only to all other known PaxB genes, but also to PaxA and PaxC genes and their relatives in triploblasts (namely Pax2/5/8, Pax4/6 and PoxNeuro). TriPaxB is expressed in distinct cell patches near the outer edge of the animal body, where undifferentiated and possibly multi-potent cells are found. This expression pattern indicates a developmental role in cell type specification and/or differentiation, probably in specifying/determining fibre cells, which are regarded as proto-neural/muscle cells in Trichoplax. While PaxB, Pax2/5/8, and Pax6 genes, have been linked to nerve cell and sensory system/organ development in virtually all animals investigated so far, our study suggests that Pax genes predate the origin of nerve and sensory cells.

Keywords: PaxB; Pax gene evolution; Proto-Pax; Trichoplax; Placozoa.
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