Mitochondrial genome of Trichoplax adhaerens supports Placozoa as the basal lower metazoan phylum

*Department of Molecular, Cellular, and Developmental Biology, Yale University, New Haven, CT 06520-8104;
^‡Division of Ecology and Evolution, Institut für Tierökologie und Zellbiologie, Tieraerztliche Hochschule Hannover, Bünteweg 17d, D-30559 Hannover, Germany; and Departments of
^§Ecology and Evolutionary Biology and
^¶Geology and Geophysics, Yale University, New Haven, CT 06520-8106

Edited by W. Ford Doolittle, Dalhousie University, Halifax, NS, Canada, and approved April 24, 2006 (received for review March 15, 2006)

Abstract

Mitochondrial genomes of multicellular animals are typically 15- to 24-kb circular molecules that encode a nearly identical set of 12–14 proteins for oxidative phosphorylation and 24–25 structural RNAs (16S rRNA, 12S rRNA, and tRNAs). These genomes lack significant intragenic spacers and are generally without introns. Here, we report the complete mitochondrial genome sequence of the placozoan Trichoplax adhaerens, a metazoan with the simplest known body plan of any animal, possessing no organs, no basal membrane, and only four different somatic cell types. Our analysis shows that the Trichoplax mitochondrion contains the largest known metazoan mtDNA genome at 43,079 bp, more than twice the size of the typical metazoan mtDNA. The mitochondrion’s size is due to numerous intragenic spacers, several introns and ORFs of unknown function, and protein-coding regions that are generally larger than those found in other animals. Not only does the Trichoplax mtDNA have characteristics of the mitochondrial genomes of known metazoan outgroups, such as chytrid fungi and choanoflagellates, but, more importantly, it shares derived features unique to the Metazoa. Phylogenetic analyses of mitochondrial proteins provide strong support for the placement of the phylum Placozoa at the root of the Metazoa.

Footnotes

^†To whom correspondence should be addressed. E-mail: stephen.dellaporta{at}yale.edu
Author contributions: S.L.D. designed research; S.L.D., A.X., S.S., W.J., and M.A.M. performed research; M.A.M. contributed new reagents/analytic tools; S.L.D., A.X., M.A.M., L.W.B., and B.S. analyzed data; and S.L.D., L.W.B., and B.S. wrote the paper.
Conflict of interest statement: No conflicts declared.
This paper was submitted directly (Track II) to the PNAS office.
Data deposition: The sequence reported in this paper has been deposited in the GenBank database (accession no. DQ112541).
Abbreviations:

Abbreviations:

ML,

maximum likelihood.
Freely available online through the PNAS open access option.