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

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.animal evolution ͉ phylogenetics T richoplax adhaerens [Shulze 1883] is a marine invertebrate distributed in tropical waters worldwide (1-3). It is the simplest known free-living animal, displaying no axis of symmetry, lacking a basal membrane, possessing only four somatic cell types (4-6), and having one of the smallest known animal genomes (7-9). Until recently, T. adhaerens was the sole representative of the phylum Placozoa, but recent field studies and molecular analyses indicate genetic diversity underlying apparent morphological uniformity within the Placozoa (3, 10). In the laboratory, placozoans reproduce asexually by either binary fission or budding dispersive propagules called swarmers. Eggs have been observed, and recent DNA polymorphism analysis has provided evidence for sexual reproduction within the Placozoa (10).The phylogenetic placement of Placozoa among the metazoans, i.e., the animals, remains unresolved. In particular, its placement among lower metazoans, that is, the phyla Cnidaria, Ctenophora, and Porifera, has been controversial. Most studies place Porifera at the base of the metazoan tree of life (11-15), but others support placozoans as one of the earliest branching lineages of . Conflicting data, including 18S, 28S, and 16S analysis, have suggested that Placozoa form a sister clade to all bilaterians or a sister clade to both cnidarians and bilaterians (14,(21)(22)(23)(24)(25)(26)(27).Comparative mitochondrial genomics is becoming an effective tool to resolve phylogenetic placements because of several unique properties of mitochondrial genomes, including uniparental inheritance, orthologous genes, and lack of substantial intermolecular recombination (reviewed in refs. 28-30). Although some have questioned th...

Genetic aspects of mitochondrial genome evolution

“…In the metazoan key phylum Placozoa (Schierwater, 2005) the mitochondrial genomes display several ancestral metazoan features (Dellaporta et al, 2006; Signorovitch et al, 2007). The extended genome size of up to 43kb, the presence of introns, large intergenic regions and several ORFs of unknown function are shared characteristics with known metazoan outgroups like choanoflagellates such as Monosiga brevicollis (Burger et al, 2003) and therefore highlight a basal position of Placozoa at the root of the Metazoa.…”

Animals and Their Unicellular Ancestors

“…However, molecular phylogenies have shown placozoans in every possible position among the diploblasts, from sister group to Cnidaria (Bridge et al, 1995), to basal diploblasts (Dellaporta et al, 2006;Schierwater et al, 2009), to an intermediate position between sponges and cnidarians (Srivastava et al, 2008). Finally, the diploblast phylum 'Ctenophora' has recently been shown to be the earliestbranching metazoans (Dunn et al, 2008;Hejnol et al, 2009).…”

The Proto-MHC of Placozoans, a Region Specialized in Cellular Stress and Ubiquitination/Proteasome Pathways

“…1A); it is generally accepted that they are a sister group to bilaterians and cnidarians, such as corals and jellyfish (36,37), and there is increasing evidence they are the most basal branches of metazoans (38)(39)(40).…”