Mitochondrial Genomes in Invertebrate Animals

Abstract

Animal mtDNA is commonly described as a small, circular molecule, remarkably uniform in size, gene content, and genomic organization. The results of recent studies contradict this view and reveal substantial diversity in animal mtDNA organization. As should be expected, most of this diversity is found in non-bilaterian animals: phyla Cnidaria, Ctenophora, Placozoa, and Porifera, each of which displays a unique mode and tempo of mitochondrial genome evolution. Mitochondrial DNA in the phylum Cnidaria is characterized by low rate of sequence evolution and loss of all but one or two tRNA genes. In addition, linear mitochondrial genome architecture has evolved within this phylum. In the phylum Ctenophora, mtDNA is characterized by small size, high rate of sequence evolution, and loss of at least 24 genes, including all tRNA genes and two protein-coding genes. In the phylum Placozoa, mtDNA is large in size and contains a substantial number of introns. It also displays such unusual features as mRNA editing and group I intron trans-splicing. Finally, mtDNA patterns are remarkably different among the four main lineages of sponges, with variation in size, genome organization, genetic code, gene content, presence/absence of introns, tRNA structures and editing, rates of evolution, and proliferation of repetitive elements.