title: Mitochondrial genomes of two demosponges provide insights into an early stage of animal evolution.
abstract: Mitochondrial DNA (mtDNA) of multicellular animals (Metazoa) is typically a small ( approximately 16 kbp), circular-mapping molecule that encodes 37 tightly packed genes. The structures of mtDNA-encoded transfer RNAs (tRNAs) and ribosomal RNAs (rRNAs) are usually highly unorthodox, and proteins are translated with multiple deviations from the standard genetic code. In contrast, mtDNA of the choanoflagellate Monosiga brevicollis, the closest unicellular relative of animals, is four times larger, contains 1.5 times as many genes, and lacks mentioned peculiarities of animal mtDNA. To investigate the evolutionary transition that led to the specific organization of metazoan mtDNA, we determined complete mitochondrial sequences from the demosponges Geodia neptuni and Tethya actinia, two representatives of the most basal animal phylum, the Porifera. We found that poriferan mtDNAs resemble those of other animals in their compact organization, lack of introns, and a well-conserved animal-like gene order. Yet, they contain several extra genes, encode bacterial-like rRNAs and tRNAs, and use a minimally derived genetic code. Our findings suggest that the evolution of the typical metazoan mtDNA has been a multistep process in which the compact genome organization and the reduced gene content were established prior to the reduction of tRNA and rRNA structures and the introduction of multiple changes of the translation code.
authors.affiliation: Département de Biochimie, Université de Montréal, Montréal, Québec, Canada. dlavrov@iastate.edu
authors.forename: Dennis V
authors.initials: DV
authors.lastname: Lavrov
authors.forename: Lise
authors.initials: L
authors.lastname: Forget
authors.forename: Michelle
authors.initials: M
authors.lastname: Kelly
authors.forename: B Franz
authors.initials: BF
authors.lastname: Lang
mesh_terms: D000595:Amino Acid Sequence; D000818:Animals; D005075:Biological Evolution; D004272:DNA, Mitochondrial / Q000235:genetics*; D005815:Genetic Code; D016678:Genome; D007438:Introns; D007448:Invertebrates / Q000235:genetics*; D008928:Mitochondria / Q000235:genetics*; D008969:Molecular Sequence Data; D010802:Phylogeny; D011161:Porifera / Q000235:genetics*; D012335:RNA, Ribosomal / Q000235:genetics*; D012343:RNA, Transfer / Q000235:genetics*; D017386:Sequence Homology, Amino Acid
chemical_list: D004272:DNA, Mitochondrial; D012335:RNA, Ribosomal; D012343:RNA, Transfer