MBE Advance Access originally published online on February 9, 2005
Molecular Biology and Evolution 2005 22(5):1231-1239; doi:10.1093/molbev/msi108
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Research Article |
Mitochondrial Genomes of Two Demosponges Provide Insights into An Early Stage of Animal Evolution


* Département de Biochimie, Université de Montréal, Montréal, Québec, Canada;
National Institute of Water & Atmospheric Research (NIWA) Ltd., Newmarket, Auckland, New Zealand; and
Canadian Institute for Advanced Research, Centre Robert Cedergren
E-mail: dlavrov{at}iastate.edu.
Mitochondrial DNA (mtDNA) of multicellular animals (Metazoa) is typically a small (
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.
Key Words: sponge mitochondrial DNAs mitochondrial evolution evolutionary rates
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