Complete Mitochondrial DNA Sequence of the Scallop Placopecten magellanicus: Evidence of Transposition Leading to an Uncharacteristically Large Mitochondrial Genome
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Complete sequence determination of the mitochondrial (mt) genome of the sea scallop Placopecten magellanicus reveals a molecule radically different from that of the standard metazoan. With a minimum length of 30,680 nucleotides (nt; with one copy of a 1.4 kilobase (kb) repeat) and a maximum of 40,725 nt, it is the longest reported metazoan mitochondrial DNA (mtDNA). More than 50% of the genome is noncoding (NC), consisting of dispersed, imperfectly repeated sequences that are associated with tRNAs or tRNA-like structures. Although the genes for atp8 and two tRNAs were not discovered, the genome still has the potential for encoding 46 genes (the additional genes are all tRNAs), 9 of which encode tRNAs for methionine. The coding portions appear to be evolving at a rate consistent with other members of the pectinid clade. When the NC regions containing “dispersed repeat families” are examined in detail, we reach the conclusion that transposition involving tRNAs or tRNA-like structures is occurring and is responsible for the large size and abundance of noncoding DNA in the molecule. The rarity of enlarged mt genomes in the face of a demonstration that they can exist suggests that a small, compact organization is an actively maintained feature of metazoan mtDNA.
KeywordsPlacopecten magellanicus Mitochondrial genome Transposition Bivalve Mollusk Dispersed repeats
We wish to thank Claude Lemieux, Monique Turmel, and Christian Otis for generous assistance during a sabbatical year at Université Laval for M.S.; Mike Dadswell for supplying scallops; and Michael Smith, Kathy Fuller, and Ian Paterson for comments on the manuscript. This work was supported by an AUFA 25.55 grant and by funding from the Howard Gould Trust.
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