Complete mitochondrial DNA sequence of the eastern oyster Crassostrea virginica

The complete mitochondrial genome of the eastern oyster Crassostrea virginica (GenBank accession number AY905542) is 17,243 bp in length and contains 2 ribosomal genes, 12 protein-coding genes, and 23 transfer RNAs. The arrangement of protein-coding genes is identical to that of the congeneric Pacific oyster C. gigas, but tRNA genes show several duplications and extensive rearrangements between the species. Unique features in C. virginica include an additional trnM gene, the absence of an ATPase subunit 8 (atp8) gene, and an inferred translational frameshift within the cytochrome b (cob) gene. In both species the large subunit ribosomal RNA gene is encoded by 2 separate regions of the mitochondrial genome, the first reported case of a split ribosomal RNA gene in a metazoan. Translation of protein-coding genes in both species is initiated with methionine, with the exception of cob, which uses leucine. In C. virginica translation of all protein-coding genes (except possibly cob) terminates with TAA, with polyadenylation completing the primary transcript in cytochrome oxidase subunit III (cox3) and NADH dehydrogenase subunit 4L (nad4L), whereas C. gigas employs stop codons TAA and TAG equally. Interspecific divergence of mitochondrially encoded proteins is considerable, with amino acid identities ranging from 47% to 92%. A single major noncoding region representing the putative control region is found in both species.