Molecular evolution of bivalvian mtDNA and its systematic significance

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Metazoan mitochondrial genome encodes 13 protein genes, 2 rRNA genes and 22 tRNA genes. The gene arrangements and gene contents are usually conserved among different phyla. However, mtDNA of Mytilus edulis, a bivalve mollusca, shows a distinct gene arrangements with the unusual gene contents lacking ATPase 8 gene. In addition, Mytilus mtDNA is demonstrated to be inherited biparentally, while mtDNA is inherited maternally in all other metazoans. It is remained to be solved whether such unusual genome structure or mode of inheritance are universal for bivalve mollusca or specific to Mytilus. In the present study, we examined mt-gene arrangement and mtDNA inheritance for various bivalve taxa of the subclass Protobranchia, Pteriomorpha, Heterodonta, Palaeoheterodonta and Anomalodesmata.We detected presence of sex-specific mtDNAs in heterodont bivalves. Their gender association and tissue specificity are concordant with those found in Mytilus. Molecular phylogenetic analysis suggests frequent parallel evolution of male-specific mtDNA lineages.We could determine partial mt-gene arrangement for 8 different bivalve taxa. In higher bivalves, the gene arrangements and gene contents differs substantially among taxa. On the other hand, protobranch bivalves retain ancestral genome structure which shares many gene arrangements and normal gene contents with other phyla. In bivalves, mitochondrial genome structures should be derived from protobranch-like ancester and the unique gene orders and contents may be evolved independently in various higher taxa.