The Mitochondrial Genomes of Two Scallops, Argopecten irradians and Chlamys farreri (Mollusca: Bivalvia): The Most Highly Rearranged Gene Order in the Family Pectinidae

Molluscs in general and bivalves in particular, exhibit an extraordinary degree of mitochondrial gene order variation when compared with other metazoans. Here, we determined the mitochondrial genomes of two scallops Argopecten irradians and Chlamys farreri. The complete mitochondrial genome of A. irradians is 16,211 nts in length and the nearly complete mitochondrial genome of C. farreri is 20,789 nts in length. Both of the genomes contain 35 genes including 12 protein-coding genes, 2 ribosomal RNAs, and 21 transfer RNAs. In contrast to the typical animal mitochondrial genome, both of them lack one protein-coding gene atp8 and two trnSs, but show an additional copy of trnF in A. irradians and of trnM in C. farreri, respectively. Gene order and genome content were compared among the four sequenced scallops. Gene arrangement of C. farreri closely resembles that of Mizuhopecten yessoensis. However, two genomes of C. farreri and A. irradians show only three small identical gene blocks and two genomes of A. irradians and Placopecten magellanicus share only one gene block. Comparison of the gene arrangement demonstrated that the four scallops share few identical gene blocks although they belong to the same family. This feature is seldom observed in Metazoa, even in other molluscan classes. The dramatic gene rearrangement often occurs in bivalves, especially in marine bivalves. In addition, comparisons of genomic character among bivalves are also presented.