The Complete Mitochondrial Genome of <em>Hyotissa hyotis</em> (Bivalvia: Gryphaeidae) Reveals a Unique Gene Order within Ostreoidea

The mitochondrial (mt) genome is an important tool when studying the evolution of metazoan animals. The oyster family Gryphaeidae, together with Ostreidae, is one of the two extant taxa of superfamily Ostreoidea. Up until now, the available mitochondrial genomes of oysters were all limited to family Ostreidae. In the present study, the first complete mtDNA of family Gryphaeidae represented by <i>Hyotissa hyotis</i> was sequenced and compared with other available ostreoid mtDNA. The mtDNA of <i>H. hyotis</i> is 22,185 bp in length, encoding 13 protein-coding-genes (PCGs), two ribosomal RNA (rRNA) and 23 transfer RNA (tRNA) genes. Within all the intergenic regions that range from 2 to 1528 bp, two large non-coding regions were identified. The first large non-coding region, located between <i>Cox1</i> and <i>trnA</i>, contains 1528 nucleotides, while the second one is 1191 bp in length and positioned between <i>Cytb</i> and <i>Nad2</i>. The nucleotide composition of the whole mtDNA is A + T biased, accounting for 59.2%, with a negative AT skew value of −0.20 and a positive GC skew value of 0.33. In contrast to the mtDNA of Ostreidae, neither the split of <i>rrnL</i> nor <i>rrnS</i> was detected in that of <i>H. hyotis</i>. The duplication of <i>trnW</i> of <i>H. hyotis</i> was also discovered for the first time within Ostreoidea. The gene order of <i>H. hyotis</i> is quite different from those of ostreids, indicating extensive rearrangements within superfamily Ostreoidea. The reconstructed phylogeny supported <i>H. hyotis</i> as sister to Ostreidae, with the latter clade formed by <i>Ostrea</i> + (<i>Saccostrea</i> + <i>Crassostrea</i>). This study could provide important information for further understanding the mitochondrial evolution of oysters.