Mitochondrial Genomes of Tapes Dorsatus and Cardita Variegata: Insights into Heteroconchia Phylogeny

Heteroconchia, a widespread and abundant aquatic invertebrate, is an important clade of bivalve mollusks. The relationship between the three branches of Heteroconchia, Palaeoheterodonta, Archiheterodonta, and Euheterodonta has become a main controversy in molecular studies of the relationships between bivalves. In the present study, we assembled the complete mitochondrial genomes of Tapes dorsatus (Veneridae) and Cardita variegata (Carditidae) using high-throughput sequencing. C. variegata is the first mitochondrial genome belonging to the family Carditidae to be reported. We used 12 protein coding genes (excluding atp8) from the complete mitochondrial genomes of 146 species to recover the internal relationships of Heteroconchia. Our results support the traditional view of early branching of Palaeoheterodonta and the recovery of the monophyly of Palaeoheterodonta, Anomalodesmata, Imparidentia. Rearrangement analysis show that gene arrangement within Venerida was highly variable. Time-calibrated phylogenetic studies based on a relaxed molecular clock model suggested that Veneridae originated approximately 337.62 million years ago (Ma) and split into two major clades, whereas Carditidae originated approximately 510.09 Ma. Our results provide evidence of the internal relationships of Heteroconchia.