Sequencing of the Complete Mitochondrial Genome of the Big Brown Mactra Clam, IMactra grandis/I

Mitochondrial genomes have become a powerful tool for studying molecular genetics and phylogeny of mollusks. In this study, the complete mitochondrial genome of Mactra grandis was characterized for the first time. The newly sequenced mitochondrial genome fits the typical composition pattern of mollusks with 37 functional genes. Among the Mactridae species with reported mitochondrial genomes, Mactra grandis has the closest relationship with Mactra cygnus. The gene arrangement, genetic distance, and selective pressure of protein-coding genes among Mactra species were also analyzed. This study provides a molecular basis for taxonomy and germplasm research on Mactridae species. Mitochondrial genomes are playing an increasingly important role in molluscan taxonomy, germplasm, and evolution studies. The first complete mitochondrial genome of the commercial big brown mactra clam, Mactra grandis, was characterized using Illumina next-generation sequencing in this study. The 17,289 bp circular genome has a typical gene organization of 13 protein-coding genes (PCGs), 2 rRNAs, and 22 tRNAs, with an obvious (A + T)-bias of 64.54%. All PCGs exhibited a homogeneous bias in nucleotide composition with a (A + T)-bias, a positive GC skew, and a negative AT skew. Results of phylogenetic analysis showed that Mactra grandis was most closely related to Mactra cygnus. The functional gene arrangement of the two species was identical but different from other Mactra species. The congeneric relationships among Mactra species were demonstrated by genetic distance analysis. Additionally, the selective pressure analysis suggested that cox1 was highly efficient for discriminating closely related species in genus Mactra, while nad2 was the most appropriate marker for population genetic analysis.