Mitochondrial Genome Sequencing And Phylogenetic Evolution Analysis Of Meretrix Meretrix

Hard clam (Meretrix meretrix) belongs to Mollusca, Lamellibranchia, Heterodonta, Veneroida, Veneridae, Meretrix, which is a natural resource-rich shellfish, and possess high nutritional and medical value. In this research, after amplifying mtDNA using Long-PCR and normal PCR methods, the complete mitochondrial genomes of Meretrix meretrix was determined and analyzed through directly sequencing PCR products or sequencing cloned PCR products. The organization and characteristic of the mitogenome, including gene order, base compositon, 13 protein coding genes, rRNA genes, 22tRNA genes, codon usage and amino-acid composition were surveyed. Finally, the rearrangement of mitochondrial genes in Bivalvia was analyzed and phylogenetic tree was constructed based on amino acid data. The conclusions drawn from the study as follows.1. The entire mitochondrial (mt genome) sequence of M. meretrix is 19,826 bp in length, and contains 37 genes including 12 protein-coding genes, 2 ribosomal RNAs, and 23 tRNAs. All genes are encoded on the heavy strand. In contrast to the typical animal mitochondrial genome, it lacks the protein-coding gene ATP8.2. Mt genes commonly use several alternatives to ATG as start codons. Eight of twelve protein-coding genes (COII,COIII, ATP6, ND1, ND2, ND3, ND4, ND4L) of the clam initiate with ATG start codon, while Cyt b, ND5 and ND6 start with ATA and cox1 with TTG. Eight protein-coding genes use TAA as stop codon(COI, COII, ND2, ND3, ND4, ND4L, cob, atp6), and the remaining four genes terminate with TAG.3. The clam mtDNA encodes 23 tRNA genes, ranging in size from 62(trnH) to 71(trnA) nucleotides. All 23 tRNAs have the typical clover-leaf structure. In contrast to the standard complement of 22 tRNA genes, the clam mt genome lacks tRN^ASer(AGN), and has two duplication of tRNA^Gln, that is the first report in Bivalvia.4. Comparing the gene arrangement among M. meretrix, M. petechialis and Venerupis philippinarum, we observed that the gene arrangement between M. petechialis and M. meretrix are the same except for one more tRNA^Gln gene in M. meretrix mt genome than in M. petechialis and the gene arrangement between two species of the genus Meretrix and V. philippinarum share three completely large identical gene blocks. 5. Phylogenetic analyses based on amino acid data show that twenty three bivalve species are divided into 4 clades. The M. meretrix is placed in a clade containing of Veneridae, Solenidae, Cardiidae and Saxicavidae, which places the grouping composed with Ostridae and Pectinidae as the sister groups.