Complete Mitochondrial Genome of King Threadfin, Polydactylus macrochir (Günther, 1867): Genome Characterization and Phylogenetic Analysis

Background: Polydactylus macrochir (Günther; 1867) is a member of the family Polynemidae. The placement of Polynemidae among teleosts has varied over the years. Methods: Therefore, in this study, we sequenced the complete mitochondrial genome of P. macrochir, analyzed the characterization of the mitochondrial genome, and investigated the phylogenetic relationships of Polynemidae. Results: The length of the P. macrochir mitogenome was 16,738 bp, with a typical order. Nucleotide composition analysis showed that the P. macrochir mitogenome was AT-biased (54.15%), and the PCGs tended to use A and C rather than T and G at the third codon. All the PCGs started with the regular codon ATG, except for cox1, which started with GTG. The termination codon varied across the PCGs. It was shown that the ka/ks ratios of all the PCGs were less than one. Phylogenetic analysis, based on the maximum likelihood (ML) and Bayesian inference (BI) methods, indicated that eight threadfins formed a well-supported monophyletic cluster. Polynemidae and Sphyraenidae clustered together as a monophyletic group. According to TimeTree analyses, the most recent common ancestor (MRCA) of Polynemidae was traced back to about 52.81 million years ago (MYA), while six species within Polynemidae diverged from 11.70 MYA to 20.05 MYA. Conclusions: The present study provides valuable mitochondrial information for the classification of P. macrochir and new insights into the phylogenetic relationships of Polynemidae.