THE COMPLETE MITOCHONDRIAL DNA OF COBIA (RACHYCENTRON CANADUM) AND PHYLOGENETICS OF CARANGOID

Cobia Rachycentron canadum is the only species of Rachycentridae distributing widely in subtropical and tropical oceans and seasonally in temperate waters.Cobia has the qualities that define an excellent candidate and emerging global potential for mariculture.Numerous comparative studies have been conducted in attempt to resolve the relationship(to be monophyletic) between Rachycentridae and the other teleosts using anatomical and behavioral methods.The Coryphaenidae and the Rachycentridae and Echeneidae were thought to form a monophyletic group,with Coryphaenidae being the sister group to the clade of Rachycentridae and Echeneidae.Nematistiidae and Carangidae were also hypothesized to be part of a clade with these three families,perhaps forming a trichotomy.However,molecu-lar research regarding the evolutionary relationship of these families remained scarce.Adequate resolution of relation-ships in any organisms required longer DNA sequences.In this study,the whole mitogenome sequence was determined for cobia,using an approach that employs a long polymerase chain reaction technique and primer walking.It was the first complete mtDNA sequence of cobia(Accession No.FJ154956 and NC_011219,16758 bp) in GenBank.The mito-chondrial genomic sequence contained the same 37 mitochondrial structural genes(two ribosomal RNA,22 transfer RNA,and 13 protein-coding genes) as found in other vertebrates,with the gene order identical to that in typical verte-brates.All following analyses were based on two datasets.Dataset 1 consisted of highly similar DNA sequences of mi-togenomes(score=10055-30213) from GenBank;and dataset 2 included the 20 ND2 gene complete sequences of the representative species within Coryphaenidae,Nematistiidae,Echeneidae and all branches with high support values in the trees based on dataset 1.The phylogenetic relationship among cobia and other teleost were reconstructed using maxi-mum parsimony(MP),neighbor-joining(NJ),and maximum likelihood(ML) with dataset 1.MP bootstrap scores were much lower than those of NJ or ML,and the MP resolution among higher-level classification were also the lowest.While the NJ and ML analyses provided strong support for a sister group relationship between two clades of Perciformes,in which one clade contained cobia,Carangidae(Carangoides armatus,Trachurus japonicas,and T.trachurus),Istio-phoridae(Makaira indica,Istiophorus platypterus)and Xiphiidae(Xiphias gladius)(bootstrap=78 and 97). Unexpect-edly,another clade was composed of the other taxa of order Perciformes and two typical species(Ostracion immaculatus and triacanthodes anomalus)of order Tetraodontiformes.The origin time of cobia was estimated by Bayesian analysis of the dataset 2 under the GTR+G+I+relaxed model for 8.0×106 generations resulted in a posterior probability distribution containing 1000 samples per analysis.The results support that:(1) the relative between cobia and dolphinfish is the closest(posterior probability=0.997),and identical with the period of first occurrence of cobia fossil,their most recent common ancestor(MRCA) lived in 56 million years ago(Ma),i.e.Thanetian. (2) Rachycentridae,Coryphaenidae and Echeneidae of the superfamily Echeneoidea proposed by Johnson(1984) cluster in a monophyletic group with low pos-terior probability(0.593). However,Nematistiidae and Carangidae are placed in clusters of scombridae and Tetraodonti-dae,repectively.Conclusively,Carangoid lineage is not a monophyly.