Cytochrome b (Cyt-b) gene sequence analysis in six flatfish species (Teleostei, Pleuronectidae), with phylogenetic and taxonomic insights

Mitochondrial DNA (mtDNA) at Cyt-b gene region was sequenced for six flatfish species (in total nine sequences of at least 1,117 bp) from the Far East of Russia and compared with other sequences of Pleuronectiformes comprising altogether 34 species and 3 outgroup species (Perciformes). Analysis of the protein-coding Cyt-b gene revealed a statistically substantiated bias in (T + C):(A + G) content, supporting earlier findings. Values of P-distances, as summarized for different scales of the evolutionary history at Cyt-b gene, revealed a clear pattern of increased nucleotide diversity at four different phylogenetic levels: (1) intraspecies, (2) intragenus, (3) intrafamily, and (4) intraorder. Scores of average P-distances of the four categories of comparison in flatfishes were (1) 0.46 ± 0.19%, (2) 11.74 ± 2.26%, (3) 17.51 ± 3.13%, and (4) 25.60 ± 0.42%, respectively (mean ± SE). These data support the concept that speciation in the order Pleuronectiformes, in most cases, follows a geographic mode through the accumulation of numerous small genetic changes over a long period of time. A phylogenetic tree for 64 sequences of flatfishes and 3 other fishes belonging to ray-finned fishes (Actinopterigii) was developed using Cyt-b gene and four different analytical approaches: neighbour-joining (NJ), Bayesian (BA), maximum parsimony (MP), and maximum likelihood (ML). The analysis revealed a monophyletic origin for the representatives of Pleuronectidae, which is the principal flatfish family investigated (99% repetition level in our BA analysis). A well supported property of the phylogenetic tree was the monophyletic placement of all the five flatfish families and the order Pleuronectiformes among other representatives of ray-finned fishes of the class Actinopterigii. Species identification on per individual basis (barcoding tagging) was generally high. However, there were taxonomic complications that arose during analysis, and they are discussed. Major outcomes of this discussion are the necessity of synonymy acceptance for Hippoglossoides elassodon, and H. robustus, and for Pseudopleuronectes yokohamae and P. schrenki. Priority considerations suggest H. elassodon and P. yokohamae as valid species names.