Molecular phylogeny of seven pleuronectid species inferred from the sequence of their cytochrome oxidase subunit I gene.

Abstract

A 1213 base pair region of mitochondrial DNA corresponding to the cytochrome oxidase subunit I gene (COI) from 7 pleuronectid flatfishes (Order Pleuronectiformes) and one outgroup (Paralichthys dentatus) was PCR-cloned and sequenced. Pleuronectid taxa were chosen so that the main branching points in a cladogram of almost all species of Pleuronectidae (Cooper 1996) could be tested using an independent data set. Our results show that the COI sequence is highly conserved and that most of the substitutions that occur between species are synonymous changes at the third codon position. Analyses of the substitutional spectrum reveals a bias favouring the occurrence of transitions over transversions. Furthermore, transitions between C and T and transversions between A and C or T are more frequent than transitions or transversions involving G. COI sequence composition shows a bias against the occurrence of G and a preference for A at the third position of fourfold degenerate codons. Phylogenetic analyses based on all substitutions and on transversions only using parsimony and distance methods were congruent in most respects but were unable to resolve the branching order of Pseudopleuronectes americanus and Limanda ferruginea due either to a shortage of characters or homoplasy. The topology of the molecular trees provide support for the subfamily relationships in Cooper's classification (1996). However, the basal position of Microstomus pacificus in the molecular trees is largely incongruent with the morphology tree. This result may question the monophyletic status of the Pleuronectinae (sensu Cooper 1996) or may be an indication of mitochondrial introgression involving Microstomus pacificus.