Orthodox and unorthodox phylogenetic relationships among tunas revealed by the nucleotide sequence analysis of the mitochondrial DNA control region

A phylogeny of all eight recognized taxa of the genus Thunnus was constructed from approximately 400 base pairs of sequence of the mitochondrial DNA (mtDNA) control region. The PCR-amplified control region I segment studied contained a total of 186 variable sites and 159 phylogenetically informative sites. Diagnostic sequences for every taxon were identified. Neighbour-joining phylogenies supported monophyletic origins of the temperate subgenus Thunnus and of the tropical subgenus Neothunnus. Similar results were obtained by maximum parsimony analyses except that there was no support for a monophyletic origin of the subgenus Thunnus. Bigeye tuna, which have been difficult to place in either subgenus using conventional morphological data, was identified as the sister species of Neothunnus. Within the subgenus Thunnus, the Atlantic bluefin and Southern bluefin tunas were shown to be sister taxa of the highly divergent monophyletic clade formed by the Pacific northern bluefin and the Albacore tunas. The conspecific Atlantic (T. thynnus thynnus) and Pacific (T. t. orientalis) northern bluefin tunas were more divergent (Tamura-Nei distance 0.145 plus or minus 0.019) from each other than the average distance separating most species-pairs within the genus. Thus, a re-examination of their status as subspecies of T. thunnus is warranted.