Multiregional introgressions inferred from the mitochondrial DNA phylogeny of a hybridizing species complex of gobiid fishes, genus Tridentiger

Partial sequences of the cytochrome b gene (402 bp) in mtDNA were determined for brackishwater gobiid fishes, genus Tridentiger, collected from geographically distant locations in the Japanese Archipelago, and their interspecific and geographic variations were analyzed and compared. Contrary to the results of a previous allozyme analysis which revealed the existence of considerable genetic divergence (Nei's genetic distance > 0.5) between T. obscurus and T. brevispinis, the mtDNA haplotypes (mitotypes) of these two species were very similar and could not be distinguished by any of the neighbor-joining, maximum-likelihood or parsimony analyses. Hybrid individuals between the two species were also found, with several mitotypes being shared by both species and their hybrids. The phylogenetic relationships of mitotypes were divided into three subgroups, the geographical distributions of the latter being allied to geographical features of the Archipelago. These results suggested the occurrence of multiregional introgression between the two species, with mitotypes transferring from one species to the other.