Molecular Phylogeny and Geographic Distribution of Wood-Feeding Cockroaches in East Asian Islands

Molecular phylogenetic relationships of the wood-feeding cockroach genera Salganea and Panesthia (Blaberidae; Panesthiinae) in East Asian Islands (Ryukyu archipelago and Taiwan Island) were analyzed based on the DNA sequence of the complete mitochondrial cytochrome oxidase II gene. Unweighted parsimony analysis resulted in high bootstrap support for relationships within Panesthia taxa; however, some nodes were unresolved between members of Salganea. Comparison of the number of transitions and transversions with genetic distance at each codon position suggested that saturation of third-codon substitutions has occurred between certain pairs of taxa. Consequently, differential weighting of substitutions at these sites was performed, which resulted in a substantial increase in resolution of Salganea relationships. The inferred phylogenies for both genera displayed good correspondence to the geographical locations of populations but however did not agree with previous subspecies designations based on morphological characters. It appears that both cockroach genera invaded the Ryukyu archipelago from the Taiwan region via a land-bridge present in the Miocene period. Invasion of the main islands of Japan by these cockroaches most likely occurred before the formation of the Tokara Tectonic Strait. Our study suggests that several barriers to gene flow have arisen and persisted over the past ~10 million years, which have caused segregation and vicariant speciation of the cockroach taxa of this region.