Natural hybridization among species of the genus Gekko (Squamata: Gekkonidae) in Japan: the roles of pre- and postmating isolation

Among the eight Japanese species of the genus Gekko, natural hybridization was reported in four pairs at various frequencies. A recent behavioral study suggested that the species-specific courtship calls of the males act as premating isolation for some species of these geckos; however, others lack species-specific calls, which results in hybridizations. No comprehensive study has been conducted to identify a general pattern in the development of isolating barriers in this group. We examined the hybridization status between eight pairs of seven species using population genetic analyses based on microsatellite markers and allozyme data. We then examined the changes in the frequency of hybrids and the proportion of F₁ hybrids against the total number of hybrids along with increasing genetic distances in a phylogenetic framework. The contribution of the courtship call type as a determinant of the presence/absence of hybridization was evaluated by statistical modeling. The results of the genetic analyses revealed hybridizations between two pairs of three species, each having a species-specific call, in addition to previously reported species pairs. Statistical analyses indicated that the frequency of hybrids between species was negatively correlated with genetic distance, and the proportion of F₁ hybrids increased with genetic distance. The results suggest the progressive development of postmating isolation with genetic divergence in this group. The contribution of call type to the presence/absence of hybridization was not fully supported, in part, because of the hybridizations of one of the species exhibiting species-specific calls. A possible reason for the exceptional hybridizations between the species with species-specific calls is discussed.