Demographic imbalance in the hybrid zone led to asymmetric gene flow between two closely related geckos, Gekko hokouensis and Gekko yakuensis (Squamata: Gekkonidae)

Abstract Introgressive hybridization caused by invasive species may lead to the extinction of pure native species, but a theoretical model of the invasion process with hybridization (the neutral demographic model) suggests that introgression occurs mainly from native to invasive species. Clarifying the direction of introgression is important to evaluate the impact of hybridization on native species. In southern Japan, a widely disseminated gecko, Gekko hokouensis, and a local gecko, Gekko yakuensis, have formed mosaic hybrid zones. Given the presumed invasiveness of Gekko hokouensis, the neutral demographic model predicts that introgression would mainly occur in the direction to Gekko hokouensis, whereas possible asymmetric assortative mating suggested by a previous study predicts the opposite direction of introgression. Our investigations using microsatellite markers and mitochondrial DNA for the gecko assemblages in 12 localities and comparisons with allozyme data collected 18 years ago indicated that Gekko hokouensis expanded its range and tended to have replaced hybrids and Gekko yakuensis. Introgression was skewed towards Gekko hokouensis, but interspecific mating between the parental species was reciprocal. These results suggest that their hybridization progress follows the neutral demographic model and that the higher population growth rate of Gekko hokouensis is the major cause of species replacement, although we cannot rule out the possibility that asymmetric assortative mating also influences the later stages of species replacement.