High microsatellite but no mitochondrial DNA variation in an invasive Japanese mainland population of the parasitoid wasp Melittobia sosui

Theory predicts that bottleneck events reduce genetic diversity in invasive populations. The parasitoid wasp Melittobia sosui was only identified in the subtropical area of Japanese south islands and Taiwan, but recently found also in the temperate area of the Japanese mainland. The species may expand their distribution northward due to factors such as recent global warming. We investigated population genetics in both the native and invasive areas using mitochondrial and nuclear microsatellite DNA. As expected, there was mitochondrial variation in the native area, but not in the invasive area, which only had one haplotype. However, the two areas had a similar level of microsatellite variation, with averagely 43% and 38% alleles uniquely found in the native and invasive populations, respectively. The difference of genetic variation between mitochondrial and microsatellite DNA in the invasive populations may be explained by the faster mutation rate of microsatellites, as well as the population structure of Melittobia, in which subdivided small inbreeding lineages may facilitate the accumulation of mutations. The high proportion of private alleles suggests that the mainland population diverged from the native populations at least 100 years ago, ruling out the possibility that the mainland population was established recently. Instead, the present study suggests that M. sosui might have already existed in the mainland but with a low frequency, or the mainland population was derived from a third population which diverged from the native populations over 100 years ago.