High Microsatellite but No Mitochondrial <scp>DNA</scp> Variation in an Invasive Japanese Mainland Population of the Parasitoid Wasp <i>Melittobia sosui</i>

Invasive populations are predicted to have reduced genetic diversity due to bottleneck events. The parasitoid wasp <i>Melittobia sosui</i> was previously identified only in the subtropical area of the southern Japanese islands and Taiwan but was recently found in the temperate area of the Japanese mainland. The distribution of this species may have recently expanded northward due to factors such as climatic events and global warming. The population genetics of both the native and invasive regions were investigated using mitochondrial and nuclear microsatellite DNA. As expected, mitochondrial variation was observed in the native region but not in the invasive region, which had only one haplotype. However, the two regions exhibited similar levels of microsatellite variation, and an average of 43% and 38% of alleles were uniquely found in the native and invasive populations, respectively. The difference in 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 <i>Melittobia</i>, in which the subdivision into 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. The present study suggests that <i>M. sosui</i> might have already existed on the mainland but at a low frequency or that the mainland population was derived from a ghost population that diverged from the native populations more than 100 years ago.