Rapid genetic structuring of populations of the invasive fall webworm in relation to spatial expansion and control campaigns.
Aim: To establish patterns of genetic variation associated with the well-documented history of invasion and expansion of the fall webworm (FWW), Hyphantria cunea, into China. Patterns are expected to be affected by geographical and environmental factors as well as a history of control campaigns. Location: Northern and eastern China. Methods: We genotyped 18 polymorphic microsatellite loci derived from 657 individuals (25 populations) collected from across the distribution range of FWW in China and a native population in USA, and we also sequenced mitochondrial DNA from a subset of 70 individuals. Population genetic structure was analysed by using three Bayesian cluster methods (STRUCTURE, GENELAND and TESS) and a multivariate principal component analysis (PCA). The scenarios of invasion history were tested using an approximate Bayesian computation (ABC) approach. A multiple matrix regression was performed to assess the relative contributions of geographical and environmental isolation. Results: The average allelic richness and mitochondrial haplotype diversity of the FWW populations in China was very low compared to native populations from USA, pointing to a significant bottleneck during colonization. Based on the nuclear markers, two genetically separated groups were identified along with a contact zone between them. ABC analyses suggested a single introduction into the east with subsequent westward expansion. Geographic distance was more important than environmental distance in contributing to genetic differentiation, and geographical barriers were likely to have restricted gene flow, with the contact zone occurring in a mountainous area. Main conclusions: The combination of genetic data and historical records of the expansion suggests that geographical barriers and control campaigns have affected the formation and persistence of the two genetic groups. Our study points to ways in which genetic differentiation can develop rapidly in an invasive species after its introduction.