MOLECULAR ECOLOGY AND EVOLUTION Population Genetic Structure of Aphis glycines

The soybean aphid (Aphis glycinesMatsumura) is an invasive pest of cultivated soybean (Glycine max L.) in North America. After the initial invasion in 2000, the aphid has quickly spread across most of the United States and Canada, suggesting large-scale dispersal and rapid adaptation to newenvironments.Usingmicrosatellitemarkers fromclosely related species,wecompared thegenetic diversity and the amount of genetic differentiation within and among 2 South Korean and 10 North American populations. Overall allelic polymorphism was low, never exceeding four alleles per locus. However, differences in genetic diversity were seen among South Korean and North American populations in terms of heterozygote excesses and genotypic richness. Within North America, two populations (Michigan and Ontario), had lower genetic diversities and exhibited high genetic differentiation compared with the remaining eight populations. The earlier collection time of Michigan andOntario samples explained the genetic differences better thangeographic subdivisions. Thesedata indicate a pattern of small colonizing populations on soybeans, followed by rapid clonal ampliÞcation and subsequent large-scale dispersal across North America.