Differences in the genetic structure of spatial and temporally distributed populations of the invasive pest soybean aphid, Aphis glycines

Background/Question/Methods : Since its introduction in 2000 to North America, the soybean aphid (Aphis glycines) has spread across much of Southeast Canada, the Midwest and Northeast U.S. This aphid is currently one of the most damaging pests of soybean, showing a remarkable ability to quickly disperse and adapt to ecological pressures. The soybean aphid’s life cycle involves multiple colonization and migration events based on host distribution and time of year. Therefore, it is important to understand the migration and genetic events that drive its adaptation. We conducted a population genetic study using populations from eight widely distributed North American sites. Collections were done twice in each location (the initial soybean colonizing population and after late-season migration) this with the purpose of evaluating genetic differences in both time and space. To analyze genetic structure, we used six microsatellite markers and twenty-four SNP markers to calculate within-population frequency parameters and genetic distance among populations. Results/Conclusions: A pairwise comparison showed significant variability in the genetic distance of early vs. late populations (P=0.002), but no significant differences among late populations. A further test using Principal Component Analysis showed strong differentiation among early populations while little differentiation among late populations. When testing the early populations for Hardy-Weinberg equilibrium (P<0.05), five out of the eight populations showed a significant deviation, while only two late populations showed disequilibrium due to heterozygosity excess.  From these results we can conclude that early in the season soybean aphid populations undergo structured migration and a bottleneck that suppresses diversity. As the summer progresses, asexual reproduction leads to an increase of population size and widespread migration. This recombination among populations leads to diversity restoration and homogenization among populations.