Genetic diversity of spatial and temporally distributed populations of soybean aphid (Aphis glycines) in the Midwest

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 overcome ecological boundaries. We conducted population genetic study using populations from 2009 from eight North American sites distributed throughout Ohio, Michigan, Minnesota, Wisconsin, South Dakota and Ontario. Collections were done twice in each location, a first time shortly after soybean was planted and a second time right before senescence of the fields, this with the purpose of evaluating genetic differences in both time and space. To analyze genetic structure, we used six microsatellite markers and developed twenty-four SNP markers to calculate within-population frequency parameters and genetic distance among populations. A pairwise comparison showed significant variability in the genetic distance of early vs. late populations (P=0.002). A further test using Principal Component Analysis showed strong differentiation among early populations while closer genetic relationship among late populations. When testing for Hardy-Weinberg equilibrium (P<0.05), five out of the eight early populations showed significant deviation, while only two late populations showed disequilibrium consequence of heterozygous excess. From these results we can conclude that early in the season soybean aphid populations undergo limited migration and a bottleneck that suppresses diversity at the moment of colonization. Nevertheless, as the summer progresses, asexual reproduction leads to an increase of population size which induces migration among populations and an overall homogenization.