Host-associated genetic structure of Mexican populations of the cabbage aphid Brevicoryne brassicae L. (Homoptera: Aphididae)

Phytophagous insects can use different host plant species across their geographic distribution. Within a locality, however, their feeding can be restricted to one or two plant species. If host species constitute different selective regimes to herbivorous insects, genetic differentiation and host-associated local adaptation may occur. In this study, we describe the genetic structure of the aphid Brevicoryne brassicae L. associated to Brassica campestris L. and B. oleraceae var. capitata L., two plant species that occur sympatrically in four localities in the highlands of Chiapas, Mexico. The aim was to determine if the aphid populations are genetically structured in relation to the plant host species, and if such differentiation is consistent among localities. The genetic description of populations was made using 11 enzyme loci using cellulose acetate electrophoresis. Aphid genotypes were surveyed in two host plant species-associated populations within each of four localities at seven polymorphic loci (eight subpopulations in total). The genetic structure was assessed at the level of subpopulations, among localities, between hosts, and pairwise comparisons of hosts within locality, using Wright F-statistics. Genetic distance among localities and between host-associated populations within each locality was also estimated. We found that overall genetic differentiation was high (F-ST = 0.22), and that differentiation among localities (F-ST = 0.13) was higher than differentiation between hosts (F-ST = 0.03). All FST estimates were statistically significant. Pairwise comparisons of FST between hosts in each locality suggest high differentiation in two of them, and low but still significant differentiation in two other localities. Given that general environmental conditions are similar within localities, selection on each host species may produce genetic divergence within and among subpopulations of B. brassicae.