A HIERARCHICAL ANALYSIS OF GENETIC DIFFERENTIATION IN A MONTANE LEAF BEETLE CHRYSOMELA AENEICOLLIS (COLEOPTERA: CHRYSOMELIDAE).

Herbivorous insects that use the same host plants as larvae and adults can have a subdivided population structure that corresponds to the distribution of their hosts. Having a subdivided population structure favors local adaptation of subpopulations to small-scale environmental differences and it may promote their genetic divergence. In this paper, I present the results of a hierarchical study of population structure in a montane willow leaf beetle, Chrysomela aeneicollis (Coleoptera: Chrysomelidae). This species spends its entire life associated with the larval host (Salix spp.), which occurs in patches along high-elevation streams and in montane bogs. I analyzed the genetic differentiation of C. aeneicollis populations along three drainages in the Sierra Nevada mountains of California at five enzyme loci: ak-1, idh-2, mpi-1, pgi-1, and pgm-1, using recent modifications of Wright's F-statistics. My results demonstrated significant differentiation (F = 0.043) among drainages that are less than 40 kilometers apart. One locus, pgi-1, showed much greater differentiation than the other four (F = 0.412), suggesting that it is under natural selection. C. aeneicollis populations were also subdivided within drainages, with significant differentiation 1) among patches of willows (spanning less than three kilometers) and 2) in some cases, among trees within a willow patch. My results demonstrate that this species has the capacity to adapt to local environmental variation at small spatial scales.