Hierarchical population structure analysis of the milkweed beetle, Tetraopes tetraophthalmus (Forster)

Abstract

The population structure of the milkweed beetle Tetraopes tetraophthalmus (Forster) is tied intimately to the biology of its one host plant, Asclepias syriaca. The patchy distribution of the host plant and the limited dispersal of the beetle combine to organise the herbivore into numerous very localised populations. An analysis of hierarchical patterns of spatial variation in allozyme frequencies by means of Wright's F-statistics reveals that in some parts of the beetle's range this population structure can result in moderate genetic differentiation (F = 0·03–0·06) among beetle populations inhabiting milkweed patches separated by only a few kilometres. Substantial differentiation occurs across the range of the species (F = 0·154). A review of similar studies shows the values of F calculated for T. tetraophthalmus to be among the highest found for a flying insect. It is argued that the local differentiation results, in part, from a reduction in effective population sizes associated with the ephemeral nature of milkweed patches and that this population structure is favourable for the operation of Wright's Shifting Balance mode of evolution.