Demic structure and its relation with the distribution of an adaptive trait in Danish flea beetles

The flea beetle Phyllotreta nemorum is an oligophagous species using crucif ers as host plants. In Denmark two populations have been found which use Ba rbarea vulgaris ssp. arcuata (G-type) as a host plant, whereas this plant i s unsuitable for-the survival of the majority of P. nemorum. In the locatio ns in which these two populations occur, alternative host plants are also p resent. The plants occur in patches, some of which contain a mixture of hos t plants.,In this study of allozyme variation, genetic differentiation betw een P. nemorum using different host plants in patches in the two different localities was studied hierarchically to assess substructuring of the popul ations. Evidence was found for low, but significant, amounts of genetic dif ferentiation between (sub)populations using spatially separated plant patch es at a distance of approximate to 100 m to 1 km (theta = 0.009) and betwee n localities approximate to 44 km apart (theta = 0.026), and there was an a ssociation between genetic differentiation and geographical distance. No ge netic differentiation was found between beetles from different host plants with overlapping local distributions. No evidence was thus found for sympat ric host race formation. The geographical distribution of genes enabling P. nemorum to use B. vulgaris as a host plant (100% 'resistant' beetles in sa mples from B. vulgaris, but much fewer on patches containing only alternati ve host plants) contrasts with the relatively low amount of genetic differe ntiation at the neutral allozyme loci. This distribution of 'resistant' bee tles (to B. vulgaris defence) is likely to be influenced by local differenc es in selection and asymmetric gene flow.