Genetics of resistance against defences of the host plant Barbarea vulgaris in a Danish flea beetle population

One essential aspect of the study of the evolution of host-plant use by ins ects is (variation in) its genetic basis. The genetic basis of the ability of a flea beetle (Phyllotreta nemorum) to use the crucifer Barbarea vulgari s ssp. arcuata (G type) as a host plant was studied in a Danish population (Kvaerkeby) occurring naturally on this atypical host plant. Evidence was f ound that this ability was determined by a single, major, autosomal gene, a lthough the presence of genes at additional loci at lower frequencies could not be excluded. No evidence was found for sex-linked inheritance, which w as common in a second population in Denmark (Ejby) using Barbarea as a host plant. All beetles in the Kvaerkeby sample were homozygous 'resistant' to Bardarea defence. After crossing resistant F-1 offspring from pairs consist ing of a field-collected beetle and a susceptible one amongst each other, g enotyping the F-2 (reared on radish) showed a 1:2:1 ratio of homozgous resi stant, heterozygous and susceptible beetles. No evidence was found for a re duction in the viability of beetles that were homozygous resistant at the a utosomal locus, in contrast to what had been found earlier for two backcros sed lines founded by beetles from Ejby The results show that there is varia tion in the genetic basis of host-plant use across local populations and im ply that population structure should form part of the study of the interact ion between P. nemorum and its host plants.