SELECTIVE MAINTENANCE OF ALLOZYME DIFFERENCES AMONG SYMPATRIC HOST RACES OF THE APPLE MAGGOT FLY

Whether phytophagous insects can speciate in sympatry when they shift and adapt to new host plants is a controversial question, One essentia l requirement for sympatric speciation is that disruptive selection ou tweighs gene flow between insect populations using different host plan ts. Empirical support for host-related selection (i.e., fitness trade- offs) is scant, however. Here, we test for host-dependent selection ac ting on apple (Malus pumila)- and hawthorn (Crataegus spp.)-infesting races of Rhagoletis pomonella (Diptera: Tephritidae). In particular, w e examine whether the earlier fruiting phenology of apple trees favors pupae in deeper states of diapause (or with slower metabolisms/develo pment rates) in the apple fly race. By experimentally lengthening the time period preceding winter, we exposed hawthorn race pupae to enviro nmental conditions typically faced by apple flies. This exposure induc ed a significant genetic response at six allozyme loci in surviving ha wthorn fly adults toward allele frequencies found in the apple race. T he sensitivity of hawthorn fly pupae to extended periods of warm weath er therefore selects against hawthorn flies that infest apples and hel ps to maintain the genetic integrity of the apple race by counteractin g gene flow from sympatric hawthorn populations. Our findings confirm that postzygotic reproductive isolation can evolve as a pleiotropic co nsequence of host-associated adaptation, a central tenet of nonallopat ric speciation. They also suggest that one reason for the paucity of r eported fitness trade-offs is a failure to consider adequately costs a ssociated with coordinating an insect's life cycle with the phenology of its host plant.