THE EFFECTS OF WINTER LENGTH ON THE GENETICS OF APPLE AND HAWTHORN RACES OF RHAGOLETIS-POMONELLA (DIPTERA, TEPHRITIDAE)

Host plant-associated fitness trade-offs are central to models of symp atric speciation proposed for certain phytophagous insects. But empiri cal evidence for such trade-offs is scant, which has called into quest ion the likelihood of nonallopatric speciation. Here, we report on the second in a series of studies testing for host-related selection on p upal life-history characteristics of apple- (Malus pumila L.) and hawt horn- (Crataegus mollis L. spp.) infesting races of the Tephritid frui t fly, Rhagoletis pomonella (Walsh). In particular we examine the effe cts of winter length on the genetics of these flies. We have previousl y found that the earlier fruiting phenology of apple trees exposes app le-fly pupae to longer periods of warm weather preceding winter than h awthorn-fly pupae. Because R. pomonella has a facultative diapause, we hypothesized that this selects for pupae with more recalcitrant pupal diapauses (or slower metabolic/development rates) in the apple-fly ra ce. A study in which we experimentally manipulated the length of the p rewintering period for hawthorn-origin pupae supported this prediction . If the period preceding winter is important for apple-and hawthorn-f ly pupae, then so too should be the length (duration) of winter; the r ationale for this prediction is hat ''fast developing'' pupae that bre ak diapause too early will deplete their energy reserves and dispropor tionately die during long winters. To test this possibility, we chille d apple-and hawthorn-origin pupae collected from a field site near Gra nt, Michigan, in a refrigerator at 4 degrees C for time periods rangin g from one week to two years. Our a priori expectation was that longer periods of cold storage would select against allozyme markers that we re associated with faster rates of development in our earlier study. S ince these electromorphs are typically found at higher frequencies in hawthorn flies, extending the overwintering period should favor ''appl e-fly alleles'' in both races, The results from this ''overwinter'' ex periment supported the diapause hypothesis. The anticipated genetic re sponse was observed in both apple and hawthorn races, as allele freque ncies became significantly more ''apple-fly-like'' in eclosing adults surviving longer chilling periods. This indicates that it is the combi nation of environmental conditions before and during winter that selec ts an the host races, Many tests for trade-offs fail to adequately con sider the interplay between insect development, host plant phenology, and local climatic conditions. Our findings suggest that such oversigh t may help to explain the paucity of reported fitness trade-offs.