The population genetics of the apple maggot fly, Rhagoletis pomonella and the snowberry maggot, R-zephyria: implications for models of sympatric speciation

True fruit flies belonging to the Rhagoletis pomonella (Diptera:Tephritidae ) sibling species complex possess several attributes consistent with a hist ory of sympatric divergence via host plant shifts. Hen, we investigate whet her hybridization and genetic introgression is occurring between two member s of the group, Rhagoletis pomonella (Walsh), whose primary hosts are domes tic apples (Malus pumila) and hawthorns (Crataegus spp., and R, zephyria (S now) whose host is snowberries (Symphoricarpos spp.). These flies are impor tant because they appear to be at a transition stage between taxa reproduct ively isolated solely on the basis of host plant-related adaptations and th ose that have evolved additional non-host dependent sterility and inviabili ty. Observing extensive hybridization and introgression between R. pomonell a and R. zephyria would have major repercussions for current models of symp atric speciation. In a survey of allozyme and mitochondrial DNA (mtDNA) variation for 1105 fl ies collected throughout the northern United States we found two results su ggesting that low level hybridization occurs between R. pomonella and R. ze phyria. (1) Two flies reared from snowberries and one fly reared from hawth oms had genotypes indicative of them being R, pomonella and R. zephyria, re spectively. Rhagoletis pomonella and R. zephyria adults map therefore occas ionally frequent each others host plant, providing the opportunity for hybr idization, (2) Four flies collected from hawthorns and one from snowberries had genotypes that made them likely to be F1 hybrids. Likelihood analysis revealed the data were also consistent with an hypothes is of shared ancestral alleles (i,e., the pattern of genetic variation coul d also be explained by R. pomonella and R, zephyria sharing alleles/haploty pes whose origins date to a common ancestor). We estimated that, in the abs ence of interspecific mating, random assortment of genes within R. pomonell a and R. zephyria populations would produce an average of 5.4 flies with ge notypes suggesting they were F1 hybrids - a number equivalent to the 5 puta tive F1 hybrids observed in the study. Our results therefore underscore the difficulty in distinguishing between hypotheses of low level introgression and shared ancestral polymorphism. But even if hybridization is occurring, the data suggest that it is happening at a very low and probably evolution arily insignificant level (perhaps 0.09% per generation), consistent with s ympatric speciation theory, Future tests are discussed that could help reso lve the hybridization issue for R, pomonella and R. zephyria.