are not directly germane to resolving the status of R. electromorpha sequences. However, sequence data available for the more distantly related R. cingulata and R. suavis for P661, P309, P2620, and P3060 (loci with shallow RNDs) place R. electromorpha between these two species and R. pomonella. The lack of interspersed clades of sequences containing all or subsets of the four species implies that variation at P661, P309, P2620, and P3060 is allelic and not paralogous.

Recently, we have discovered a second population of R. pomonella-like flies that infest hawthorns in the Sierra Madre Oriental Mountains of Mexico (Fig. 9.1). The genetics, biogeography, and phenology of the Sierra Oriental population suggest that it may have been a conduit for gene flow between the Altiplano and the North in the past (J.R., J.L.F., X.X., S. Berlocher, and M.A., unpublished data). DNA sequence analysis indicates that Sierra flies are differentiated but, overall, appear to be most closely related to southern U.S. populations (X.X. and J.L.F., unpublished data). The Sierra population abuts the Altiplano population through parts of the states of Veracruz, Puebla, and Hidalgo, Mexico (Fig. 9.1) (J.R., J.L.F., X.X., S. Berlocher, and M.A., unpublished data). We do not know whether the Sierra population contacts U.S. flies. However, if it does, this contact zone is spotty and ephemeral. Hawthorns are rare through the border region but are present in isolated patches in southern New Mexico and, possibly, the Davis Mountains of Texas. The primary hawthorn host for Sierra flies is C. rosei var. parrayana, which is infested from September to early October (J.R., J.L.F., S. Berlocher, and M.A., unpublished data). As is the case for Altiplano and U.S. flies, the diapause characteristics of the Sierra population match host phenology. Sierra flies eclose significantly earlier than Altiplano flies, resulting in potentially substantial allochronic isolation (J.R., J.L.F., X.X., S. Berlocher, and M.A., unpublished data). However, host specificity is not absolute in Mexico. In the transition region between the Altiplano and Sierra, C. mexicana and C. rosei rosei cooccur with C. rosei parrayana and can be found infested by genetically Sierra populations of flies. Here, C. mexicana and C. rosei rosei fruit earlier than they do on the Altiplano and are infested from late September to early November. Thus, host specificity is not as critical a factor isolating Mexican flies as it is for the R. pomonella complex in the United States. However, the spatial and temporal overlap of hawthorns in the transition zone provides a potential bridge for past introgression between the Altiplano and North via the Sierra population.