Precopulatory mating and postzygotic isolation between two walnut-infesting species of Rhagoletis from Mexican highlands

Abstract

Pleistocene glacial and postglacial cycles producing contraction and expansion of temperate habitats have resulted in substantial diversification among several plant and animal taxa of Neararctic origin undergoing periods of isolation and secondary contact in high-elevation areas of Mexico. One of such groups are walnut-infesting fruit flies in the genus Rhagoletis (Diptera: Tephritidae), comprised of six recently derived species among which phylogenetic relationships have been difficult to unravel using conventional molecular methods. Here, we examined pre- and postzygotic isolation between two genetically similar and morphologically distinct species which are currently parapatric in central Mexico. Local adaptation driven by differences in host plant phenology between Rhagoletis zoqui Bush and Rhagoletis ramosae Hernández-Ortíz resulted in allochronic isolation. Despite the existence of precopulatory mating isolation, there was a substantial number of hybrid matings in field cages where conspecific and heterospecific males and females were simultaneously released. The bulk of mating activity took place on host fruit. Rhagoletis zoqui females were more reluctant to mate with R. ramosae males than with males of their own species. Distinctive behavioral differences were observed between males during contests, fruit guarding, and approach to conspecifics on fruit. There was also some asymmetric postzygotic isolation, with the hybrid combination of R. zoqui males and R. ramosae females resulting in lower egg hatch than other mating combinations. Results were consistent with those of a phylogenetic study suggesting recent divergence of R. ramosae from Rhagoletis completa Cresson and R. zoqui in the Sierra Madre Oriental and the Mexican Trans Volcanic Belt.