Sexual and ecologically based reproductive isolation in the walnut-infesting Rhagoletis suavis species group

The evolution of reproductive isolation is essential to the formation of new species. The Rhagoletis pomonella sibling species complex has been widely studied as a model system for sympatric speciation via host-shifting. It is suggested that reproductive isolation is maintained initially through ecological isolation, with sexual isolation evolving later. Pre-zygotic isolation is weak for R. pomonella and absent in the apple and hawthorn infesting host races, while ecological factors such as host choice and diapause timing act as reproductive barriers. By comparison, in the allopatrically speciating, walnut-infesting Rhagoletis suavis species group, pre-zygotic isolation is thought to be an important factor during secondary contact, particularly due to the differences in morphology and courtship behaviors between the species.  However, previous studies have shown no evidence of pre-mating isolation in R. zoqui and R. completa, the most closely related of the walnut-infesting species, and hybrids are common in their small area of overlap. Our results suggest that R. suavis species are, in part, ecologically isolated by post diapause emergence timing in areas of co-occurrence. In observing walnut infesting flies endemic to the United States, we found that pre-mating isolation is significant but surprisingly incomplete between the walnut flies. Additionally, post-mating reproductive isolation mechanisms, including hybrid egg inviability, provide a barrier to gene flow. These results may have important implication as the ranges of walnut-infesting flies shows signs of shifting over the last few decades.