Cuticular hydrocarbon variation among Rhagoletis fruit flies (Diptera: Tephritidae): implications for premating reproductive isolation and ecological speciation

1. Fruit flies in the genus Rhagoletis are a model for ecological speciation via sympatric host plant shifting. Rhagoletis mate on or near the fruit of their respective host plants, generating premating reproductive isolation between taxa specialised on different hosts. However, non-host-related premating isolation has been observed between some Rhagoletis species associated with morphological differences in body colour and wing patterns. 2. Here, the extent of epicuticular hydrocarbon (CHC) variation as a possible, additional determinant of mate choice in adults of six Rhagoletis taxa, including the apple and hawthorn-infesting host races of R. pomonella is investigated. 3. Gas-chromatography-mass-spectrometry revealed 36 repeatable and quantifiable hydrocarbon components present on the epicuticles of all six fly taxa, comprised of at least 53 different CHC compounds, with chain lengths varying from 27 to 34 carbon atoms, consisting of n-alkanes, mono-, dimethyl-, and trimethyl-alkanes, alkenes, and alkadienes. There were significant differences in the relative proportions of CHCs between adult R. cingulata, R. cornivora, R. zephyria, R. mendax, and R. pomonella, as well as between the apple- and hawthorn-infesting host races of R. pomonella. Furthermore, within the R. pomonella host races and R. mendax, significant CHC differences were observed between the sexes and across collecting sites. 4. The results are consistent with variation in CHCs potentially contributing to observed patterns of premating isolation between Rhagoletis taxa, possibly due to combination of sexual and host-related selection, which will necessitate further, in depth chemical analyses and future mating trials to substantiate.

Automated summary

The study found variation in epicuticular hydrocarbons (CHCs) as a potential factor in mate choice among adults of six Rhagoletis taxa, including the apple and hawthorn-infesting host races. Gas-chromatography-mass-spectrometry revealed 36 repeatable and quantifiable hydrocarbon components across all six fly taxa, with at least 53 different CHC compounds present, consisting of n-alkanes, mono-, dimethyl-, and trimethyl-alkanes, alkenes, and alkadienes. The results suggest that CHC variation may contribute to patterns of premating isolation between Rhagoletis taxa, potentially driven by sexual and host-related selection.