PREMATING ISOLATION IS DETERMINED BY LARVAL REARING SUBSTRATES IN CACTOPHILIC Drosophila mojavensis. III. EPICUTICULAR HYDROCARBON VARIATION IS DETERMINED BY USE OF DIFFERENT HOST PLANTS IN Drosophila mojavensis AND Drosophila arizonae

Abstract—Adult epicuticular hydrocarbon profiles of male and female Dro-sophila mojavensis have been implicated as determinants of mate choice lead-ing to premating isolation between geographically isolated populations. Hydrocarbon profiles of a Baja California and a mainland Sonora population of Drosophila mojavensis, a yellow body mutant strain of D. mojavensis, and a population of D. arizonae were compared among flies that had been reared on two cactus substrates and a synthetic laboratory growth medium in order to assess the degree to which natural rearing substrates influence adult hydro-carbon composition. Twenty epicuticular hydrocarbon components, ranging from C29 to C41, were recovered by gas chromatography that represented major classes of alkanes, alkenes, and alkadienes. We found differences in relative amounts of epicuticular hydrocarbons among Baja and mainland D. mojaven-sis, and the yellow body mutants. There were few differences between D. mojavensis and D. arizonae. The effects of rearing substrates were remark-able: most of the differences were due to the effects of lab food vs. cactus, but there were significant rearing substrate effects due to differences in the two cacti used. Eleven hydrocarbon components differed in abundance between males and females or showed significant sex x rearing substrate interactions from ANOVA. The effects of rearing substrates on epicuticular hydrocarbon composition in D. mojavensis are concordant with changes in the intensity of premating isolation between populations, implicating host ecology as a major determinant in patterns of mate choice in this species. *To whom correspondence should be addressed