Rhonda Snook, H. S. Crudgington, and S. J. Martin. Department of Animal and Plant Sciences, University of Sheffield, Western Bank, Alfred Denny Building, Sheffield, S10 2TN, United Kingdom
Sexual selection operates on male and female traits that improve mating success. Such traits may be identified by manipulating the opportunity for sexual selection to operate. Insect cuticular hydrocarbons (CHCs) have been shown to be involved in sexual communication, functioning as signaling pheromones. Using experimental evolution, we show that male cuticular hydrocarbon profiles change depending on whether sexual selection is allowed to act. In the naturally promiscuous fruitfly, Drosophila pseudoobscura, we experimentally enforced monogamy by randomly assigning one female to one male, resulting in the complete relaxation of sexual selection. In another treatment, we elevated promiscuity by housing six males with one female, which results in increased opportunity for variance in male reproductive success. After ca. 9, 20, and 45 generations of selection, we sampled male and female CHCs by hexane extraction and characterized the chemical profiles of males and females using standard gas chromatography and mass spectrophotometry. The CHC profile of D. pseudoobscura has around 16 peaks. We found that, as a consequence of elevated sexual selection, the proportion of only one CHC increased in males through time and that, within this elevated sexual selection treatment, the sexes had diverged in the amount of this hydrocarbon being produced with males producing more than females. Moreover, this response was consistent across four replicate populations, implying parallel evolution and thus strong directional sexual selection to increase the amount of this chemical produced by males. We suggest that the altered compound functions during pre-mating sexual selection, either through male-male competition or female choice. By examining the biochemical pathways, we also identified functional adjustments which allow males to increase production of the altered compound. Future work will identify the functional significance of the divergent CHC.
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