In Drosophila melanogaster, the DDT resistance allele (DDT‐R) is beneficial in the presence of DDT. Interestingly, DDT‐R also elevates female fitness in the absence of DDT and existed in populations before DDT use. However, DDT‐R did not spread regardless of DDT‐independent selective advantages in females. We ask whether sexual antagonism could explain why DDT‐R did not spread before pesticide use. We tested pre‐ and post‐copulatory male fitness correlates in two genetic backgrounds into which we backcrossed the DDT‐R allele. We found costs to DDT‐R that depended on the genetic background in which DDT‐R was found and documented strong epistasis between genetic background and DDT‐R that influenced male size. Although it remains unclear whether DDT‐R is generally sexually antagonistic, or whether the fitness costs noted would be sufficient to retard the spread of DDT‐R in the absence of DDT, general fitness advantages to DDT‐R in the absence of DDT may be unlikely.

Male reproductive phenotypes can evolve in response to the social and sexual environment. The expression of many such phenotypes may also be plastic within an individual's lifetime. For example, male Drosophila melanogaster show significantly extended mating duration following a period of exposure to conspecific male rivals. The costs and benefits of reproductive investment, and plasticity itself, can be shaped by the prevailing sociosexual environment and by resource availability. We investigated these ideas using experimental evolution lines of D. melanogaster evolving under three fixed sex ratios (high, medium, and low male‐male competition) on either rich or poor adult diets. We found that males evolving in high‐competition environments evolved longer mating durations overall. In addition, these males expressed a novel type of plastic behavioral response following exposure to rival males: they both significantly reduced and showed altered courtship delivery, and exhibited significantly longer mating latencies. Plasticity in male mating duration in response to rivals was maintained in all of the lines, suggesting that the costs of plasticity were minimal. None of the evolutionary responses tested were consistently affected by dietary resource regimes. Collectively, the results show that fixed behavioral changes and new augmentations to the repertoire of reproductive behaviors can evolve rapidly.

BackgroundThe maintenance of genetic variation through sexually antagonistic selection is controversial, partly because specific sexually-antagonistic alleles have not been identified. The Drosophila DDT resistance allele (DDT-R) is an exception. This allele increases female fitness, but simultaneously decreases male fitness, and it has been suggested that this sexual antagonism could explain why polymorphism was maintained at the locus prior to DDT use. We tested this possibility using a genetic model and then used evolving fly populations to test model predictions.ResultsTheory predicted that sexual antagonism is able to maintain genetic variation at this locus, hence explaining why DDT-R did not fix prior to DDT use despite increasing female fitness, and experimentally evolving fly populations verified theoretical predictions.ConclusionsThis demonstrates that sexually antagonistic selection can maintain genetic variation and explains the DDT-R frequencies observed in nature.Electronic supplementary materialThe online version of this article (doi:10.1186/s12915-015-0143-3) contains supplementary material, which is available to authorized users.