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Intralocus sexual conflict arises when the expression of shared alleles at a single locus generates opposite fitness effects in each sex (i.e. sexually antagonistic alleles), preventing each sex from reaching its sex-specific optimum. Despite its importance to reproductive success, the relative contribution of intralocus sexual conflict to male pre- and post-copulatory success is not well-understood. Here, we used a female-limited X-chromosome (FLX) evolution experiment to limit the inheritance of the X-chromosome to the matriline, eliminating possible counter-selection in males and allowing the X-chromosome to accumulate female-benefit alleles. After greater than 100 generations of FLX evolution, we studied the effect of the evolved X-chromosome on male attractiveness and sperm competitiveness. We found a non-significant increase in attractiveness and decrease in sperm offense ability in males expressing the evolved X-chromosomes, but a significant increase in their ability to avoid displacement by other males' sperm. This is consistent with a trade-off between these traits, perhaps mediated by differences in body size, causing a small net reduction in overall male fitness in the FLX lines. These results indicate that the X-chromosome in Drosophila melanogaster is subject to selection via intralocus sexual conflict in males.

In this project, we are studying the nature of X-linked sexually antagonistic loci by doing a female-limited X chromosome (FLX) experimental evolution in a laboratory-adapted population of D. melanogaster. we predict that expressing the experimentally evolved X chromosome in the selected sex will result in more feminized phenotype expression of shared traits. There are four replicate populations of each of three experimental treatments: Female-limited X-chromosome (FLX), control wild type (Cwt), and a methodological control treatment to control for the confounding effect of the FM balancer chromosomein the FLX populations (CFM).The CFM treatment is handled in the same way as the FLX treatment, except that the X chromosome goes through repeating cycles of two generations in females followed by one generation in males.The mating attractiveness: This experiment was done in 12 blocks within a day, with 20 flies per replicate population per treatment in each block, estimating attractiveness of a total of 80 males per treatment.The sperm compitation: At generation 143, these tests were performed in 15 replicates per treatment per replicate population. In total, 360 test tubes of offspring were scored. The body size: Body size was estimated using measurements of thorax length. On day 13 from oviposition, 20 flies from each genotype were placed in 95 % ethanol. Air-dried flies were measured using a Nikon SMZ800 dissecting microscope at 63x magnification fitted with an eyepiece graticule. This assay was done at generations 18 and 72.