Local Adaptation to the Sex Environment: Reciprocal Sex-Limited Selection in Different Thermal Regimes.

The shared genome prevents independent responses to sex differences in selection. Each sex does not evolve in response to its own sex-specific selection but is influenced by selection on the other sex via the shared genome. We used experimental evolution in Drosophila melanogaster to create separate pools of Chromosome 3s for males (male-limited "ML" chromosomes) and females (female-limited "FL" chromosomes), thereby separating the gene pool, for 15 generations. Viewing each sex as a separate environment, this separation of the genome allows sex-specific adaptation to each sex environment. We performed a reciprocal transplant between the sexes to quantify the strength of local adaptation to each sex. We found each chromosome type was more beneficial in the sex it had been selected for (i.e., local adaptation to sex). Because it has been postulated that sex-differences in selection may depend on how well adapted a population is to the abiotic environment, we performed experimental evolution at two thermal regimes (3 replicate populations each): one benign temperature to which the populations were well-adapted and one novel temperature. Female-specific adaptation was stronger at the benign temperature whereas male-specific adaptation was stronger in the novel temperature. Within chromosome pools, male and female fitness were more positively correlated in the novel compared to the benign conditions. Though males carrying ML chromosomes were typically more fit than males carrying FL chromosomes, they were also more harmful to their female mating partners.