The microevolutionary response to male-limited X-chromosome evolution in Drosophila melanogaster reflects macroevolutionary patterns

Due to its hemizygous inheritance and role in sex determination, the X
chromosome is expected to play an important role in the evolution of
sexual dimorphism, and to be enriched for sexually antagonistic genetic
variation. By forcing the X chromosome to only be expressed in males over
>40 generations, we changed the selection pressures on the X to
become similar to those experienced by the Y. This releases the X from any
constraints arising from selection in females, and should lead to
specialization for male fitness, which could occur either via direct
effects of X-linked loci or trans-regulation of autosomal loci by the X.
We found evidence of masculinization via upregulation of male-benefit
sexually antagonistic genes, and downregulation of X-linked female benefit
genes. Potential artifacts of the experimental evolution protocol are
discussed and cannot be wholly discounted, leading to several caveats.
Interestingly, we could detect evidence of microevolutionary changes
consistent with previously documented macroevolutionary patterns, such as
changes in expression consistent with previously established patterns of
sexual dimorphism, an increase in the expression of metabolic genes
related to mitonuclear conflict, and evidence that dosage compensation
effects can be rapidly altered. These results confirm the importance of
the X in the evolution of sexual dimorphism and as a source for sexually
antagonistic genetic variation, and demonstrate that experimental
evolution can be a fruitful method for testing theories of sex chromosome
evolution.