Experimental evolution reveals sex-specific dominance for surviving bacterial infection in laboratory populations of Drosophila melanogaster

Males and females are subjected to distinct kinds of selection pressures,
often leading to the evolution of sex-specific genetic architecture, an
example being sex-specific dominance. Sex-specific dominance reversals
(SSDRs), where alleles at sexually antagonistic loci are at least
partially dominant in the sex they benefit, have been documented in
Atlantic salmon, rainbow trout and beetles. Another interesting feature of
many sexually reproducing organisms is the asymmetric inheritance pattern
of X chromosomes, which often leads to distinct evolutionary outcomes on X
chromosomes compared to autosomes. Examples include, the higher efficacy
of sexually concordant selection on X chromosomes, and X chromosomes being
more conducive to the maintenance of sexually antagonistic polymorphisms
under certain conditions. Immunocompetence is a trait that has been
extensively investigated for sexual dimorphism with growing evidence for
sex-specific or sexually antagonistic variation. X chromosomes have been
shown to harbour substantial immunity-related genetic variation in the
fruit-fly, Drosophila melanogaster. Here, using interpopulation crosses
and cytogenetic cloning, we investigated sex-specific dominance and the
role of the X chromosome in improved post-infection survivorship of
laboratory populations of D. melanogaster selected against pathogenic
challenge by Pseudomonas entomophila. We could not detect any contribution
of the X chromosome to the evolved immunocompetence of our selected
populations as well as to within population variation in immunocompetence.
However, we found strong evidence of sex-specific dominance related to
surviving bacterial infection. Our results indicate that alleles that
confer a survival advantage to the selected populations are, on average,
partially dominant in females but partially recessive in males. This could
also imply a sex-specific dominance reversal for overall fitness, given
the putative evidence for sexually antagonistic selection affecting
immunocompetence in Drosophila melanogaster. We also highlight
sex-specific dominance as a potential mechanism of sex differences in
immunocompetence, with population-level sex differences primarily driven
by sex differences in heterozygotes.