Adaptation to a bacterial pathogen in Drosophila melanogaster is not
aided by sexual selection
Abstract
Theory predicts that sexual selection should aid adaptation to novel
environments, but empirical support for this idea is limited. Pathogens
are a major driver of host evolution and, unlike abiotic selection
pressures, undergo epidemiological and co-evolutionary cycles with the
host involving adaptation and counteradaptation. Because of this,
populations harbor ample genetic variation underlying immunity and the
opportunity for sexual selection based on condition-dependent “good
genes” is expected to be large. In this study, we evolved populations
of Drosophila melanogaster in a 2-way factorial design manipulating
sexual selection and pathogen presence, using a gram-negative insect
pathogen Pseudomonas entomophila, for 14 generations. We then examined
how the presence of sexual selection and the pathogen, as well as any
potential interaction, affected the evolution of pathogen resistance. We
found increased resistance to P. entomophila in populations that evolved
under pathogen pressure, driven primarily by increased female survival
after infection despite selection for resistance acting only on males
over the course of experimental evolution. This result suggests that the
genetic basis of resistance is in part shared between the sexes. We did
not find any evidence of sexual selection aiding adaptation to pathogen,
however, a finding contrary to the predictions of “good genes” theory.
Our results therefore provide no support for a role for sexual selection
in the evolution of immunity in this experimental system.