Results
To assess the potential for sexual selection to act on pathogen
resistance, we first compared the paternity success of infected and
sham-treated males in competition with males from a reference strain. We
found that infected males had lower competitive mating success than
uninfected males, as evidenced by a lower proportion of offspring sired
by the focal males (treatment effect:
χ2df=1 = 4.45; p = 0.03;Figure 1, Table 1 ). Infected males sired on average 59.2% of
progeny in competition with the competitive standard, while uninfected
males sired on average 68.5% of progeny. This result indicated that
infection harms male mating success and suggested that genetic variation
contributing to infection resistance might be favored by sexual
selection.
We also verified that the infected males had cleared the pathogen from
their gut by the time they were placed with females. Although males
harbored many live P. entomophila 4h after the onset of the
infection treatment, no live bacteria were detected at 8 or 24 h
(Supplemental Figure S1), in agreement with earlier results (Bou Sleimanet al. , 2015). Thus, there was little opportunity for the males
to transmit the infection to the females.
We next evolved replicate populations with and without both sexual
selection and pathogen for fourteen generations. Over the course of
experimental evolution, P. entomophila virulence varied; the
pathogen reliably killed a substantial fraction of the relishmutants (mean survival post infection 43% ± 10.7 (s.e.) inrelish mutants, Supplemental Figure S2 ). Survival at 24
hours was lower in experimental populations exposed to the pathogen
(+P), averaging 92.4%, than it was in populations not exposed to the
pathogen (-P), in which survival was 99.9%.
To compare resistance to P. entomophila in the populations
subject to the different regimes, we measured their survival following
infection after fourteen generations of experimental evolution and one
generation of common garden rearing. In general, females survived less
well after infection than males (Figure 2 ). Populations evolved
under pathogen pressure (+P evolutionary regimes) showed better
post-infection survival than populations evolved without pathogen
exposure (-P evolutionary regimes) (pathogen selection effect:
χ2df=1 = 8.89; p = 0.002;Figure 2, Table 1 ). A significant three-way interaction between
sexual selection, pathogen, and sex (SS*Pathogen*Sex,
χ2df=1 = 5.91; p = 0.01)
indicates a difference between males and females in how the interaction
between sexual selection and pathogen presence affects post-infection
survival, which we further explored in sex-specific analyses.
In females, the sex-specific analyses showed that post-infection
survival under +P regimes was better than that in the –P regimes
(Figure 2A , pathogen selection effect:
χ2df=1 = 4.92; p = 0.026), but
we detected no effect of sexual selection
(χ2df=1 = 0.04; p = 0.82) or
any interaction between sexual selection and pathogen
(χ2df=1 = 0.93; p = 0.33). In
males there was neither a significant effect of sexual selection
(Figure 2B , χ2df=1 =
0.094; p = 0.75) nor pathogen
(χ2df=1 = 3.40; p = 0.06).
However, there was a significant interaction between sexual selection
treatment and pathogen presence
(χ2df=1 = 4.71; p = 0.029). For
the good genes hypothesis to be true in our case, the +SS +P populations
should have elevated survivorship compared to -SS +P regimes. However,
in our study we see the opposite effect, with the -SS +P regimes
surviving significantly better than +SS +P (Figure 2B , Tukey’s
post hoc comparison p = 0.02). At the same time, there is no difference
between +SS populations evolved with and without pathogen. This
difference in the effect of sexual selection that depended on whether
pathogen was present or not during the course of experimental evolution
is what drives the significant interaction between sexual selection and
pathogen.