Female and male genetic contributions to post-mating immune defence in female Drosophila melanogaster

Abstract

Post-mating reduction in immune defence is common in female insects, and a trade-off between mating and immunity could affect the evolution of immunity. In this work, we tested the capacity of virgin and mated female Drosophila melanogaster to defend against infection by four bacterial pathogens. We found that female D. melanogaster suffer post-mating immunosuppression in a pathogen-dependent manner. The effect of mating was seen after infection with two bacterial pathogens (Providencia rettgeri and Providencia alcalifaciens), though not after infection with two other bacteria (Enterococcus faecalis and Pseudomonas aeruginosa). We then asked whether the evolution of post-mating immunosuppression is primarily a 'female' or 'male' trait by assaying for genetic variation among females for the degree of post-mating immune suppression they experience and among males for the level of post-mating immunosuppression they elicit in their mates. We also assayed for an interaction between male and female genotypes to test the specific hypothesis that the evolution of a trade-off between mating and immune defence in females might be being driven by sexual conflict. We found that females, but not males, harbour significant genetic variation for post-mating immunosuppression, and we did not detect an interaction between female and male genotypes. We thus conclude that post-mating immune depression is predominantly a 'female' trait, and find no evidence that it is evolving under sexual conflict.

Figure 1.

The effect of mating on female bacterial load after infection with four bacterial pathogens. Bacterial loads of wild-type (Canton S) females mated to wild-type (Canton S) males were significantly higher than those of virgin wild-type (Canton S) females after infection with (a) P. rettgeri (F_1,67 = 28.77, p < 0.0001) and (b) P. alcalifaciens (F_1,77 = 9.86, p = 0.0024), but not after infection with (c) E. faecalis (F_1,52 = 1.20, p = 0.279) or (d) Ps. aeruginosa (F_1,32 = 0.17, p = 0.6804). We infected virgin and mated females in parallel 2–3 h after mated females completed copulation. Total sample sizes were as follows: for P. rettgeri, n_mated = 36 and n_virgin = 35 for P. alcalifaciens, n_mated = 43 and n_virgin = 38, for E. faecalis, n_mated = 28 and n_virgin = 28, and for Ps. aeruginosa, n_mated = 17 and n_virgin = 18. Each data point consists of three pooled females, and data were collected over three replicates for each bacterial species with the exception of Ps. aeruginosa, for which only two replicates were collected. Uninfected controls (not shown) were sham-infected with a sterile needle and always yielded zero bacteria.

Figure 2.

The effect of mating on female survival after infection with four bacterial pathogens. Survival over time of wild-type (Canton S) females mated to wild-type (Canton S) males was significantly lower than that of virgin wild-type (Canton S) females after infection with P. rettgeri (panel (a), p < 0.0001) and P. alcalifaciens (panel (b), p < 0.0001), but not after infection with E. faecalis (panel (c), p = 0.0811) or P. aeruginosa (panel (d), p = 0.3466). Survival curves were estimated using the Kaplan–Meier method. Significance values are for the effect of mating treatment in infected females and were determined by Cox regression analysis. We infected both mated and virgin females in parallel 2–3 h after mated females complete copulation. N = 44–75 infected females per mating status per replicate, and two to four replicates were performed for each survival experiment. Uninfected controls pierced with a sterile needle (lines shown in grey) showed negligible mortality for both mated (dashed grey line) and virgin (solid grey line) treatments.

Figure 3.

Variation in the effect of mating across female genotypes. We calculated corrected mean bacterial load for mated females (black diamonds) and virgin females (open squares) of each female genotype pooled across all male genotypes. For example, the black diamond for female 1F corresponds to the mean load sustained by 1F females after mating to males from genotypes 1M–9M, and the open square corresponds to loads sustained by virgin 1F females infected and plated alongside mated 1F females. Mean refers to geometric mean, and error bars represent a 95% confidence interval. The parenthetical numbers on the x-axis are the DGRP stock identity number.

Figure 4.

Variation in the effect of mating across male genotypes. We calculated corrected mean bacterial load for all females mated to each individual male genotype (black diamonds) and the mean bacterial load for the corresponding virgin controls from all female genotypes (open squares). For example, the black diamond for 1M corresponds to the mean of females from genotypes 1F–9F mated to 1M males, and the open square corresponds to the mean of virgin control females from lines 1F–9F infected and plated alongside the females mated to 1M males. Mean refers to geometric mean, and error bars represent a 95% confidence interval. The parenthetical numbers on the x-axis are the DGRP stock identity number.