Thermal phenotypic plasticity of pre- and post-copulatory male harm buffers sexual conflict in wild Drosophila melanogaster

Abstract

Strong sexual selection frequently leads to sexual conflict and ensuing male harm, whereby males increase their reproductive success at the expense of harming females. Male harm is a widespread evolutionary phenomenon with a strong bearing on population viability. Thus, understanding how it unfolds in the wild is a current priority. Here, we sampled a wild Drosophila melanogaster population and studied male harm across the normal range of temperatures under which it reproduces optimally in nature by comparing female lifetime reproductive success and underlying male harm mechanisms under monogamy (i.e. low male competition/harm) vs. polyandry (i.e. high male competition/harm). While females had equal lifetime reproductive success across temperatures under monogamy, polyandry resulted in a maximum decrease of female fitness at 24°C (35%), reducing its impact at both 20°C (22%), and 28°C (10%). Furthermore, female fitness components and pre- (i.e. harassment) and post-copulatory (i.e. ejaculate toxicity) mechanisms of male harm were asymmetrically affected by temperature. At 20°C, male harassment of females was reduced, and polyandry accelerated female actuarial aging. In contrast, the effect of mating on female receptivity (a component of ejaculate toxicity) was affected at 28°C, where the mating costs for females decreased and polyandry mostly resulted in accelerated reproductive aging. We thus show that, across a natural thermal range, sexual conflict processes and their effects on female fitness components are plastic and complex. As a result, the net effect of male harm on overall population viability is likely to be lower than previously surmised. We discuss how such plasticity may affect selection, adaptation and, ultimately, evolutionary rescue under a warming climate.

Keywords: D. melanogaster; climate change; ecology; evolutionary biology; male harm; sexual conflict; temperature; thermal ecology.

Figure 1.. Schematic overview of the study.

(A) Our aim was to study how temperature variation, across a range at which reproduction is optimum in the wild, may affect: the net decrease in female fitness resulting from male harm, what female fitness components are mainly affected by male harm, and pre-copulatory (i.e. sexual harassment) and post-copulatory (i.e. ejaculate effects on female receptivity, short-term fecundity, and survival) mechanism of harm. (B) General design of the study: (1) We sampled a wild population of Drosophila melanogaster flies that reproduce optimally between 20°C and 28°C, (2) We setup a population in the lab and left it to acclimate for a few generations under a programmed fluctuating temperature regime that mimics wild conditions in late spring-early summer (20–28°C range with mean at 24°C), (3) We run a series of five experiments (each repeated at 20°C, 24°C, and 28°C) to study temperature effects on net male harm, female fitness components and male pre- and post-copulatory mechanisms of harm.

Figure 1—figure supplement 1.. Fitness and behavioural assay design (Experiment 1).

Figure 1—figure supplement 2.. Receptivity assay design (Short treatment duration – 48 hr, experiment 2).

Figure 1—figure supplement 3.. Receptivity assay design (Long treatment duration – 13 days, experiment 3).

Figure 1—figure supplement 4.. Fecundity and survival assay design (Short treatment duration – 48 hr, experiment 4).

Figure 1—figure supplement 5.. Fecundity and survival assay design (Long treatment duration – 13 days, experiment 5).

Figure 2.. Female lifetime reproductive success (mean ± SEM) across temperature and mating system treatments.

20°C: n_polyandry = 73 and n_monogamy = 74. 24°C: n_polyandry = 71 and n_monogamy = 74. 28°C: n_polyandry = 66 and n_monogamy = 71.

Figure 2—figure supplement 1.. Violin plot for female reproductive success across temperature and mating system treatments.

Figure 2—figure supplement 2.. Early reproductive rate (number of offspring produced during the first two weeks of age), late reproductive rate (number of offspring produced during the second two weeks of age), and reproductive aging (number of offspring produced over weeks 1–2 vs. 3-4) plots.

(a) Mean ± SEM number of offspring in monogamy vs. polyandry mating system treatments, across the different temperature treatments. 20°C: n_polyandry = 73 and n_monogamy = 74. 24°C: n_polyandry = 71 and n_monogamy = 74. 28°C: n_polyandry = 66 and n_monogamy = 71. (b) Violin plots.

Figure 3.. Rate-sensitive fitness estimates.

(a) Average rate-sensitive index fitness estimate of individual females (Mean ω_ind) for different population growth rates across temperature and mating system treatments (shaded areas denote SEM). 20°C: n_polyandry = 73 and n_monogamy = 74. 24°C: n_polyandry = 71 and n_monogamy = 74. 28°C: n_polyandry = 66 and n_monogamy = 71. (b) Relative cost (C_r) of polyandry (vs. monogamy) for each temperature treatment for different population growth rates. Cr was calculated based on rate-sensitive index fitness estimates for populations (ω_pop), whereby population costs are shown as 1 – Cr, thus reflecting the relative decrease in population growth rate.

Figure 4.. Male harm effect on female lifespan (mean ± SEM) across temperature and mating system treatments.

20°C: n_polyandry = 73 and n_monogamy = 74. 24°C: n_polyandry = 71 and n_monogamy = 73. 28°C: n_polyandry = 66 and n_monogamy = 73.

Figure 4—figure supplement 1.. Male harm effect on female lifespan across temperature and mating system treatments.

(a) Violin plot. (b) Survival plot from the Cox proportional hazard model as a complementary analysis.

Figure 5.. Reproductive behaviors (mean ± SEM) across temperature and mating system treatments.

(a) Courtships per female per hour, (b) Female rejections per hour, and (c) Aggressions male-male per hour. 20°C: n_polyandry = 74 and n_monogamy = 76. 24°C: n_polyandry = 72 and n_monogamy = 77. 28°C: n_polyandry = 70 and n_monogamy = 75.

Figure 5—figure supplement 1.. Violin plot for male harm effect on: (a) Courtship rate and (b) Rejection rate across temperature and mating system treatments; (c) Violin plot for polyandry mating system effect on aggression rate.

Figure 5—figure supplement 2.. Total number of matings across the 8 hr of observations.

(a) Mean ± SEM across temperature and mating system treatments. (b) Violin plot. Data from reproductive behaviour measures. 20°C: n_polyandry = 74 and n_monogamy = 76. 24°C: n_polyandry = 72 and n_monogamy = 77. 28°C: n_polyandry = 70 and n_monogamy = 75.

Figure 6.. Mean ± SEM for mating duration and remating latency.

(a) Mating duration of males exposed to high (8 males per vial) or low sperm competition risk (1 male per vial) for 48 hr or 13 days prior to mating at different temperatures. 20°C: n_high/48hr = 91, n_low/48hr = 96, n_high/13days = 121 and n_low/13days = 117. 24°C: n_high/48hr = 85, n_low/48hr = 88, n_high/13days = 119 and n_low/13days = 115. 28°C: n_high/48hr = 92, n_low/48hr = 104, n_high/13days = 99 and n_low/13days = 117. (b) Female remating latency following a single mating with either a male from a high or low sperm competition risk level for 48 hr or 13 days before mating across temperature treatments. 20°C: n_high/48hr = 75, n_low/48hr = 73, n_high/13days = 119 and n_low/13days = 113. 24°C: n_high/48hr = 61, n_low/48hr = 70, n_high/13days = 116 and n_low/13days = 113. 28°C: n_high/48hr = 63, n_low/48hr = 82, n_high/13days = 98 and n_low/13days = 117.

Figure 6—figure supplement 1.. Violin plots for (a) Mating duration of males exposed to a high (8 males per vial) or low sperm competition risk (1 male per vial) level 48 hr (experiment 2) and 13 days (experiment 3) before mating across temperature treatments, and (b) Female remating latency following a single mating with either a male from a high or low sperm competition risk level, for both 48 hr and 13 days of temperature treatment duration before mating in a common garden.

Figure 6—figure supplement 2.. Eggs produced by females during the first three days following a single mating with treated males.

(a) Mean ± SEM, 48 hr treatment duration. 20°C: n_{high/day 1} = 88, n_{low/day 1} = 86, n_{high/day 2} = 88, n_{low/day 2} = 86, n_{high/day 3} = 88 and n_{low/day 3} = 86. 24°C: n_{high/day 1} = 87, n_{low/day 1} = 88, n_{high/day 2} = 87, n_{low/day 2} = 88, n_{high/day 3} = 87 and n_{low/day 3} = 88. 28°C: n_{high/day 1} = 86, n_{low/day 1} = 86, n_{high/day 2} = 86, n_{low/day 2} = 86, n_{high/day 3} = 86 and n_{low/day 3} = 86. (b) Violin plot, 48 hr treatment duration. (c) Mean ± SEM, 13 days treatment duration. 20°C: n_{high/day 1} = 74, n_{low/day 1} = 76, n_{high/day 2} = 74, n_{low/day 2} = 76, n_{high/day 3} = 74 and n_{low/day 3} = 76. 24°C: n_{high/day 1} = 72, n_{low/day 1} = 76, n_{high/day 2} = 72, n_{low/day 2} = 76, n_{high/day 3} = 72 and n_{low/day 3} = 76. 28°C: n_{high/day 1} = 75, n_{low/day 1} = 65, n_{high/day 2} = 75, n_{low/day 2} = 63, n_{high/day 3} = 75 and n_{low/day 3} = 63. (d) Violin plot, 13 days treatment duration.

Figure 6—figure supplement 3.. Total of offspring produced by females during the days 1, 2, 3, 4, 5, and 8 after mating following a single mating with treated males.

(a) Mean ± SEM. 20°C: n_high/48hr = 88, n_low/48hr = 86, n_high/13days = 74 and n_low/13days = 76. 24°C: n_high/48hr = 87, n_low/48hr = 88, n_high/13days = 72 and n_low/13days = 76. 28°C: n_high/48hr = 86, n_low/48hr = 86, n_high/13days = 75 and n_low/13days = 63.(b) Violin plot.

Figure 6—figure supplement 4.. Female lifespan after mating following a single mating with treated males.

(a) Mean ± SEM. 20°C: n_high/48hr = 80, n_low/48hr = 80, n_high/13days = 71 and n_low/13days = 70. 24°C: n_high/48hr = 81, n_low/48hr = 82, n_high/13days = 60 and n_low/13days = 73. 28°C: n_high/48hr = 80, n_low/48hr = 84, n_high/13days = 69 and n_low/13days = 45. (b) Violin plot (c) Survival plot from the Cox proportional hazard model.