Heat stress but not inbreeding affects offensive sperm competitiveness in Callosobruchus maculatus

Abstract

Environmental and genetic stress have well-known detrimental effects on ejaculate quality, but their concomitant effect on male fitness remains poorly understood. We used competitive fertilization assays to expose the effects of stress on offensive sperm competitive ability in the beetle Callosobruchus maculatus, a species where ejaculates make up more than 5% of male body mass. To examine the effects of environmental and genetic stress, males derived from outcrosses or sib matings were heat shocked at 50°C for 50 min during the pupal stage, while their siblings were maintained at a standard rearing temperature of 28°C. Heat-shocked males achieved only half the offensive paternity success of their siblings. While this population exhibited inbreeding depression in body size, sperm competitiveness was unaffected by inbreeding, nor did the effect of heat shock stress on sperm competitiveness depend on inbreeding status. In contrast, pupal emergence success was increased by 34% among heat-stressed individuals, regardless of their inbreeding status. Heat-shocked males' ejaculate size was 19% reduced, but they exhibited 25% increased mating duration in single mating trials. Our results highlight both the importance of stress in postcopulatory sexual selection, and the variability among stressors in affecting male fitness.

Male-biased sex ratio does not promote increased sperm competitiveness in the seed beetle, Callosobruchus maculatus

“…Typically, there is strong last-male sperm precedence in C. maculatus , which is consistent over a number of female mating frequencies (P 2  = 0.78; P 3  = 0.8340), and is consistent when male morphological markers, radiation and genetic markers are used to assign paternity404142. High last-male sperm precedence has also been reported in the source population for this experiment31. In laboratory populations, males benefit from larger ejaculates: the number of sperm transferred by a male increases his paternity share, but only when mating in the last position43.…”

“…Although we did not have an equal sex ratio (control) population in this experiment, it is possible to compare ejaculate traits from our experimental individuals with those from the original source population. Males from our experimental lines had relatively smaller ejaculates for their body weight than source population males (focal male ejaculate weight (mg) = 0.16 ± 0.005; source population males = 0.19 ± 0.01; F 1,164  = 10.53, P = 0.001)31. This comparison must be made cautiously, however, as the source population was maintained on a different diet (black-eyed beans).…”

“…Here, a virgin stock female was first mated to a 1 day old, virgin, stock male. Stock males had been sterilized with a 60 Gy dose of radiation from a Cobalt-60 source, under nitrogen anesthesia (5 L min −1 ), 24 h prior to mating (for detailed methods, see31). We isolated mated females in 1.5 mL microtubes with 4 mung beans to encourage oviposition and remating.…”

Temperature fluctuations during development reduce male fitness and may limit adaptive potential in tropical rainforest Drosophila

“…Sperm formation is particularly heat sensitive (David et al ., ), with temperature thresholds at which males can produce sperm being narrower than those that allow survival or limit the expression of other stress‐related traits (Jorgensen et al ., ). Other studies have documented the impacts of heat stress on sperm viability and performance in tropical ectotherms (Zeh et al ., ; Breckels & Neff, ) and trade‐offs between sperm quantity and investment in heat‐shock proteins (Hsps) (van Lieshout et al ., ). Although the physiological basis of our results is unknown, such studies offer avenues for future work on the costs of fluctuating thermal stress for male reproductive success.…”

Evolution of pre- and post-copulatory traits in female Drosophila melanogaster as a correlated response to selection for resistance to cold stress

“…The FSB and FCB females were not different in any of these traits when assayed under no-shock condition. Multiple studies have documented the negative effects of temperature shock on male reproductive traits (Rohmer et al, 2004, Zizzari and Ellers, 2011, Lieshout et al, 2013, Nguyen et al, 2013, Rinehart et al, 2000and Singh et al, 2015, 2016 and female fecundity (Irwin andLee, 2003, reviewed in Hance et al, 2007;Sinclair, 2010 andRinehart et al, 2000). To the best of our knowledge, our study is the first to assess the evolution of female reproductive traits in response to cold shock.…”