Effects of inbreeding and elevated rearing temperatures on strategic sperm production

Abstract Males often strategically modify their rate of sperm production based on the social context, but it remains unclear how environmental and genetic factors shape this plasticity. In freshwater ecosystems, high ambient temperatures often lead to isolated pools of hotter water in which inbreeding occurs. Higher water temperatures and inbreeding can impair fish development, potentially disrupting sperm production. We used guppies ( Poecilia reticulata ) to investigate how developmental temperature (26 °C or 30 °C) and male inbreeding status (inbred, outbred) influence sperm production in the absence or presence of a female (i.e., sperm priming response). We also tested if sperm priming was affected by whether the female was a relative (sister), and whether she was inbred or outbred. A higher rearing temperature had no effect on the rate of sperm production or the priming response. Inbred males produced significantly more sperm in the presence of an unrelated, outbred female than when no female was present. Conversely, outbred males did not alter sperm production in response to female presence or relatedness. In addition, inbred males showed marginally greater sperm production when exposed to an unrelated female that was outbred rather than inbred, but no difference when exposed to an inbred female that was unrelated versus related. Together, only inbred males increased sperm production in response to the presence of a female, but this depended on her being outbred. This suggests stronger sexual selection on inbred males to allocate ejaculate resources, perhaps due to greater benefits when mating with outbred females in better condition.