Temperature affects conspecific and heterospecific mating rates in Drosophila

Behavioural mating choices and mating success are important factors in the development of reproductive isolation during speciation. Environmental conditions, especially temperature, can affect these key traits. Environmental conditions can vary across, and frequently delimit, species' geographical ranges. Pairing suboptimal conditions with relative rarity of conspecifics at range margins may set the stage for hybridization. Despite the importance of mating behaviours as a reproductive barrier, a general understanding of the interaction between behavioural choices and the environment is lacking, in part because systematic studies are rare. With this report, we begin to bridge that gap by providing evidence that temperature has a significant but inconsistent influence on mating choices and success and, thus, on reproductive isolation in Drosophila. We studied mating propensity and success at four different temperatures among 14 Drosophila species in no-choice conspecific mating trials and in heterospecific trials among two Drosophila species triads that are known to regularly hybridize in the wild. We found that mating frequency varied significantly across a 10 °C range (from 18 °C to 28 °C), both in 1:1 mating trials and in high-density en masse trials, but that the effect of temperature was highly species specific. We also found that mating frequency was consistently low and that temperature had a moderate effect on some heterospecific crosses. As conspecific mating propensity decreased outside of the optimal thermal range, while heterospecific matings remained constant, the proportion of heterospecific matings at suboptimal temperatures was relatively high. This result indicates that temperature can modulate behavioural choices that impose reproductive barriers and influence the rate of hybridization. More broadly, our results demonstrate that to truly understand how mating choice and reproductive isolation occur in nature, they need to be studied in an environmental context.