Facultative polyandry under heat stress and the evolutionary potential for climate-driven shifts in mating systems

The ecology of mating interactions determines a species’ mating system,
yet whether environmental change can alter the mating system of a species
remains unclear. Elevated temperatures can cause male sterility, prompting
females to remate for fertility assurance. In monandrous systems,
heat-induced male infertility poses a significant extinction risk, as
females may mate exclusively with infertile males. A key question is
whether male sterility could drive polyandry in a typically monandrous
system. Here we address this by examining genetic variance underlying both
male fertility resilience to heat stress and facultative polyandry, and
assessing the fitness consequences of each mating system. We used
isofemales lines of Drosophila subobscura, a monandrous species, exposing
males to developmental heat stress. Male heat stress generated sterility
and females mated to these males typically remated. While significant
genetic variation in male fertility sen sitivity and female remating
emerged at moderate to high temperatures, we found little genetic
variation in plasticity for polyandry. These results indicate evolutionary
potential in both traits, but that a shift in mating system would arise
through selection on genes associated with polyandry, rather than
plasticity. Polyandry improved offspring production after initially mating
to a sterile male, but did not fully restore reproductive output relative
to fertile monandrous pairs, and mating with heat-stressed males increased
female mortality. Heat stress also altered mating behaviour which could
impact female mate choice. Together, these findings show that increasing
temperatures may shape species’ mating systems and the interplay between
thermal ecology and sexual selection under climate change.