Plastic responses of survival and fertility following heat stress in pupal and adult Drosophila virilis

The impact of rising global temperatures on survival and reproduction is
putting many species at risk of extinction. In particular, it has recently
been shown that thermal effects on reproduction, particularly limits to
male fertility, can underpin species distributions in insects. However,
the physiological factors influencing fertility at high temperatures are
poorly understood. Key factors that affect somatic thermal tolerance such
as hardening, the ability to phenotypically increase thermal tolerance
after a mild heat shock, and the differential impact of temperature on
different life stages, are largely unexplored for thermal fertility
tolerance. Here, we examine the impact of high temperatures on male
fertility in the cosmopolitan fruit fly Drosophila virilis. We first
determined whether temperature stress at either the pupal or adult
life-history stage impacts fertility. We then tested the capacity for
heat-hardening to mitigate heat-induced sterility. We found that thermal
stress reduces fertility in different ways in pupae and adults. Pupal heat
stress delays sexual maturity, whereas males heated as adults can
reproduce initially following heat stress, but lose the ability to produce
offspring. We also found evidence that while heat-hardening in D. virilis
can improve high temperature survival, there is no significant protective
impact of this same hardening treatment on fertility. These results
suggest that males may be unable to prevent the costs of high temperature
stress on fertility through heat-hardening which limits a species’ ability
to quickly and effectively reduce fertility loss in the face of short-term
high temperature events.