Conclusion
Under predicted climate change scenarios, an increase in temperature is
expected next to an occurrence of longer and more severe heat waves
(Meehl and Tebaldi, 2004). Temperatures in the range of four to six
degrees above the optimum temperature as tested here are easily
reachable in many areas worldwide, especially during the summer (Solomon
et al., 2007). From an ecological point of view, this could lead to
severe consequences for species distributions and persistence,
particularly as recent research highlights a lack of genetic variability
in male sub-lethal fertility limits (van Heerwaarden and Sgrò, 2021;
Zwoinska et al., 2020). This lack would severely hamper a species
ability to mitigate escape from this predicament through evolutionary
adaptation to the novel conditions. Already now, a species thermal
fertility limit is a better predictor of species ranges than the
critical thermal limit across 43 Drosophila species (Parratt et al.,
2020). Our findings echo this recent interest in understanding the
impact of temperature on male reproduction. We here add insights on the
fitness costs of heat stress and mechanisms allowing recovery. In sum,
we show that sub-lethal thermal sterility and the subsequent fertility
reduction could be caused by a combination of malfunctioning
reproductive traits: inefficient functionality of the accessory gland
and alteration of spermatogenesis. In addition, we show that the
possibility of recovery after exposure, even when facing a
mid-challenge, does not mitigate the damage imposed on reproduction by
elevated thermal stress during development. Moreover, five days of
recovery is not enough to rescue SFP functionality, and the AGs fate is
mainly determined during development, which could explain the inability
of heat-damaged males to recover and reach the fitness of control males.
We found AG functionality more thermosensitive than spermatogenesis as
SFPs induced female post-mating responses were already impaired at the
mid-challenging temperature of 29°C. Mature sperm though was found in
males raised at 29°C and particularly in those allowed to recover, which
could explain the progressive increase in fertility observed in
recovering males.