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.