Expression of heat shock proteins and thermal sensitivity of male fertility across six <i>Drosophila</i> species

Understanding the mechanisms that confer resilience to thermal stress is crucial in the context of climate change. Recently there has been increasing focus on sublethal effects of high temperatures, such as on reproduction. Male fertility is particularly sensitive to heat, and the upper Thermal Fertility Limit (TFL) is a better predictor of species' geographical ranges than lethal limits (LT) alone. Drosophila fruit fly species vary in their TFL and in the magnitude of difference between TFL and LT, but what drives this variation is unknown. We hypothesised that expression of Heat shock proteins (Hsps), known to play a role in both the heat stress response and spermatogenesis, may explain these species differences. We compared the effects of a short, moderate thermal shock on the expression of seven Hsps in the male reproductive tract versus the rest of the body, across six drosophilid species. Patterns of expression varied across tissues and species both before and after heat shock. There is some indication that species with lower lethal limits show greater upregulation in response to heat shock in somatic tissue. There was no clear pattern of differential regulation in relation to absolute TFL, but a suggestion that species with a larger TFL-LT gap lack differential regulation in reproductive tissue. Hence, whilst Hsp expression may play a role, there are clearly other mechanisms that underlie the sensitivity of species' fertility to elevated temperatures which need to be assessed.