Age-specific sensitivity of sperm length and testes size to developmental temperature.
Spermatogenesis is sensitive to thermal stress. Much of biodiversity is threatened by globally rising temperatures as the link between temperature stress, infertility and biodiversity declines is becoming apparent. Recent studies, across a broad range of taxa, have shown the temperature experienced during development to affect sperm form and function. However, little is known about the duration of spermatozoal sensitivity to temperature stress during development. Here, we used a temperature-switch protocol to identify when during development temperature affects the expression of sperm length and testes size in the bruchid beetle Callosobruchus maculatus. Egg-to-adult development took place at 17°C, 27°C or 33°C for either the full duration of pre-imago development or it was switched at different stages during development. Full development at 17°C or 33°C resulted in significantly shorter sperm than at 27°C. However, when developing larvae were switched to higher or lower temperatures, we observed switch-specific phenotypic expression of sperm length and testes size. Our key finding was that sperm length was sensitive to high temperature stress during the early stages of ontogeny and to low temperature stress during the latter stages of ontogeny. The interaction between developmental stage and temperature on spermatogenesis suggests that infertility resulting from transient heatwaves could be mitigated by age (developmental) heterogeneity within populations, owing to age-specific developmental sensitivity. Those able to avoid severe effects of heat stress on fertility, through serendipitously being at less sensitive stages of development, could potentially compensate for the loss of fertility experienced by those at more sensitive stages of development.