Abstract
AbstractIn an era of global warming, the negative effects of temperature stress on fertility could intensify predicted losses of biodiversity. Male fertility is particularly sensitive to temperature stress, yet we have an incomplete understanding of when, during reproductive ontogeny, spermatogenesis is most affected. 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. Such age‐specific developmental sensitivity suggests that infertility resulting from transient heat stress (i.e. heatwaves) could be mitigated by age‐related developmental heterogeneity within populations. Those individuals 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 individuals at more sensitive stages of development.
Highlights
Insect ontogeny encompasses the processes by which an egg transforms into a sexually mature adult
Testes size and sperm length in imagoes were influenced by the temperature experienced at earlier developmental stages
Using a temperature-switch protocol, we show plasticity in sperm length and testes size to be age-specific and that periods of temperature sensitivity depended on the direction of the temperature switch, up or down, from normal (27°C)
Summary
Insect ontogeny encompasses the processes by which an egg transforms into a sexually mature adult. Insect ontogeny is strongly influenced by temperature whereby developmental processes are greatest (in terms of rates or successful completion) around a given thermal optimum and decline as critical thermal maxima and minima are reached (Ratte, 1985; Angilletta Jr., 2009; Gullan & Cranston, 2014). Elevated temperatures experienced during development have been shown to reduce both male and female fertility, placing populations at greater risk of extinction in the face of global climate change (David et al, 2005; Colinet et al, 2015; Sage et al, 2015; Sales et al, 2018; Walsh et al, 2019; Parratt et al, 2020; Zwoinska et al, 2020)
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