Experimental evolution reveals that males evolving within warmer thermal regimes improve reproductive performance under heatwave conditions in a model insect

Climate change is increasing mean temperatures, and intensifying
heatwaves. Natural populations may respond to stress through shorter-term
acclimation via plasticity and/or longer-term inter-generational
evolution. However, if the pace and/or extent of thermal change is too
great, local extinctions occur; one potential cause in ectotherms is
identified to be the heat-liability of male reproductive biology. Recent
data from several species, including the beetle Tribolium castaneum,
confirmed that male reproductive biology is vulnerable to heatwaves, which
may constrain populations. However, such reproductive-damage may be
overestimated, if there is potential to adapt to elevated mean
temperatures associated with climate change via evolution and/or
acclimation. Here, we tested this to evaluate whether pre-exposures could
improve heatwave tolerance (adaptation or acclimation), by experimentally
evolving T. castaneum populations to divergent thermal regimes (30°C
versus 38°C). Findings across assays revealed that relative to 30°C-regime
males, males from the 38°C regime, maintained constantly at 8°C warmer for
25 generations, displayed an increase; i) in post heatwave (42°C)
reproductive fitness by 55%, ii) survival by 33% and iii) 32% larger
testes volumes. Unexpectedly, in the acclimation assay, warm-adapted
males’ post-heatwave survival and reproduction were best if they
experienced cool developmental acclimation beforehand, suggesting a cost
to adapting to 38°C.These results help progress knowledge of the potential
for survival and reproduction to adapt to climate change; trait specific
adaptation to divergent thermal regimes can occur over relatively few
generations, but this capacity depended on the interaction of evolutionary
and thermal acclimatory processes.