Adaptation in an unpredictable world: consequences for insect responses to environmental stress

Climate change is driving temperatures to become more variable and less predictable,
which is increasing the threat of extreme thermal stress in natural populations. In this
thesis, I investigated how historical exposure to different magnitudes of unpredictable
thermal variation affected population responses to future stress.

Using the moth, Plodia interpunctella, I established three experimental selection lines,
which experienced either constant, or low or high variation stochastic temperatures. After
multiple generations of exposure, the lines expressed altered phenotypes, and responded differently to acute heat stress, which had strong negative effects on life history traits. Building on this, I investigated the effects of acute heat stress, and chronic resource stress, across developmental stages. The stochastic, variable lines showed evidence of increased resistance, as well as vulnerability to stress, depending on the stress combination, developmental timing, and trait. This indicated historical, stochastic thermal variation could drive distinct life history strategies, depending on the thermal variance. To explore the mechanisms underpinning these different strategies, I performed ddRAD sequencing on each selection line, to characterise the genetic differences between them. The lines were genetically differentiated, indicating that stochastic thermal variation was driving genetic adaptation. I also found evidence for alleles under selection, which were associated with development and adult body plan specification. Finally, I investigated male fertility, by assessing the sensitivity of sperm production and mating behaviours to heat stress, between the three lines. I showed exposure to historical thermal variation altered both behavioural and sperm responses to stress, which were overall, highly negative, and suggest that fertility would be degraded at sub-lethal levels of heat stress in this species. These results suggest that stochastic thermal variation can shape complex phenotypic and genetic changes, depending on the level of temperature variance, which can prepare, or impair populations to damaging climate stress.