Exposure to environmental stress is a common feature of the life of most organisms, making the ability to cope with such stress imperative to survival and persistence. In particular, small populations that experience inbreeding and inbreeding depression may be particularly vulnerable to the effects of environmental stress. However, we lack and understanding of the factors that contribute to the considerable variation in effects of stress on the expression of inbreeding depression. In this dissertation I examined the role of four factors in determining the outcome of inbreeding-stress interactions.
First I investigated the relationship between stress level, measured as the reduction in fitness a stress causes relative to benign conditions, and the magnitude of inbreeding depression. I found a strong positive linear relationship both in laboratory populations and populations exposed to field conditions, whereby inbreeding depression increases as the level of stress increases. Contrary to initial predictions, in a three year field study I found winter conditions were more stressful and induced greater inbreeding depression relative to summer conditions.
Second, I examined how different types of abiotic and biotic stresses affect the expression of inbreeding depression both during exposure (larval survival) and after exposure (male mating). I found that during exposure, only heat and competitive stress amplified inbreeding depression relative to benign conditions, while ethanol and bacterial stress did not. In addition, I found that exposure to stress during development significantly reduced inbreeding depression for competitive male mating success, but only for the two biotic stresses.
Finally, I investigated how stress and sexual selection affect the expression of inbreeding depression in males versus females. I found a striking difference in the magnitude of inbreeding depression expressed in the sexes, with males suffering a two-fold higher cost to being inbred that females. This is presumably due sexual selection, via female choice and/or male competitive interactions, increasing selection against inbred males expressing deleterious alleles.