Inbreeding-environment interactions for fitness: Complex relationships between inbreeding depression and temperature stress in a seed-feeding beetle

It is commonly argued that inbred individuals should be more sensitive to environmental stress than are outbred individuals, presumably because stress increases the expression of deleterious recessive alleles. However, the degree to which inbreeding depression is dependent on environmental conditions is not clear. We use two populations of the seed-feeding beetle, Callosobruchus maculatus, to test the hypotheses that (a) inbreeding depression varies among rearing temperatures, (b) inbreeding depression is greatest at the more stressful rearing temperatures, (c) the degree to which high or low temperature is stressful for larval development varies with inbreeding level, and (d) inbreeding depression is positively correlated between different environments. Inbreeding depression (δ) on larval development varied among temperatures (i. e., there was a significant inbreeding-environment interaction). Positive correlations for degree of inbreeding depression were consistently found between all pairs of temperatures, suggesting that at least some loci affected inbreeding depression across all temperatures examined. Despite variation in inbreeding depression among temperatures, inbreeding depression did not increase consistently with our proxy for developmental stress. However, inbreeding changed which environments are benign versus stressful for beetles; although 20°C was not a stressful rearing temperature for outbred beetles, it became the most stressful environment for inbred larvae. The finding that inbreeding-environment interactions can cause normally benign environments to become stressful for inbred populations has important consequences for many areas of evolutionary genetics, artificial breeding (for conservation or food production), and conservation of natural populations.