The Genetic Architecture of Life Span and Mortality Rates: Gender and Species Differences in Inbreeding Load of Two Seed-Feeding Beetles

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Abstract
We examine the inbreeding load for adult life span and mortality rates of two seed beetle species, Callosobruchus maculatus and Stator limbatus. Inbreeding load differs substantially between males and females in both study populations of C. maculatus—life span of inbred females was 9–13% shorter than the life span of outbred females, whereas the life span of inbred males did not differ from the life span of outbred males. The effect of inbreeding on female life span was largely due to an increase in the slope of the mortality curve. In contrast, inbreeding had only a small effect on the life span of S. limbatus—life spans of inbred beetles were ∼5% shorter than those of outbred beetles, and there was no difference in inbreeding load between the sexes. The inbreeding load for mean life span was ∼0.4–0.6 lethal equivalents per haploid gamete for female C. maculatus and ∼0.2–0.3 for both males and females of S. limbatus, all within the range of estimates commonly obtained for Drosophila. However, contrary to the predictions of mutation-accumulation models, inbreeding load for loci affecting mortality rates did not increase with age in either species, despite an effect of inbreeding on the initial rate of increase in mortality. This was because mortality rates decelerated with age and converged to a mortality plateau for both outbred and inbred beetles.