Effect of inbreeding on host discrimination and other fitness components in a seed beetle

Inbreeding is well known to have adverse effects on fitness-related traits in insects, but less is known about its effect on behavior, particularly outside the context of mating success. We used the seed beetle Callosobruchus maculatus (F.) (Coleoptera: Chrysomelidae: Bruchinae) to determine the effect of moderate inbreeding (inbreeding coefficient F = 0.25) on host discrimination, that is, the ability of ovipositing females to avoid occupied hosts and thereby reduce competition among larval offspring. Inbreeding depression was simultaneously estimated for morphological (body mass), physiological (egg hatch), and life-history (realized fecundity) traits in the same population. Compared with outbred females, inbred females exhibited an 11% reduction in mass, a 9% reduction in fecundity, and a small (4%) but significant reduction in egg hatching success. Nearly all of the decline in fecundity among inbred females could be explained by the decline in body mass. In contrast to the other three traits, host discrimination was unaffected by inbreeding. Inbred females actually distributed their eggs slightly more uniformly among seeds than did outbred females, but this difference disappeared after we corrected for inbreeding effects on fecundity. An assay of doubly inbred females (F = 0.375) confirmed that the tendency to avoid occupied hosts is immune to moderate inbreeding. The lack of inbreeding depression for host discrimination suggests that heritable variation for the trait within populations (as detected in previous studies) is largely caused by alleles with additive rather than dominant effects, and contrasts with predictions based on population crosses.