A genetic perspective on mating systems and sex ratios of parasitoid wasps

Parasitoid sex ratios are influenced by mating systems, whether complete in breeding, partial inbreeding, complete inbreeding avoidance, or production of all-male broods by unmated females. Population genetic theory demonstrat es that inbreeding is possible in haplodiploids because the purging of dele terious and lethal mutations through haploid males reduces inbreeding depre ssion. However, this purging does not act quickly for deleterious mutations or female-limited traits (e.g, fecundity, host searching, sex ratio). The relationship between sex ratio: inbreeding, and inbreeding depression has n ot been explored in depth in parasitoids. The gregarious egg parasitoid, Tr ichogramma pretiosum Riley, collected from Riverside, CA (USA) produced a f emale-biased sex ratio of 0.24 (proportion of males). Six generations of si bling mating in the laboratory uncovered considerable inbreeding depression (similar to 20%) in fecundity and sex ratio. A population genetic study (b ased upon allozymes) showed the population was inbred (F-it = 0.246), which corresponds to 56.6% sib-mating. However, average relatedness among female s emerging from the same host egg was only 0.646, which is less than expect ed (0.75) if ovipositing females mate randomly. This lower relatedness coul d arise from inbreeding avoidance, multiple mating by females, or superpara sitism. A review of the literature in general shows relatively low inbreedi ng depression in haplodiploid species, but indicates that inbreeding depres sion can be as high as that found in Drosophila. Finally, mating systems an d inbreeding depression are thought to evolve in concert (in plants), but s imilar dynamic models of the joint evolution of sex ratio, mating systems, and inbreeding depression have not been developed for parasitoid wasps.