Quantitative genetics of body size and development time in the parasitoid wasp Aphidius ervi (Hymenoptera: Aphidiidae)

Abstract

Body size and development time are key components of life-history strategies and fitness in parasitoid wasps. To assess the relative importance of phenotypic variability for fitness, we determined the heritabilities and reaction norms of body size (= dry mass) and development time in Aphidius ervi, a solitary parasitoid of the pea aphid. We estimated the variance components for body size from an ANOVA model for haplodiploidy, using a half-sib design, with each of 18 sires mated to 2 or 3 dams. Phenotypic expression of body size was strongly influenced by host size (= instar) at the time of parasitization. Heritability for body size in female A. ervi, averaged over sire and dam components, was 0.38. Although the heritability for development time could not be estimated precisely, a larger dam than sire component suggests that development time has lower heritability than body size. Differences between the heritability estimates for body size in males and females indicate that the mode of inheritance and phenotypic expression may be asymmetrical. These results suggest that, in a stochastic environment, aphid parasitoids experience strong selection for rapid development; however, host-size effects are likely to mask differences in genetically determined body size. Genotype–environment interactions may play an important role in maintaining genetic variability in body size in natural populations of A. ervi.