Primary and secondary sex ratios in a gregarious parasitoid with local mate competition

Abstract

Sex allocation is one of the most productive domains of behavioral ecology and has led to a sophisticated understanding of factors influencing an organism’s reproductive decisions. The production of strongly female-biased sex ratios, with low between group variance, is selected for when single mothers produce groups of sibmating progeny. Although the sex of progeny is determined at oviposition (primary sex ratio), the selective value of given sexual compositions is often only apparent when offspring mature and mate (secondary sex ratio). As developmental mortality can alter the sexual composition of given offspring groups, its occurrence can select for mothers to adjust their (primary) sex allocation strategies as insurance against female-only secondary sex ratios. Empirical assessment of primary sex ratios is problematic when male and female eggs are indistinguishable. Here, we apply DNA microsatellite markers to evaluate primary sex ratios in the gregarious parasitoid Goniozus legneri Gordh (Hymenoptera: Bethylidae) and compare these with secondary sex ratios. We find that sexually differential mortality is absent or weak, but mortality acts to increase sex ratio variance and to obscure initially present relationships between sex ratio and group size. In some groups of offspring, there is a tendency for males and females to be laid in spatial separation. Our direct assessments of the sex of eggs avoid widespread problems inherent in utilizing subsets of matured offspring groups with no mortality as representative of overall primary sex ratios but, in this instance, it also confirms interpretations made by studies constrained to employ methods that are strictly incorrect.