SEX‐RATIO CONTROL IN A PARASITIC WASP,NASONIA VITRIPENNIS. I. GENETIC VARIATION IN FACULTATIVE SEX‐RATIO ADJUSTMENT

The precision of sex-ratio control in a parasitic wasp, Nasonia vitripennis (Pteromalidae) was investigated using a series of "sequential oviposition" experiments in which a female oviposits on hosts previously visited by another female. There are clear genotypic differences in the averages of these "second" sex ratios. Indeed, one genotype does not alter sex ratio detectably. Two genotypes with identical average sex ratios on previously unparasitized hosts have distinct second sex ratios. Artificial selection on one strain to produce less female-biased "first" sex ratios in previously unparasitized hosts has not affected the ability of that strain to adjust sex ratio, suggesting that the proximal mechanisms controlling first and second sex ratios can change independently. Adjustment of sex ratios is accomplished by the production of fewer females and more males. Sex ratios produced in control hosts indicate that sex ratios are adjusted in hosts independently of one another. Second sex-ratio response also appears to be independent of host clumping. These results represent both a challenge to our present understanding of sex-ratio evolution (see Orzack, 1986) and a foundation for experimental description of this important adaptation.