Precise Sex Ratios in Highly Inbred Parasitic Wasps

Abstract

When Williams (1979) considered the question of adaptive sex ratios in out-crossed vertebrates he concluded that the evidence favored Mendelian (random, binomial) sex determination. This may be because vertebrates possess no efficient mechanism for regulating the sex of their offspring. Parasitic wasps do possess such a mechanism, namely arrhenotoky, and they show deviation from random sex determination. Outcrossed wasps may vary sex ratio with host size and this has been modeled by Charnov (1979) and Charnov et al. (1981). Hamilton (1967) has explained why highly inbred parasitic wasps should produce highly female-biased sex ratios. This paper shows that not only should the sex ratios be female biased, but they should also be precise. The selective advantage of a precise sex ratio can be calculated and is seen to be about 5%-30% for the cases considered here. Laboratory studies show that parasitic wasps do, in fact, produce precise sex ratios. Goniozus gordhi produces exactly one male as a rule in small clutches of eggs, and tends to produce more only in larger clutches. Goniozus emigratus, which has larger clutches, generally produces more than one male per clutch, but the variance in the number of males is half what it would be if sex determination were binomial. It is suggested that an evolutionary advantage of arrhenotoky is that it makes possible precise sex ratios, which provide a selective advantage in highly inbred parasitic wasps.