SEX-RATIO AND MATING STRUCTURE IN THE PARASITOID HYMENOPTERA

Assumptions about mating structure have been important in the developm ent of sex ratio theory. Fisher's theory of frequency dependent select ion for sex ratio equality, and Charnov's theory of sex allocation whe n host quality varies, assume panmixis, whereas Hamilton's theory of l ocal mate competition assumes mating occurs only at the natal patch. A n important group of organisms in stimulating the development of sex r atio theory, and in its empirical testing, have been the parasitoid Hy menoptera. These exhibit many life history Variations and possess a me chanism by which females can adaptively control their progeny sex rati o. Although many parasitoid species may conform closely to both the as sumptions and predictions of the above models, others do not. Here I r eview the mating structure assumptions in sex allocation theories and the evidence for mating structure and sex ratio among parasitoid Hymen optera. Detailed information about parasitoid mating structures is sca rce, but the available knowledge indicates that mating structures fall ing between the extremes of panmixis and fully local mating are not un common. Stimulated by such observations, recent theory has incorporate d partial local mating into Hamilton's model, and also into the combin ation of Hamilton's and Charnov's models. The resulting general predic tion is that the proportion of local mating can strongly influence opt imal sex ratio. Tests of these new theories are almost entirely lackin g, although some comparative evidence is in qualitative agreement with their predictions. It is concluded that fully local mating is probabl y the exception rather than the rule, and that the understanding of se x ratio evolution will be improved by further exploration of intermedi ate mating structures.