SEX RATIO IN A NEW SPECIES OF NASONIA WITH FULLY-WINGED MALES.

Abstract

One of the better-studied sex ratio theories is mate competition (LMC) theory, first developed by Hamilton in 1967 (for reviews see Chamov, 1982; Waage, 1986; King, 1987). LMC models assume a subdivided population structure with emergence and then random mating taking place within patches followed by female dispersal to new patches to lay offspring. Under these conditions, offspring sex ratio (proportion sons) is expected to increase with increasing number of ovipositing mothers in a patch, eventually reaching an asymptote (Hamilton, 1967, 1979). One of the best-studied species with regards to LMC theory is the parasitoid wasp Nasonia vitripennis. N. vitripennis's sex ratio as it relates to LMC theory has been examined both genetically and behaviorally, both in the laboratory and in the field (e.g., Orzack and Parker, 1986; Werren, 1980, 1983). Although current sex ratio theory cannot completely explain offspring sex ratios in N. vitripennis (e.g., Orzack, 1986), empirical evidence on N. vitripennis supports LMC model predictions in general. For example, most N. vitripennis females produce female-biased offspring sex ratios when alone and increase the proportion of sons they produce when other ovipositing females are present (Walker, 1967; Wylie, 1965; Velthius et al., 1965; Werren, 1983). Here we examine offspring sex ratios in a newly discovered species of Nasonia, N. giraulti (Darling and Werren, 1990), and compare it to that of N. vitripennis. In N. vitripennis, the explanation for why the species meets the population structure assumed by most LMC models has been that 1) males lack full wings; 2) thus, males cannot disperse to mate, and so mating must take place at the emergence site; and 3) because mating takes place at the site of emergence, there will be competition for mates among males emerging from the same or nearby hosts (i.e., local mate competition). N. giraulti is almost identical morphologically to N. vitripennis, except that N. giraulti males have full wings and can fly. Thus, our expectation was that in N. giraulti, males disperse and so their populations, relative to N. vitripennis, should experience both less competition among brothers for mates and less inbreeding. Either or both of which should have selected for less female-biased sex ratios in N. giraulti than in N. vitripennis (Frank, 1985; Herre, 1985). We present results showing that N. giraulti does manipulate offspring sex ratio in response to the number of other mothers present, but that N. giraulti produces