Are Mating and Mate Competition by the Fig Wasp Pegoscapus assuetus (Agaonidae) Random within a Fig

Female-biased sex ratios are often observed in parasitic Hymenoptera that mate within small, isolated groups before dispersing to colonize new hosts (Hamilton 1967, Charnov 1982). In these groups there is both mating between sibs and competition among brothers for mates. Much theoretical attention has been devoted to predicting the sex ratio within the groups for the population structure that occurs in these parasitic wasps (reviewed by Charnov 1982, Frank 1983a). Recent theoretical studies indicate that the proportion of males is negatively correlated with the relatedness among males competing for mates and with the relatedness between mates (Borgia 1982, Colwell 1982). The predicted sex ratio (males : females) for diploid organisms is 1 — a: 1 + f3, where a is the proportion of competitive interactions between brothers, and is the proportion of matings between sibs (Frank, unpub. ms.). An untested assumption of previous quantitative models (e.g., Hamilton 1967, Taylor and Bulmer 1980, Colwell 1981) is that mate competition among males and mating occur randomly within a local group. Random mate competition and mating within a group means that if the proportion of sibs in the group is p, then sib mating is expected with probability p, and p of the interactions among males will be between brothers. If a group is founded by n egglaying females, each producing the same number of eggs, then p = 1/n, and the predicted sex ratio is 1 — 1/n : 1 + 1/n, or, given as males/total, (n — 1)/2n. This is Hamilton's (1967) widely cited result for diploid organisms. The predicted sex ratio for haplodiploid organisms such as fig wasps is slightly more female-biased than for diploids (Hamilton 1979, Taylor and Bulmer 1980, Frank 1983a). Two empirical studies have been published that report female-biased sex ratios in fig wasps correlated with mate competition and inbreeding (Hamilton 1979, Frank 1983b). Neither study concerned itself with the assumptions of random mate competition and mating in local groups. In this note I report the results of an experiment that yielded correlative evidence inconsistent with these two assumptions. A monoecious fig (Ficus spp.) is a globular inflorescence containing hundreds of staminate and pistillate florets lining a nearly sealed cavity. A few female fig wasps (Agaonidae) enter the fig through a narrow passage (ostiole), and once inside, never leave. They pollinate many of the pistillate florets with pollen carried from the fig from which they emerged, lay an egg in the single-seeded ovary of some of the pistillate florets, and then die. Each larva develops by eating the tissues associated with the developing seed or the seed itself. About one month later the next generation of wingless males emerges and mates with the eclosed females while the females are still in the individual florets. A male gains access to a female within a floret by chewing a hole through the wall of the ovary and inserting his telescoping abdomen. Usually 1-4 females enter each fig (97% of the figs of Ficus citrifolia, Frank 1983a; 98% of the figs of F. cotinifolia, F. morazaniana, and F. ovalis, and 29% of the figs of F. insipida, Janzen 1979a), and one-quarter or more of the progeny within a fig are likely to be sibs. After mating, the males chew an exit tunnel through the wall of the fig. The females leave their florets, collect pollen, then depart through the exit tunnel to find other figs and begin the cycle again. See Janzen (1979b), Wiebes (1979), and Frank (in press) for further details of the natural history. The assumptions of random mate competition and mating within figs are difficult to test directly for fig wasps. (Here 'random' means uncorrelated with respect to genetic relatedness.) The fig, which contains the local mating group, must remain sealed until the males emerge and begin to mate. During mating, the movement of the males may be observed by cutting open a fig; however, no method has been developed to determine which wasps are sibs. In order to test for a possible searching pattern used by the males, a simple experiment was performed. The null hypothesis tested was that male movement while searching for mates is uncorrelated with the location of (i) related females, (ii) related males, and (iii) any other cues that allow a male to discriminate between a fig in which he was born and a foreign fig. Ficus citrifolia figs containing Pegoscapus assuetus wasps searching for mates were selected haphazardly from trees in the Everglades Park, Florida, and brought into the laboratory during June, 1982. Two figs were chosen, and only one-half of each of the two figs was used in each trial. From one-half of each of the two figs a male was chosen and marked with Testors enamel paint. Each marked male was then returned to the fig-half from which it was taken (designated 'own fig,' or '0'), or was placed in the fig-half from which the other marked male was obtained (designated `not own fig,' or 'N'). The two fig-halves, each containing one marked male and many unmarked males, were then joined with insect pins for 60 minutes. After this time elapsed, the location of each marked male was determined as either 'own fig-half (0) or 'not own fig-half (N).