OLFACTORY CUES AS A FACTOR IN FREQUENCY-DEPENDENT MATE SELECTION IN MORMONIELLA VITRIPENNIS.

Abstract

Ironically, the phenomenon of frequencydependent mate selection, so far welldocumented only in the class Insecta, seems rather to have its greatest intuitive appeal in a consideration of human characteristics. The suggestion, capitalized on by advertising executives, that blondes (as a minority type) have more fun, would appear to illuminate a facet of human sexual experience; what Claudine Petit (1972) has referred to as the charm of the exotic. For evolutionists, however, the interest in frequency-dependent mate selection lies beyond a simple description of unique sexual behavior. Though little is known about the existence of a frequency-dependent response in natural populations (Borisov, 1970), this mode of sexual selection may prove to play an important role in the maintenance of certain balanced polymorphisms. Petit (1958) and Ehrman et al. (1965) independently observed that for Drosophila melanogaster and D. pseudoobscura, respectively, mating success in the laboratory was dependent on the relative frequencies of the two types of males. The minority male type showed an increased mating efficiency. The advantage of the rare male has since been described in eight more species of the genus Drosophila; D. equinoxialis, D. funebris, D. gaucha, D. immigrans, D. pavani, D. persimilis, D. tropicalis, and D. willistoni (Ehrman and Petit, 1968; Ehrman, 1966, 1972b; Ehrman et al., 1972; Spiess, 1968; Spiess and Spiess, 1969; and Borisov, 1970); the parasitic wasp Mormoniella vitripennis (Grant et al., 1974); and suggested in the red flour beetle Tribolium castaneum (Sinnock, 1970). Females have been shown to possess the ability to distinguish a variety of differences among conspecific males, e.g., strains of males carrying different chromosomal inversions, of different geographic origins, reared at different temperatures, and wild versus mutant types (Ehrman, 1967, 1970b; Petit and Ehrman, 1969). Recent work has been concerned with the behavioral components of frequencydependent mate selection, the mechanism(s) by which males in the minority may gain a mating advantage over those in the majority, thus making a disproportionate contribution to the gene pool. Ehrman (1969, 1970a, 1972a) has demonstrated that frequency-dependent behavior depends most heavily on information transferred to the females by airborne olfactory cues. Frequency-dependent mate selection has been experimentally induced with equal frequencies of male types or, conversely, reduced when unequal frequencies were present. This has been accomplished through use of a double chamber technique (see Ehrman, 1969, 1970a, for details), in which flies in the mating chamber perceive the odor of flies in an adjacent compensation chamber. Although a pheromone would appear to be the major mediating factor (Ehrman et al., 1973), it probably acts in conjunction with auditory, tactile, and visual sensory stimulation (Leonard et al., 1974). The situation is further complicated because of the age and experience dependence involved in mate selection (Pruzan and Ehrman, 1974; Pruzan, 1976). Unlike the studies of the Ehrman group with D. pseudoobscura, Petit and Nouaud (1975) found that olfactory stimulation appeared to be of no importance as a