FEMALE MATE DISCRIMINATION OR MALE RESPONSES TO FEMALE STIMULATION?

Abstract

Determining which partner is responsible for any given effect in a dyadic interaction is extremely difficult. In Drosophila, even complex multifactorial studies may be unable to discover the basis for patterns of courtship and mating (Cobb et al. 1990; for a review see Cobb and Ferveur 1996). Scott (1994) claims to have found evidence for female mate discrimination in Drosophila melanogaster. We consider that his data do not necessarily justify his conclusions and that other interpretations could usefully be taken into account. In D. melanogaster, mating is preceded by courtship: the male will orient to the female, follow her if she moves, vibrate his wing, lick her genitalia, and eventually copulate with her. Since the beginning of the 1980s, it has been known that female cuticular hydrocarbons induce male courtship (Antony and Jallon 1982; Jallon 1984). The principal sex-specific hydrocarbon on females of most laboratory strains, the 27 carbon diene 7,11 heptacosadiene (7,11 HD), is a pheromone and induces male wing vibration in a dose-response fashion (Antony et al. 1985). We have carried out a world-wide survey of D. melanogaster and found that in sub-saharan Africa and the Caribbean, 7,11 HD is largely replaced by its position isomer, 5,9 heptacosadiene (5,9 HD), which is behaviorally inactive. This polymorphism is controlled by gene(s) on chromosome III (Ferveur et al. 1996). Scott (1994) studied courtship and mating between two strains of D. melanogaster, Canton-S (CS), a US laboratory strain, and Tai-Y, a strain from West Africa. Although he found that Tai females mated with an equal frequency with males from either strain, CS females mated at relatively low levels with Tai-Y males while mating at normal frequencies with CS males. Scott's interpretation of this result is that CS females were discriminating between the two types of males on the basis of variations in the males' cuticular hydrocarbons. Males from Scott's CS strain show high levels of the 23 carbon monoene 7-tricosene (7-T) and low levels of the 25 carbon monoene 7-pentacosene (7-P), whereas the proportions are inversed in Tai-Y males. As yet, there is no decisive evidence for any male cuticular hydrocarbons acting as a pheromone from males to females in this species. The conclusion presented by Scott (1994) would appear to represent an important proof of this hypothesis. However, we consider that there are four weaknesses in both the data and the interpretation. (1) What is the role of the female? The biggest interpretative weakness is the lack of any information as to the role of the