Courtship signals and mate choice of the ̄ies of inbred Drosophila montana strains

Many Drosophila species exhibit genetic variation in traits affecting mate choice (Scott & Richmond, 1988; MarõÂn, 1994; Aspi & Hoikkala, 1995). This variation forms the basis for sexual selection (exercised) by the females within a species and may also give rise to isolation between geographically separated populations (Markow, 1991). Studies on genetic variation in traits affecting mate choice and progeny production are thus essential for understanding how species-speci®c courtship behaviour has evolved. Mating success of the ̄ies of different Drosophila species depends on a variety of factors including male courtship vigour, female receptivity, male size and the signals emitted during the courtship (studies reviewed by Spiess, 1987). A diallel crossing design (a set of n possible crosses and selves) between homozygous lines makes it possible to study the effects of trait variation between strains on ̄y mating success. Our study species, Drosophila montana Stone, Griffen, Patterson, belongs to the D. virilis group. It is distributed throughout the northern hemisphere, having populations in Fennoscandia, North America, Japan and Russia (Throckmorton, 1982). D. montana ̄ies overwinter as adults and mate in spring (Aspi et al., 1993). Songs produced by males vibrating their wings are essential for stimulating the females to mate (Liimatainen et al., 1992). Males that have overwintered vary in their song characters due to both genetic and environmental factors (Aspi & Hoikkala, 1993; Hoikkala & Isoherranen, 1997), enabling the females to use male courtship song in their mate choice (Aspi & Hoikkala, 1995). Bartelt et al. (1986) have reported that D. montana males and females have similar hydrocarbon pro®les, but whether these pro®les vary within and between populations and whether such variation has any effect on mate choice has not been studied. Cuticular hydrocarbons act as mating pheromones in several insect species, and even slight changes in their composition have been shown to selectively and signi®cantly in ̄uence mating success in other species of Drosophila (Cobb & Jallon, 1990; Markow & Toolson, 1990).