Abstract.-Incipient sexual isolation between genotypes, lines, or populations of the same species is commonly measured in Drosophila by choice tests. Results of these tests are known to be influenced, in an undetermined manner, by the mating propensity of competitors and by discriminatory factors during courtship. We have approached the problem by measuring male and female propensities in separate, independent tests, and by examining whether these estimates could explain the results of the choice tests. First, male and female choice tests were used to measure sexual isolation between populations of Drosophila melanogaster and between populations of D. simulans. Significant deviations from random mating occurred in 31 out of 48 tests, in agreement with the propensity values of the tested genotypes. We conclude that mating propensity instead of discrimination is directly involved in the estimation of sexual isolation in our populations, and advise against the application of male and female choice tests to assess intraspecific isolation without a proper knowledge of the mating propensities of competing individuals. Second, multiple choice tests were used to assess isolation between D. melanogaster populations. In examining the dynamics of matings throughout the test, we show that if competing individuals differ in mating propensities and tests are long enough to allow most matings to happen, a spurious sexual isolation can appear. We recommend that multiple choice tests be terminated once 50 percent of matings had been observed.Key words.-Drosophila, mating choice tests, mating propensity, sexual isolation.Received March 10, 1997.The role of sexual isolation in species formation is one of the most interesting questions in evolutionary biology. Sexual isolation is a form of reproductive isolation arising from behavioral differences between potential mates. The elaborate courtship of most Drosophila species provides good material for the study of sexual behavior. Since Sturtevant's (1915) pioneering work, students of Drosophila have tried to understand how mating differences and sexual selection could lead to speciation.At the intraspecific level, incipient sexual isolation can be seen as a tendency to mate homogamically rather than heterogamically. The underlying divergence in mating behavior between two conspecific strains can be studied in different ways. Unfortunately, a direct analysis of behavioral differences is laborious, time-consuming, and requires a lot of practice and experience (Spiess 1970;Burnet and Connolly 1981;Cobb et al. 1985Cobb et al. , 1986. As a result, the most commonly used designs for measuring sexual isolation in Drosophila are the male choice, female choice, and multiple choice tests (see Parsons 1973), which measure the frequencies of intra-and interstrain matings. The male choice test consists on introducing a male from one strain with a female from his own strain and a female from another strain, in a mating chamber, and recording the types of mating that occur. In the female choice tes...

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From a critical review of the literature on mating speed in Drosophila, the importance of fast mating in male fitness is questioned. The genetic architecture of male mating speed (MMS) has been evaluated in D. melanogaster through a populational analysis and a full 5 x 5 diallel cross between inbred lines. The results emphasize the fundamental role of the female genotype in both the absolute and the relative MMS performances. Somewhat different genetic architectures for MMS are revealed according to the female used in the tests. It is suggested that different parts of the complex genetic system involved in the male's "behavioral sexual phenotype" are relevant depending on the female's characteristics, thus causing the heterogeneity in the MMS genetic architecture. An overall picture reveals a genetic system characterized by additivity, dominance for fast mating, and no influence of the X chromosome. There results do not support strong natural selection favoring fast mating in Drosophila males.

Receptivity of sexually mature, virgin females, measured as the time to copulation, has been investigated. Two independent 5 x 5 diallel crosses were carried out, each one with a different male tester genotype. Interactions between male and female genotypes were detected because some lines appeared as dominant in one diallel and recessive in the other. The overall picture emerging from the analyses shows a genetic system for female receptivity characterized by additive genes, with dominance for high receptivity in one of the diallel crosses. This remarkable additive component does not support the action of natural selection favouring females with extreme values of receptivity, rather, the lack of intrasexual female competition for mates is suggested.

Females of Drosophila melanogaster and males of D. simulans hybridizing in a nonchoice condition were artificially selected for 12 generations. The frequency of hybridization increased from 10% to 79%. Response to selection occurred in both species, particularly in D. melanogaster. Female receptivity was the primary sexual trait that accounted for breaking up sexual isolation in these species, but it remained unclear which elements of the D. simulans male courtship were involved.