DISENTANGLING THE EFFECTS OF MATING PROPENSITY AND MATING CHOICE IN DROSOPHILA

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 ex amining whether these estimates could explain the results of the choic e tests. First, male and female choice tests were used to measure sexu al isolation between populations of Drosophila melanogaster and betwee n populations of D. simulans. Significant deviations from random matin g occurred in 31 out of 48 tests, in agreement with the propensity val ues of the tested genotypes. We conclude that mating propensity instea d of discrimination is directly involved in the estimation of sexual i solation in our populations, and advise against the application of mal e 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 throug hout 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 c hoice tests be terminated once 50 percent of matings had been observed .