DISENTANGLING THE EFFECTS OF MATING PROPENSITY AND MATING CHOICE INDROSOPHILA

Abstract

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...

Sexual and reproductive behaviour ofDrosophila melanogasterfrom a microclimatically interslope differentiated population of ‘Evolution Canyon’ (Mount Carmel, Israel)

“…It is known from many studies in Drosophila (Manning 1963;Kessler 1969Singh & Chatterjee 1987;Singh & Sisodia 1999) that variation in parameters of mating propensity may appear on the level of populations and strains. Recently, it was clearly shown that di¡erences in mating propensity can lead to non-random mating in mate choice situations and be erroneously interpreted as discrimination (Casares et al 1998).…”

“…Usually, male mating propensity is measured in nonchoice tests as time elapsed from introduction of mating partners into the mating chamber until copulation (Casares et al 1998) or the number of females mated (Ringo et al 1986). These de¢nitions are probably not absolutely correct for all cases.…”

“…Therefore, for correct interpretation of observed positive assortative mating it is mandatory to unravel the underlying behavioural components (Markow 1981;Dodd 1989;Casares et al 1998). Here, we describe the results of a systematic analysis of sexual behaviour in a non-choice situation, and several reproductive parameters of the sympatric populations of D. melanogaster from the opposite slopes of`Evolution Canyon'.…”

Is premating isolation inDrosophila overestimated due to uncontrolled factors?

“…The most common of these is the Malogolowkin-Cohen index, which calculates the number of homogamic minus heterogamic matings and divides it by the total number of matings, giving positive values in the case of an excess of homogamic matings, and negative values in the opposite case (Malogolowkinthat can give rise to an erroneous interpretation of the obtained data. Casares et al (1998) have shown that if the two types of males and females differ in their mating propensities, and the number of matings occuring in the tests are close to 100% of all possible matings, then the choice conditions are not maintained and there is a high risk of obtaining significant results that do not correspond to true choice and falsely suggest assortative mating. In other words, mating propensity differences between the studied genotypes can produce significant departures from random mating between different types of flies that could then be erroneously interpreted as evidence of incipient sexual isolation.…”

“…Another possible answer emerges from the design and data handling of the multiplechoice mating tests. As discussed above, Casares et al (1998) have demonstrated that mating propensity differences can give rise to deviations from random mating and false positive isolation measures. If one of the two competing types has greater male and female mating propensity than the other type, then homogamic matings will exceed heterogamic ones and the isolation index will be positive.…”