The misuse of the three-point cross for estimating chiasma interference.

The building of genetic maps in diploid organisms by crosses between different genotypes and estimation of recombination frequencies from the obtained segregation data has been successfully used since a very young step in the birth of genetics. The three-point cross methodology has facilitated this task and has demonstrated at the same time that genetic distances are not additive, as some recombinant products are not recognised in the progeny. Three-point cross also allows to examine if chiasma interference exists and its evaluation. Here I show that the classical method of this estimation is erroneous and inevitably determines the apparition of a spurious, positive interference, which has been claimed to be an almost general phenomenon. Interference can only be estimated from a precise knowledge of the number of crossing over events occurring in meiocytes.

Involvement of cuticular hydrocarbons in the mating success of Drosophila melanogaster females.

The role of Drosophila melanogaster female pheromones in mating success was investigated in parallel with female receptivity. Lines selected for high and low female receptivity showed no changes in the patterns of cuticular hydrocarbons despite the occurrence of large variation for these aphrodisiac compounds in the base population. Female mating success was clearly related to receptivity level but not to pheromone-mediated attractiveness. The role of female pheromones in sexual selection is questioned.

A corrected Haldane's map function to calculate genetic distances from recombination data.

The estimation of genetic distances from recombination data has no direct relationship due to the fact that multiple crossovers do not generate recombinant gametes that can be recognized in the progeny. The Haldane's map function is the most widely used mathematical formulation able to relate the observed recombination frequency with the actual number of crossovers. Here I show that the model in which the Haldane's correction is based on is not correct, and I present a modified map function that takes into consideration the actual number of recombinant gametes produced in cells in which different number of crossovers have occurred. My correction generates shorter genetic distances than the Haldane's one.

Is premating isolation in Drosophila overestimated due to uncontrolled factors?

Sexual isolation in Drosophila is typically measured by multiple-choice mating tests. While many environmental variables during such tests are controlled by the researcher, there are some factors that are usually uncontrolled. We demonstrate, using Drosophila melanogaster and D. pseudoobscura flies, that the temperature of rearing, pre-adult density, and level of consanguinity, can all produce differences in mating propensity between genetically equivalent flies. These differences in mating propensity, in turn, can give rise to statistically significant results in multiple-choice mating tests, leading to positive isolation values and the artifactual inference of sexual isolation between populations. This fact agrees with a nonrandom excess of significant positive tests found in a review of the literature of Drosophila intraspecific mating choice. An overestimate of true cases of sexual isolation in Drosophila in the literature can, therefore, not be ruled out.

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 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.