Studies on crossover-specific mutants and the distribution of crossing over in Drosophila females.

In Drosophila females, the majority of recombination events do not become crossovers and those that do occur are nonrandomly distributed. Furthermore, a group of Drosophila mutants specifically reduce crossing over, suggesting that crossovers depend on different gene products than noncrossovers. In mei-218 mutants, crossing over is reduced by approximately 90% while noncrossovers and the initiation of recombination remain unchanged. Importantly, the residual crossovers have a more random distribution than wild-type. It has been proposed that mei-218 has a role in establishing the crossover distribution by determining which recombination sites become crossovers. Surprisingly, a diverse group of genes, including those required for double strand break (DSB) formation or repair, have an effect on crossover distribution. Not all of these mutants, however, have a crossover-specific defect like mei-218 and it is not understood why some crossover-defective mutants alter the distribution of crossovers. Intragenic recombination experiments suggest that mei-218 is required for a molecular transition of the recombination intermediate late in the DSB repair pathway. We propose that the changes in crossover distribution in some crossover-defective mutants are a secondary consequence of the crossover reductions. This may be the activation of a regulatory system that ensures at least one crossover per chromosome, and which compensates for an absence of crossovers by attempting to generate them at random locations.