LAUSR

The number and distribution of meiotic crossovers (COs) are highly regulated, reflecting the requirement for COs during the first round of meiotic chromosome segregation. CO control includes CO assurance and CO interference, which promote at least one CO per chromosome bivalent and evenly-spaced COs, respectively. Previous studies revealed a role for the DNA damage response (DDR) clamp and the clamp loader in CO formation by promoting interfering COs and interhomolog recombination, and also by suppressing ectopic recombination. In this study, we use classical tetrad analysis of Saccharomyces cerevisiae to show that a mutant defective in RAD24, which encodes the DDR clamp loader (RAD17 in other organisms), displayed reduced CO frequencies on two shorter chromosomes (III and V), but not on a long chromosome (chromosome VII). The residual COs in the rad24 mutant do not show interference. In contrast to rad24, mutants defective in the ATR kinase homolog Mec1, including a mec1 null and a mec1 kinase-dead mutant, show slight or few defects in CO frequency. On the other hand, mec1 COs show defects in interference, similar to the rad24 mutant. Our results support a model in which the DDR clamp and clamp-loader proteins promote interfering COs by recruiting pro-CO Zip, Mer, and Msh proteins to recombination sites, while the Mec1 kinase regulates CO distribution by a distinct mechanism. Moreover, CO formation and its control are implemented in a chromosome-specific manner, which may reflect a role for chromosome size in regulation.