Genetic interactions between meiS332 and ord in the control of sister-chromatid cohesion

The Drosophila mei-S332 and ord gene products are essential for proper sist er-chromatid cohesion during meiosis in both males and females. We have con structed flies that contain null mutations for both genes. Double-mutant fl ies are viable and fertile. Therefore, the lack of an essential role for ei ther gene in mitotic cohesion cannot be explained by compensatory activity of the two proteins during mitotic divisions. Analysis of sex chromosome se gregation in the double mutant indicates that ord is epistatic to mei-S332. We demonstrate that ord is not required for MEI-S332 protein to localize t o meiotic centromeres. Although overexpression of either protein in a wild- type background does not interfere with normal meiotic chromosome segregati on, extra ORD+ protein in mei-S332 mutant males enhances nondisjunction at meiosis II. Our results suggest that a balance between the activity of mei- S332 and ord is required for proper regulation of meiotic cohesion and demo nstrate that additional proteins must be functioning to ensure mitotic sist er-chromatid cohesion.