Meiosis in asynaptic yeast.

The Saccharomyces cerevisiae red1 mutant fails to assemble synaptonemal complex during meiotic prophase. This mutant displays locus-specific reductions in interchromosomal gene conversion and a moderate reduction in crossing over. The occurrence of a significant amount of meiotically induced recombination in the red1 mutant indicates that the synaptonemal complex is not absolutely required for meiotic exchange. The RED1 gene product is required for intrachromosomal recombination in some assays but not others. Chromosomes that have undergone reciprocal exchange nevertheless nondisjoin in red1 mutants, indicating that crossovers are not sufficient for disjunction. Epistasis studies reveal that HOP1 is epistatic to RED1, and that RED1 acts in an independent pathway from MER1. A model for the function of the RED1 gene product in chromosome synapsis is discussed.