Meiotic gene conversion and crossing over: their relationship to each other and to chromosome synapsis and segregation.

The yeast mer1 mutant produces inviable spores and is defective in both meiotic recombination and chromosome pairing. A gene called MER2 partially suppresses the mer1 phenotype when present in high copy number. Both gene conversion and chromosome pairing are completely restored in mer1 strains overexpressing MER2; however, reciprocal crossing over and spore viability are not restored. The data presented are consistent with a model in which chromosome pairing is a direct consequence of a homology search mediated through gene conversion. Analysis of random viable spores indicates that the crossovers that occur in mer1 strains overexpressing MER2 are more effective in ensuring meiosis I disjunction than those that occur in mer1 strains. One interpretation of this result is that only those crossovers that occur in the context of the synaptonemal complex lead to the establishment of functional chiasmata. The MER2 gene product is essential for meiosis.