Coordination of the initiation of recombination and the reductional division in meiosis in Saccharomyces cerevisiae

Early exchange (EE) genes are required for the initiation of meiotic recomb ination in Saccharomyces cerevisiae. Cells with mutations in several EE gen es undergo an earlier reductional division (MI), which suggests that the in itiation of meiotic recombination is involved in determining proper timing of the division. The different effects of null mutations on the timing of r eductional division allow EE genes to be assorted into three classes: mutat ions in RAD50 or REC102 that confer a very early reductional division; muta tions in REC104 or REC114 that confer a division earlier than that of wild- type (WT) cells, but later than that of mutants of the first class; and mut ations in ME14 that do not significantly alter the timing of MI. The very e arly mutations are epistatic to mutations in the other two classes. We prop ose a model that accounts for the epistatic relationships and the communica tion between recombination initiation and the first division. Data in this article indicate that double-strand breaks (DSBs) are not the signal for th e normal delay of reductional division; these experiments also confirm that ME14 is required for the formation of meiotic DSBs. Finally, if a DSB is p rovided by the HO endonuclease, recombination can occur in the absence of M EI4 and REC104.