Crossover homeostasis in yeast meiosis.

Crossovers produced by homologous recombination promote accurate chromosome segregation in meiosis and are controlled such that at least one forms per chromosome pair and multiple crossovers are widely spaced. Recombination initiates with an excess number of double-strand breaks made by Spo11 protein. Thus, crossover control involves a decision by which some breaks give crossovers while others follow a predominantly noncrossover pathway(s). To understand this decision, we examined recombination when breaks are reduced in yeast spo11 hypomorphs. We find that crossover levels tend to be maintained at the expense of noncrossovers and that genomic loci differ in expression of this "crossover homeostasis." These findings define a previously unsuspected manifestation of crossover control, i.e., that the crossover/noncrossover ratio can change to maintain crossovers. Our results distinguish between existing models of crossover control and support the hypothesis that an obligate crossover is a genetically programmed event tied to crossover interference.