Crossover Homeostasis and Chromosome Pairing in Maize

Meiotic recombination is required for generating variation in the progeny as well as ensuring proper segregation of chromosomes into daughter cells.  Recombination is also tightly linked to other processes taking places during meiosis, such as pairing of homologous chromosomes.  The process of recombination is initiated by formation of double-strand breaks (DSBs) in chromosomal DNA early in meiosis and prior to pairing of homologous chromosomes.  The breaks are subsequently repaired as crossovers (COs) and non-crossovers (gene conversions).  In most eukaryotes, including plants, DSBs vastly outnumber COs.  The reason for this disparity is not fully understood.  Using a maize mutant with vastly reduced number of meiotic DSBs, we found that plants exhibit CO homeostasis, a phenomenon in which the number of COs is kept stable even when the number of DSBs changes dramatically.  However, the reduction of the DSB number leads to major defects in chromosome pairing, suggesting that the excess of DSBs is needed to facilitate chromosome interactions that lead to pairing.  Using a diverse set of maize inbred lines with varying numbers of meiotic DSBs and COs, we found that homeostasis acts to ensure that each chromosome pair has at least one CO.  However, once this minimum requirement is met, homeostasis no longer operates and the number of COs is proportional to the number of DSBs.  This finding may provide clues on how to manipulate recombination rates for plant breeding.