Crossover Homeostasis and Chromosome Pairing in Maize

Meiotic recombination is essential for correct segregation of chromosomes to the progeny and for generating genetic variation.  In most eukaryotes, including plants, recombination is also tightly linked to pairing of homologous chromosomes.  Many mutants defective in early steps of meiotic recombination exhibit defects in homologous chromosome pairing.  Recombination is initiated by formation of double-strand breaks (DSBs) in meiotic chromosomes.  DSB repair eventually leads to formation of crossing overs (COs).  In most species DSBs far outnumber COs, a phenomenon whose biological consequences are not well understood.  We have identified a mutant in maize in a gene known to be required for DSB formation that exhibits only about 2% of the normal number of meiotic DSBs.  We found, however, that despite this dramatic reduction in the DSB number, the mutant retains at least 30% of the normal number of COs.  These data suggest that COs homeostasis, a mechanism which keeps a stable number of COs despite a reduced number of DSBs, also operates in maize.  In contrast to CO formation, we found that homologous chromosome pairing is largely abolished in the mutant and many COs are formed between non-homologous partners.  These observations imply that recombination pathway intermediates that aid chromosome pairing are not subject to homeostasis.  We hypothesize that recombination-facilitated chromosome interactions at multiple locations along each chromosome may be required to ensure proper pairing and recombination fidelity.