DMC1, RAD51 and Meiotic Recombination in Arabidopsis

Eukaryotic sexual reproduction is based on the production of gametes of halved ploidy, the fusion of two of which regenerates the original ploidy in the subsequent generation. This halving of ploidy in gametogenesis is carried out by a meiosis, a specialized cell division in which the two successive nuclear divisions follow a single round of replication. Homologous recombination during prophase of the first meiotic division ensures synapsis and proper segregation of homologous chromosomes of maternal and paternal origins and shuffles the genetic information, generating genetic variation. Notwithstanding tremendous advances in our understanding, many questions remain concerning the molecular mechanisms controlling meiotic recombination and synapsis.

Homologous recombination involves the repair of a broken DNA through invasion and copying of a homologous DNA template molecule. The strand-transfer enzymes which catalyse this key invasion step are ubiquitous and highly conserved through evolution. In addition to RAD51, which acts in both mitotic and meiotic cells, with a few exceptions all studied eukaryotes have a second enzyme, DMC1, which acts specifically in meiosis. Thus, both RAD51 and DMC1 act in meiosis to catalyse homologous recombination. To better understand their specificities and the relationship between recombination and meiotic homologue pairing, we are characterizing the roles of the two recombinases RAD51 and DMC1 in the plant Arabidopsis thaliana. We have previously described a dominant-negative RAD51-GFP fusion protein that rescues the meiotic chromosome fragmentation of rad51 mutants, but is inactive for catalysing recombination (Da Ines et al, PLoS Genet. 2013. 9: e1003787). Meiotic recombination in rad51 mutants expressing this protein is thus catalysed only by DMC1 and we are using this fact to test for effects on crossing-over rates, both genome-wide and in different chromosomal regions. Data from immunofluorescence, chiasmata counting and crossing-over rate measurements in centromere-spanning and chromosome-arm genetic intervals will be presented and discussed in the context of the specificities of the roles of the DMC1 and RAD51 strand-exchange activities in meiosis.