Meiotic recombination occurs preferentially between homologous nonsister chromatids rather than between sisters, opposite to the bias of mitotic recombinational repair. We have examined formation of joint molecule recombination intermediates (JMs) between homologs and between sisters in yeast strains lacking the meiotic chromosomal protein Red1, the meiotic recA homolog Dmc1, and/or mitotic recA homolog(s), Rad51, Rad55, and Rad57. Mutant phenotypes imply that most meiotic recombination occurs via an interhomolog-only pathway along which interhomolog bias is established early, prior to or during double strand break (DSB) formation, and then enforced, just at the time when DSBs initiate JM formation. A parallel, less differentiated pathway yields intersister and, probably, a few interhomolog events. Coordinate action of mitotic recA homologs as one functional unit, two functions of RED1, and an interhomolog interaction function of DMC1 are also revealed.
