Caenorhabditis elegans msh-5 is required for both normal and radiation-induced meiotic crossing over but not for completion of meiosis

Crossing over and chiasma formation during Caenorhabditis elegans meiosis r equire msh-5 which encodes a conserved germline-specific MutS family member , msh-5 mutant oocytes lack chiasmata between homologous chromosomes, and c rossover frequencies are severely reduced in both oocyte and spermatocyte m eiosis. Artificially induced DNA breaks do not bypass the requirement for m sh-5, suggesting that msh-5 functions after the initiation step of meiotic recombination. msh-5 mutants are apparently competent to repair breaks indu ced during meiosis, but accomplish repair in a way that does not lead to cr ossovers between homologs. These results combine with data from budding yea st to establish a conserved role for Msh5 proteins in promoting the crossov er outcome of meiotic recombination events. Apart from the crossover defici t, progression through meiotic prophase is largely unperturbed in msh-5 mut ants. Homologous chromosomes are fully aligned at the pachytene stage, and germ cells survive to complete meiosis and gametogenesis with high efficien cy. Our demonstration that artificially induced breaks generate crossovers and chiasmata using the normal meiotic recombination machinery suggests (1) that association of breaks with a preinitiation complex is not a prerequis ite for entering the meiotic recombination path way and (2) that the decisi on for a subset of recombination events to become crossovers is made after the initiation step.