An analysis of genes required for the generation and function of meiotic crossing over: mei-218, mei-910, and mei-1794. E.A. Manheim , K. Giunta , K.S. McKim. Waksman Institute, Rutgers University, Piscataway, NJ.

   Meiosis is essential to sexual reproduction in all multicellular organisms because it is the process whereby the chromosome complement is precisely divided in half. Meiotic recombination, which occurs during prophase of Meiosis I, serves several important functions. For example, some recombination events mature into chiasmata, ensuring the proper segregation of homologous chromosomes during the reductional division. In addition to the crossovers, the formation of the bipolar spindle is also essential for disjunction of the chromosomes. If either the crossover or the spindle does not form properly, the chromosomes do not segregate, resulting in aneuploidy and usually death of the embryo. As originally described, the precondition genes, including mei-218, are required for a step prior to the actual recombination event that produces a crossover. Immunocytochemical analyses demonstrate that the MEI-218 protein is found predominantly in the cytoplasm, suggesting that precondition genes act in a regulatory role. We will describe the results of a large-scale gene conversion experiment to investigate the differences between the gene conversion and crossover pathways, i.e. how and if the genes required for the crossover pathway promote Holliday junction formation as a repair intermediate, and if so, at which point it diverges from the gene conversion pathway. We will also report on our genetic and molecular characterization of two new genes identified in a recent screen for meiotic mutants, mei-910 and mei-1794. mei-910 is only the second precondition gene isolated, and the first autosomal, with as severe a defect as mei-218. mei-1794, while not precondition defective, is required for spindle assembly, and specifically, the formation of the two spindle poles. We believe that both genes play integral roles in creating a proper environment, in which both crossovers and normal bipolar spindles form, which promotes the proper segregation of homologous chromosomes.