NONHOMOLOGOUS SYNAPSIS AND REDUCED CROSSING-OVER IN A HETEROZYGOUS PARACENTRIC INVERSION IN SACCHAROMYCES-CEREVISIAE

Homologous chromosome synapsis (''homosynapsis'') and crossing over ar e well-conserved aspects of meiotic chromosome behavior. The long-stan ding assumption that these two processes are causally related has been challenged recently by observations in Saccharomyces cerevisiae of si gnificant levels of crossing over (1) between small sequences at nonho mologous locations and (2) in mutants where synapsis is abnormal or ab sent. In order to avoid problems of local sequence effects and of muta tion pleiotropy, we have perturbed synapsis by making a set of isogeni c strains that are heterozygous and homozygous for a large chromosomal paracentric inversion covering a well marked genetic interval and the n measured recombination. We find that reciprocal recombination in the marked interval in heterozygotes is reduced variably across the inter val, on average to similar to 55% of that in the homozygotes, and that positive interference still modulates crossing over. Cytologically, s table synapsis across the interval is apparently heterologous rather t han homologous, consistent with the interpretation that stable homosyn apsis is required to initiate or consummate a large fraction of the cr ossing over observed in wild-type strains. When crossing over does occ ur in heterozygotes, dicentric and acentric chromosomes are formed and can be visualized and quantitated on blots though not demonstrated in viable spores. We find that there is no loss of dicentric chromosomes during the two meiotic divisions and that the acentric chromosome is recovered at only 1/3 to 1/2 of the expected level.