Abstract
In order for two heterothallic MATalpha haploids of Saccharomyces cerevisiae to mate, one parent must apparently become, at least transiently, an a-like cell. Only about 25% of the matings result from an actual transposition of MATa sequences to replace MATalpha, and about 1% result from a deletion joining MAT to the normally silent HMRa allele. The majority of matings occur after an apparent chromosome break that deletes MATalpha and all of the known markers more distal on the right arm of chromosome III.--The chromosome break occurs at or very near MAT, invariably leaving the distal marker tsm1 hemizygous, but the closely linked proximal marker cry1 usually is heterozygous. The resulting diploid containing the broken chromosome is mitotically unstable; about 10% of the colonies contain visible sectors in which the rest of the broken chromosome is lost. The region close to the breakpoint (i.e., cry1) is unusually active in recombination. About 20% of the intact homologues remaining after chromosome loss were gene-converted for cry1. In addition, the broken end participated in reciprocal recombination events that joined the chromosome to the distal portion of the intact homologous chromosome.--The unstable diploids may also become stable and no longer give rise to mitotic segregants. We have found two distinct ways in which stabilization occurs. Most often the diploid becomes euploid by a recombination event that yields a cell homozygous for all markers distal to (and sometimes including) cry1. In one of 9 cases so far analyzed, the stable diploid was still hemizygous for MATalpha and for other markers distal to MAT. This last case is similar to the healing of broken chromosomes in maize described by McClintock (1939, 1941, 1951).