MEIOTIC CROSSING-OVER BETWEEN NONHOMOLOGOUS CHROMOSOMES AFFECTS CHROMOSOME SEGREGATION IN YEAST

Meiotic recombination between artificial repeats positioned on nonhomo logous chromosomes occurs efficiently in the yeast Saccharomyces cerev isiae. Both gene conversion and crossover events have been observed, w ith crossovers yielding reciprocal translocations. In the current stud y, 5.5-kb ura3 repeats positioned on chromosomes V and XV were used to examine the effect of ectopic recombination on meiotic chromosome seg regation. Ura(+) random spores were selected and gene conversion vs. c rossover events were distinguished by Southern blot analysis. Approxim ately 15% of the crossover events between chromosomes V and XV were as sociated with missegregation of one of these chromosomes. The missegre gation was manifest as hyperploid spores containing either both transl ocations plus a normal chromosome, or both normal chromosomes plus one of the translocations. In those cases where it could be analyzed, mis segregation occurred at the first meiotic division. These data are dis cussed in terms of a model in which ectopic crossovers compete efficie ntly with normal allelic crossovers in directing meiotic chromosome se gregation.