Coincident chromosomal disomy in meiotic dyads from triploid yeast

Among meiotic asci produced by triploid (3N) Saccharomyces cerevisiae are cases in which exactly two of the four ascospores proliferate into colonies. Given the unique asymmetry problems inherent in distributing three chromosome homologues in meiosis, these ascospore dyads are of special interest. We have tested 40 of these dyads (80 ascospores) for their chromosome content by ascertaining whether they have inherited one or two copies of each of the sixteen yeast chromosomes from the parental triploid. Overall, then, ascospores in these dyads can be either haploid (N) or disomic (N + 1) for each chromosome. The principal results of this analysis include: (1) Coincident disomy (inheritance of two copies of a given chromosome in both members of an ascospore dyad) was detected for 15 of the 16 yeast chromosomes, and at least once in every dyad. (2) Coincident disomy increased as a function of the mean number of disomic chromosomes per spore in each dyad, but this increase differed functionally from that expected if coincident disomy in the two ascospores were a simple, meiotically independent, concomitant of multiple disomy. We conclude from these results that: (1) The ascospore dyads, as the two proliferating spores of single meioses from the triploid, represent meiotic sisters. That is, they stem from the same half of the first meiotic division. (2) Multiply-disomic meiotic segregants of yeast triploids proliferate at the expense of their multiple disomy, as cells in spore colonies experience repeated and independent disomic chromosome losses (N + 1----N).(ABSTRACT TRUNCATED AT 250 WORDS)