Abstract
Heterokaryons of Saccharomyces cerevisiae have been constructed utilizing the kar1-1 mutation, which prevents nuclear fusion during conjugation (J. Conde and G. Fink, Proc. Natl. Acad. Sci. U.S.A. 73:3651-3655, 1976). Each heterokaryon contained two haploid nuclei that were marked on several chromosomes. They segregated haploid progeny (cytoductants), most of which have the nuclear genotype of one or the other of the heterokaryon parents, but they occasionally segregated progeny having a recombinant genotype (exceptional cytoductants). Exceptional cytoductants receive the majority of their genome from one parent (the recipient) and a minority from the other (the donor). Transfer of two markers from the donor nucleus to the recipient is rarely coincident for markers located on different chromosomes but is nearly always coincident for those markers located on the same chromosome, suggesting that whole chromosomes are transferred from the donor nucleus to the recipient. In crosses of kar1-1 X KAR1 parents, either nucleus may act as a recipient or donor with equal probability. Recipient nuclei acquired 9 of the 10 chromosomes examined, with frequencies which were inversely correlated with the size of the chromosome. When a chromosome is acquired by the recipient nucleus, it either replaces its homolog or exists in a disomic condition. Haploid progeny emanating from kar1 X KAR1 crosses are frequently inviable. I tested whether this inviability might be the result of chromosome loss by donor nuclei. Viability of progeny from kar1 X KAR1 heterokaryons was improved when the parental nuclei were diploid to an extent consistent with the hypothesis, and diploid progeny which had become monosomic were recovered from these heterokaryons. The following sequence of events accounts for chromosome transfer in kar1 X KAR1 heterokaryons. After cell fusion, each nucleus in the heterokaryon has a probability of about 0.38 of losing one or more chromosomes. A nucleus sustaining such a loss can become a donor in a chromosome transfer event. If the other nucleus does not sustain a mortal chromosome loss, it can become a recipient in a transfer event. The chance of acquiring a chromosome lost by the donor is greater for smaller chromosomes than for larger ones and is about 0.05 for the average chromosome.