By means of special selective conditions we analyzed the temporal relationship of diploidization and haploidization in the yeast Yarrowia lipolytica. These processes could be divided into several steps. Twelve hours after mixing parental strains dissociation products of unstable heterokaryons are selectable. Four hours later karyogamy takes place as shown by isolation of mitotic recombinants. At this time spontaneous haploidization starts which leads to an enrichment of haploid and aneuploid segregants in the cell population. A high portion of chimeric clones which represents the dynamics of segregation were selected at time intervals 16,23 and 29 hours.
We concluded that an unstable heterokaryon should be a connecting link between sexual and parasexual processes in Y. lipolytica.
Formation of stable heterokaryons has been described in several lower eukaryotes, for example in Aspergillus nidulane and Neurospora crassa (BURNETT 1968, CATEN 1981).
Recently heterokaryon formation, chromosome transfer and haploidization in the yeast Sacch. cerevieiae were reported. This opens a new field for genetic analysis (CONDE and FINK 1976, NILSSON-TILLGREN et al. 1980, WOOD 1982, YURCHENKO 1982).
We have shown that in the dimorphic yeast Yarrowia lipolytica haploidization and mitotic crossing over can occur soon after conjugation of haploid cells (KURISCHKO 1986). The detailed analysis of several steps in the life cycle of this organism should give some insight into basic events of parasexual and sexual processes. Therefore, the aim of this work was to analyze genetically by means of a special selection system the temporal relationship of diploidization (formation of heterokaryons, karyogamy) and haploidization (mitotic segregation).