Tetraploid Saccharomyces

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Abstract
Cultures (unlike the diploid cells heterozygous for mating type) are capable of mating, and when mated with similar cells of complementary mating type produced tetra- ploid yeasts. Triploids were produced by mating diploids with haploids. The tetra- ploid segregates to produce the expected tetrad types and gametic ratios. Some sporulating cultures produced unexpected types of asci for characters controlling sugar fermentation but interpretation of their significance is not feasible since our customary control of breeding procedure does not apply in the analysis of diploid cultures which sporulate on ordinary agar slants. METHODS Safeguards in the breeding procedure. We have developed a breeding pro- cedure which excludes errors due to intervening sporulations and copulations between the generations which are analysed. In the earlier phases of this work single ascospore cultures which produced spores were not unusual (Lindegren & Lindegren, 1944). A rigid programme of selection was established and sporulating haplophase cultures were never used as parent breeding stocks. Diplophase cultures were never maintained as sources for breeding stocks since sporulation would be expected to occur and would quickly render them heterogeneous. Non-sporulating single ascospore haploid cultures were pre- ferred as breeding stocks, since a minimum of variation occurs when they are carried on a rich nutrient medium by heavy transfers. A mating is made by mixing two heavy inocula of these cultures in a small amount of broth and transferring the cells to presporulation agar as soon as zygotes appear. On the presporulation agar, growth of both haplophase and diplophase occurs but spores are not produced. Most of the cells from the presporulation agar are transferred to gypsum. The immediate descendants of the zygotes sporulate while the haplophases do not. Each haploid culture used in a mating is carefully tested for its ability to sporulate by growth on presporulation agar followed by transfer to gypsum. Those which show any tendency to sporulate under these conditions are not used.
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