Radiobiological and Genetic Studies on a Polyploid Series (Haploid to Hexaploid) of Saccharomyces cerevisiae

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Abstract
A related series of Saccharomyces cerevisiae varying in ploidy from haploid to hexaploid was developed by a series of controlled matings. Measurements of linear dimensions of the various cultures indicated a linear increase of cellular volume with ploidy. Genetic analysis of the hybrid cultures revealed normal segregations for the diploid, and segregations for the tetraploid that were consistent with previous findings and with theoretical predictions based on the assumption of random bivalent pairing. Low viability of the triploid meiotic products precluded adequate genetic analyses of these cultures. The hexaploid was found to yield an expected low frequency of phenotypically recessive spores, and the distributions of segregation ratios in different asci agreed quite well with theoretical predictions for three of the four loci studied. It was necessary to assume the fourth locus to be in duplex constitution in order to account for the results obtained for it. The radiation studies on the different cultures showed an increase in resistance to X-ray inactivation from haploid to diploid, followed by a progressive decrease in radioresistance with further increased in ploidy from diploid through hexaploid. Considerable variability of radiation resistance of diploid cultures was encountered, and the results suggested that this may be at least in part due to variations in degree of genetic homozygosity. The results of the radiation studies are consistent with the assumption that the majority of inactivation of haploid cultures is brought about by recessive lethals, whereas higher-ploidy cultures are inactivated mostly by dominant lethals.