Inaktivierungsversuche mit homozygoten Hefestämmen verschiedenen Ploidiegrades

Abstract

A series of extensively homozygous and isogenic Saccharomyces strains of different ploidy (haploid —hexaploid) was developed by controlled matings. The degree of ploidy was further checked by analysis of the segregation of mating type alleles and in case of the hexaploid strain also of two other markers. The genetic analysis was consistent with previous findings. Measurements of linear dimensions of the various strains indicated an increase of cellular volume proportional with ploidy. Radiation inactivation studies on the different strains with UV showed a maximum of resistance of the triploid strains and a decrease of resistance with further increase of ploidy from triploid to hexaploid. In X-ray and Po-a-ray inactivation, diploid strains heterozygous in the mating type alleles showed a maximum of resistance, whereas diploids homozygous in the mating type alleles proved to be less resistant than triploids. Additional to the effect of the mating type alleles on the resistance to X-rays and Po-α-rays (aa-effect) there could be shown an effect of alleles controlling the dependence on the aminoacids serine and valine-isoleucine (AS-effect). Strains dependent on serine or valine-isoleucine proved to be more X-ray and Po-a-ray resistant than related prototrophic strains. There is, however, no difference in the UV resistance. This AS effect is not present in haploid strains but only in strains of higher ploidy. The dose effect curves of strains exhibiting an AS effect or an aa effect show a characteristic difference. The AS effect is expressed by a constant dose reduction factor. The aa effect, however, produces dose effect curves that have a more expressed shoulder in the lower dose range than comparable curves of aa or αα strains but run parallel to these curves in the higher dose ranges. Possible meanings of these findings are discussed.