Radiosensitivity of haplont yeast cells irradiated with sparsely and densely ionizing radiations.

Five haploid and three diploid yeast strains of various species (Yarrowia lipolytica, Pichia pinus and Pichia guilliermondii) were irradiated with alpha-particles from 239Pu and gamma-rays from 137Cs or 60Co in the stationary phase of growth. A common feature of these species is that they exhibit a haploid state as a normal vegetative state in natural conditions. It was shown that the transition from the haploid to the diploid state is not accompanied by increased radioresistance, and diploid strains were unable to perform liquid-holding recovery. The absence of diploid-specific recovery in diploid strains was also supported by the fact that the RBE of alpha-particles was almost identical for haploid and the corresponding diploid strains being much smaller than that observed in typical wild-type diploid strains capable of diploid-specific recovery. The results suggest that haplont yeast may have evolved to diplont yeast via the development of a specific repair system conferring specific resistance in the diploid state.