The effects of different mutagens on self-incompatibility in Nicotiana alata link and otto—I. Chronic gamma irradiation*

An analysis was made of the efficiency of chronic gamma irradiation for inducing mutations at the S-locus of the self-incompatible species Nicotiana alata . A clonal population derived from a single S 2 S 3 individual was exposed, at the flowering stage, to different dose rates of gamma rays ranging from 0·3 to 14·3 rad/hr. With a daily interruption of 3 hr, irradiation was continuous and lasted 3 months. In all more than 25 million pollen grains were screened, via repeated self-pollination in the irradiated populations for either self-compatibility mutations or new specificities. Whereas the results obtained clearly indicated that chronic irradiation was very effective for inducing seed set upon selfing, only one unequivocal mutant could be recorded among the M 2 progenies. This mutant produced compatible pollen and clearly displayed a centric fragment. Not a single case of a new specificity at the S-locus could be detected. These results are compared to those that have been obtained elsewhere after acute exposure to X-rays and the conclusion is reached that chronic irradiation is far less effective than acute treatment for inducing self-compatibility mutations. The apparent incapacity of both chronic and acute irradiation to generate new self-incompatibility alleles is discussed in relation to the spontaneous occurrence of such mutations in inbred populations.