Mutability of the self-incompatibility locus and identification of the S-bearing chromosome in Nicotiana alata

γrays, X rays, fast neutrons and ethyl methanesulfonate (EMS) were used for inducing mutations at the self-incompatibility locus of Nicotiana alata.Chronic gamma irradiation and EMS treatment neither induced selfcompatibility mutations nor led to changes from one S allele to another. X rays and fast neutrons induced many self-compatibility mutations, but did not generate new self-incompatibility alleles.Triploid individuals were male sterile.Tri(S)allelic aneuploid plants were self-incompatible because heterogenic di(S)allelic pollen grains are not functional.Self-compatibility in pollen-part mutants with and without a centric fragment was explained by complementation of the mutant S allele by a fragment or duplication. Deviations of expected segregation ratios were explained by lethality of S homozygotes.It was shown that the longest unsatellited acrocentric chromosome is the S-bearing chromosome.