A molecular description of mutations affecting the pollen component of theNicotiana alata S locus

Mutations affecting the self-incompatibility response of Nicotiana alata we re generated by irradiation. Mutants in the M-1 generation were selected on the basis uf pollen tube growth through an otherwise. incompatible pistil. Twelve of the 18 M-1 plants obtained from the Mutagenesis screen were self -compatible. Eleven self-compatible plants had mutations affecting only the pollen function of thr S locus (pollen-part mutants), The remaining self-c ompatible plant had a mutation affecting only the style function of the S l ocus (style-part mutant). Cytological examination of the pollen-part mutant plants revealed that 8 had an extra chromosome (2n + 1) and 3 did not. The pollen-part mutation in 7 M-1 plants was followed in a series of crosses. DNA blot analysis using probes for S-RNase genes (encoding the style functi on of the S locus) indicated that the pollen-part mutation was associated w ith an extra S allele in 4 M-1 plants. In 3 of these plants, the extra S al lele was located on the additional chromosome. There was no evidence of an extra S allele in the 3 remaining M-1 plants. The breakdown of self-incompa tibility in plants with an extra S allele is discussed with reference to cu rrent models of the molecular basis of self-incompatibility.