Cultivated plants of Nicotiana alata are self-incompatible and are of two kinds: normal (N); and exceptional (M). N plants are reciprocally compatible with N. langsdorffii; M plants are compatible only as males. M plants contain an unusual allele, SFI, which has a dual action in the style: it rejects both self-pollen, and Sf pollen from N. langsdorffii. The overall results agree with the assumption that the SFI gene produces two kinds of specificity in the style: primary specificity, which is responsible for the rejection of Sf pollen; and secondary specificity, which is responsible for the rejection of self-pollen as in SI alleles generally. The genetic sub-units concerned must be closely linked; there was no evidence for their dissociation in the 599 plants studied.
In both compatible and incompatible pollinations, SFI pollen grows more slowly than SI and, in addition, appears to depress the normal rate of growth of SI pollen. In consequence, crosses SfSf × SISFI ♂ yielded significantly fewer S.I. plants than the 50% expected. The two kinds of pollen grew at comparable rates, however, when F1 (M × M) plants involving parents from different original sources were backcrossed to SfSf ♀. Progenies then showed the expected 1:1 ratio of S.I. to S.C. plants. These results are assumed to be due to differential behaviour of the SFI allele according to its genetic background. The change in background would be from a degree of homozygosity, in plants from the same source, to a degree of heterozygosity, in crosses between plants from different sources.
The high incidence of the SFI gene in N. alata is considered to be due to the advantage it confers on a self-incompatible population when it is overlapping with a related self-compatible population (having the Sf gene). Plants carrying an SFI allele, by rejecting the Sf pollen, will restrict the spread of inbreeding and so be favoured by selection.
The origin of the SFI and Sf alleles are discussed in relation to the author's hypothesis of S-gene structure.