Elements of theS-gene complex

Abstract

Cultivated plants ofNicotiana alataare self-incompatible and are of two kinds: normal (N); and exceptional (M). N plants are reciprocally compatible withN.langsdorffii; M plants are compatible only as males. M plants contain an unusual allele,S_FI, which has a dual action in the style: it rejects both self-pollen, andS_fpollen fromN. langsdorffii. The overall results agree with the assumption that theS_FIgene produces two kinds of specificity in the style:primaryspecificity, which is responsible for the rejection ofS_fpollen; andsecondaryspecificity, which is responsible for the rejection of self-pollen as inS_Ialleles 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,S_FIpollen grows more slowly thanS_Iand, in addition, appears to depress the normal rate of growth ofS_Ipollen. In consequence, crossesS_fS_f×S_IS_FI♂ 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 toS_fS_f♀. 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 theS_FIallele 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 theS_FIgene inN. alatais 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 theS_fgene). Plants carrying anS_FIallele, by rejecting the S_fpollen, will restrict the spread of inbreeding and so be favoured by selection.The origin of theS_FIandS_falleles are discussed in relation to the author's hypothesis ofS-gene structure.