Self-sterility in Nicotiana

Multiple alleles were documented in plants in connection with self-sterility, also called self-incompatibility, a genetic mechanism that actively prevents a plant's own pollen from fertilising its own ovules and so forces the plant to outcross with pollen from a genetically different individual. East (1925) worked out this mechanism in the tobacco relative Nicotiana, identifying a single self-incompatibility gene, designated S, that exists in the population as an extended allelic series (S1, S2, S3, S4, S5, and so on). Crucially, cross-fertilising Nicotiana plants are never found as homozygotes like S1S1; every individual is heterozygous for two different S alleles, such as S1S2, S3S4 or S5S6. The rule governing fertilisation is simple but strict: a pollen grain fails to grow its pollen tube down the style, and so fails to fertilise, whenever the single S allele it carries matches either of the two S alleles already present in the stigma's own genotype; a pollen grain succeeds whenever its S allele is genuinely new to that stigma. So a cross between two plants sharing the identical S-genotype (say S1S2 x S1S2) is entirely self-sterile, because neither S1 nor S2 pollen is new to the stigma; a cross between plants that share one S allele but not the other (say S1S2 x S2S3) is only partly fertile, since …