The breeding system in Nicotiana glauca is unique in that the species comprises mainly self-compatible (S.C.) green-flowered plants but also includes rare self-incompatible (S.I.) red-flowered races. The S.C. plants of Nicotiana glauca show unilateral interspecific incompatibility with other S.I. species of Nicotiana, a widely occurring characteristic normally associated with S.C. species with relatively long-established self-compatibility.

The present study of S.I. and S.C. strains of Nicotiana glauca showed that the ♀ S.I. × S.C. ♂ type of unilateral incompatibility (U.I.) occurs within this species, and the U.I. is a function of the S alleles. Of the nine SI alleles studied seven, when present in the homozygous state in the style, made that style reject Sc (Nicotiana glauca) pollen whereas two alleles allowed styles to accept this pollen. Similar polymorphism at the Nicotiana glauca S locus was also found in relation to Sf pollen from the S.C. species Nicotiana langsdorffii. Sl alleles in Nicotiana glauca were classified differently for the polymorphisms in relation to Sc and Sf pollen.

The results show that the Sc allele is sporophytically determined causing allelic interaction in the pollen and style. Dominance, co-dominance and competitive interaction were observed. In contrast the St allele of Nicotiana langsdorffii is gametophytically determined, stylar alleles always showing independent action.

Within the same species Nicotiana glauca, there is both gametophytic and sporophytic determination of 5 alleles. Whereas in species with sporophytic self-incompatibility allelic interaction is predetermined in the pollen and style, in this case allelic interaction is not determined until after pollen/stigma contact and is complementary.

The occurrence within the same species of (1) the unilateral S.I. × S.C. type of incompatibility, (2) both gametophytic and sporophytic determination, (3) late determination of allelic interaction after pollen/stigma contact, and (4) competitive interaction of 5 alleles, is all explained on the basis of the author's theory of ‘twin specificities’ controlling intra-and interspecific incompatibility in flowering plants.

Wider implications of the theory are discussed. Early evolution of specificity determination in prokaryotes provided a basis for cell to cell interaction that eventually led, on the one hand, to evolution of sex and genetic recombination systems and, on the other, to mechanisms for infection/disease protection, and graft-incompatibility control. These phenomena are evident in both plant and animal kingdoms.