Compatibility Relationships in Flowering Plants: Role of the S-Gene Complex

Interspecific hybridization between 17 species of Nicotiana, including those of both South American and Australian origins, revealed a complex but highly systematic pattern of unilateral incompatibility behavior. A critical review of the earlier observations on unilateral incompatibility and S-gene polymorphism and a detailed consideration of the present data have led the author to conclude that different patterns of interspecific pollen-style compatibility relationships are highly specific and are mainly controlled by different forms of S alleles which have arisen from mutation of various genetic elements comprising the primitive S-gene complex. Unilateral incompatibility owes its origin primarily to the mutational independence of the pollen and stylar controlling elements of the S complex. It is argued that the supremacy of the S gene in the determination of compatibility behavior is dependent upon the maintenance of the native polygenic background in which it normally acts. A hybrid or disturbed genetic background may lead to the loss of this supremacy; as a result, major genes, or polygenic combinations, which could potentially alter the compatibility behavior but which are normally incapable of expression may now be able to express themselves. Large species complexes, like that of Lycopersicon hirsutum or L. peruvianum, comprising many distinct ecotypes that often have different forms of S alleles, and yet all together forming one large interbreeding population, would have such an unsettled genetic background in relation to their various S genes. The failure to appreciate this factor may lie at the root of much of the confusion as to the role of the S-gene complex in the control of interspecific compatibility relationships in flowering plants.