Self-incompatibility (SI) was defined as ‘the inability of a fertile hermaphrodite seed plant to produce zygotes after self-pollination’. SI is one of the most important systems used by many flowering plants to prevent self-fertilization. SI is purely an intraspecific prezygotic reproductive barrier aimed at generating and maintaining genetic diversity within a species. The SI response comprised of a self- and nonself-recognition process between pollen and pistil that is followed by selective inhibition of the self-pollen tube. Self-/nonself-recognition in most species is controlled by a single multiallelic locus, the S-locus, and that pollen inhibition occurs when the same “S-allele” specificity is expressed by both pollen and pistil3.

S-locus consists of at least two linked transcriptional units arranged in pairs, with one functioning as the female determinant and the other as the male. SI does not represent one system, but rather a collection of divergent mechanisms, suggesting that SI evolved independently in several lineages. In the Solanaceae, Rosaceae, and Scrophulariaceae, the determinants are a ribonuclease and an F-box protein, suggesting the involvement of RNA degradation and protein degradation within the system. In the Papaveraceae, the only identified female determinant induces a Ca2+-dependent signalling network that ultimately results in the death of incompatible pollen. The term pseudo-self-compatibility has been used to describe individuals or lines of self-compatible plants identified in species that are generally self-incompatible.

In tripartite model of SI, S locus is composed of a complex of three closely linked parts- the S allele specificity part, which determines the allelic specificity of pollen and pistil; the pollen activity part, which activates the S allele specificity of pollen; and the style activity part, which activates the S allele specificity of the style. Mutations in the pollen activity part affect pollen behaviour only, while mutations in the style activity part affect stylar behaviour only leading to self-compatibility2. Inter-specific pollen rejection as a result of unilateral incompatibility or incongruity (UI) is less well understood than intra-specific SI.

SI is mainly used in hybrid seed productions, establishing polyclonal gardens and in the evolution of crops. Transfer of S alleles from one variety or more particularly species to another species is tedious and complicated. This has prevented the use of self-incompatibility in hybrid seed production in Solanaceae and Compositae. Orchards of SI fruit trees have to contain at least one cultivar that serves as pollen donor. A typical pear orchard contains at least two cultivars planted adjacent to one another and that serve as each other’s pollenizers1. The pollenizer must be genetically compatible and has to flower synchronically with the pollen recipient. To achieve satisfactory yields, sufficient pollination has to be guaranteed. Insects, mainly honeybees, are the main vector for transferring the pollen. Temporary suspension and even elimination of SI can be achieved through various methods.

Self-incompatibility is a major outbreeding mechanism that is widespread in angiosperm but not found in gymnosperm, it is thought to have evolved very early in the evolution of angiosperm and to have been responsible for their rapid expansion. Despite a number of setbacks and encounters with unexpected complexities a picture is starting to emerge of how rejection of self-pollen is mediated at the molecular level, at least on the female side. It is high time to identify and characterize the S-alleles in the germplasm and utilize the strong alleles to develop stable self-incompatible parents. Cloning genes of the GSI system and understanding their role also provides tools for manipulating the system for agricultural needs.

REFERENCES:

 1. GOLDWAY, M., SAPIR, G.AND RAPHAEL A., Molecular Basis and Horticultural Application of the Gametophytic Self-incompatibility System in Rosaceous Tree Fruits. Plant Breeding Reviews 28:215-237 

2. SINGH, A., AND KAO, T. H., 1992. Gametophytic self-incompatibility: biochemical, molecular genetic, and evolutionary aspects. Int. review of cytology, 140, 449-483. 3. TAKAYAMA, SEIJI, AND ISOGAI, A., "Self-incompatibility in plants." Annu. Rev. Plant Biol. 56 2005: 467-489.