Self-incompatibility (SI), is the most important and widespread mechanism promoting out breeding and gene flow through pollen in flowering plants. SI is a genetically determined pollen–pistil recognition system that prevents self-fertilization and cross-fertilization between individuals sharing the same incompatibility type. SI therefore, prevents inbreeding at two levels:
(i) by making selfing impossible and (ii) by preventing mating between close relatives that have inherited the same incompatibility
SI, ‘The inability of a fertile hermaphrodite seed plant toproduce zygotes after self-pollination’ (D De Nettancourt & Croix, 1997)is one of the mechanisms that has evolved to encourage outbreeding in flowering plants. The effectiveness of self-incompatibility in promoting outbreeding is believed to be one of the most important factors which ensured the evolutionary success of flowering plants (Whitehouse, 1950).
It has been suggested that SI was a feature of the earliest flowering plants and may have played a significant role in their extraordinary evolutionary success by optimizing the benefits afforded by co-sexual flowers and insect pollination (Whitehouse, 1950).Today it is estimated that SI is present in ca. 60% of all flowering plants(Hiscock & Kües, 1999).
Two genetic mechanisms of Self Incompatibility have been recognized GSI (Gametophytic self-incompatibility) and SSI (Sporophytic self-incompatibility). In GSI the incompatibility type of the pollen is controlled by its own haploid genotype, whereas in SSI, the pollen incompatibility type is controlled by the diploid (sporophyte) genotype of the parental anther in which it was produced.
Self-incompatibility are of two types: homomorphic and heteromorphic self-incompatibility. In heteromorphic self-incompatibility, morphologically distinct flowers are produced with difference in length of style and anther. This causes physical barrier in pollination which results in outcrossing(Clarke & Newbigin, 1993).
In homomorphic self-incompatibility, morphologically same flowers are produced within inbreeding species. Homomorphic self-incompatibility is further divided into two sub groups: Sporophytic and Gametophytic self-incompatibility. In homomorphic self-incompatibility, self- pollination is not affected by physical barrier. But self-pollination is controlled by a single locus (the S-locus) with multiple alleles which arrest the growth of self-pollen tubes before fertilization of the ovules (Clarke & Newbigin, 1993). In homomorphic self-incompatibility both the pollen and pistil express the S-locus.
In gametophytic self-incompatible plants phenotype of pollen is determined by its own (haploid) S genotype. But in case of sporophytic self-incompatible plants the phenotype of pollen is determined by the diploid S genotype of the plant producing pollen (Clarke & Newbigin, 1993). It is estimated that SI is present in more than half of all the species of angiosperms(Brewbaker, 1959; Dreux de Nettancourt, 1997). In most of the fruit species, gametophytic self-incompatibility occurs. In gametophytic self-incompatibility, two separate genes present in S locus control male and female specificities (Yamane & Tao, 2009).
The variants of S-locus are called as S-haplophyte whereas the variant of S-locus genes, pistil-S and pollen-S are known as allele and either pistil-S or pollen-S represent their S haplotype (Yamane & Tao, 2009). In GSI, pollen tube (having either one of the two S-alleles carried by recipient pistil) growth is arrested in the style (Dreux de Nettancourt, 1997). In most fruit species, ribonuclease (S-RNase) were pistil-S determinants and S haplotype-specific F-box gene (SFB) were pollen-S determinant or the S-locus F-box gene (SLF) (Entani et al., 2003).