This system is found in a majority of the self-incompatible plant species and incompatibility is not related to the morphological difference among flowers as in the heteromorphic system (B. D. Singh, 2015). In this homomorphic type, there are sporophytic and gametophytic systems, the incompatibility reaction of pollen may be controlled by the genotype of the plant which is sporophytic control, or by its genotype which is termed as gametophytic control (B. D. Singh, 2015). These are defined by the action of the S gene. The behavior of pollen tubes is determined in the sporophytic system by the phenotype of the sporophyte from which the pollen is generated.. Thus, dominant or recessive relations exist between S alleles in some cases, though codominance relations are also frequent. Dominance relations are also found in the expression of S alleles in pistils. Self-pollinated pollen tubes are generally arrested on the surface of the stigma. Different plants in Brassicaceae, Asteraceae, and Convolvulaceae, etc., are known to have sporophytic self-incompatibility (Hinata, Watanabe, Toriyama, & Isogai, 1993). Generally speaking, they have the dry stigma, and pollen at emission has three nuclei in two reproduction cells and a vegetative cell.
The gametophytic system of self-incompatibility was first described in Nicotina sanderae in 1925 by East and Mangelsdorf( B. D. Singh, 2015). Plants in Solanaceae, Rosaceae, Mimosaceae, Papaveraceae, Poaceae, and probably, Liliaceae, etc., have a gametophytic self-incompatibility system (Hinata et al., 1993). In GSI, the behavior of a pollen tube is determined by its gene and not by the genotype of the plant on which it is produced.
Incompatible pollen successfully germinates on the stigma in most gametophytic SI systems, penetrates the stigma, and grows in style. There, the tube grows between the longitudinal files of cells of the central transmitting tract, and at some point, pollen tube growth through the transmitting tract toward the ovary is arrested (Newbigin, Anderson, & Clarke, 1993).
The incompatibility system is controlled by one or two genes, giving rise to (1) Monofactorial and (2) Bifactorial system of GSI. But some species more than two genes e.g.Beta vulgaris has genes, papaver has three genes; in these cases, polyploidy does not overcome incompatibility reaction (B. D. Singh, 2015)
It is the most widely distributed SI system and is generally considered to be an ancestral type. SI reaction is controlled by a single gene, recognized as the S gene, which usually has 50 or more alleles. This system is found in Trifolium, Nicotiana, Lycopersicon, Solanum, Petunia, etc. Various mating may be grouped as:
- Fully incompatible mating, e.g. S1S2 * S1S2
- Fully compatible mating, e.g. S1S2 * S3S4
- Partially compatible mating e.g. S1S2 * S1S3 or S1S2 * S2S3
This system is very efficient at preventing fertilization on self-pollination but it is not very efficient at preventing fertilization of sib- population.
In this system, the SI reaction is controlled by two genes designated as S and Z, which are unlinked and exhibit multiple alleles. Loci S and Z behave cooperatively, and in order for inhibition to occur, the alleles at both loci must match in the pollen stigma.
In the Sporophytic SI system, self-incompatibility is governed by a single gene, S, with multiple alleles: often the number of alleles is 50 or more. In general, the number of S alleles is considerably larger in gametophytic than in the sporophytic system (Singh, 2015). The incompatibility reaction of pollen is governed by the genotype of the plant on which the pollen is produced, and not by the genotype of pollen. This system was first reported by Hughes and Babcock(1950) on Crepis foetida and Gerstel (1950) on Parthenium argentatum.
Lewis (1954) has summarized the following characteristic of this system:
- There are frequent reciprocal differences.
- Incompatibility with a female parent may occur.
- A family can consist of three categories of incompatibility.
- 4. Homozygotes are a normal feature of the system.
- An incompatible group may contain 2 different genotypes.
Four families exhibit gametophytic SI (GSI), Solanaceae, Rosaceae, Scrophulariaceae, and Papaveraceae, and one of the families that exhibit sporophytic SI (SSI), Brassicaceae, have been studied extensively at the molecular level. A single polymorphic locus, termed the S-locus, controls the SI response in all five of these families (Kao & Tsukamoto, 2004).