SELF-INCOMPATIBILITY

Definition 

Self incompatibility is the inability of plant to set seed with functional pollen after self pollination.

The pollen from a flower is not responsive on the stigma of the same plant. In this case it is incapable of making seeds. It is the prevention of self fertilization.

There are two type of self incompatibility.

I   Heteromorphic incompatibility

II  Homomorphic incompatibility 

I   Heteromorphic incompatibility

    

It is based on the morphological characters of the flower .

The physical arrangement of reproductive organs (stigma and stamens ) varies between flowers on different plants. the two common types are pin (long pistil and short stamen length) and thrum (short pistil and long stamen length ) flowers , which differ in the relative length of stamens and pistils . 

Cross pollination between different flower type is compatible.

pollination between pin x pin / thrum x thrum  is incompatible. 

pollination of pin x thrum is compatible .

The genotype of pin is ss homozygote, and that of thrum is Ss heterozygote. 

                                 
Photograph (A) and schematic model (B) of heteromorphic SI in primrose. (A) The pin (long pistil and short stamen length) form is shown on the left, and thrum (short pistil and long stamen length) is on the right. (B) Selfed pollen and pollen derived from the same flower form are incompatible. Crossed pollen and pollen derived from the different flower form are compatible.
(Picture from NLM article )

II  Homomorphic incompatibility 

Involves plants with similar floral morphs where the reproductive organs (stigma and stamens) are arranged similarly in all flowers, but fertilization is controlled by genetic mechanisms, rather than physical differences.

Types:

Gametophytic Self-Incompatibility (GSI): 

Determined by the genetic makeup (S-alleles) of the pollen grain itself (haploid). The interaction between the pollen and the pistil's S-alleles prevents fertilization if they share the same alleles.

(Picture from Theagricos.com)

Sporophytic Self-Incompatibility (SSI): 

Determined by the S-alleles of the diploid parent plant producing the pollen. If the alleles of the pollen parent and the pistil match, fertilization is blocked.

(Picture from Theagricos.com)

Importance of self incompatibility

1. Self incompatibility may be used in hybrid seed production.

2. It is necessary to cross-compatible varieties to ensure fruitfulness.

3. It effectively prevents self pollination, as a result a profound effect on plant breeding approaches.

4. Increases of genetic exchange.

5. Continued selfing leads to a depression in self incompatibility.