Self infertility or SI is a genetically controlled cell-cell recognition system that acts as a barrier to self fertilization in a wide range of flowering plants or angiosperms. It is defined as the inability of the plant producing functional gametes to set seed upon self pollination. Self incompatibility or intra specific incompatibility is a pre-fertilization barrier where certain plants recognize and reject their own pollen thus forcing out breeding. The significance of the SI in the evolutionary context cannot be overstated since its possession leads to obligate out-breeding and the maintenance of heterozygosity within the species. SI is commonly seen in members of the Solanaceae, Brassicacease, Asteraceae and Rosaceae families and also in most of the perennial grasses.

Nature has several mechanisms to prevent self pollination and subsequent self fertilization. Several features or out-breeding devices are present in the angiosperms that prevent auto gamy or self pollination as follows:

A. Dicliny or Unisexuality: In diclinous flowers cross pollination is the rule.
B. Dichogmay: in dichogamous species anther dehiscence and stigma receptivity
are temporally separated. The anthers and the stigma mature at
different time.
C. Herkogamy: Herkogamous species show spatial separation of the anthers and stigma.
Their relative position is such that auto gamy or self pollination cannot
D. Heteromorphism: In certain plants there are flowers of two or three forms with the
anthers and stigmas at different levels.

All of the above features prevent the pollen grains from landing on the stigmatic head of the same flower. But in case self pollination does occur then the SI reaction takes over and prevents self fertilization. Thus self incompatibility or intra specific incompatibility is a major mechanism by which flowering plants prevent self fertilization and maintain genetic diversity.

Heteromorphic Self Incompatibility

Heteromorphic SI plants bear flowers with two or three different forms. These distinct morphological differences are mainly due to heterostyly meaning styles of different lengths and different anther positions. These create a physical barrier that in conjunction with the biochemical mating barrier that exists prevents self fertilization. In distyly the plants produce two forms of flowers, called long styled or 'pin' morph and short styled or 'thrum' morph. Distyly is controlled by a single gene with two alleles. The tristyly plants produce three floral morphs like long styled, mid styled and short styled. Tristyly is controlled by two genes each of which has two alleles. This type of heteromorphic SI is reported in 24 families and over 164 genera such as Primulaceae, Rubiaceae, and Plumbaginaceae.

Gametophytic Self Incompatibility

When the pollen phenotype is determined by own haploid genotype it is known as gametotyphic self incompatibility or GSI. The pollen rejection mechanism generally operates in the style leading to the inhibition of the pollen tube growth. Gametophytic self incompatibility is common in the Solanaceae, Fabaceae, Papavaraceae, Rosaceae, Rubiceae, Liliaceae, Poaceae and Commelinaceae families. Most of the gametophytic self incompatibility is controlled by two independent loci, S and Z, notably in the Poaceae family. In the case of gametophytic self incompatibility segregation of the incompatible alleles occurs.

Sporophytic Self Incompatibility

The control is described to be sporophytic self incompatibility when the diploid genotype of the pollen parent specifies the pollen phenotype. The stigma is the site for the pollen rejection in this case. Sporophytic self incompatibility is common in the Brassicaceae, Asteraceae and Convolvulaceae families. So in sporophytic self incompatibility all pollen from an individual plant possesses the same incompatibility phenotype.

Homomorphic Self Incompatibility

In homomorphic self incompatibility all the flowers have similar morphological structures. In this type of self incompatibility self fertilization inhibition depends on genetic, biochemical or physiological conditions of the plants. This type of homomorphic self incompatibility in plants has immense importance in plants.

Homomorphic self incompatibility can occur in many ways as below:

• Same flowers do not produce pollen grains
• In case of germination the germinated pollens fail to penetrate the stigma
• In case of successful germination the fruit does not develop or a very slow growth
stunts the growth

Significance of Self Incompatibility

Self incompatibility is one of the most efficient out breeding mechanism. Self incompatibility has been envisaged as one of the main cause for the rapid evolution of angiosperms. Even though cross pollination involves a great deal of pollen wastage because of its uncertainty more than 50% of the flowering plants are self incompatible. The flowering plants undergo this complex interaction because the self incompatibility results in genetic heterogeneity.

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