Self-incompatibility

Self-incompatibility

It is the failure of pollen from a flower to fertilize the same flower or other flowers on the same plant. It has been reported in more than 300 species of 20 families of angiosperms.

Self – incompatibility is of two types: Sporophytic and Gametophytic.

In both the cases, flowers do not set seed on selfing. It appears to be a biochemical reaction, but the genetic control is simpler to understand.

       

Self – incompatibility is of two types:

      

(1)  Heteromorphic system,

      

(2) Homomorphic system - (a) Sporophytic control and (b) Gametophytic control.

(1)  Heteromorphic system –

In this system, flowers of different incompatibility group are  of different morphology. E.g. As we have discussed under the heading of heterostyly. Here, there are two types of lowers pin and thrum.

These two are self – incompatible, but compatible for each other. This characteristic is governed by a single gene s; Ss produces thrum lower and ss produces pin flowers.  Allele S is dominant over allele s. e.g. sweet potato and buck wheat.

(2) Homomorphic system –

In this system, the flowers are morphologically similar but the incompatibility reaction may be controlled by the genotype of the plant on which it is produced or by its own type.

(a) Sporophytic control -

In the sporophytic system, the self – incompatibility is governed by a single gene S, with multiple alleles; more than 30 alleles are known in Brassica oleracea. It was first reported by Hughes and Babcock in 1950 in Crespis foetida and by Gerstel in Parthenium. The incompatibility reaction of pollen is controlled by the genotype of the plant on which the pollen is produced, and not by the genotype of the pollen. In the sporophytic system, the S alleles may show dominance, individual action or competition.

 (b) Gametophytic control –

It was first described by East and Mangelsdorf in 1925 in Nicotiana sanderae.  The incompatibility reaction of pollen is determined by its own genotype and not by the genotype of the plant on which the pollen is produced. The reaction is controlled by a single gene having multiple alleles .The gametophytic system is found in pineapple, coffee.

The mechanism of self –

Incompatibility is quite complex and involve various phenomenon. These may be grouped into three categories:-

pollen – stigma interaction

pollen tube – style interaction

pollen tube  – ovule interaction

4.2.8  Pollen-stigma interaction

1. Pollen – stigma interaction -

when a pollen grain reaches at stigma, the interaction between pollen and stigma, prevent the pollen grain to germinate. The stigmatic surface appears to be involved in this interaction.

      

In the gametophytic system, the stigma is plumose with elongated receptive cells and is termed as wet stigma. Here, pollen grain germinates after reaching the stigma and incompatibility reaction occurs at a later stage. The pollen grains show clear serological differences due to the different S genotypes.

  

In the sporophytic system, the stigma is papillate and dry, and is covered by a hydrated layer of proteins known as ‘pellicle’. The ‘pellicle’ is involved in incompatibility reaction. As soon as the pollen grain reaches to stigma, it secretes exine exudates which are either protein or glycoprotein in nature. This exudate induces immediate callose formation in the papillae of incompatible stigma. Often, callose is also formed on the young developing pollen tubes preventing further germination of it.  

2. Pollen tube – style interaction –

In most cases of the gametophytic system, pollen grain germinates and pollen tubes penetrate the stigmatic surface. But in incompatibility reactions the growth of   pollen tubes is retarded within the stigma e.g. in Oenothera or a little later in style e.g. in Petunia, Lillium . In the latter cases, there is a cessation of protein and polysaccharide synthesis in the pollen tubes, which leads to the degeneration of tube wall and bursting of pollen tubes.

3. Pollen tube – ovule interaction –

In some cases, e.g. Theobroma, pollen tubes reaches the ovule and effect fertilization. However, in incompatibility combinations, embryos degenerate at an early stage of development.