Self incompatibility: Definition, Meaning, Formation, Types, Functions, Characteristics

What Is Self-Incompatibility (SI)?

Self-incompatibility in flowering plants is a genetic process to prevent self-fertilisation and promote cross-pollination. It allows a plant to recognise and reject its pollen or pollen from individuals that are similar genetically. In such a way, this provides the certainty that only pollen from different plants will fertilise the ovules. This is a complex interaction at the surface of the pistil between pollen and the surface of the pistil, controlled by special genes often called S-genes.

Self-incompatibility is, therefore, of fundamental value to the gene diversity of plant populations, and a low degree of it equates to reduced adaptability and survival in the long term. Through the process of preventing inbreeding, this process reduces the chances of occurrence of genetic defects and enhances the chances of the introduction of new variations. There has to be such a mechanism for the evolution of plant species for better survival in changing environments.

Historical Background And Discovery

The phenomenon of self-incompatibility was first discovered in the early 20th century by scientists working on plant breeding. Since then, research into SI mechanisms has outlined several molecular and genetic procedures that plants use to recognise and reject their pollen, thus contributing to our general understanding of the reproduction in plants.

Types Of Self Incompatibility

In gametophytic self-incompatibility, pollen compatibility would be based on its haploid genotype

Gametophytic Self Incompatibility (GSI)

If the genotype of the pollen is the same as the genotype of the stigma, it is recognised as self and rejected; thus, blocking fertilisation. This involves specific interacting proteins which trigger pollen tube inhibition

Examples:

Gametophytic self-incompatibility is found in plants such as petunias, tobacco, and some grasses - all of which rely on this mechanism to ensure cross-pollination for genetic diversity.

Sporophytic Self Incompatibility (SSI)

Sporophytic self-incompatibility is dependent on the diploid genotype of the parent plant producing the pollen. The interaction takes place at the surface of the stigma where proteins from the pollen and the stigma determine the compatibility. If the pollen is identified as 'self', it is inhibited from germinating or penetrating the stigma.

Examples:

Sporophytic self-incompatibility is seen in Brassica species (which include cabbage and mustard) as well as some Asteraceae members. This ensures that there is genetic diversity in populations of these plants.

S-Genes and Molecular Mechanism

The components of S-locus and the molecular mechanism are:

• Pollen S-gene: The pollen S-gene encodes a specific S-protein carried by the pollen grain, which acts to signal its genetic “self” to the stigma.

• Stigma S-gene: The stigma expresses its own S-protein receptor that can recognize incoming pollen S-proteins and determine whether they match the plant’s self-identity.

• Interaction: When the pollen S-protein matches the stigma’s S-receptor, the system triggers a rejection response, preventing fertilisation. If they do not match, pollen is accepted and fertilisation can proceed.

Self-Incompatibility vs Self-Pollination

The difference between self-incompatibility and self pollination is:

Advantages of Self-Incompatibility

The advantages of self-incompatibility are:

• Promotes outbreeding: Ensures pollen from genetically different individuals fertilises the plant which increases genetic diversity.

• Maintains genetic diversity: Helps preserve a wide range of alleles in a population.

• Prevents inbreeding: Reduces the chances of harmful recessive traits expressed during close relative mating.

• Supports plant adaptations: Provides diverse genetic diversity that improve the ability of a plant to adapt in an environment.

• Encourage hybrid: Favors cross-pollination that can produce stronger, more resilient offspring.

Disadvantages of Self-Incompatibility

The disadvantages of self-incompatibility are very few and mostly rare. Some of the disadvantages includes:

• In self-incompatible plants, the ability to self-fertilize means that a shortage of pollinators can lead to poor or no pollination.

• When plants grow far from compatible mates, self-incompatibility restricts reproduction, resulting in few or no seeds.

Self-Incompatibility NEET MCQs (With Answers & Explanations)

Important topics for NEET are:

• Types of Self-incompatibility

• S-genes and its molecular mechanism

• Advantages of Self-incompatibility

Practice Questions for NEET

Q1. What are the genetic mechanisms that regulate self-incompatibility in plants?

1. Gametophytic and sporophytic

2. Heterozygotic and homozygotic

3. Dominant and recessive

4. Polygenic and monogenic

Correct answer: 1) Gametophytic and sporophytic

Explanation:

Self-incompatibility in plants is regulated by two main genetic mechanisms: gametophytic and sporophytic.

Gametophytic self-incompatibility involves the interaction between the pollen and the stigma. The S-allele gene is expressed in the pollen, and the S-receptor gene is expressed in the stigma. If the pollen and stigma express the same S-allele, the pollen will not be able to fertilize the ovules.

In sporophytic self-incompatibility, the interaction occurs between the pollen and the maternal tissues of the plant. The S-allele gene is expressed in the sporophyte, and the S-receptor gene is expressed in the stigma. If the pollen and sporophyte express the same S-allele, the pollen will not be able to fertilize the ovules.

Overall, these mechanisms ensure that plants cannot self-fertilize, promoting genetic diversity within the species.

Hence, the correct answer is option 1) Gametophytic and sporophytic.

Q2. Which of the following is not true about the mechanism to control inbreeding depression?

1. Pollen release & stigma receptively are synchronised.

2. Anther & stigma are placed at different positions.

3. Self incompatibility

4. Production of unisexual flowers

Correct answer: 1) Pollen release & stigma receptively are synchronised.

Explanation:

If both male and female flowers are present on the same plant, such as castor and maize (monoecious), it prevents autogamy but not geitonogamy. In several species, such as papaya, male and female flowers are present on different plants, that is, each plant is either male or female (dioecy). This condition prevents both autogamy and geitonogamy.

Inbreeding depression can be controlled by preventing the fusion of gametes of the same plant.

Mechanisms to control inbreeding depression

1. Pollen release and stigma receptivity are not synchronised
2. The anther and stigma are placed at different positions so that the pollen cannot come in contact with the stigma of the same flower
3. Self-incompatibility
4. Production of unisexual flowers

Hence, the correct answer is option 1) Pollen release & stigma receptively are synchronised.

Q3. Self-incompatibility in plants is a genetic mechanism that promotes:

1. Self-pollination

2. Cross-pollination

3. Inbreeding

4. Vegetative propagation

Correct answer: 2) Cross-pollination

Explanation:

Self-incompatibility in plants is a genetic mechanism that prevents self-pollination and promotes cross-pollination. It is a well-designed mechanism by which certain plants recognize and reject their pollen, thus encouraging outbreeding. This mechanism ensures genetic diversity in plant populations by promoting the transfer of pollen from one plant to another, typically through the assistance of pollinators like insects, birds, or wind. Self-incompatibility helps to prevent inbreeding and maintain genetic variability within plant species, leading to healthier and more adaptable populations.

Hence, the correct answer is option 2) Cross-pollination.

Also Read:

• MCQs on Self-Incompatibility

• Double Fertilization

• Ovule

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