💔Self-Incompatibility: Types, Mechanisms, and Breeding Use

Why Self-Incompatibility Matters in Agriculture

In Brassica crops (cabbage, cauliflower, mustard), plant breeders exploit self-incompatibility to produce hybrid seed without manual emasculation. The plant’s own genetic mechanism prevents self-pollination, ensuring that all seeds produced are true hybrids when two compatible but self-incompatible lines are planted together. Understanding SI systems is essential for hybrid breeding in several vegetable and oilseed crops.

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Self-Incompatibility

• Self-incompatibility (SI) is a genetic mechanism that prevents self-fertilization in plants. A self-incompatible pollen grain does not germinate and if pollen grain germinates, its pollen tube fails to enter into stigma. If pollen tube enters into stigma and fertilization takes place but ultimately embryo aborts. It means any how embryo does not develop and occurrence of self-incompatibility is due to biochemical reaction but its precise nature is not clearly understood. This mechanism promotes outcrossing and maintains genetic diversity within plant populations.

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• On the basis of the interaction between pollen grains and pistil, it is of two types:
  • Complementary i.e. stimulatory type where pistil & pollen stimulate each other or either. e.g. Dendrobium. In this type, compatible pollen is actively stimulated to grow.
  • Oppositional i.e. inhibitory type where pistil & pollen inhibit each other or either. Almost all the cases of self-incompatibility are inhibitory type. Here, incompatible pollen is actively prevented from completing fertilization.
• Lewis (1954) suggested various types of classifications of self-incompatibility (SI). A simpler classification is:

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Heteromorphic System

• Flowers of different incompatibility groups are different in morphology. This means you can visually distinguish flowers belonging to different incompatibility groups — for example, differences in style length, anther position, or pollen size.
• The compatibility reaction of pollen is determined by the genotype of the plant producing them. The incompatibility system is heteromorphic-sporophytic. A classic example is pin and thrum flowers in Primula, where pin flowers have long styles and short stamens, while thrum flowers have short styles and long stamens.

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Homomorphic System

• Here morphological differences between flowers is not associated with incompatibility. All flowers look alike regardless of their incompatibility group. The incompatibility reaction of pollen is either controlled by the genotype of the plant on which it is produced (sporophytic control) or by its own genotype (gametophytic control).
  • Gametophytic system: East and Mangelsdorf (1925) firstly described Gametophytic incompatibility in Nicotiana sanderae. The incompatibility reaction of pollen is determined by its own genotype and not by the genotype of the plant on which it is produced. This means each pollen grain acts independently based on its own S-allele. e.g. pineapple, ryegrass, diploid coffee, diploid clovers (Trifolium sp.) etc. The gametophytic system is controlled by a single S-locus with multiple alleles.
  • Sporophytic system: First time this system was reported, by Hughes & Babcock (1950) in Crepis foetida and by Gerstel (1950) in Parthenium argentatum. The incompatibility reaction of pollen is determined by the genotype of the plant on which the pollen is produced and not by the genotype of the pollen. In this system, the diploid genotype of the pollen parent determines compatibility, and dominance interactions between S-alleles are common.

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Elimination of self-incompatibility

• By doubling the chromosome number in single locus gametophytic system. When the chromosome number is doubled, each pollen grain carries two different S-alleles, which can overcome the incompatibility barrier.
• Isolation of self-fertile mutations (very useful tool). Naturally occurring or induced mutations at the S-locus can break down self-incompatibility.
• The transfer of self-compatibility alleles from a related species through a back cross programme. This approach introduces functional self-compatible alleles into the breeding population.

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Temporary suppression of self-incompatibility

• For the production of inbred (for hybrid seed production) it is essential to achieve self-fertility, where self-incompatibility is fully functional in the selfed progeny. This self-fertility is called as pseudo fertility. This pseudo fertility is achieved through temporary suppression of incompatibility by one of the following techniques:
• Bud pollination: Means application of mature pollen to immature non-receptive stigma generally 1-2 days before anthesis. Bud pollination is the most practicable & successful method in both gametophytic and sporophytic systems. The SI proteins have not yet accumulated on the immature stigma, allowing the pollen tube to grow successfully.
• Surgical techniques: Removal of stigma is very useful in sporophytic system. Pollen is directly dropped on to the ovules. By bypassing the stigma entirely, the SI barrier is physically removed.
• End-of-season pollination controversial technique.
• High temperature: May induce pseudo fertility E.g. in Trifolium, Lycopersicon, Brassica, Oenothera exposure of pistils upto 60 degrees C induces pseudo fertility. High temperatures can denature the SI proteins on the stigma surface.

• Increased CO2 conc. in sporophytic system
• High humidity
• Salt, (NaCl) sprays are used by the Chinese
• Irradiation: In a single locus gametophytic system. e.g. in solanaceae, acute irradiation with x-rays or gamma-rays induces pseudo-fertility. Radiation can damage the S-gene products, temporarily disabling the SI mechanism.
• Grafting
• Double pollination
• Other techniques like use of phytohormones, treatment of flowers with carbon-monoxide etc.

Summary Cheat Sheet