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

Edited By Irshad Anwar | Updated on Jul 02, 2025 07:12 PM IST

Definition of Self Incompatibility

Self-incompatibility in flowering plants is a genetic process to prevents self-fertilisation and favours 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.

Basics Of Plant Reproduction

Sexual reproduction of plants involves the fusion of male and female gametes to form seeds that grow into new plants.

Overview Of Sexual Reproduction In Plants

Generally, this is a result of pollination, wherein pollen from the male part of the flower is transferred to the stigma, which is the female part. Hence, fertilisation takes place, followed by development into seeds in the ovary.

Self-Pollination Vs. Cross-Pollination

Self-pollination is a process by which another produces pollen and transfers it to the same flower or another flower on the same plant. On the other hand, the transfer of pollen from the anther of one plant onto the stigma of another plant is cross-pollination. Cross-pollination ensures variability while self-pollination results in inbreeding.

What Is Self-Incompatibility?

SI is a genetic mechanism in flowering plants that prevents self-fertilisation but allows for the possibility of cross-pollination.

SI means that it achieves genetic diversity by rejecting pollen from the same plant or similar individuals. This ensures healthy populations of plants, avoiding inbreeding.

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 reproductive biology of 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 of Plants with GSI

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 of Plants with SSI

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.

Recommended video on "Self Incompatibility"

Frequently Asked Questions (FAQs)

1. What is self-incompatibility in plants?

Self-incompatibility is the genetic attribute in flowering plants that prevents self-pollination and thus promotes inter-accession pollen exchange by rejecting pollen from the same plant or grouped individuals with identical genetic makeup.

2. What is self-incompatibility in plants?

Self-incompatibility is a genetic mechanism in flowering plants that prevents self-fertilization by rejecting pollen from the same plant or closely related individuals. This promotes genetic diversity and outbreeding.

3. How does gametophytic self-incompatibility work?

Gametophytic self-incompatibility: This system depends on the haploid genotype of the pollen to determine its compatibility. If the genotype of the pollen is identical to the genotype of the stigma, that pollen is recognised as self and is prevented from fertilising the ovule.

4. How does gametophytic self-incompatibility work?

In gametophytic self-incompatibility, the pollen's compatibility is determined by its own haploid genotype. If the pollen's S-allele matches either of the pistil's S-alleles, pollen tube growth is inhibited, preventing fertilization.

5. Which plants exhibit sporophytic self-incompatibility?

Plants like the Brassica species, cabbage, mustard, and certain members of the Asteraceae family have sporophytic self-incompatibility.

6. What is the genetic basis of self-incompatibility?

The genetic basis of self-incompatibility lies in specific genes located in the S-locus, which encode proteins responsible for recognising and rejecting self-pollen at the stigma or style.

7. How does self-incompatibility contribute to genetic diversity?

Self-incompatibility avoids self-fertilisation and promotes cross-pollination, a process that enhances genetic variation and reduces the risk of inbreeding in plant populations.

8. How do self-incompatibility systems affect pollination syndromes?

Self-incompatibility systems can influence the evolution of pollination syndromes by promoting adaptations that enhance cross-pollination, such as attractive flowers, nectar production, and synchronized flowering times within populations.

9. How do self-incompatibility systems affect plant population genetics?

Self-incompatibility systems influence population genetics by maintaining high levels of heterozygosity, promoting gene flow between individuals, and contributing to the maintenance of rare alleles in the population.

10. How do self-incompatibility systems affect the evolution of flower morphology?

Self-incompatibility can drive the evolution of flower morphology by favoring traits that promote efficient cross-pollination, such as spatial separation of anthers and stigma, or the development of different flower morphs in heteromorphic systems.

11. How does self-incompatibility affect pollen competition?

Self-incompatibility affects pollen competition by eliminating self-pollen, thereby reducing the pool of competing pollen grains. This can increase the chances of fertilization by the most fit or compatible non-self pollen.

12. How do self-incompatibility systems affect plant speciation?

Self-incompatibility systems can contribute to plant speciation by creating reproductive barriers between populations. As S-alleles diverge, cross-compatibility between populations may decrease, potentially leading to reproductive isolation and speciation.

13. What is the difference between "self-incompatibility" and "cross-incompatibility"?

Self-incompatibility prevents fertilization between genetically similar individuals of the same species, while cross-incompatibility refers to barriers to fertilization between different species or distant populations within a species.

14. What is the "S-haplotype" in self-incompatibility?

The S-haplotype refers to the specific combination of alleles at the S-locus that determines self-incompatibility. It includes genes for both male and female components of the self-incompatibility system.

15. How do self-incompatibility systems affect the evolution of plant mating systems?

Self-incompatibility systems play a crucial role in the evolution of plant mating systems by promoting outcrossing. They can influence the evolution of traits related to pollination, floral display, and reproductive allocation.

16. What are the two main types of self-incompatibility?

The two main types of self-incompatibility are gametophytic self-incompatibility (GSI) and sporophytic self-incompatibility (SSI). GSI is determined by the pollen's own genotype, while SSI is determined by the genotype of the pollen-producing plant.

17. What is the role of S-locus in self-incompatibility?

The S-locus (Self-incompatibility locus) is a genetic region that contains genes encoding proteins involved in the self-incompatibility response. These genes produce molecules that recognize and reject self-pollen, ensuring cross-pollination.

18. How does sporophytic self-incompatibility differ from gametophytic self-incompatibility?

In sporophytic self-incompatibility, the pollen's compatibility is determined by the diploid genotype of its parent plant (sporophyte), not its own haploid genotype. This affects the entire pollen grain, including its coat, rather than just the pollen tube.

19. Can self-incompatibility be overcome in nature?

Yes, self-incompatibility can sometimes be overcome through various mechanisms such as polyploidy, mutations in S-alleles, or environmental stress. This allows for some self-fertilization when cross-pollination is not possible.

20. How does self-incompatibility differ from self-sterility?

Self-incompatibility is a genetically controlled mechanism that prevents self-fertilization, while self-sterility refers to the inability to produce functional gametes. Self-incompatible plants produce viable pollen and ovules but have mechanisms to prevent their fusion.

21. What is the evolutionary advantage of self-incompatibility?

Self-incompatibility promotes genetic diversity by ensuring cross-pollination between different individuals. This increases the population's adaptability to environmental changes and reduces the expression of harmful recessive alleles.

22. How does self-incompatibility affect plant breeding and agriculture?

Self-incompatibility can be both beneficial and challenging in agriculture. It ensures genetic diversity in crop populations but can make it difficult to produce pure breeding lines. Breeders may need to use techniques to overcome self-incompatibility for certain crop improvements.

23. How does self-incompatibility affect pollen tube growth?

In self-incompatible reactions, pollen tube growth is inhibited shortly after it begins. This can occur through various mechanisms, such as callose deposition in the pollen tube tip or degradation of pollen tube RNA.

24. What is the difference between homomorphic and heteromorphic self-incompatibility?

Homomorphic self-incompatibility occurs in plants with similar flower morphology, relying solely on genetic mechanisms. Heteromorphic self-incompatibility involves both genetic and physical barriers, with plants producing different flower morphs (e.g., pin and thrum flowers in primroses).

25. How do S-RNases function in gametophytic self-incompatibility?

S-RNases are proteins produced by the pistil that enter the growing pollen tube. In self-pollen, these S-RNases degrade RNA, inhibiting pollen tube growth. In compatible pollen, S-RNases are inactivated, allowing fertilization to occur.

26. What is the role of SLF/SFB proteins in self-incompatibility?

SLF (S-Locus F-box) or SFB (S-haplotype-specific F-box) proteins are produced by the pollen and act as part of a complex that targets non-self S-RNases for degradation, allowing compatible pollen tubes to grow.

27. What is the "one-locus gametophytic" self-incompatibility system?

The one-locus gametophytic system is the most common form of self-incompatibility, where compatibility is determined by a single S-locus with multiple alleles. If the pollen's S-allele matches either of the pistil's S-alleles, it is rejected.

28. How do plants with sporophytic self-incompatibility recognize self-pollen?

In sporophytic self-incompatibility, proteins from the pollen parent's S-alleles are deposited on the pollen coat. The stigma recognizes these proteins and initiates a rejection response if they match its own S-alleles.

29. What is the significance of dominance relationships in sporophytic self-incompatibility?

In sporophytic self-incompatibility, S-alleles can show dominance relationships. This means that in heterozygous plants, one S-allele may mask the effect of the other, influencing which pollen is recognized as self or non-self.

30. What is the role of calcium signaling in self-incompatibility responses?

Calcium signaling plays a crucial role in self-incompatibility responses. In incompatible interactions, there is a rapid increase in cytosolic calcium in the pollen tube, leading to changes in actin cytoskeleton and eventual pollen tube rupture.

31. How do self-incompatibility systems evolve?

Self-incompatibility systems evolve through balancing selection, which maintains many S-alleles in a population. New S-alleles arise through mutation and are favored due to their rarity, leading to a diverse array of incompatibility types.

32. What is pseudo-self-compatibility?

Pseudo-self-compatibility is a phenomenon where plants with self-incompatibility systems occasionally allow self-fertilization, especially under certain environmental conditions or as flowers age. This provides a balance between outbreeding and reproductive assurance.

33. How does temperature affect self-incompatibility?

Temperature can influence self-incompatibility reactions. High temperatures can sometimes weaken or break down self-incompatibility responses, potentially allowing self-fertilization that would normally be prevented.

34. What is the difference between pre-zygotic and post-zygotic self-incompatibility?

Pre-zygotic self-incompatibility occurs before fertilization, typically by preventing pollen tube growth. Post-zygotic self-incompatibility, which is rarer, allows fertilization but leads to embryo abortion or reduced fitness of selfed offspring.

35. How do self-incompatibility systems maintain genetic diversity in plant populations?

Self-incompatibility systems promote outcrossing, which increases genetic recombination and maintains high levels of heterozygosity in populations. This genetic diversity helps populations adapt to changing environments and resist diseases.

36. What is the role of ubiquitin in self-incompatibility responses?

Ubiquitin plays a role in self-incompatibility by targeting proteins for degradation. In compatible pollen-pistil interactions, ubiquitin-mediated degradation of S-RNases allows pollen tube growth, while in incompatible interactions, this degradation is prevented.

37. What is the "collaborative non-self recognition" model in self-incompatibility?

The collaborative non-self recognition model proposes that in gametophytic self-incompatibility, multiple SLF proteins work together to recognize and detoxify non-self S-RNases, allowing compatible pollen tubes to grow.

38. What is the significance of glycoproteins in self-incompatibility responses?

Glycoproteins play important roles in self-incompatibility responses. For example, in Brassicaceae, the female S-locus glycoprotein (SLG) and male S-locus cysteine-rich protein (SCR) are key players in the recognition and rejection of self-pollen.

39. What is the "S-locus receptor kinase" (SRK) and its role in self-incompatibility?

The S-locus receptor kinase (SRK) is a protein found in the stigma cells of some plants with sporophytic self-incompatibility. It acts as a receptor that recognizes self-pollen and initiates the self-incompatibility response through a signaling cascade.

40. How do plants balance the benefits of self-incompatibility with the need for reproductive assurance?

Plants balance self-incompatibility and reproductive assurance through mechanisms like pseudo-self-compatibility, delayed self-incompatibility, or the production of some self-compatible flowers. These allow for some self-fertilization when cross-pollination fails.

41. What is the role of programmed cell death in self-incompatibility responses?

Programmed cell death can be triggered in self-incompatible pollen tubes, leading to their controlled destruction. This prevents self-fertilization by ensuring that self-pollen tubes do not reach the ovules.

42. How do self-incompatibility systems affect plant-pollinator interactions?

Self-incompatibility systems can influence plant-pollinator interactions by favoring traits that attract and reward diverse pollinators, ensuring effective cross-pollination. This can lead to co-evolution between plants and their pollinators.

43. What is the "S-locus F-box brothers" (SFBB) hypothesis in self-incompatibility?

The SFBB hypothesis proposes that multiple F-box genes at the S-locus collectively function to recognize and detoxify non-self S-RNases in gametophytic self-incompatibility, allowing for a more flexible recognition system.

44. How do self-incompatibility systems affect inbreeding depression in plant populations?

Self-incompatibility systems reduce inbreeding depression by preventing self-fertilization and promoting outcrossing. This helps maintain genetic diversity and reduces the expression of deleterious recessive alleles in the population.

45. What is the role of small RNA in self-incompatibility responses?

Small RNAs, such as siRNAs and miRNAs, have been implicated in regulating self-incompatibility responses. They may be involved in fine-tuning gene expression during pollen-pistil interactions and in the rejection of self-pollen.

46. How do self-incompatibility systems affect pollen dispersal strategies in plants?

Self-incompatibility systems can influence pollen dispersal strategies by favoring mechanisms that promote long-distance pollen transfer, such as wind pollination or specialized relationships with animal pollinators that travel between distant plants.

47. What is the "compartmentalization model" in gametophytic self-incompatibility?

The compartmentalization model proposes that in gametophytic self-incompatibility, S-RNases are sequestered in vacuoles within the pollen tube. In self-pollen, these vacuoles break down, releasing S-RNases that degrade RNA and inhibit pollen tube growth.

48. What is the role of actin cytoskeleton reorganization in self-incompatibility responses?

Actin cytoskeleton reorganization is a key event in self-incompatibility responses. In incompatible interactions, rapid depolymerization of actin filaments occurs, leading to disruption of pollen tube growth and eventual tube rupture.

49. How do self-incompatibility systems affect gene flow in plant populations?

Self-incompatibility systems promote gene flow by necessitating cross-pollination between different individuals. This increases genetic exchange within and between populations, contributing to genetic diversity and population connectivity.

50. What is the significance of S-locus polymorphism in self-incompatibility?

S-locus polymorphism, or the presence of multiple S-alleles in a population, is crucial for the functioning of self-incompatibility systems. High polymorphism ensures that plants can find compatible mates and maintains genetic diversity in the population.

51. How do self-incompatibility systems interact with other plant reproductive barriers?

Self-incompatibility systems can interact with other reproductive barriers, such as flowering time differences or pollen-pistil interactions, to shape overall reproductive isolation between plant populations or species.

52. What is the role of reactive oxygen species (ROS) in self-incompatibility responses?

Reactive oxygen species play a role in self-incompatibility responses by contributing to signaling cascades and potentially damaging cellular components in incompatible pollen tubes, leading to their growth arrest or rupture.

53. How do self-incompatibility systems affect the evolution of plant genome size?

Self-incompatibility systems may indirectly affect genome size evolution by promoting outcrossing, which can increase effective population sizes and the efficiency of selection against the accumulation of repetitive DNA and transposable elements.

54. What is the "non-self recognition by multiple factors" model in self-incompatibility?

This model proposes that in some self-incompatibility systems, multiple factors work together to recognize non-self pollen, rather than a single receptor-ligand interaction. This allows for a more robust and flexible recognition system.

55. How do self-incompatibility systems contribute to the maintenance of genetic load in plant populations?

Self-incompatibility systems contribute to the maintenance of genetic load by promoting heterozygosity and preventing the purging of deleterious recessive alleles through inbreeding. This preserves genetic variation but can also maintain harmful alleles in the population.