Self-Incompatibility PPT

1. Introduction

Self-incompatibility (SI) is a genetically controlled mechanism in flowering plants that prevents self-fertilization and promotes outcrossing. SI blocks pollen from the same plant or from plants carrying identical incompatibility alleles from achieving fertilization, thereby increasing genetic diversity and reducing inbreeding depression.

2. Historical background

Botanists in the 19th century noticed that certain species failed to set seed by self-pollination but produced seed after cross-pollination. Later genetic and molecular studies revealed a highly polymorphic S-locus as the main determinant in many species and identified proteins such as S-RNases, SLF/SFB, SRK and SCR/SP11 that mediate recognition and rejection.

3. Biological significance

• Promotes outcrossing and maintains heterozygosity.
• Reduces inbreeding depression by preventing selfing.
• Maintains many S-alleles in populations via negative frequency-dependent selection.
• Contributes to reproductive isolation and can influence speciation.

4. Types of self-incompatibility

4.1 Gametophytic self-incompatibility (GSI)

• Compatibility of pollen is determined by its own haploid genotype (the pollen's S-allele).
• Recognition occurs within the style; matching S-alleles between pollen and pistil inhibit pollen-tube growth.
• In many GSI species (Solanaceae, Rosaceae, Plantaginaceae) the pistil expresses S-RNases that degrade RNA in incompatible pollen tubes. Pollen expresses SLF (S-locus F-box) proteins that target nonself S-RNases for degradation, but cannot neutralize self S-RNase.
• Examples: Nicotiana, Petunia, many Rosaceae species, some Plantaginaceae.

4.2 Sporophytic self-incompatibility (SSI)

• Pollen compatibility is determined by the diploid genotype of the parent plant (sporophyte) that produced the pollen.
• Recognition generally occurs on the stigma surface; pollen germination or hydration is prevented if incompatibility is detected.
• In Brassicaceae, the well-studied SRK (S-Receptor Kinase) in the stigma binds the pollen ligand SCR/SP11; matching alleles activate SRK and trigger rejection via downstream effectors (for example ARC1, an E3 ubiquitin ligase).
• Examples: Brassica species, some Arabidopsis relatives and other families with SSI.

4.3 Other mechanisms

• Heteromorphic incompatibility (heterostyly) — incompatibility linked with floral morphs (e.g., Primula distyly).
• Late-acting self-incompatibility — rejection occurs after pollen tube entry into the ovary or at fertilization (sometimes hard to distinguish from inbreeding depression).

5. Genetics of the S-locus

• The S-locus is usually highly polymorphic (many alleles maintained in populations).
• Complex dominance relationships can exist among alleles, particularly in SSI systems.
• Genes for pistil and pollen determinants are often tightly linked to prevent recombination that would break functional recognition pairs (for example SRK and SCR in Brassicaceae; S-RNase and SLF in Solanaceae).

6. Molecular nisms (deeper look)

6.1 GSI — S-RNase based system

S-RNases secreted in the style enter pollen tubes. Pollen SLF proteins (components of SCF E3 ubiquitin ligases) can recognize and target nonself S-RNases for ubiquitination and degradation. Self S-RNase is not neutralized and degrades pollen RNA, arresting tube growth.

6.2 SSI — SRK and SCR/SP11 system

Pollen carries SCR/SP11 peptides that bind the SRK receptor kinase on stigma papilla cells. Cognate (self) interactions activate SRK kinase activity and downstream effectors (e.g., ARC1) that block pollen hydration/germination and prevent successful fertilization.

6.3 Other molecular players

Calcium signaling, reactive oxygen species, callose deposition, proteases and modifier genes outside the S-locus influence SI strength, dominance and outcome.

7. Physiological events during incompatibility

• Pollen adhesion and hydration (often blocked in SSI).
• Pollen germination (blocked on stigma in SSI or in style for GSI).
• Pollen tube growth arrested in the style (GSI) showing callose deposition and morphological abnormalities.
• Molecular degradation (for example RNA degradation by S-RNases in GSI).

8. Evolutionary considerations

• Negative frequency-dependent selection maintains many S-alleles in populations; rare alleles have a mating advantage.
• Loss of SI (transition to self-compatibility) can occur via mutation in S-locus genes or modifiers — often selected for when mates or pollinators are scarce (reproductive assurance).
• Trade-offs exist: SI preserves genetic diversity but may be costly when pollination opportunities are limited.

9. Ecological correlates

• Dependence on pollinators: SI species are vulnerable to pollinator decline.
• Population structure: Small or fragmented populations can suffer mate limitation if S-allele diversity is low.
• Breeding system syndromes: SI often co-occurs with traits that favor cross-pollination (large showy flowers, nectar, specialized pollinators).

10. Practical implications for plant breeding and agriculture

• SI is exploited in hybrid seed production to enforce outcrossing without emasculation in some crops.
• Managing S-alleles is critical in orchards (apples, pears) to ensure compatible pollinators and optimal fruit set.
• Overcoming SI is sometimes necessary to obtain selfed lines or to facilitate crosses; methods are described below.

11. Methods to overcome self-incompatibility

• Bud pollination: pollinate immature flowers before SI is active.
• Mentor pollen technique: use compatible pollen to assist self pollen.
• Heat or chemical treatments: temporarily reduce SI responses.
• Mutagenesis or genetic transformation: create self-compatible mutants by altering S-locus or modifier genes.
• In vitro pollination or tissue-culture methods that bypass stigma/style recognition.

12. Tests and assays for SI

• Controlled pollination: compare seed set after self and cross pollination.
• Pollen germination assays: in vitro or on-stigma assays with fluorescence staining (aniline blue) to track pollen tubes.
• Molecular genotyping: PCR and sequencing to identify S-alleles.
• Paternity analysis using genetic markers to estimate selfing rates.

13. Examples in crops and model species

• Brassicaceae (SSI): Brassica species — SRK/SCR system, important for hybrid seed production.
• Solanaceae (GSI): Nicotiana, Petunia — S-RNase system; cultivated tomato (Solanum lycopersicum) is self-compatible due to mutation.
• Rosaceae (GSI): Apple and pear — orchard compatibility planning is essential for fruit set.
• Primula (heterostyly): floral morph linked incompatibility enforcing outcrossing.

14. SI breakdown and consequences of self-compatibility

• Breakdown commonly occurs through mutations in S-locus genes or modifiers that suppress the SI response.
• Consequences include increased selfing, reduced heterozygosity and effective population size, and altered adaptive potential.
• Agronomically, self-compatibility can simplify seed production and provide reproductive assurance but may reduce genetic variability.

15. Research frontiers

• Molecular detail of SLF–S-RNase interactions and specificity mechanisms.
• Downstream signaling of SRK beyond ARC1 and stigma response complexity.
• Gene editing of S-locus for crop improvement and robust hybrid systems.
• Population genomics of S-allele diversity and responses to environmental change.

17. Summary — Key points

• SI is an S-locus mediated system preventing self-fertilization and promoting outcrossing.
• GSI: pollen determined by haploid genotype; common S-RNase/SLF mechanism. SSI: pollen determined by sporophyte; SRK/SCR mechanism.
• SI has ecological, evolutionary and practical importance; breeders often manage or overcome it for crop production.

18. Suggested short questions

1. Compare and contrast GSI and SSI.
2. Explain the role of S-RNases and SLF proteins in GSI.
3. Describe SRK–SCR interaction and downstream rejection in Brassicaceae.
4. Discuss ecological drivers favoring loss of SI.
5. List three methods breeders use to bypass SI for seed production.