PB Ch 6. Self Incompatability

Introduction: What is Self-Incompatibility?

• In many plants, a flower may have perfectly viable pollen and a fully functional pistil (female organ), yet the pollen will entirely fail to fertilize the flower it came from. This biological phenomenon is known as Self-Incompatibility (SI).
• Currently, more than 300 species across 20 families of angiosperms (flowering plants) are known to exhibit SI. At its core, SI is a biochemical and molecular "recognition and rejection" system.
• The plant essentially recognizes its own pollen as "self" and actively stops it from fertilizing the egg, thereby forcing the plant to cross-pollinate with others. While the precise molecular mechanisms are highly complex, the genetic control is usually quite simple: it is governed by the S-locus (self-incompatibility locus) which contains multiple alleles.

How does SI physically manifest?

When incompatible "self" pollen lands on a flower, the rejection can happen at several stages:

• The pollen grains simply fail to germinate on the stigma.
• The pollen germinates, but the pollen tube is physically blocked from entering the stigma.
• The pollen tube enters the style but grows so slowly that the flower naturally drops off before fertilization can occur.
• Fertilization successfully occurs, but the resulting embryo degenerates at an early stage.

5.1 Classification of Self-Incompatibility Systems

According to the Lewis classification system (1954), self-incompatibility is divided into two major categories: Heteromorphic and Homomorphic.

A. Heteromorphic Self-Incompatibility

In the heteromorphic system, you can actually see a physical, morphological difference between flowers of different incompatibility groups. Specifically, the lengths of the styles (female part) and stamens (male part) are different. There are two subtypes:

1. Distyly (Two style lengths)

Certain plant species produce two entirely different types of flowers, usually found on separate plants:

• Pin flowers: Have long styles and short stamens. The stigma sits high up at the top of the flower tube.
• Thrum flowers: Have short styles and long stamens. The stigma sits deep down inside the flower tube.
• The Rule: The only successful matings are between opposite types (Pin x Thrum). Mating Pin x Pin or Thrum x Thrum is entirely incompatible.
• Genetics: This is governed by a single gene (S). The genotype Ss produces Thrum flowers, while ss produces Pin flowers. This is considered a "sporophytic" system because the plant's overall genotype dictates the reaction.
• Example: Primula (primrose).

2. Tristyly (Three style lengths)

Some species take this further by having three distinct style lengths—short, medium, and long—occurring on different plants.

• Example: Lythrum salicaria (purple loosestrife).

B. Homomorphic Self-Incompatibility

In the homomorphic system, there are absolutely no physical differences between the flowers. All plants look perfectly identical. The rejection of self-pollen happens entirely on a microscopic, biochemical level. This system is divided into two highly distinct types: Gametophytic and Sporophytic.

1. Gametophytic Self-Incompatibility (GSI)

Discovered by: East and Mangelsdorf in 1925 (in the plant Nicotiana sanderae).

• In GSI, the incompatibility reaction is determined strictly by the pollen grain's OWN genetic code (the gametophyte), completely independent of the parent plant that produced it.
• Genetics: GSI is governed by a single gene with multiple alleles (e.g., S1, S2, S3... up to S40 or more).
• The Rejection Rule: If the specific S-allele inside the pollen grain matches either of the S-alleles present in the female pistil, the pollen tube is inhibited.

  • Example: If an S1S2 plant produces pollen, half the pollen is S1 and half is S2. If that pollen lands on a female pistil with an S1S3 genotype, the S1 pollen will be blocked (because the female also has S1), but the S2 pollen will grow normally and fertilize the plant.

The Three Mating Types of single-gene GSI:

• Fully Incompatible: S1S2 (Male) x S1S2 (Female). Result: All pollen is rejected.
• Fully Compatible: S1S2 (Male) x S3S4 (Female). Result: All pollen grows normally.
• Partially Compatible: S1S2 (Male) x S2S3 (Female). Result: Only 50% of the pollen (the S1 pollen) grows normally; the S2 pollen is rejected.

• Biological Mechanism: GSI flowers typically feature a "plumose" (wet) stigma. Pollen grains land and germinate just fine. However, as the pollen tube travels down the style, the incompatibility reaction kicks in. The synthesis of vital proteins and polysaccharides inside the pollen tube stops, causing the tube wall to degenerate and eventually burst.
• The Sf Allele: Occasionally, a mutation creates an Sf (self-fertility) allele. Pollen carrying this allele ignores the SI reaction. (e.g., An SfS1 plant will self-pollinate and produce SfSf and SfS1 offspring). This mutation can be artificially induced by radiating pollen.
• Crops showing GSI: Pineapple (governed by 2 loci), ryegrass (2 loci), diploid coffee, clover (Trifolium sp.). It is heavily present in the families Solanaceae, Rosaceae, Gramineae, Leguminosae, and Chenopodiaceae.

2. Sporophytic Self-Incompatibility (SSI)

Discovered by: Hughes and Babcock in 1950 (in Crepis foetida) and simultaneously by Gerstel (in Parthenium argentatum).

• In SSI, the pollen grain's individual genetic code does not matter. Instead, the pollen's incompatibility behavior is dictated entirely by the genotype of the PARENT PLANT (the sporophyte) that produced it.
• Why does this happen? During pollen development, the anther's tapetal cells deposit S-allele protein products directly onto the outer surface of the pollen grain. Therefore, all pollen grains from an S1S2 plant wear an "S1S2 jacket," regardless of whether their internal DNA says S1 or S2.
• Genetics: Governed by a single S-gene with multiple alleles (over 30 alleles are known in Brassica oleracea). These alleles can exhibit complex relationships like dominance, codominance, or competition.

Five Unique Characteristics of SSI:

• Reciprocal Differences: A cross of Plant A x Plant B might be successful, but crossing Plant B x Plant A might fail.
• Female Parent Incompatibility: A plant can be incompatible with its own mother.
• Family Groupings: A single family can consist of up to three distinct incompatibility groups.
• Homozygotes: Homozygous plants (e.g., S1S1) occur naturally in this system (unlike GSI, where all plants are forced to be heterozygous).
• Genotypic Grouping: A single incompatibility group may contain two different genotypes.

• Biological Mechanism: SSI flowers typically have a "papillate" (dry) stigma covered by a hydrated protein layer called a pellicle. When pollen lands, it releases exudates from its outer shell (exine) within minutes. If recognized as "self," the female stigma instantly reacts by forming a wall of callose, physically blocking the pollen from germinating or entering. The entire battle happens right on the surface of the stigma.
• Crops showing SSI: Radish (Raphanus sativus), diploid Brassica crops, and Sinapis (mustards). The Compositae family also possesses SSI, though most cultivated crops in this family are self-fertile.

5.2 Relevance of Self-Incompatibility in Plant Breeding

A. Problems Caused by SI

• Orchard Management: In SI fruit trees, farmers must plant at least two distinct, cross-compatible varieties in the same orchard. If they plant a solid block of just one variety, zero fruit will grow. Furthermore, if bad weather keeps bees away, cross-pollination fails, leading to massive crop losses.
• Breeding Difficulties: To create inbred lines, breeders must force a plant to self-pollinate. SI actively prevents this, forcing breeders to use tedious workarounds like bud pollination (detailed in 5.3). Alternatively, breeders use "sib-mating" (crossing brothers and sisters), but this takes twice as long to achieve the required genetic purity as selfing.
• Genetic Transfers: Attempting to transfer specific S-alleles between varieties is a highly complex and tedious breeding endeavor.

B. Exploitation of SI in Hybrid Seed Production

Rather than fight it, breeders use SI to produce commercial F1 hybrid seeds without needing expensive hand-emasculation (removing male organs).

Three Schemes for Hybrid Production:

• Scheme 1: Two different, self-incompatible but mutually cross-compatible inbred lines are planted side-by-side. The bees cross them. Because neither can self-pollinate, the seeds harvested from both lines are 100% hybrid. (Note: Both lines must be equally productive for this to be economical).
• Scheme 2: An SI line is planted next to a self-compatible (SC) line. The SC line pollinates the SI line. Breeders then harvest seed only from the SI line, guaranteeing it is hybrid seed.
• Scheme 3: Double-cross and triple-cross hybrids using multiple SI lines are frequently used in Brassica crops.

Current Commercial Usage:

• Gametophytic (GSI): Used minimally, mainly for hybrid clover (Trifolium). Not used in cultivated Solanaceae crops because most have evolved to be self-fertile (SI is mostly left in their wild relatives).
• Sporophytic (SSI): Highly exploited, largely spearheaded by Japanese seed companies. SSI is the primary engine behind global hybrid seed production for cabbage, cauliflower, broccoli, and Brussels sprouts.

C. Limitations and Problems in Hybrid Seed Production

• High Costs: Maintaining the pure parent lines requires manual bud-pollination, which is labor-intensive and raises the cost of the final seed.
• Genetic Depression: Continuously forcing SI lines to self-pollinate eventually depresses the SI trait—it unintentionally breeds for self-fertility (Sf alleles), ruining the hybrid purity.
• New Reactions: In the GSI system, continuous inbreeding can spontaneously generate entirely new incompatibility reactions.
• Environmental Sensitivity: High temperatures and high humidity can temporarily "break" the SI mechanism. This can result in up to 30% of the harvest being useless self-pollinated seeds rather than true hybrids.
• Bee Preferences: Bees are smart; if two parent lines look slightly different, bees will often prefer to visit just one type of flower, drastically reducing the cross-pollination rate.

5.3 Temporary Suppression of SI (Breeding Techniques)

When breeders need to create pure inbred lines, they have to temporarily turn off the SI system without destroying it permanently. They use several clever techniques to "trick" the plant:

• Bud Pollination (The Gold Standard): Breeders manually apply mature pollen to an immature flower bud (usually 1 to 2 days before the flower naturally opens, or anthesis). Because the bud is young, its SI chemical defense system hasn't fully booted up yet, allowing the pollen tube to sneak in and fertilize the egg. This is the most successful and widely used method for both GSI and SSI.
• Surgical Techniques: Breeders take a scalpel to the flower. In SSI (Brassica), the barrier is the stigma; slicing off the stigma removes the barrier entirely. In GSI (Petunia), the barrier is the style; breeders can cut away the entire style and dump pollen directly onto the ovules.
• End-of-Season Pollination: In some species, the SI system simply tires out as the plant ages or the season ends, allowing self-pollination. (Results with this method are inconsistent).
• High Temperature: Exposing pistils to high heat (up to 60°C) can induce "pseudofertility" in crops like Trifolium, tomato, Brassica, and Oenothera. Care must be taken not to cook the ovules.
• Irradiation of Pollen: In single-locus GSI plants (Solanaceae), blasting pollen with X-rays or gamma rays temporarily stuns the SI reaction. The damaged pollen can germinate, but fertilization may fail, so this is usually combined with other methods.
• Double Pollination (Mentor Pollen): Breeders mix incompatible "self" pollen with compatible "foreign" pollen. The compatible pollen initiates a positive reaction, allowing the "self" pollen to sneak through the barrier alongside it.
• Grafting: Grafting a branch onto another plant has been shown to reduce SI in Trifolium pratense, though it is rarely used commercially.
• Other Niche Methods: Exposing flowers to carbon monoxide, injecting immunosuppressants, applying a 100V electrical shock between the stigma and pollen, using phytohormones, or scraping the stigma with a steel brush.

5.4 Permanent Elimination of Self-Incompatibility

In some industries—particularly fruit orchards—breeders want to completely and permanently destroy the SI trait to create crops that reliably yield fruit on their own.

• Chromosome Doubling: In GSI plants, artificially doubling the chromosomes (turning a diploid into a tetraploid) can permanently erase incompatibility. For example, in SI diploid potatoes, tetraploid conversion creates pollen grains that carry two different S-alleles at once. The internal competition between these two alleles suppresses the SI reaction (acting like built-in mentor pollen).
• Isolation of Mutations (Sf alleles): Breeders deliberately irradiate flower buds at the Pollen Mother Cell (PMC) stage to induce mutations, hoping to create a self-fertile (Sf) allele. The frequency of natural Sf mutations is about 1 in 100 million ($10^{-8}$) in Oenothera, and exposing them to X-rays increases this rate by approximately $1.6 \times 10^{-8}$ per roentgen. This is easier to track in GSI systems than in SSI systems.
• Transfer of SC Alleles: If a wild relative or similar variety possesses natural self-compatibility (SC) alleles, breeders can attempt to move those genes into the commercial crop using a repeated backcrossing program. While straightforward in theory, this takes years of practical labor.