Sexual incompatibility, inter specific and intra-specific compatibility , Homo- & Hetero-morphic compatibility, GSI &SSI. click : https://syedbasharat123.blogspot.com/

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Self-incompatibility in plants
(NOTES)
Sexual incompatibility
If a pistil, carrying functional female gametes, fails to set seeds after pollination with
viable and fertile pollens, the two are called incompatible. This phenomenon is called as
sexual incompatibility.
Inter-specific Incompatibility Intra-specific Incompatibility
(Self Incompatibility)
Between individuals of different species Between individuals of the same species
Inter-specific Incompatibility
 It is heterogenic incompatibility controlled by more than one genes at different
loci on the chromosome
 It prevents cross pollination
 It creates new races and species
CAUSES: (Inter-specific incompatibility)
1. Fertilization (fusion of male & female gametes) does not occur
2. Hybrid embryo is aborted
3. Inadequate development of endosperm
4. Endosperm-embryo incompatibility
Intra-specific Incompatibility (Self Incompatibility or SI)
Many species of flowering plant are bisexual, suggesting that they generally set the
seeds by self-pollination. However, a number of mechanisms exist to prevent the self-
pollination.
Self-incompatibility (SI) is the mechanism by which plants prevent self-fertilization and
maintain genetic diversity. Self-incompatible plants are not able to produce seeds when

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its flowers are pollinated from its own flowers or flowers from plants that are
genetically the same.
SI is an active process where self-incompatible pollen tubes are killed in the female
flower-parts to prevent fertilization. In plants with SI, when a pollen grain, produced by
a plant, reaches the stigma of the same plant or of another plant with a similar
genotype, the process of pollen germination, pollen-tube growth,
ovule fertilization and embryo development is halted at one of its stages and,
consequently, no seeds are produced due to the following reasons:
1. Pollen grain fails to germinate on the stigma.
2. Pollen grain germinates, but pollen tube fails to enter into stigma and style.
3. Pollen tube enters into the style but its growth is very slow.
4. Pollen tube enters the ovule but there is no fertilization due to degeneration of
egg cells.
5. Fertilization is affected but embryo is degenerated at very early stage.
FACTS:
 SI is estimated to occur in 30–50% of flowering plant species
 It is one of the most important means of preventing self-fertilization and
promoting the generation of new genotypes in plants.
 It is considered as one of the causes for the spread and success of angiosperms
on the earth.
Lewis (1954) has suggested the following classification of self-incompatibility in plants:
1. Heteromorphic System (different morphology of flowers)
2. Homomorphic System (Two kinds)
o Gametophytic Control
o Sporophytic Control

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Heteromorphic System:
In Heteromorphic System, flowers of different incompatibility groups have different
morphology. For example, relative length of styles and stamens in the flowers of the same
species, e.g. Distyly (Primula species) and Tristyly (Oenothera) has been reported as
Heteromorphic Systems.
Diagrammatic representation of heteromorphic incompatibility
Distyly Tristyly
DISTYLY
Primula has two types of flowers, viz. Pin and Thrum; this situation is referred as Distyly.
Pin flowers: bear long styles and short stamens Thrum flowers: bear short styles and long stamens.
In the case of Distyly, the only compatible mating is possible between Pin and Thrum flowers.
The characteristic is governed by single S-gene (self-incompatibility gene); Ss produce Thrum
flowers and ss produce Pin flowers.
Mating Phenotype Mating Genotype Genotype Phenotype
Pin × Pin ss× ss Incompatible mating -
Thrum × Thrum Ss × Ss Incompatible mating -
Pin × Thrum ss × Ss 1Ss : 1 ss 1 Thrum : 1 Pin
Thrum × Pin Ss× ss 1Ss : 1 ss 1 Thrum : 1 Pin

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The incompatibility reaction of pollen is controlled by the genotype (genetic makeup) of
the plant producing them. Allele S dominates over allele s. Incompatibility system, thus,
represents Heteromorphic-Sporophytic type. Such type of system has been reported in
plants like sweet potato and buckwheat.
In Tristyly, the style of a flower may be of short, long or of medium length. It is known in
some plant species, e.g. Lythrum. This characteristic is also governed by a single S-gene
with alleles Ss and ss. Allele Ss produces Thrum, while ss allele produces Pin flowers.
The incompatibility reaction of pollen is determined by the genotype of the plant
producing them. Allele S is dominant over s. Here also, the incompatibility system is
Heteromorphic-Sporophytic.
Homomorphic System
In the Homomorphic System, incompatibility is not associated with morphological
differences among flowers. The incompatibility reaction of pollen may be controlled by:
1. The genotype of the sporophyte (the plant on which it is produced) →
Sporophytic control or
2. The own genotype of male gamete/pollen grain → Gametophytic control.

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In Homomorphic flowers, all flowers have exactly the same structure. Avoidance of self-
fertilization depends on genetic/biochemical mechanisms. There are two quite
different types of self-incompatibility.
o Sporophytic self-incompatibility (SSI)
o Gametophytic self-incompatibility (GSI)
Sporophytic self-incompatibility (SSI) Gametophytic self-incompatibility (GSI)
 Incompatibility reaction between
the pollen-tube and the style is
determined by the genotype of the
sporophyte (diploid tissue).
 S-locus products are synthesized
before completion of meiosis.
 Growth of the pollen tube is
arrested at the surface of the
stigma.
 Incompatibility reaction between
the pollen tube and the style is
determined by the genotype of the
pollen (gamete).
 S-locus products are synthesized
after completion of meiosis.
 Growth of the pollen tube is
arrested in the style.
Sporophytic Self-incompatibility (SSI)
In the Sporophytic System, the stigma is papillate and dry, and is covered with a
hydrated layer of proteins known as ‘pellicle’. Within few minutes of reaching the
stigmatic surface, the pollen releases an exine-exudate, which is either protein or
glycoprotein in nature. This exudate induces immediately the callose formation in the
papillae of incompatible stigma. Thus, in Sporophytic System, stigma is the site of
incompatibility reaction; once the pollen tube crosses the stigmatic barrier, there is not
further inhibition of the pollen tube growth. This form of self-incompatibility has been
studied intensively in members of the mustard family (Brassicaceae), including turnips,
rape-seed, cabbage, broccoli, and cauliflower.

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Sporophytic SI: Stigmatic papilla in cross-pollination (A) and self-pollination (B).
Note the deposition of callose at the tip of pollen tube and in the papilla in self-pollinated stigma
The S-locus
In dicots, it is the single gene of self-incompatibility on the chromosome called as S-
locus. In contrast, grasses (monocots) have two (unlinked) S-loci.
While a species may have as many as 50 or 60 different S-alleles, each plant has only
two of them, one inherited from each parent.
 Pollen grains are haploid; they contain only a single set of chromosomes and, thus,
each pollen grain contains only one of the two S-alleles of the parent plant.
 Pistil (style) is diploid; it has two sets of chromosomes (one from each parent) and,
therefore, pistil has both S-alleles of the parent plant.
Molecular dissection of the S-locus in several plant species has revealed that:
 The S-locus consists of multiple, tightly-linked genes, encoding male and female
compatibility determinants.
 Divergent mechanisms of self-incompatibility are encoded by the S-loci of different
plant species.

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 Individual plants carry a pair of different S-loci (e. g. S1 and S2).
However, dozens of different S-alleles may be present in a population of the
species; that is, the S-locus in the species is extremely polymorphic (having
multiple alleles).
Homorphic-Sporophytic SI (Figure) Rules for Homomorphic SSI:
1. Pollen will not germinate on the stigma
(diploid tissue) of a flower that
contains either of the two alleles present
in the pollen-producing sporophyte-parent
(Case 1).
2. Pollen will also not germinate if the haploid
pollen-grain contains only one of these
alleles. (Case 2).
3. Pollen will germinate successfully on the
stigma of a flower that contains none of
the two alleles present in the pollen-
producing sporophyte-parent. (Case 3).
 In Homographic-Sporophytic Self-incompatibility (SSI), recognition as well as
rejection reactions for pollen-tube growth take place on the stigma.
 The rejection of self-pollen (with the same genotype as that of stigma-bearing
plant) is controlled by the diploid genotype of the sporophyte generation (stigma).
Biochemical Mechanisms of Sporophytic Self Incompatibility (SSI)
The control lies in the "S-locus", which is actually a cluster of three tightly-linked loci.
The three genes of the S-locus, produce the following three proteins, respectively:
 SLG (S-Locus Glycoprotein). It encodes the part of a receptor-protein present in
the cell wall of the stigma.
 SCR (S-locus Cysteine-rich Protein). It encodes a soluble ligand for
the same receptor-protein, which is secreted by the pollen.

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 SRK (S-Receptor Kinase). It encodes the kinase domain of the receptor-protein.
SRK is transmembrane protein embedded in the plasma membrane of
the stigma cell.
S1
S2
Biochemical Mechanism of SSI  The sporophyte, producing S1S2 pollen,
synthesizes both SCR1 and SCR2 for
incorporation in (and later release
from) both S1 and S2 pollen grains.
 If either of the SCR molecule (SCR1 or
SCR2) binds to either receptor-protein
(SLG1 or SLG2) on the pistil, the enzyme
Kinase attaches the phosphate group to
other proteins, which trigger a series of
events that lead to failure of the stigma to
support germination of the pollen grain.
 If this path is not triggered (e.g., when
pollen from an S1S2 lands on an
S3S4 stigma), the pollen germinates
successfully.

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Sporophytic Self-Incompatibility
(SSI)
Gametophytic Self-Incompatibility
(GSI)
(SSI) (GSI)
(1) Fully Incompatible (S1 S2 × S1 S2)
(2) Fully Incompatible (S1 S2 × S1 S3)
(3) Fully Compatible (S1 S2 × S3 S4)
(1) Fully Incompatible (S1 S2 × S1 S2)
(2) Partially Compatible (S1 S2 × S2 S3)
(3) Fully Compatible (S1 S2 × S3 S4)
GSI rules:
1. In Homorphic-Gametophytic Self-incompatibility (GSI), recognition as well as rejection
reactions occur inside the pistil.
2. The rejection of self-pollen is controlled by the haploid genotype of the Gametophytic
generation (Pollen).
3. The S-loci (as those present in SSI plants) are extremely polymorphic; that is, there is an
abundance of multiple alleles in the population.
4. Incompatibility is controlled by the single S-allele present in the haploid pollen grain.
5. Thus, in GSI system, a pollen grain will grow into any pistil that does not contain the
same allele (as that of pollen) in contrast to what happens in SSI system, where a pollen
grain will not grow into a pistil that contains any one of the S-allele of the pollen grain.

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Biochemical Mechanisms of Gametophytic Self Incompatibility (GSI)
The best studied mechanisms of self incompatibility (SI) are the following:
1. inhibiting the germination of pollen on the stigmas
2. Inhibiting the elongation of the pollen tube in the styles
These mechanisms are based on protein-protein interactions, and the best-understood
mechanisms are controlled by a “single S-gene”, termed as S-locus, which has many
different alleles in the species population. Despite their similar morphological and
genetic manifestations, these mechanisms have evolved independently, and are based
on different cellular components; therefore, each mechanism has its own, unique S-
genes.
The S-locus contains two basic protein-coding regions:
1. Expressed in the pistil (Female Determinant)
2. Expressed in the anther and/or in pollen (male determinant)
Because of their physical closeness, these S-loci are genetically linked, and are inherited
together as single unit. These units are called S-haplotypes.
As a result of interaction of the proteins encoded by the two regions of the S-locus of
the same haplotype, the pollen germination and/or pollen-tube elongation is arrested,
preventing fertilization.
However, when a female determinant interacts with a male determinant of a different
haplotype, no self-incompatibility occurs, and fertilization ensues. This is a simplistic
description of the general mechanism of self-incompatibility. Following is the detailed
description of the different known mechanisms of self-incompatibility in plants.
In Gametophytic Self-incompatibility (GSI), the self-incompatibility phenotype of the
pollen is determined by its own Gametophytic haploid genotype. This is the more
common type of self-incompatibility, existing in the families such
as Solanaceae, Rosaceae and Papaveraceae.

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Two different mechanisms of GSI have been described in detail at the molecular level,
and their description follows.
(1) The RNase mechanism
The RNase mechanism was discovered in Petunia (Solanaceae). In this mechanism,
pollen tube elongation is stopped when it has moved approximately one third of the
way through the style. The female component ribonuclease, termed as S-RNase, causes
degradation of the ribosomal RNA (rRNA) inside the pollen tube, in the case of identical
male and female S-alleles. Consequently, pollen tube elongation is arrested and the
pollen grain dies.
STEPS:
 All pollen grains — incompatible as well as compatible — germinate on stigma,
forming pollen tubes that begin to grow down the style.
 However, growth of incompatible pollen tubes stops in the style; while,
compatible pollen tubes grows normally and fertilizes the egg inside the ovary.

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The growth-blockage within incompatible pollen tubes is created by a ribonuclease (S-
RNase) encoded by an S-locus.
The S-RNase
 It is synthesized within the style
 It enters the pollen tube
 It destroys the RNA molecules in the pollen tube
 As a result, the pollen tube growth is arrested.
Self-Pollen Mechanism: In the self-incompatible pollen-tubes, the interaction of the SLF
(male determinant) with the S-RNase (Female determinant) blocks the degradation of
S-RNas (in proteasomes; a barrel-shaped cell organelle for protein breakdown); so, the
RNAs of the pollen tube are destroyed and pollen tube growth is halted.
Cross-Pollen Mechanism: In compatible pollen-tubes, a protein-complex (Ubiquitin-
Ligase Complex), designated as “SCF(SLF)”, triggers the degradation of the S-RNase thus
permitting RNAs in the pollen tube to survive and growth to continue.

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(2) The S-glycoprotein mechanism (discovered in Papaver rhoeas)
The following mechanism was described in detail in Papaver rhoeas. In this mechanism,
pollen growth is inhibited within minutes of its placement on the stigma.
The female determinant is a small, extracellular molecule, expressed in the stigma.
The male determinant is probably some cell membrane receptor-protein.
The interaction between male and female determinants transmits a cellular signal into
the pollen tube, resulting in strong influx of calcium cations; this interferes with the
intracellular concentration gradient of calcium ions, which exists inside the pollen tube,
essential for pollen-tube elongation. The influx of calcium ions arrests tube elongation
within 1–2 minutes.
 Within 10 minutes from the placement of pollen-grain on the stigma, the pollen is
committed to a physiological-process, which leads to its death.
 At 3–4 hours past pollination, fragmentation of pollen DNA begins, and finally (at
10–14 hours), the cell dies apoptotically.
Significance of Self Incompatibility
1. Nature has got balanced inbreeding and outbreeding regulated by inter-specific
and intra-specific incompatibilities.
2. Extensive selfing leads to highly homozygous individuals that carry very low
survival value.
3. Inter-specific incompatibility brings about reproduction-isolation, which is
responsible for emergence of new races.
4. Self-incompatibility stops free flow of genes, enabling fairly large differences in
populations.
5. Self-incompatibility prevents self-pollination effectively. As a result, it has a
profound effect on plant-breeding approaches and objectives as given below:

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6. Self-incompatibility may be used in hybrid seed-production. For that, two self-
incompatible but cross-compatible lines are to be interpolated; seeds obtained
from both the lines would be hybrid seed.
7. Self-incompatibility provides a way for hybrid seed-production without
emasculation and without resorting to genetic or cytoplasmic male sterility.
8. Self-incompatibility system permits combining of desirable genes in a single
genotype from two or more different sources through natural cross pollination,
which is not possible in self compatible species.
9. In case of pineapple, commercial clones are self-incompatible. As a result, their
fruits develop parthenocarpically and are seedless.
10. In self-incompatible fruit trees, it is necessary to grow two cross-compatible
varieties to ensure fruitfulness (formation of fruits).
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