Self incompatibility in Plants

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ANAND AGRICLTURAL UNIVERSITYB. A. COLLEGE OF AGRICULTURE

Topic: Self-incompatibility in Plants.

Course: GP-502 - Principles of Cytogenetics(2+1)Course Teacher: Dr. H. L. Dhaduk

Prepared By, Dhanya A J, [ Reg. No: 04-2348-2014 ], M. Sc. (Agri) Plant Molecular - Biology & Biotechnology

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Self-incompatibility (SI) It refers to the inability of a plant with functional pollen to set seeds when self pollinated. It is the failure of pollen from a flower to fertilize the same flower or other flowers of the same plant.

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Self-incompatibility (SI) is a general name for several genetic mechanisms in angiosperms, which prevent self-fertilization and thus encourage out-crossing and allogamy.

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In plants with SI, when a pollen grain produced in a plant reaches a stigma of the same plant or 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. SI is one of the most important means to prevent selfing and promote the generation of new genotypes in plants, and it is considered as one of the causes for the spread and success of the angiosperms on the earth.

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Criteria Types

Flower morphology

Heteromorphic self incompatibility

Distyly

Tristyly

Homomorphic self incompatibility

Sporophytic self incompatibilityGametophytic self incompatibility

Classification of Self-incompatibility

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Criteria Types

Genes involved (number)

Monoallelic (governed by single gene)Diallelic (governed by two genes)Polyallelic (governed by many genes)

Cytology of pollen

Binucleate (pollens with two nuclei)Trinucleate (pollens with three nuclei)

Expression siteOvarian (expression site is ovary)Stylar (expression site is style)Stigmatic (expression site is stigma)

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General features of Self-incompatibilityPrevents selfing and promotes out-breeding so

increases the probability of new gene combinations.

Causes may be morphological, physiological, genetical or biochemical.

Normal seed set on cross pollination. May operate at any stage between pollination and

fertilization. Reduces homozygosity. In plants, self-incompatibility is often inherited by

a single gene (S) with different alleles (e.g. S1, S2, S3 etc.) in the species population

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1) Single-locus self-incompatibility Gametophytic self-incompatibility (GSI)

Sporophytic self-incompatibility (SSI)

2) 2-locus gametophytic self-incompatibility.

3) Heteromorphic self-incompatibility

4) Cryptic self-incompatibility (CSI)

5) Late-acting self-incompatibility (LSI)

Types of self incompatibility

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Mechanisms of single-locus self-incompatibility

The best studied mechanisms of SI act by inhibiting the germination of pollen on stigmas, or 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 locus termed S, 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.

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The S-locus contains two basic protein coding regions - one expressed in the pistil, and the other in the anther and/or pollen (referred to as the female and male determinants, respectively). Because of their physical proximity, these are genetically linked, and are inherited as a unit.The units are called S-haplotypes.

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The translation products of the two regions of the S-locus are two proteins which, by interacting with one another, lead to the arrest of pollen germination and/or pollen tube elongation, and thereby generate an SI response, preventing fertilization. However, when a female determinant interacts with a male determinant of a different haplotype, no SI is created, and fertilization ensues.

This is a simplistic description of the general mechanism of SI, which is more complicated, and in some species the S-haplotype contains more than two protein coding regions.

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Gametophytic Self-Incompatibility (GSI)

This form of self-incompatibility is more common than SSI but not so well understood. It occurs in nearly one-half of all the families of angiosperms, including the Solanaceae (potatoes, tomatoes [wild, not cultivated], and tobacco) petunias beets (Beta vulgaris) buttercups (Ranunculus) Lilies roses many grasses

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The rules: The S loci are

extremely polymorphic; that is, there is an abundance of multiple alleles in the population.

Incompatibility is controlled by the single S allele in the haploid pollen grain.

Thus a pollen grain will grow in any pistil that does not contain the same

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This appears to be the mechanism in the Petunia: All pollen grains — incompatible as well as

compatible — germinate forming pollen tubes that begin to grow down the style.

However, growth of incompatible pollen tubes stops in the style while compatible tubes go on to fertilize the egg in the ovary.

The block within incompatible pollen tubes is created by an S-locus-encoded ribonuclease (S-RNase), which is synthesized within the style; enters the pollen tube and destroys its RNA molecules halting pollen tube growth.

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The RNase molecules contain a hypervariable region, each encoded by a different allele, which establishes each S specificity (S1, S2, S3, etc.).

The pollen tube expresses a protein designated SLF(S-locus F-box protein) that binds S-RNase. SLF also exists in different S specificities (S1, S2, S3, etc.).

In compatible ("nonself") tubes, the SLF or SCF(Skp1–Cul1–F-box-protein ubiquitin ligase) triggers the degradation (in proteasomes) of the S-RNase thus permitting RNAs in the pollen tube to survive and growth to continue.

In incompatible ("self") tubes the interaction of, for example, the S1 SCF with the S1 S-RNase blocks its degradation so the RNAs of the pollen tube are destroyed and growth is halted.

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Cross CompatibilityS1S2 X S1S2 Fully incompatibleS1S2 X S1S3 Partially compatibleS1S2 X S3S4 Fully compatible

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Sporophytic Self-Incompatibility (SSI)

This form of self-incompatibility has been studied intensively in members of the mustard family (Brassica), including turnips, rape, cabbage, broccoli, and cauliflower.

In this system,•Rejection of self pollen is controlled by the diploid genotype of the sporophyte generation.

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•The control lies in the "S-locus", which is actually a cluster of three tightly-linked loci:

• SLG (S-Locus Glycoprotein) which encodes part of a receptor present in the cell wall of the stigma;

• SRK (S-Receptor Kinase), which encodes the other part of the receptor. Kinases attach phosphate groups to other proteins. SRK is transmembrane protein embedded in the plasma membrane of the stigma cell.

• SCR (S-locus Cysteine-Rich protein), which encodes a soluble ligand for the same receptor which is secreted by the pollen.

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•Because the plants cannot fertilize themselves, they tend to be heterozygous; that is, carry a pair of different S loci (here designated S1 and S2).

•However, dozens of different S alleles may be present in the population of the species; that is; the S-locus in the species is extremely polymorphic.

•The difference between the alleles is concentrated in certain "hypervariable regions" of the receptor .

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The rules:

• Pollen will not germinate on the stigma (diploid) of a flower that contains either of the two alleles in the sporophyte parent that produced the pollen.

• This holds true even though each pollen grain — being haploid — contains only one of the alleles.

• For example, the S2 pollen, which was produced by a S1S2 parent, cannot germinate on an S1S3 stigma.

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The explanation:

•The S1S2 pollen-producing sporophyte synthesizes both SCR1 and SCR2 for incorporation in (and later release from) both S1 and S2 pollen grains.

•If either SCR molecule can bind to either receptor on the pistil, the kinase triggers a series of events that lead to failure of the stigma to support germination of the pollen grain. Among these events is the ubiquination of proteins targeting them for destruction in proteasomes.

•If this path is not triggered (e.g., pollen from an S1S2 parent on an S3S4 stigma, the pollen germinates successfully.

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The grass subfamily Pooideae, and perhaps all of the family Poaceae, have a gametophytic self-incompatibility system that involves two unlinked loci referred to as S and Z. If the alleles expressed at these two loci in the pollen grain both match the corresponding alleles in the pistil, the pollen grain will be recognized as incompatible.

2) 2-locus gametophytic self-incompatibility

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The genes responsible for self-incompatibility in heterostylous flowers are strongly linked to the genes responsible for flower polymorphism, so these traits are inherited together.The associated concepts are distyly and tristyly.

3) Heteromorphic self-incompatibility

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What is Distyly ? Here, both stamens and styles are of two

types. Stamens may be low and high styles short and long. It is determined by a single gene, with two

alleles. The flower with short style and high stamen

is called as thrum type and flower with long style and low stamen is called as pin type. Both thrum and pin flowers differ for six characters in addition to stamen and style length.

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StigmaAnther

Cross Result

Ss (thrum) X Ss (thrum) Incompatible

ss (pin) X ss (pin) Incompatible

Ss(thrum) X ss (pin) 1:1

ss (pin) X Ss(thrum) 1:1

Distyly

Thrum Pin

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What is Tristyly?

In tristyly, styles and stamens have three different positions.

It is determined by two genes S and M, each with two alleles. S gives rise to short style, S and M to medium style and s and m to long style. The number of possible genotypes is greater,

but a 1:1 ratio exists between individuals of each SI type.

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Short Style Medium Style Long Style

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Cryptic self-incompatibility (CSI)

It exists in a limited number of taxa (for example, there is evidence for CSI in 

Bladder Campion-Silene vulgaris (Caryophyllaceae),

Viper's Bugloss or Blueweed -Echium vulgare(Boraginaceae),

Waterwillow or swamp loosestrife -Decodon verticillatus (Lythraceae),

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Bladder Campion(Silene vulgaris)

Waterwillow or Swamp loosestrife

(Decodon verticillatus)

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Viper's Bugloss or Blueweed

(Echium vulgare)

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In this mechanism, the simultaneous presence of cross and self pollen on the same stigma, results in higher seed set from cross pollen, relative to self pollen. However, as opposed to 'complete' or 'absolute' SI, in CSI, self-pollination without the presence of competing cross pollen, results in successive fertilization and seed set; in this way, reproduction is assured, even in the absence of cross-pollination. Contd…

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CSI acts, at least in some species, at the stage of pollen tube elongation, and leads to faster elongation of cross pollen tubes, relative to self pollen tubes. The cellular and molecular mechanisms of CSI have not been described.The strength of a CSI response can be defined, as the ratio of crossed to selfed ovules, formed when equal amounts of cross and self pollen, are placed upon the stigma; in the taxa described up to this day, this ratio ranges between 3.2 and 11.5

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It is also termed ovarian self-incompatibility (OSI).

In this mechanism, self pollen germinates and reaches the ovules, but no fruit is set.

 LSI can be pre-zygotic(e.g. deterioration of the embryo sac prior to pollen tube entry, as in Narcissus triandrus) or

post-zygotic (malformation of the zygote or embryo, as in certain species of Asclepias and in Spathodea campanulata).

Late-acting self-incompatibility (LSI)

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 Narcissus triandrus

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Spathodea campanulata

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The existence of the LSI mechanism among different taxa and in general, is subject for scientific debate. Criticizers claim, that absence of fruit set is due to genetic defects (homozygosity for lethal recessive alleles), which are the direct result of self-fertilization (inbreeding depression).

 Supporters, on the other hand, argue for the existence of several basic criteria, which differentiate certain cases of LSI from the inbreeding depression phenomenon.

Late-acting self-incompatibility (LSI)

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 Importance of Self-Incompatibility In Plant Breeding

Self-incompatibility effectively prevents self-pollination; as a result, it has a profound effect on plant breeding approaches and objectives.

(1)  In self incompatible fruit trees, it is necessary to plant two cross-compatible varieties to ensure fruitfulness.

 (2) 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.

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(3) Self incompatibility provides a way for hybrid seed production without emasculation and without resorting to genetic or cytoplasmic male sterility.

(4) 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 .

(5) In case of pineapple, commercial clones are self-incompatible. As a result, their fruits develop parthenocarpically & are seedless.

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1. It is very difficult to produce homozygous inbred lines in a self incompatible species.

2. Bud pollination has to be made to maintain the parental lines.

3. Self incompatibility is affected by environmental factors such as temperature and humidity. Incompatibility is reduced or broken down at high temperature and hu midity.

4. There is a limited use of self-incompatibility due to problems associated with the maintenance of inbred lines through hand pollination as it is tedious and costly.

Limitations

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Reference Cryptic self-incompatibility in Echium vulgare

(Boraginaceae)- Korbecka G. and P.G.L. Klinkhamer

Cryptic Self-incompatibility In Tristylous Decodon Verticillatus (Lythraceae) -

Christopher G. Eckert2 And Maryl Allen Gametophytic self incompatibility Systems -

Ed Newbigin, Marilyn A. Anderson, and Adrienne E. Clarke’

www.wikipedia.com www.theagricos.com Principles of Plant Breeding by B. D. Singh.

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Thank You