Self-incompatibility 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. This presentation includes, Single-locus self-incompatibility- {Gametophytic self-incompatibility (GSI) and Sporophytic self-incompatibility (SSI)},2-locus gametophytic self-incompatibility, Heteromorphic self-incompatibility,Cryptic self-incompatibility (CSI) and Late-acting self-incompatibility (LSI).

1. ANAND AGRICLTURAL UNIVERSITY
B. 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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2. 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.
2

3. Self-incompatibility (SI) is a general
name for several genetic mechanisms
in angiosperms, which prevent self-
fertilization and thus encourage out-
crossing and allogamy.
3

4. 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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5. Criteria Types
Flower
morphology
Heteromorphic
self
incompatibility
Distyly
Tristyly
Homomorphic
self
incompatibility
Sporophytic self
incompatibility
Gametophytic self
incompatibility
Classification of Self-incompatibility
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6. 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 site
Ovarian (expression site is ovary)
Stylar (expression site is style)
Stigmatic (expression site is stigma)
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7. General features of Self-incompatibility
Prevents 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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8. 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
8

9. 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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10. 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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11. 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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13. 13

14. 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
14

15. 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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16. 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. 16

17.  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. 17

18. Cross Compatibility
S1S2 X S1S2 Fully incompatible
S1S2 X S1S3 Partially compatible
S1S2 X S3S4 Fully compatible
18

19. 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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20. •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.
20

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22. 22

23. •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 .
23

24. 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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25. 25

26. 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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27. 27

28. 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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29. 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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30. 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.
30

31. Stigma
Anther
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 31

32. 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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33. Short Style Medium Style Long Style
33

34. 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),
34

35. Bladder Campion
(Silene vulgaris)
Waterwillow or
Swamp loosestrife
(Decodon verticillatus)
35

36. Viper's Bugloss or
Blueweed
(Echium vulgare)
36

37. 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…37

38. 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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39.  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)
39

40. Narcissus triandrus
40

41. Spathodea
campanulata
41

42. 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)
42

43. 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.
43

44. (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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45. 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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46. 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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