Self incompatibility PART 1 Plant breeding K Vanangamudi TNPSC AO, AAO, HO, ADH, AHO exams ICAR AIEEA JRF & SRF for PG admissions exams ICAR NET, ARS & STO (T-6) exams IBPS – AFO exams interspecific incompability General features of Self-incompatibility Classification of Self-incompatibility Types of self - incompatibility Mechanism of self - incompatibility

1. Self – incompatibility: PART 1
Dr. K. Vanangamudi
Formerly Dean (Agriculture), AC & RI, Coimbatore,
Dean, Adhiparashakthi Agricultural College, Kalavai,
Professor and Head - Seed Science and Technology,
Tamil Nadu Agricultural University, Coimbatore.
Self - incompatibility
 Self-incompatibility refers to the inability of fertile pollen to fertilize its own egg
cell resulting in failure of seed set.
 Self-incompatibility occurs at any stage between pollination and fertilization.
 Self-incompatibility is reported in more than 6000 species of plants covering
Leguminosae, Rosacae, Solanaceae, Compositae, Brassicacae, Graminacae.
Distribution of self - incompability
 In plants with SI, when a pollen grain reaches a stigma of the same plant or
another plant with a similar genotype, pollen germination, pollen tube growth,
ovule fertilization, and embryo development is halted at one of its stages, and
consequently no seeds are produced.
Illustration of self – incompability and interspecific incompability in Brassica sp.
SI: self – incompability; C: Compability; ISI: interspecific incompability

2. 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
SI was confirmed by:
Classification of Self-incompatibility
Genes involved
(number)
Cytology of
pollen
Expression
site
Monoallelic
(governed by single
gene)
Binucleate
(pollens with
two nuclei)
Ovarian
(expression
site is ovary)
Diallelic (governed
by two genes)
Trinucleate
(pollens with
three nuclei)
Stylar
(expression
site is style)
Polyallelic (governed
by many genes)
Stigmatic
(expression
site is stigma)

3. Types of self - incompatibility
1) Single locus self-incompatibility
o Gametophytic self-incompatibility (GSI)
o 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)
1). Single-locus self-incompatibility
1. Homomorphic system
 Only one type of flower.
 Compatible and incompatible plants cannot be differentiated on the basis of
flower morphology.
 Two types of incompatibility systems
1. Gametophytic incompatibility
 First reported by East & Mangelsdorf (1925) in Nicotiana sanderae.
 Governed by multiple alleles of S gene pollen.
 About 21 multiple alleles have been found to control incompatibility.
 These are independent in their action and do not show any dominance
relationship.
 Pollen and ovule carrying dissimilar S alleles can set seed, if the style does not
inhibit the tube growth.
 Tissue is diploid and if there is any S alleles are common in pollen, it arrests the
tube growth.

4. Example
 Styles of S1 genotype would not
allow any pollen tube either S1 or
alleles to penetrate whereas, it will
not inhibit S3 and S3 pollen tubes.
 Thus, there are three situations
Fully incompatibility. E.g.,
S1S2 x S1S2
Partially incompatible. E.g.,
S1S2 x S1S3
Fully compatable. E.g., S1S2 x
S3S4.
Examples
 One-half of all the families of angiosperms
Solanaceae (potatoes, tomatoes [wild, not cultivated], and tobacco)
Petunias
Beets (Beta vulgaris)
Buttercups (Ranunculus)
Lilies
Roses
Many grasses
Pear
Peach
Lucerne
2. Sporophytic incompatibility
 First described by Hughes and Babcock (1950) in Crepis and by Gerstel (1950) in
Parthenium.
 This system is also governed by multiple alleles S gene.
 Tube reaction is governed by genotype where they are produced
o For example, both type of pollen produced by S1S2 plant would behave
similar.
 Inhibition occurs at the stigmatic surface and the pollen does not germinate on
stigma.
 In this system, rejection of self-pollen is controlled by the diploid genotype of the
sporophyte generation.

5. Examples
 Members of the mustard family (Brassica), including turnips, rape,
cabbage, broccoli, and cauliflower.
Mechanism of self - incompatibility
 Three categories.
1. Pollen - Stigma interaction
2. Pollen tube - Style interaction.
3. Pollen tube - Ovule interaction.
1. Pollen - Stigma interaction
 Occurs just after the pollen grains reach the stigma and generally prevents pollen
from germination.
 Prviously it was thought that binucleate condition of pollen in gamatophytic SI
and trinucleate condition in sporophytic SI was the reason for self
incompatability.
 But later on, it was observed that they are not the reason for SI.
 Under homomorphic incompatability, there are differences in the stigmatic
surface which prevents pollen germination.
 In gametophytic system, the stigma surface is plumose having elongated
receptive cells which is commonly known as wet stigma. The pollen grain
germinates on reaching the stigma and incompatability reaction occurs at a later
stage.

6.  In the sporophytic system, the stigma is papillate and dry and covered with
hydrated layer of protein known as pellicle which is involved in incompatability.
 Within few minutes of reaching the stigmatic surface, the pollen releases an
exince excudate which is either protein or glycero protein.
 This reacts with pellicel and induces callose formation which further prevents the
growth of pollen tube.
2. Pollen Tube - Style interaction
 Pollen grains germinate and pollen tube penetrates the stigmatic surface.
 But, the growth of pollen tube is retarded with in the style as in Petunia,
Lycopersicon, Lilium.
 Protein and poly saccharine synthesis in the pollen tube stops resulting in
bursting up of pollen tube and leading to death of nuclei.
3. Pollen tube - Ovule interaction
 In Theobroma cacao, pollen tube reaches the ovule and fertilisation occurs, but
the embryo degenerates later due to some biochemical reaction.
Pseudo-compatibility
 Refers that incompatibility plants show some degree of self-fertilization.
 This is used for creating homozygous S lines and maintain them by selfing.