1. PLANT BREEDING
Self-incompatibility 2
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.
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)
2). 2-locus gametophytic self- incompatibility
Grass subfamily Pooideae, have a gametophytic self- incompatibility 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.
3). Heteromorphic self-incompatibility
Genes responsible for self-incompatibility in heterostylous flowers are strongly linked to the genes
responsible for flower polymorphism, so these traits are inherited together.
Two concepts
o Distyly
o Tristyly
Distyly
Here, both stamens and styles are of two types.
Stamens may be low and high
Styles short and long.
2. It is determined by a single gene, with two alleles.
Flower with short style and high
stamen is called as thrum type
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.
Distyly
Tristyly
Styles and stamens have three different positions.
Determined by two genes S and M, each with two alleles.
S gives rise to short style
S and M to medium style
s and m to long style
Long style Medium style Short style
4). Cryptic self-incompatibility (CSI)
Exists in a limited number
o Bladder Campion- Silene vulgaris (Caryophyllaceae)
o Viper's Bugloss or Blueweed -Echium vulgare (Boraginaceae)
o Water willow or swamp loosestrife - Decodon verticillatus (Lythraceae)
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
3. Bladder Campion Water willow or swamp loosestrife Viper's Bugloss or Blueweed
In this mechanism, the simultaneous presence of cross and self pollen on the same stigma, results
in higher seed set from cross pollen than self pollen.
However in CSI, self - pollination without the presence of competing cross pollen results in
successive fertilization and seed set.
In some species, pollen tube elongation leads to faster elongation of cross pollen tubes than self
pollen tubes.
Ratio of crossed to selfed ovules ranges between 3.2 and 11.5, when equal amounts of cross and
self pollen are placed upon the stigma.
5. Late-acting self-incompatibility (LSI)
Also termed ovarian self-incompatibility (OSI).
In this mechanism, self pollen germinates and reaches the ovules, but no fruit set.
Absence of fruit set is due to genetic defects (homozygosity for lethal recessive alleles), which are
the direct result of self-fertilization (inbreeding depression).
LSI can be
o Pre-zygotic: Deterioration of the embryo sac prior to pollen tube entry, e.g., Narcissus
triandrus
Narcissus triandrus
o Post-zygotic: Malformation of the zygote or embryo, e.g., Asclepias and Spathodea
campanulata)
o
Spathodea campanulata
4. Maintenance of self-incompatibility
1. Bud pollination
It is supposed that some compound which inhibits the pollen germination and tube growth is
produced by stigma and style at the time of flowering.
If pollination is done 24 hours before the natural flower opening, there is compatibility.
Probably the inhibitor compound is not produced by that time.
Widely used in maintaining lines in grapes, cabbage, cauliflower, etc.
2. End of season pollinations
Usually when the crop season is about to end, incompatibility mechanisms weaken.
This probably due to less metabolic activity of the plant and failure of the plant to produce enough
inhibitory compounds.
3. Stigma removal
In cauliflower, cabbage and radish, failure of pollen to germinate of tube growth occurs.
So, removal of stigma and then pollinations results in perfect seed set.
4. Grafting
In Trefoilparense, grafting of vegetative parts, either within a single plant (homograft) or between
different plants (heterograft), have also improved the level of compatibility.
Pollination can be done after grafting.
Making polyploidy of incompatible strains, they may become compatible because the altered genic
balance at the tetraploid level.
The method has been utilized in peach and potato.
Importance of Self-Incompatibility in Plant Breeding
In SI fruit trees, plant two cross-compatible varieties to ensure fruitfulness.
SI 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.
SI provides a way for hybrid seed production without emasculation and without resorting to
genetic or cytoplasmic male sterility.
SI 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.
In pineapple, since commercial clones are self-incompatible, fruits are parthenocarpic (seedless).
Limitations
It is very difficult to produce homozygous inbred lines in a self incompatible species.
Bud pollination has to be made to maintain the parental lines.
Incompatibility is reduced or broken down at high temperature and humidity.
Limited use of self-incompatibility due to problems associated with the maintenance of inbred
lines through hand pollination as it is tedious and costly.
Thank you