Self-incompatibility
and sterility are the two mechanisms, which encourage cross pollination. More
than 300 species belonging to 20 families of angiosperms show self incompatibility.
Definition:-
In self incompatibility plants, the
flowers will produce functional or viable pollen grains which are unable to
fertilize the same flower or any other flower of the same plant.
- Self
incompatible pollen grain may fail to germinate on the stigmatic surface.
- Some
may germinate but fails to penetrate the stigmatic surface.
- Some
pollen grains may produce pollen tube, which enters through stigmatic surface,
but its growth will be too slow. By the time the pollen tube enters the ovule
the flower will drop.
- Some
time fertilization is effected but embryo degenerates early.
Classification of Self Incompatibility
According
to Lewis (1954) the self incompatibility is classified:-
(A).
Heteromorphic System:- It is further divided
into two parts:-
(1).
DISTYLY:- In this case there will be difference
in the morphology of the flowers, namely PIN and THRUM flowers. PIN
flowers have long style and short stamens while THRUM
flowers have short style and long stamens. This type of
difference is known as Distyly. In case of distyly the only compatible
mating is between PIN and THRUM.
The
incompatibility reaction of pollen is determined by the genotype of the plant
producing them. Allele S is dominant over s. This system is also
known as heteromorphic - sporophytic system. The mating between PIN and THRUM
would produce Ss and ss progeny in equal frequencies.
(2).
TRISTYLY:- is known in some plants like Lythrum
salicaria. In this case the style of the flower may be short, long or
medium length of the anthers.
(B).
Homomorphic System:- Here the
incompatibility is not associated with morphological difference among flower.
The incompatibility reaction of pollen may be controlled by the genotype of the
plant on which it is produced – (Sporophytic Incompatibilty) or by its own
genotype – (Gametophytic Incompatibilty).
(1).
Gametophytic System:- First discovered by East
and Mangelsdorf in 1925 in Nicotiana sanderae. Here
the incompatible reaction of pollen is determined by its own genotype and not
by the genotype of the plant on which pollen is produced. Genetically the
incompatibility reaction is determined by a single gene having multiple allele.
Eg. Trifolium Nicotiana, Lycopersicon, Solanum ,& Petunia.
(2).
Sporophytic System:- Here also the self
incompatibility is governed by a single gene S with multiple alleles.. The
incompatibility reaction is determined by the genotype of the plant on which
pollen grain is produced and not by the genotype of the pollen. This system was
first reported by Hugues and Babcock in 1950 in Crepis
foetida and by Gerstal in Parthenium argentatum. The
sporophytic system is found in radish, brassicas and spinach.
Mechanism
of Self Incompatibility:-
The various phenomena observed in Self incompatibility is grouped into three
categories.
1.
Pollen – Stigma interaction
2.
Pollen tube – Style interaction
3.
Pollen tube – Ovule interaction
1.
Pollen – Stigma Interaction:- This phenomenon occurs
just after the pollen grains reach on the stigma and generally prevents pollen
from 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 incompatibility reaction occurs at a later stage.
- In
the sporophytic system the stigma is papillate and dry and covered with
hydrated layer of protein known as pellicle. After few minutes of reaching to
the stigma surface the pollen releases an exine exudates which is either
protein or glycerol protein. This reacts with pellicle 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 in incompatible
combinations the growth of pollen tube is retarded with in the style as in
Petunia, Lycopersicon. The protein and poly saccharine synthesis in the pollen
tube stops resulting in bursting up of pollen tube.
3.
Pollen tube – Ovule Interaction:- In Theobroma cacao
pollen tube reaches the ovule and fertilization occurs but the embryo
degenerates later due to some biochemical reaction.
Temporary
Suppression of Self-Incompatibility
In
many situations, e.g., during the production of inbreds for use as parents in
hybrid seed production, it is essential that temporary self-fertility is
achieved in a manner so that self-incompatibility is fully functional in the
selfed progeny.
Such
self-fertility is known as pseudofertility and is achieved by temporarily
suppressing the incompatibility reaction using one of the following techniques:-
(1).
Bud Pollination:- Bud pollination means application
of mature pollen to immature non-receptive stigma, generally 1-2 days prior to
the anthesis of flowers. This is the most practicable and successful method
both in the gametophytic and sporophytic systems.
(2).
Surgical Techniques:- Removal of the surface
of the stigma, the whole of stigma or a part or whole of the style may permit
an otherwise incomptible mating. Removal of the stigma is very useful in the
sporophytic system, e.g., Brassica, while removal of the style is helpful in
some cases of gametophytic incompatibility, e.g., Petunia. In Petunia, the
whole of the style may be removed and the pollen grains may be directly dropped
on the the ovules in the ovarian cavity.
(3).
End-of-Season Pollination:- In some species, the
degree of incompatibility is reduced towards the end of the flowering season or
in mature plants. But there are controversial reports on the usefulness of this
technique.
(4).
High Temperature:- In some species, e.g., Trifolium,
Lycopersicon etc., exposure of pistils to temperatures upto 600 C induce pesudofertility.
(5).
Irradiation:- In the single-locus gametophytic
system, e.g., in Solanaceae, acute irradiation with X[1]rays or gamma-rays
induces a temporary loss of self-incompatibility.
(6).
Grafting:- Grafting of a branch onto another
branch of the same plant or of another plant is reported to reduce the degree
of self-incompatibility in Trifolium Pratense.
(7).
Double Pollination:- In some species, self-incompatible
mating become possible when incompatible pollen in applied as a mixture with a
compatible pollen, or it is applied after pollination with a compatible pollen.
(8).
Other Techniques:- A number of other techniques have
been tried with varying degrees of success. All the other techniques are
mentioned below are not used commonly. These techniques are following:
- Treatment
of flowers with carbon monoxide.
- Injecting
styles with munosuppressants.
- Application
of electrical potential difference of about 100 V between the stigma and pollen
grains.
- Treatment
of pistil with phytohormones and with protein synthesis inhibitors.
- Steel
brush pollination.
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