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CLASSIFICATION OF SI (heteromorphic system, (In this system, flowers of…
CLASSIFICATION OF SI
In this system, flowers of different incompatibility groups are
different in morphology
For example, in Primula there are two types of flowers, pin and
thrum;
Pin flowers have long styles and short stamens, while thrum
flowers have short styles and long stamens. This situation is
referred to as distyly.
Tristyly is known in some plant species, e.g. Lythrum
in such
cases, the style of a flower may be either short, long or of
medium length.
In the case of distyly, the only compatible mating is between pin
and thrum flowers
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(2a) gametophytic control,
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In the homomorphic system, incompatibility is not associated with morphological differences among flowers.
The incompatibility reaction of
pollen may be controlled by the genotype of the plant on which it is produced or by its own genotype.
(2a) gametophytic control,
This incompatibility reaction of pollen is determined by its own
genotype, and not by the genotype of the plant on which it is produced.
Generally, incompatibility reaction is determined by a single gene
having multiple alleles,
If same allele as that of pollen is present in the
stylar tissues, it opposes the growth of pollen tube in the style
In a single gene system, there are three types of mating;
i) Fully incompatible, e.g., S1S2 x S1S2
ii) Fully compatible, e.g., S1S2 x S3S4
ii) Partially (i.e., 50% of the pollen) compatible, eg S1S2 X S1S3
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In the case of distyly, the only compatible mating is between pin
and thrum flowers
This characteristic is governed by a single
gene s; Ss produces thrum, while ss produces pin flowers.
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Allele S is dominant over s.
The incompatibility system, therefore, is heteromorphic sporophytic
The pollen grains produced by pin flowers, would all be s in genotype as well as incompatibility reaction.
The pollen produced in thrum flowers would be of two types genotypically, S and s, but all of them would be S phenotypically.
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