CLASSIFICATION OF SI
- heteromorphic system,
- homomorphic system,
(2a) gametophytic control,
(2b) sporophytic control.
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
(2b) sporophytic control.
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.
The incompatibility reaction of pollen is determined by the genotype of the plant producing them.
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.
The mating between pin and thrum plants would produce Ss and ss progeny in equal frequencies.
This system is of little importance in crop plants;it occurs in sweet potato and buckwheat
(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.
In the homomorphic system, incompatibility is not associated with morphological differences among flowers.
Generally, incompatibility reaction is determined by a single gene
having multiple alleles,
The incompatibility reaction of
pollen may be controlled by the genotype of the plant on which it is produced or by its own genotype.
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
In the sporophytic system also, the self - incompatibility is
governed by a single gene, S, with multiple alleles-more than
30 alleles are known in Brassica oleracea.
In general, the number of S alleles is considerably larger in the
gametophytic than in the sporophytic system.
The incompatibility reaction of pollen is governed by the
genotype of the plant on which the pollen is produced, and not
by the genotype of the pollen.
In the sporophytic system, the S alleles may exhibit
dominance, individual action (codominance) or competition.Consequently, there may be many complex incompatibility
relationships.
characteristics of this
system.
. 1.There are frequent reciprocal differences
- Incompatibility can occur with the female parent
- A family can consist of three incompatibility groups
4.Homozygotes are a normal part of the system
5.An incompatibility group may contain two genotypes