Gametophytic Self Incompatibility/Self Incompatibility (PART-2)

Classification of S.I Based on Flower morphology
Heteromorphic : S.I is associated with differences in flower morphology. Example:
Distyly:Style & Stamens are of two types(Pin & thrum flowers)
Tristyly: Style& Stamens have three positions short, medium and long.
Homomorphic
The flowers do not differ in their morphology
Gametophytic: SI is govern by genetic constitution of gametes.
Sporophytic: SI is govern by genetic constitution of pollen producing plant.
Gametophytic Self Incompatibility System
Gametophytic incompatibility was first described by East and Mangelsdorf in 1925 in Nicotiana sanderae.
In this system, the incompatibility reaction of pollen is determined by its own genotype, and not by the genotype of the plant on which it is produced.
This is because the biochemical substance involved in SI reaction of the pollen is produced after meiosis.
In style, generally both the S alleles express themselves, ie., show co-dominance.
The incompatibility reaction may be controlled by one or two genes, giving rise t(l) monofactorial and (2) bifactorial systems of gametophytic self-incompatibility.
But some species have more than two genes, e.g., Papaver has three genes, while Beta vulgaris has four genes, in these cases. polyploidy does not overcome the incompatibility reaction.
Monofactorial SI reaction is controlled by a single gene designated by S which usually has 50 or more alleles.
Bifactorial Gametophytic S.I. In grasses, SI reaction is controlled by two genes designated as S and Z, which are unlinked and exhibit multiple alleles
This system is found in Trifolium, Nicotiana, Lycopersicon, Solanum, Petunia, etc.
The various matings may be grouped into the following three types.
1. Fully incompatible matings, e.g. S1S2 x S1S2
2. Fully compatible matings, e.g, S1S2 x S3S4
3. Partially or half-compatible (50% of the pollen compatible) matings, e.g. S1S2 x S1S3 and S1S2 x S2S3

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