i. Inability of certain otherwise viable gametes from the genetically specific individuals to fuse with each other.

ii. Reported in 66 families of angiosperms.

iii. Can be morphological or physiological.

(1) Morphological self-incompatibility:

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Two or three mating types viz. distyly (e.g., Primula), tristyly (e.g., Lyhrum).

(2) Physiological self-incompatibility:

Due to physiology. Morphological differences absent. May be:

(A) Gametophytic self-incompatibility (GSI):

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Due to the genotype of the pollen e.g., in Poaceae, Solanaceae, Liliaceae.

(B) Sporophytic self-incompatibility (SSI):

Due to the genotype of the sporophytic/stigmatic tissues e.g., Asteraceae, Brassicaceae.

i. It prevent pollen germination, retardation of growth, deorientation of pollen tube or even failure of nuclear fusion.

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ii. This incompatibility is due to a single multiallelic gene (S-allele). Usually, in develops with the maturation of stigma.