Question
What is self-incompatibility? Why does self-pollination not lead to seed formation in self-incompatible species?

Answer

Self-incompatibility is a genetic mechanism to prevent self pollen from fertilizing the ovules by inhibiting pollen germination or pollen tube growth in the pistil. 
Self-pollination does not lead to seed formation in self-incompatible species because of interaction between the chemicals of the pollen and those of the stigma.

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