How is Nicotiana exhibit self-incompatibility? Explain its mechanism.

Explanation

(i) In plants, multiple alleles have been reported in association with self-sterility or self-incompatibility. Self-sterility means that the pollen from a plant is unable to germinate on its own stigma and will not be able to bring about fertilization in the ovules of the same plant.

(ii) East (1925) observed multiple alleles in Nicotiana which are responsible for self-incompatibility or self-sterility. The gene for self-incompatibility can be designated as S, which has allelic series `S_(1), S_(2) , S_(3), S_(4) and S_(5)`,
(iii) The cross-fertilizing tobacco plants were not always homozygous as `S_(1)S_(1)" or "S_(2)S_(2)`, but all plants were heterozygous as `S_(1) S_(2) , S_(3) S_(4), S_(5)S_(6)`.
(iv) When crosses were made between different `S_(1)S_(2)` plants, the pollen tube did not develop normally. But effective pollen tube development was observed when crossing was made with other than `S_(1) S_(2)` for example `S_(3)S_(4)`.