Self-sterility in Nicotiana
In plants, multiple alleles have been reported in association with self-sterility or selfincompatibility. 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. 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 S1, S2, S3, S4 and S5
The cross-fertilizing tobacco plants were not always homozygous as S1S1 or S2S2, but all plants were heterozygous as S1S2, S3S4, S5S6. When crosses were made between different S1S2 plants, the pollen tube did not develop normally. But effective pollen tube development was observed when crossing was made with other than S1S2 for example S3S4.
When crosses were made between seed parents with S1S2 and pollen parents with S2S3, two kinds of pollen tubes were distinguished. Pollen grains carrying S2 were not effective, but the pollen grains carrying S3 were capable of fertilization. Thus, from the cross S1S2XS3S4, all the pollens were effective and four kinds of progeny resulted: S1S3, S1S4, S2S3 and S2S4. Some combinations are showed in the table.