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A series of multiple alleles for self-sterility in jobacco was reported by E.M. East. Self sterility would mean that the pollen from a plant will not be able to bring about fertilization in the ovules of the same plant. Alleles in such a series can be designated as S1,S2, S3, etc. A plant can never be homozygous. If there are only three alleles, S1/S2, S1 /S3 and S2/S3 are the only possible combinations. In such a situation, a pollen carrying an allele different from the two alleles present in the female plant will be able to function. For instance, if S1/S2 is the genotype of female plant, pollen carrying S3 or any other allele except S1 and S2 will function. The principle involved in self incompatibility alleles is shown in Table 5.6. This puts a great restriction. Therefore, a selection pressure in favour of a long series of multiple alleles on the functioning ability of pollen would be exerted. In some cases, as many as 40 such alleles are known to be present in the same series. |
