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
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
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.
The mechanism of action of 5-allele is not very clearly understood. However, recently an effort has been made to study this mechanism at the molecular level. Nasrallah et al.
(1985), cloned DNA from S6
allele of Brassica oleracea
and showed that the glycoprotein synthesized by S6
allele can be detected by an antibody. One does not know whether S-specificity is determined by protein or carbohydrate part of glycoprotein, but it is believed that both should be important. It has also been shown that different S
alleles may have different DNA sequences.