Interactions of S alleles in sporophytically controlled self incompatibility of Brassica.

The expressed activity in pollen and stigma was determined for both S alleles of sixteen S-alíele heterozygous genotypes and for one of the two S alleles of two additional heterozygotes. Activities were measured using pollen tube penetration and seed set data from reciprocal crosses between each S-allele heterozygote and its two corresponding S-allele homozygotes.In pollen the S-allele activities ranged from zero to 100% inhibition of pollen tube penetration and seed set, and in the stigma they ranged from 8 to 100% inhibition. Of the sixty-eight S-allele activities measured, thirty-three (48%) were 90 to 100% inhibition, nine (13%) were 80 to 89% inhibition and one to five were within each ten-unit range below 80% inhibition.In an S-allele heterozygote, each subset of two S alleles had an activity for each allele in both pollen and stigma which was highly repeatable among duplicate pollinations within and among successive years. Each subset of two S alleles had a specific S-allele interaction in the pollen, and the same or another specific interaction in the stigma. In pairings with six other S alleles, allele S 2 had four calculated levels of activity in pollen that ranged from 88 to 94%, and five levels in the stigmas between 15 and 94%. When paired in a heterozygote, alleles S 3 and S 5 had activities ranging between 42 and 59%, representing mutual weakening of S-allele activity. Also, heterozygote S 15 S 3 had pollen activities, respectively, of 25 and 6%, i.e. mutual weakening in the pollen.These results indicate that in heterozygous combination with a series of other S alleles, each S-allele may have activity in pollen and also in stigma that potentially is between zero and 100% inhibition. They further indicate that the defined sexual-organ X S-allele-interaction Types I, II, III and IV are extremes; all intermediate variations including complete weakening of both alleles are possible. Recessiveness is weakening of the activity of but one of the two S alleles. The pollen tube penetrations into the style and seed set were highly correlated.