Genetic control of specificity and activity of the S antigen in plants

Abstract

Mutations and rare recombinations of the incompatibility S gene in Oenothera organensis, which had been selected from 10$^{9}$ pollen grains, have been tested for gene interactions and complementarity in diploid pollen grains of artificially synthesized tetraploids. Unlike the hundreds of normal alleles of this gene, each of which produces a highly specific protein in the pollen and a molecule with the same specificity in the style, the mutants S$_{4^{\prime}}$ and S$_{6^{\prime}}$ do not produce their specific protein in haploid pollen but produce their original S$_{4}$ and S$_{6}$ substances in the style. The gene interactions of the mutant alleles with S$_{2}$, S$_{3}$, S$_{4}$ and S$_{6}$ in diploid pollen show, with one exception, the same characteristic patterns of dominance, recessiveness and competition which are obtained with the original S$_{4}$ and S$_{6}$ from which the mutants were obtained. The new interaction is that the allele S$_{2}$ in diploid pollen restores the activity of S$_{4^{\prime}}$ by complementation so that the original S$_{4}$ protein as well as the S$_{2}$ protein are produced, thus showing that the specificity determining cistron of the gene is unchanged in the S$_{4^{\prime}}$ mutant. The results fit the hypothesis that the S gene is composed of two cistrons, one controlling the specific groupings of the protein that is active in the incompatibility reaction and the other controlling a half molecule or carrier responsible for the activity of this protein in the pollen and style. On this hypothesis the mutations and recombinations are all lesions or changes in this carrier cistron.