Complementary Genetic Control of Differential Compatibility in Rusts

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Abstract
A genetic analysis of pathogenic and antigenic specificity of Puccinia sorghi on four dominant alleles at the Rp locus and on two recessive alleles for resistance in lines of corn permits a theoretical interpretation of genetic control of host-obligate parasite interaction in the light of recent biochemical genetical concepts. The dominant alleles at the Rp locus condition resistance (incompatibility manifested as necrotic fleck) and score a varying spectrum of pathogenic types in the fungus. Each allele is differentially vulnerable (compatible to infection in varying degrees) to one or more pathogenic types of the fungus. The four alleles are complementary in varying dimension, i.e., both qualitatively and quantitatively, in conditioning rust reaction. The alleles apparently do not influence antigen production in the pathogen significantly. The gene structural analysis of the Rp locus is presently impractical because of the paucity of mutant markers in this region of chromosome X. The data on inheritance of pathogenecity and serotypes in the dicaryon of P. sorghi indicate a complete genie mechanism not resoluble by the application of simple Mendelian concepts alone. The four urediospore clones used were heterozygous for pathogenecity and serotypes. Analysis indicated an indeterminate inheritance with a continuous potential for variation. The genetic unit of complementary function and specificity in a dicaryon line was not constant on the host alleles but became reduced or expanded in complementarity by genetic recombination. The present data do not indicate the fraction of the total genotype that is utilized in conditioning complementarity of lit to a specific differential allele in the host. The use of the genetic linkage map of the pathogenic genotype is impractical in these dicaryotic, obligate parasites; therefore, immunochemical and biochemical methods must be developed to explore the magnitude, structure, and function of the pathogenic locus in P. sorghi and the Rp locus in the host. At this stage of investigation a framework of ideas intended to relate the data on rust reaction to chromosome organization and gene function must be theoretical. The critical review of recent investigations on the biochemical genetics of microorganisms may indicate where to probe for additional evidence on the nature of the reactions and their gene tic-regulation.