Compatibility reactions on solid medium and interstrain inhibition in Ustilago maydis.

HE fungus Ustilago m q d i s is the cause of boil smut disease of corn. Although Tthe complete sexual cycle of the fungus requires the host, rapid vegetative growth on artificial medium is easily obtained. With the work of PERKINS (1 949) and the extensive investigations of HOLLIDAY (1961a) U. may& has been established as a useful tool for genetical research. Since it is also a plant pathogen it may aid in the investigation of the genetics of pathogenicity. The symptoms of the corn smut disease, gall formation and brandspore formation, are the result of the mating of two haploid strains within the host. Mating is under the control of two gene loci, a and b. There are only two a alleles but perhaps as many as 25 b alleles. Only strains with different alleles at both loci will mate; such combinations are termed “compatible”, and the a and b loci are called incompatibility loci. As early as 1932, SLEUMER and later BOWMAN (1946) and ROWELL (1955) described what they consider a mating reaction on artificial medium. They observed fusion of compatible haploid cells under a microscope; the resultant fusion product, however, developed no further. Also in 1932, BAUCH described a macroscopic indication of mating. Two compatible strains, when spotted one upon the other on malt agar, became covered with a heavy mycelial growth. Incompatible combinations and individual haploids remained yeast-like, the usual growth pattern on artificial medium. In the present paper the reaction BAUCH described is called plate mating. Some of the internal and external restrictions on plate mating are defined. A second interaction on agar called interstrain inhibition is also described; the inheritance of this inhibition is considered in detail. It is the first reported instance of extrachromosomal inheritance in U. maydis. Both plate mating and interstrain inhibition provide information on the development of the fungus in corn.