ABSTRACT
The detection of the viruses in Ustilago stemmed from the effort to develop a plate assay for the identification of the mating alleles that control sexual interaction between haploid sporidial cultures. Initial indications on the cytoplasmic nature of the inhibitory effect were obtained by genetic studies using formal crosses and the classical heterokaryon transfer experiments for the detection of cytoplasmically inherited factors. The resistance of strains that did not express an inhibitory factor was not always transmitted as a nonchromosomal factor. The cytoplasmic transmission of the factors that determine interstrain inhibition and the resistance to the inhibitory factors led to a search for the determinants of these phenotypes. The strains displaying the cytoplasmic factors involved in interstrain inhibition were from different locations. Each strain displayed one of the inhibitory specificities. The strains displaying the cytoplasmic factors involved in interstrain inhibition were from different locations. Each strain displayed one of the inhibitory specificities.
