The killer systems of Ustilago Maydis

110The 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. 1 In general, the confrontations led to direct contact between the tested strains with no apparent change in morphology if the mates were incompatible, or the confrontation led to the development of mycelial hyphae typical of the heterokaryotic state of this organism if the mates were compatible. The formation of heterokaryotic mycelium is a prerequisite to infection of the host tissue, namely, the corn plant (Figure 1). A compatible interaction leading to the formation of the heter- okaryon is known to be governed by multiple alleles at one locus. 2 6 During the course of the development of the plate assay in some instances, strains known to be compatible, based on their ability to infect jointly corn seedlings, displayed in the plate assay interstrain inhibition (Figure 2). The strains displaying this effect were of independent origin and occurred at a low frequency. The inhibition was shown to result from a secreted substance 7 thought to be of a proteinaceous nature based on its sensitivity to proteolytic enzymes and its resistance to nucleases.