Fungi which have yeastlike growth have considerable advantages for genetic studies over filamentous fungi. The replica plating of colonies with velvet greatly simplifies the isolation of many types of mutants and the classification of the phenotypes of the progeny of crosses. In addition, the growth of single, uninucleate cells in liquid media facilitates many physiological and biochemical experimental procedures. A particular reason for choosing a yeastlike smut fungus of the genus Ustilago was the number of reports in the literature that the haploid chromosome number was two; this would have been a considerable advantage for genetic studies. However, subsequent genetic analysis in Ustilago maydis and Ustilago violacea has shown that these cytological reports are untrue. In the latter species at least ten linkage groups have been identified (Day and Jones, 1969). Although the vegetative cells grow vigorously on synthetic media, the sexual stage of smut fungi is parasitic. U. maydis (De Candolle) Corda was chosen for detailed studies because a few days after infection it produces diploid teliospores (brandspores) in vegetative parts of the host Zea mays, and the life cycle is completed in less than two weeks. Many other species produce teliospores only in the inflorescence of the host, many weeks or months after inoculation.
KeywordsNitrate Reductase Growth Requirement Pyrimidine Dimer Restrictive Temperature Diploid Strain
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