BIOCHEMICAL MUTANTS IN THE SMUT FUNGUS USTILAGO MAYDIS

Abstract

HE usefulness of growth-factor mutants for genetic studies was demon-T strated by BEADLE and TATUM in 1941, using the mold Neurospora.Although a number of other microorganisms have since been employed in studies of a similar nature, there has until recently been no instance of the use of biochemical mutants in sexual fungi other than Ascomycetes.This paper is concerned with the production, characterization and inheritance of requirements for essential metabolites in a parasitic basidiomycete, the smut-fungus of corn, Uslilago maydis (DC.)Cda.(Ustilago zeae, cf.STEVENSON and JOHN-SON 1944).The smut-fungi, comprising the order Ustilaginales, are characterized by the production of smut-spores or chlamydospores which constitute the zygotes and are typically the only diploid cella in the life cycle.During chlamydospore germination, meiosis occurs in a short hypha, the promycelium, from which haploid cells, the sporidia, are budded off.I n the family Ustilaginaceae these products of the reduction divisions are arranged in a linear order that reflects the events of meiosis.The sporidia multiply saprophytically as discrete, yeastlike cells.I n certain environmental circumstances monocaryotic, mycelial growth may occur.(Mycelia may replace sporidia completely in some species, as well as in some variants of species that normally produce sporidia.)Under suitable conditions, haploids of compatible mating-types fuse, giving rise to a dicaryotic mycelium which reproduces by conjugate nuclear division, characteristic of the Basidiomycetes.Formation of a dicaryon is generally essential for virulent infection.I n U .maydis, though not in many other species, infection is evidently initiat'ed by the haploids and the dicaryon is formed within the host (HANNA 1929).The life cycle is completed by production of chlamydospores within which the two nuclei fuse to form a diploid nucleus.I n the case of the maize parasite, this occurs in hypertrophied lesions, the smut-galls.References to the work of mycologists who contributed fundamentally to knowledge of life-cycles of the smuts are given by STAKMAN (1913), and the ecology of the group has been reviewed by TAPKE (1948).Genetic studies on the Ustilaginales have dealt with the inheritance of factors governing sexual compatibility (e.g.KNIEP 1929; BAUCH 1930), colony morphology (e.g.DICKINSON 1928, 1931), and pathogenicity (e.g.HANNA 1929; ALLISON 1937).Mendelian studies have hardly gone beyond the stage of demonstrating that segregation occurs, and that certain phenotypes can be attributed to single or to multiple factors.Aside from data on the relative fre-