Cell Cycle Regulation of Mating Type Alleles in the Smut Fungus Ustilago violacea

Abstract

THE sexual cycle of the smut fungus, Ustilago violacea, is initiated by conjugation between uninqcleate, yeast-like haploid sporidia of opposite mating type. The mating type is determined by a pair of alleles (a₁ and a₂). Mating is achieved by the cooperative assembly of a tube between the paired sporidia. After copulation the haploid nuclei remain separate and a dikaryotic mycelium is established. This dikaryon is an obligate parasite of a host plant (family Caryophyllaceae). Nuclear fusion (karyogamy) is induced in hyphae inhabiting the anthers of a host plant and a diploid brandspore is formed. When the brandspore germinates its nucleus divides meiotically to re-establish the haploid sporidial phase¹. In this basidiomycete plasmogamy and karyogamy are temporally separated and the mechanisms governing each can be analysed separately. We wish to report here observations on the relationship between the cell cycle and cellular morphogenesis (manufacture and assembly of the conjugation organelle) in the sporidial phase of the life cycle. We have evidence that mating depends on the stage in the cell cycle of conjugating sporidia. The mating type alleles are differently regulated during the cell cycle, as cells bearing allele a₁ seem to be under stringent control and copulate only during the G₁ phase of a cell cycle, while cells bearing the dominant a₂ allele have a relaxed control and can initiate conjugation during most of the cell cycle.