Ploidy study in Sporothrix schenkii

Sporothrix schenckii is a dimorphic fungus that is pathogenic for humans. N o sexual cycle has been reported for this fungus and little is known of its genetic constitution. To inquire into the ploidy state of Sporothrix schen ckii, different approaches were followed: DNA content during transition fro m conidia to yeast, survival to ultraviolet irradiation, chemical mutagenes is, and induction of mitotic recombination. No change in ploidy was detecte d between the conidia and yeast phases of the fungi. Resistance to cell ina ctivation by UV irradiation was higher in S. schenckii and in Sacharomyces cerevisiae in its diploid state than in isogenic haploids a and alpha from S. cerevisiae that were inactivated at lower doses. Two mutant phenotypes, auxotrophy and albinism, were screened after chemical mutagenesis. One-step mutagenesis with either nitrous acid or ultraviolet light was unsuccessful in inducing auxotrophy but was sufficient to induce albino colonies. Two-s tep mutagenesis with nitrous acid in combination with UV light was necessar y to attain two auxotrophic requirements: adenine and methionine. Prototrop hic and pigmented revertants behaved as heterozygotes; after exposure to UV light they gave rise to derivatives which resemble the original mutation. The experiments presented in this work suggest that S. schenckii is a diplo id, although aneuploidy cannot be excluded. (C) 1999 Academic Press.