Diploid strains of the pathogenic basidiomycete Cryptococcus neoformans are thermally dimorphic

Cryptococcus neoformans is an opportunistic human pathogenic fungus with a defined sexual cycle. Clinical and environmental isolates of C, neoformans are haploid, and the diploid stage of the lifecycle is thought to be transi ent and unstable. In contrast, we find that diploid strains are readily obt ained following genetic crosses of congenic MAT alpha and MATa strains. At 37 degrees C, the diploid strains grow as yeast cells with a single nucleus that is larger than a haploid nucleus, contains a 2n content of DNA by FAG S analysis, and is heterozygous for the MAT alpha and MATa loci. At 24 degr ees C, these diploid self-fertile strains filament and sporulate, producing recombinant haploid progeny in which meiotic segregation has occurred. In contrast to dikaryotic filament cells that are typically linked by fused cl amp connections during mating, self-fertile diploid strains produce monokar yotic filament cells with unfused clamp connections. We also show that thes e diploid strains can be transformed and sporulated and that an integrated selectable marker segregates in a mendelian fashion. The diploid state coul d play novel roles in the lifecycle and virulence of the organism and can b e exploited for the analysis of essential genes. Finally, the observation t hat dimorphism is thermally regulated suggests similarities between the lif ecycle of C. neoformans and other thermally dimorphic human pathogenic fung i, including Histoplasma capsulatum, Blastomyces dermatitidis, Coccidioides immitis, Paracoccidioides brasiliensis, and Sporothrix schenkii. (C) 2000 academic Press.