Serotype AD strains of Cryptococcus neoformans are diploid or aneuploid and are heterozygous at the mating-type locus

Cryptococcus neoformans is a pathogenic basidiomycete with a defined sexual cycle involving mating between haploid yeast cells with a transient diploi d state. C. neoformans occurs in four predominant serotypes (A, B, C, and D ), which represent different varieties or species. Rare clinical and enviro nmental isolates with an unusual AD serotype have been reported and suggest ed to be diploid. We found by fluorescence-activated cell sorter analysis t hat serotype AD strains are aneuploid or diploid. PCR analysis with primers specific for serotype A or D alleles of the CNA1, CLA4, and GPA1 genes rev ealed that both alleles are often present in serotype AD strains. PCR analy sis with primers specific for genes in the MATa or MAT alpha mating-type lo ci revealed that serotype AD strains are heterozygous for the mating-type l ocus. Interestingly, in several serotype AD strains, the MAT alpha locus wa s derived from the serotype D parent and the MATa locus was inherited from a serotype A parent that has been thought to be extinct. Basidiospores from a self-fertile serotype AD strain bearing the putative serotype A MATa loc us showed a very low viability (similar to5%), and no fertile serotype A MA Ta strain could be recovered. Serotype AD strains were virulent in a murine model. Hybrid AD strains could readily be isolated following a laboratory cross between a serotype A strain and a serotype I) strain. In summary, ser otype AD strains of C. neoformans are unusual aneuploid or diploid strains that result from matings between serotype A and D strains. Self-fertile iso lates fail to undergo normal meiosis because of genetic divergence. Our fin dings further suggest that serotype A MATa strains may exist in nature.