Heterozygosis and pathogenicity of Cryptococcus neoformans AD-hybrid isolates

Nineteen Cryptococcus neoformans AD-hybrid isolates were investigated to assess whether hybrid genomic background could affect virulence in a mouse model. The level of heterozygosity of each strain was analyzed using primers specific for allele A and D of 15 polymorphic genes. Virulence was tested in a mouse model of systemic infection by measuring time of survival. In addition, the putative virulence attributes, melanin, phospholipase, and capsule production, as well as growth at 39°C and UV sensitivity were investigated. Eight strains showed to be heterozygous in up to 70% of loci, other eight strains were heterozygous in less than 60% of loci, while the remaining three strains were homozygous at all tested loci. Mice infected with hybrids with a high percentage of heterozygosis showed significantly (P < 0.01) shorter survival than mice infected with the other hybrids. Mortality was not correlated with the mating-type locus pattern, as well as it was not correlated with the level of expression of the different virulence attributes investigated. The present study confirms that hybridization in C. neoformans could represent an important evolutionary driving force in increasing the fitness of this yeast in the environment and in the host.