Hybrid population of cryptococcus neoformans includes strains homozygous at mating-type locus

The recent efforts to characterize the hybrid strains of Cryptococcus neoformans has led to the identification of a cryptic population, here described as H strains, which includes hybrid strains with a double content of DNA but presenting a single mating type: Aa, Da, Aalpha, or Dalpha. A set of hypotheses can be formulated about the origin of these H strains: i) they might have lost or modified one mating type allele by a mutation event; ii) they might be homozygous originated from an incomplete mitotic event; iii) they might be homozygous originated from an incomplete meiotic event; iv) they might be homozygous originated from a post-meiotic event. To test these hypotheses we further investigated some H strains previously isolated and then we studied the F1 progeny originated from the mating between H99 (serotype A) and JEC20 (serotype D) reference strains. Fourteen clinical isolates were investigated. The double content of DNA was confirmed by flow cytometry and the presence of only one mating type by the amplification of 12 mating-type-specific genes. In addition, the amplification and the allelic characterization of URA5 gene, outside the mating-type locus, was also performed. The same analyses were performed in 10 strains originated by the mating between H99 and JEC20 strains. Two strains were identify as H strains with mating type Aalpha and further investigation by electrophoretic karyotyping confirmed that genetic recombination occurred. Therefore, the present results suggest that H strains originated from the post-meiotic random fusion of two of the four recombinant nuclei. This gives an explanation to the simultaneous homozygousity of mating type locus and the homozygosity or heterozygousity of URA5 locus in H strains, and is supported by the finding that all F1 progeny, including the two H strains, presented a recombinant karyotype. In conclusion, H strains represents a subpopulation of C. neoformans that originated from the hybridization of serotype A and D populations and that may be isolated from clinical sample. Further studies are required to better understand if homozygosity at mating-type locus may influence the virulence of C. neoformans strains.