αADα Hybrids of Cryptococcus neoformans: Evidence of Same-Sex Mating in Nature and Hybrid Fitness

Abstract

Cryptococcus neoformans is a ubiquitous human fungal pathogen that causes meningoencephalitis in predominantly immunocompromised hosts. The fungus is typically haploid, and sexual reproduction involves two individuals with opposite mating types/sexes, α and a. However, the overwhelming predominance of mating type (MAT) α over a in C. neoformans populations limits α–a mating in nature. Recently it was discovered that C. neoformans can undergo same-sex mating under laboratory conditions, especially between α isolates. Whether same-sex mating occurs in nature and contributes to the current population structure was unknown. In this study, natural αADα hybrids that arose by fusion between two α cells of different serotypes (A and D) were identified and characterized, providing definitive evidence that same-sex mating occurs naturally. A novel truncated allele of the mating-type-specific cell identity determinant SXI1α was also identified as a genetic factor likely involved in this process. In addition, laboratory-constructed αADα strains exhibited hybrid vigor both in vitro and in vivo, providing a plausible explanation for their relative abundance in nature despite the fact that AD hybrids are inefficient in meiosis/sporulation and are trapped in the diploid state. These findings provide insights on the origins, genetic mechanisms, and fitness impact of unisexual hybridization in the Cryptococcus population. Cryptococcus neoformans is a major cause of fungal meningitis, predominantly in immunocompromised individuals. This fungus has two mating types/sexes, a and α, and mating typically requires two individuals with opposite mating types. It is mysterious why the α mating type is overwhelmingly predominant in nature and how the capacity for sexual reproduction is maintained in a largely unisexual population. We postulated that same-sex mating between α isolates may occur naturally, as it does under laboratory conditions. By analyzing natural Cryptococcus diploid hybrid isolates containing two α alleles of different serotypic origins, this study demonstrates that same-sex mating transpires in nature. The observations that Sxi1α, a sex regulator encoded by the mating type locus, is frequently altered in C. neoformans hybrids but rarely in the haploid population, and that Sxi1α is also altered in the fertile VGIII group of the sibling species C. gattii by a different mutation support the hypothesis that these SXI1α mutations may enhance fertility, possibly in concert with other genomic changes. Our study provides insights on the genetic and environmental factors that play important roles in the evolution of the current population structure of this pathogenic fungus.