Isolates of Cryptococcus neoformans from Infected Animals Reveal Genetic Exchange in Unisexual, α Mating Type Populations

Abstract

Sexual reproduction and genetic exchange are important for the evolution of fungal pathogens and for producing potentially infective spores. Studies to determine whether sex occurs in the pathogenic yeast Cryptococcus neoformans var. grubii have produced enigmatic results, however: basidiospores are the most likely infective propagules, and clinical isolates are fertile and genetically diverse, consistent with a sexual species, but almost all populations examined consist of a single mating type and have little evidence for genetic recombination. The choice of population is critical when looking for recombination, particularly when significant asexual propagation is likely and when latency may complicate assessing the origin of an isolate. We therefore selected isolates from infected animals living in the region of Sydney, Australia, with the assumption that the relatively short life spans and limited travels of the animal hosts would provide a very defined population. All isolates were mating type ␣ and were of molecular genotype VNI or VNII. A lack of linkage disequilibrium among loci suggested that genetic exchange occurred within both genotype groups. Four diploid VNII isolates that produced filaments and basidium-like structures when cultured in proximity to an a mating type strain were found. Recent studies suggest that compatible ␣-␣ unions can occur in C. neoformans var. neoformans populations and in populations of the sibling species Cryptococcus gattii. As a mating type strains of C. neoformans var. grubii have never been found in Australia, or in the VNII molecular type globally, the potential for ␣-␣ unions is evidence that ␣-␣ unisexual mating maintains sexual recombination and diversity in this pathogen and may produce infectious propagules.

The Evolution of Sex: a Perspective from the Fungal Kingdom

“…Third, population genetic studies of C. neoformans isolates from tree hollows in India provide evidence for recombination in otherwise ␣ unisexual populations, and one parsimonious hypothesis is that this involves unisexual reproduction (105). Fourth, C. neoformans serotype A veterinary isolates from infected animals in Sydney, Australia, were found to define a recombining population that is exclusively of the ␣ mating type (29). Moreover, diploid ␣AA␣ isolates that represent intermediates produced via same-sex mating were identified within this population.…”

Distribution of mating-type alleles and M13 PCR markers in the black leaf spot fungus Mycosphaerella fijiensis of bananas in Brazil

“…In novel environments, recombinant pathogenic populations are able to adapt and mutate more rapidly than clonal populations (Bui et al, 2008;Hsueh and Heitman, 2008). In wheat, sexual recombination is important for generating novel allelic combinations in Mycosphaerella graminicola that are able to overcome the resistance of the host genotypes (Zhan et al, 2007).…”

Sex-Induced Silencing Operates During Opposite-Sex and Unisexual Reproduction in Cryptococcus neoformans

“…grubii lineage (Wang et al 2010). Same-sex mating that has been observed under laboratory conditions is mostly found in the C. neoformans serotype D lineage and rarely in serotype A (Lin et al , 2006Bui et al 2008). To test whether SIS operates in the C. neoformans serotype D var.…”

“…However, the natural Cryptococcus population is almost exclusively of the a mating type , and thus unisexual reproduction may be the predominant route to produce meiotic progeny and spores in nature. In fact, evidence from population genetic analyses has shown that unisexual reproduction occurs naturally and contributes to the population structure (Lin et al 2007Bui et al 2008). Thus, uncovering whether SIS occurs during a-a unisexual reproduction is a key question to understand how the Cryptococcus genome has been shaped through this unique life cycle.…”