Unisexual and Heterosexual Meiotic Reproduction Generate Aneuploidy and Phenotypic Diversity De Novo in the Yeast Cryptococcus neoformans

Abstract

Unisexual and heterosexual reproduction in the pathogenic yeast Cryptococcus neoformans enables de novo phenotypic and genotypic plasticity with frequent aneuploidy and rapid adaptation.

Abstract

Aneuploidy is known to be deleterious and underlies several common human diseases, including cancer and genetic disorders such as trisomy 21 in Down's syndrome. In contrast, aneuploidy can also be advantageous and in fungi confers antifungal drug resistance and enables rapid adaptive evolution. We report here that sexual reproduction generates phenotypic and genotypic diversity in the human pathogenic yeast Cryptococcus neoformans, which is globally distributed and commonly infects individuals with compromised immunity, such as HIV/AIDS patients, causing life-threatening meningoencephalitis. C. neoformans has a defined a-α opposite sexual cycle; however, >99% of isolates are of the α mating type. Interestingly, α cells can undergo α-α unisexual reproduction, even involving genotypically identical cells. A central question is: Why would cells mate with themselves given that sex is costly and typically serves to admix preexisting genetic diversity from genetically divergent parents? In this study, we demonstrate that α-α unisexual reproduction frequently generates phenotypic diversity, and the majority of these variant progeny are aneuploid. Aneuploidy is responsible for the observed phenotypic changes, as chromosome loss restoring euploidy results in a wild-type phenotype. Other genetic changes, including diploidization, chromosome length polymorphisms, SNPs, and indels, were also generated. Phenotypic/genotypic changes were not observed following asexual mitotic reproduction. Aneuploidy was also detected in progeny from a-α opposite-sex congenic mating; thus, both homothallic and heterothallic sexual reproduction can generate phenotypic diversity de novo. Our study suggests that the ability to undergo unisexual reproduction may be an evolutionary strategy for eukaryotic microbial pathogens, enabling de novo genotypic and phenotypic plasticity and facilitating rapid adaptation to novel environments.

Author Summary

Aneuploidy refers to increases or decreases in the copy number of individual chromosomes (rather than of the entire haploid or diploid genome). In humans, aneuploidy is well known to be deleterious, causing genetic disorders such as Down syndrome (trisomy 21), and frequently occurring during mitosis in the genesis of cancer. By contrast, aneuploidy in fungi can be advantageous, conferring antifungal drug resistance and enabling rapid adaptive evolution. Cryptococcus neoformans is a globally distributed human pathogen that often infects patients with compromised immunity. It accounts for significant morbidity and mortality associated with HIV/AIDS and is linked to more than one million infections and >600,000 deaths per year world-wide. Although C. neoformans has a defined heterosexual cycle involving a and α cells, more than 99% of clinical and environmental isolates are α. Interestingly, C. neoformans α cells undergo α-α unisexual reproduction to generate diploid intermediates and infectious haploid spores. Sex is costly, though, and the question therefore arises as to why C. neoformans would undergo selfing unisexual, meiotic reproduction as opposed to more efficient asexual, mitotic reproduction. We show here that unisexual, meiotic reproduction in C. neoformans results in aneuploidy, creating advantageous genetic diversity de novo.