Ploidy variation and spontaneous haploid-diploid switching of Candida glabrata clinical isolates

The human fungal pathogen Candida glabrata is phylogenetically closely related to Saccharomyces cerevisiae , a model eukaryotic organism. Unlike S. cerevisiae with both haploid and diploid forms and a complete sexual cycle, C. glabrata has long been considered a haploid and asexual species. In this study, we analyzed the ploidy states of 500 clinical isolates of C. glabrata from four Chinese hospitals and found that approximately 4% of the isolates were in or able to spontaneously switch to an aneuploidy (genomic DNA: 1N-2N), diploidy (2N), or hyperdiploid (>2N) form under in vivo or in vitro conditions. Stable diploid-form cells were identified in 3% of the isolates (15/500). Of particular interest, one clinical strain only existed in the diploid form. Multilocus sequence typing (MLST) assays revealed two major genetic clusters (A and B) of C. glabrata isolates. Most of the isolates (70%) from China belonged to the A cluster, whereas most of the isolates from other countries (such as Iran, Japan, USA, and European countries) belonged to the B cluster. Further investigation indicated that C. glabrata cells of different ploidy forms differed in a number of aspects, including morphologies, antifungal susceptibility, virulence, and global gene expression profiles. Additionally, C. glabrata could undergo spontaneous switching between the diploid and haploid form under both in vitro and in vivo conditions. Given the absence of an apparent sexual phase, one would expect that the ploidy shifts could function as an alternative strategy that promotes genetic diversity and benefits the ability of the fungus to rapidly adapt to the changing environment. Importance The human fungal pathogen Candida glabrata has long been thought to be a haploid organism. Here we report the population structure and ploidy states of 500 clinical isolates of C. glabrata from China. To our surprise, we found that the ploidy of a subset of clinical isolates varied dramatically. Some isolates were in or able to switch to an aneuploid, diploid, or hyperdiploid form. C. glabrata cells with different ploidy differed in a number of biological aspects, including morphologies, antifungal susceptibility, virulence, and global gene expression profiles. Given the absence of an apparent sexual phase in this fungus, we propose that ploidy switching could be a rapid adaption strategy to environmental changes and could function as an alternative strategy of sexual reproduction.