An agricultural triazole induces genomic instability and haploid cell formation in the human fungal pathogen Candida tropicalis

The human fungal pathogen Candida tropicalis is widely distributed in clinical and natural environments. It is known to be an obligate diploid organism with an incomplete and atypical sexual cycle. Azole-resistant C. tropicalis isolates have been observed with increasing prevalence in many countries in recent years. Here, we report that tebuconazole (TBZ), a triazole fungicide widely used in agriculture, can induce ploidy plasticity and the formation of haploid cells in C. tropicalis. The evolved C. tropicalis strains with ploidy variations exhibit a cross-resistance between TBZ and standard azoles used in clinical settings (such as fluconazole and voriconazole). Similar to its diploid cells, these newly discovered C. tropicalis haploid cells are capable of undergoing filamentation, white-opaque switching, and mating. However, compared to its diploid cells, these haploid C. tropicalis cells grow more slowly under in vitro culture conditions and are less virulent in a mouse model of systemic infection. Interestingly, flow cytometry analysis of a clinical strain with extremely low genome heterozygosity indicates the existence of natural C. tropicalis haploids. Discovery of this C. tropicalis haploid state sheds new light into the biology and genetic plasticity of C. tropicalis and could provide the framework for the development of new genetic tools in the field.