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White-opaque switching in natural MTLa/α isolates of Candida albicans: evolutionary implications for roles in host adaptation, pathogenesis, and sex.

  • Author(s): Xie, Jing;
  • Tao, Li;
  • Nobile, Clarissa J;
  • Tong, Yaojun;
  • Guan, Guobo;
  • Sun, Yuan;
  • Cao, Chengjun;
  • Hernday, Aaron D;
  • Johnson, Alexander D;
  • Zhang, Lixin;
  • Bai, Feng-Yan;
  • Huang, Guanghua
  • et al.
Abstract

Phenotypic transitions play critical roles in host adaptation, virulence, and sexual reproduction in pathogenic fungi. A minority of natural isolates of Candida albicans, which are homozygous at the mating type locus (MTL, a/a or α/α), are known to be able to switch between two distinct cell types: white and opaque. It is puzzling that white-opaque switching has never been observed in the majority of natural C. albicans strains that have heterozygous MTL genotypes (a/α), given that they contain all of the opaque-specific genes essential for switching. Here we report the discovery of white-opaque switching in a number of natural a/α strains of C. albicans under a condition mimicking aspects of the host environment. The optimal condition for white-to-opaque switching in a/α strains of C. albicans is to use N-acetylglucosamine (GlcNAc) as the sole carbon source and to incubate the cells in 5% CO2. Although the induction of white-to-opaque switching in a/α strains of C. albicans is not as robust as in MTL homozygotes in response to GlcNAc and CO2, opaque cells of a/α strains exhibit similar features of cellular and colony morphology to their MTL homozygous counterparts. Like MTL homozygotes, white and opaque cells of a/α strains differ in their behavior in different mouse infection models. We have further demonstrated that the transcriptional regulators Rfg1, Brg1, and Efg1 are involved in the regulation of white-to-opaque switching in a/α strains. We propose that the integration of multiple environmental cues and the activation and inactivation of a set of transcriptional regulators controls the expression of the master switching regulator WOR1, which determines the final fate of the cell type in C. albicans. Our discovery of white-opaque switching in the majority of natural a/α strains of C. albicans emphasizes its widespread nature and importance in host adaptation, pathogenesis, and parasexual reproduction.

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