"White-opaque transition": a second high-frequency switching system in Candida albicans

Abstract

A second high-frequency switching system was identified in selected pathogenic strains in the dimorphic yeast Candida albicans. In the characterized strain WO-1, cells switched heritably, reversibly, and at a high frequency (-10-2) between two phenotypes readily distinguishable by the size, shape, and color of colonies formed on agar at 25°C. In this system, referred to as the "white-opaque transition," cells formed either "white" hemispherical colonies, which were similar to the ones formed by standard laboratory strains of C. albicans, or "opaque" colonies, which were larger, flatter, and grey. At least three other heritable colony phenotypes were generated by WO-1 and included one irregular-wrinkle and two fuzzy colony phenotypes. The basis of the white-opaque transition appears to be a fundamental difference in cellular morphology. White cells were similar in shape, size, and budding pattern to cells of common laboratory strains. In dramatic contrast, opaque cells were bean shaped and exhibited three times the volume and twice the mass of white cells, even though these alternative phenotypes contained the same amount of DNA and a single nucleus in the log phase. In addition to differences in morphology, white and opaque cells differed in their generation time, in their sensitivity to low and high temperatures, and in their capacity to form hypae. The possible molecular mechanisms involved in high-frequency switching in the white-opaque transition are considered.Recently, we demonstrated that a common laboratory strain of the dimorphic yeast Candida albicans was capable of switching heritably, reversibly, and at a high frequency among at least seven general phenotypes distinguishable by colony morphology (17; D. R. Soll, B. Slutsky, S. Mackenzie, C. Langtimm, and M. Staebell, J. Oral Pathol., in press). In this system, cells of the parent strain switched spontaneously at a frequency of roughly io-4. A low dose of UV light, which killed less than 10% of the cell population, stimulated a 200-fold increase in this initial frequency. Whether spontaneous or UV induced, once the original strain switched, it continued to switch spontaneously and reversibly between variant colony phenotypes at a frequency of 10-2. Revertants to the original colony phenotype which exhibited a decrease in switching frequency from 10-2 to i0' were also obtained (17; Soll et al., in press).In examining the switching capabilities of strains of C. albicans isolated from patients with systemic infections, we have discovered a second switching system, which we will refer to as the "white-opaque transition." In this system, cells switch heritably, at a high frequency, and reversibly between two phenotypes which generate alternative colony morphologies distinguishable by colony size, shape, and color. In the "white" phenotype, cells form colonies which are white and hemispherical. In the "opaque" phenotype, cells form colonies which are larger, flatter, and opaque, or grey. The differences in colony shape and tone appear to be the result of a...

Phenotypic Switching of Cryptococcus neoformans and Cryptococcus gattii

“…For fungi phenotypic switching is defined as the reversible change manifested as altered colony morphology at a rate higher than the somatic mutation rate. Phenotypic switching has been reported first in Candida over 20 years ago [2,3] and the molecular mechanism has been studied extensively [4,5]. In Candida phenotypic switching controls mating and has also been proposed to contribute to virulence [6].…”

The Same Receptor, G Protein, and Mitogen-activated Protein Kinase Pathway Activate Different Downstream Regulators in the Alternative White and Opaque Pheromone Responses ofCandida albicans

“…Cells of the natural strains P37005 (a/a) (Lockhart et al, 2002), P57072 (␣/␣) (Lockhart et al, 2002), and WO-1 (␣/␣) (Slutsky et al, 1987), the derived mutants, and complemented strains were maintained at 25°C on agar containing modified Lee's medium (Bedell and Soll, 1979) or YPD medium (Sherman et al, 1986). For distinguishing between white and opaque phase sectors or colonies, colonies were grown on modified Lee's agar medium supplemented with 5 g/ml phloxine B, which differentially stained opaque phase cells red .…”

“…The deletion mutant ste3⌬ (␣/␣) was generated in the natural ␣/␣ strain P57072 (Lockhart et al, 2002) and a far1⌬ deletion mutant was generated in the natural ␣/␣ strain WO-1 (Slutsky et al, 1987). Each mutant and complemented derivatives were individually tested for spontaneous white-opaque switching by plating cells from single white colonies at low density on nutrient agar containing phloxine B, which differentially stained opaque cells red .…”

“…For distinguishing between white and opaque phase sectors or colonies, colonies were grown on modified Lee's agar medium supplemented with 5 g/ml phloxine B, which differentially stained opaque phase cells red . Before use, white and opaque phase cells were verified microscopically for the unique differences in cell shape and vacuole formation Slutsky et al, 1987). …”

Crystal structure of the WOPR-DNA complex and implications for Wor1 function in white-opaque switching of Candida albicans

“…Important to its pathogenesis and commensalism is its ability to switch between different morphological forms [2][3][4]. C. albicans can switch between two distinct cell types, white and opaque, which have different properties including cell shape, colonial morphology, metabolic preference, mating ability, gene expression pattern, and host tissue preference [5][6][7][8][9]. White-opaque switching is controlled through expression of a master regulator, Wor1 (whiteopaque switching regulator 1), which is highly upregulated in opaque cells and is required for both the transition to and maintenance of the opaque cell type [10][11][12][13].…”