Homothallic and heterothallic mating in the opportunistic pathogen Candida albicans

Abstract

Candida albicans is the most common fungal pathogen in humans, causing both debilitating mucosal infections and potentially life-threatening systemic infections. Until recently, C. albicans was thought to be strictly asexual, existing only as an obligate diploid. A cryptic mating cycle has since been uncovered in which diploid a and alpha cells undergo efficient cell and nuclear fusion, resulting in tetraploid a/alpha mating products. Whereas mating between a and alpha cells has been established (heterothallism), we report here two pathways for same-sex mating (homothallism) in C. albicans. First, unisexual populations of a cells were found to undergo autocrine pheromone signalling and same-sex mating in the absence of the Bar1 protease. In both C. albicans and Saccharomyces cerevisiae, Bar1 is produced by a cells and inactivates mating pheromone alpha, typically secreted by alpha cells. C. albicans Deltabar1 a cells were shown to secrete both a and alpha mating pheromones; alpha-pheromone activated self-mating in these cells in a process dependent on Ste2, the receptor for alpha-pheromone. In addition, pheromone production by alpha cells was found to promote same-sex mating between wild-type a cells. These results establish that homothallic mating can occur in C. albicans, revealing the potential for genetic exchange even within unisexual populations of the organism. Furthermore, Bar1 protease has an unexpected but pivotal role in determining whether sexual reproduction can potentially be homothallic or is exclusively heterothallic. These findings also have implications for the mode of sexual reproduction in related species that propagate unisexually, and indicate a role for specialized sexual cycles in the survival and adaptation of pathogenic fungi.

Figure 1. Deletion of BAR1 results in an autocrine mating response

a, Patches of wild-type and Δbar1 a cells after 4 days growth on YPD medium. b, Mating response of wild-type and Δbar1 a cells with and without α-pheromone in Spider medium for 36 h. Differential interference contrast (DIC, left) and GFP fluorescence (GFP, right) images are shown. Induction of the mating response was monitored using GFP reporters under the control of the mating-response genes FIG1 or FUS1. All C. albicans cells used in this study were in the opaque phase as this is the mating-competent form of the organism,. Scale bars, 16 µm.

Figure 2. Autocrine signalling in C. albicans a cells

a, Model of conventional paracrine signalling between a and α cells and two models for autocrine signalling. Note that ‘autocrine’ signalling is used here to refer to signalling within populations of a cells. b, C. albicans a cells after 4 days growth on YPD medium. Whereas Δbar1 strains showed a highly wrinkled morphology, reconstituted strains in which the BAR1 gene had been reintegrated did not show wrinkling (not shown). c, Demonstration of autocrine signalling among different populations of a cells after 4 days growth on YPD medium. Wrinkling at the edge of a test colony is indicative of autocrine signalling.

Figure 3. Homothallic mating in C. albicans

a, Flow cytometric analysis indicating the tetraploid content of progeny isolated from a–a matings. The x-axis of each graph (Sytox) represents a linear scale of fluorescence and the y-axis (Cell counts) a linear scale of cell number. For analysis of additional same-sex mating products see Supplementary Fig. 6. b, PCR confirming the mating type of a–a mating products using primers directed at OBPa and OBPα. Odd-numbered lanes are OBPa reactions, whereas even-numbered lanes are OBPα reactions. Lanes 1, 2 and 3, 4 are control reactions for a and α cells, respectively. c, Table of mating frequencies for different genetic crosses. Values are listed as mean frequency ± s.d.

Figure 4. Homothallic mating across clades in C. albicans

a, Mating frequencies of an SC5314 Δbar1 a cell crossed with naturally occurring isolates from different clades of C. albicans. 1–8 are crosses between Δbar1 a cells and clinical α cells. 9–18 are crosses between Δbar1 a cells and clinical a cells. For a complete list of strains see Methods. b, Mating zygote formed between a Δbar1 a cell derived from SC5314 (DSY906, expressing HTB–RFP; histone H2B gene–red fluorescent protein) and a clinical a cell derived from IHEM16614 (DSY908, expressing HTB–YFP). Arrows indicate fused nuclei expressing both fluorescent proteins. DIC (left), YFP fluorescence (observed using the GFP channel, middle) and RFP fluorescence (right) images are shown. Scale bar, 15 µm.