Abstract

It has been proposed that the ancestral fungus was mating competent and homothallic. However, many mating competent fungi were initially classified as asexual because their mating capacity was hidden behind layers of regulation. For efficient in vitro mating, the essentially obligate diploid ascomycete pathogen Candida albicans has to homozygose its mating type locus from MTLa/α to MTLa/a or MTLα/α, and then undergo an environmentally controlled epigenetic switch from the white form to the mating competent opaque form (white opaque switch). These requirements greatly reduce the potential for C. albicans mating. In this thesis, I used a mutant library to screen genes involved in white to opaque switching in the MTLa/α background, which allowed us to focus on the barriers of mating in C. albicans (Chapter 2 to 5). Construction of the mutant library is described as an appendix. Heterotrimeric G proteins are an important class of eukaryotic signaling molecules that have been identified as central elements in the pheromone response pathways of many fungi. As well, I did genetic studies on G proteins of the mating pheromone pathway in C. albicans (Chapter 6).
Chapter 2 discusses how deletion of the YciI domain gene OFR1 allows the bypass of the need for C. albicans cells to homozygose the mating type locus prior to switching to the opaque form and mating, and allows homothallic mating of MTL heterozygous strains. Transcriptional profiling of ofr1 mutant cells shows that in addition to regulating cell type and mating circuitry, Ofr1 is needed for proper regulation of histone and chitin biosynthesis gene expression. It appears that OFR1 is a key regulator in C. albicans, and functions in part to maintain the cryptic mating phenotype of the pathogen. Chapter 3 describes the gene termed OFR2, which affects the formation of lipid droplets, white opaque switching and mating. Chapter 4 discusses how the different transcripts of ORF19.7060 in white and opaque states play a role in white opaque switching and mating. Chapter 5 describes how the F1-ATPase chaperon ATP12 regulates white opaque switching and carbon metabolism.
In Chapter 6, disruption of the STE18 gene which encodes a potential γ subunit of a heterotrimeric G protein was shown to cause sterility of MTLa mating cells and to block pheromone-induced gene expression and shmoo formation; deletion of just the C-terminal CAAX box residues is sufficient to inactivate Ste18 function in the mating process. Intriguingly, ectopic expression behind the strong ACT1 promoter of either the Gα or the Gβ subunit of the heterotrimeric G protein is able to suppress the mating defect caused by deletion of the Gγ subunit and restore both pheromone-induced gene expression and morphology changes.