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Comparative Studies of Fungal Dimorphism in Dikarya
Fungi display diverse growth forms. Some grow as unicellular yeasts, some grow as multicellular hyphae, while others switch between these two growth forms, i.e., the dimorphic fungi. Dimorphism is found in many pathogenic fungi, and it is thought to be a strategy to maximize their fitness during different stages of life cycles. The corn smut fungus Ustilago maydis serves as a renowned model organism for studying fungal dimorphism and its role in pathogenesis. However, knowledge only from the model species may not be expanded to other species unless multispecies studies have been demonstrated. In this dissertation, I performed comparative analyses to examine if knowledge from U. maydis is translational to other dimorphic fungi. First, a physiological study was conducted to find what can serve as a common signal for dimorphic transition of several Ustilaginomycotina species. I found that the lipid serves as a potential common cue for yeast-to-hyphal transition in most dimorphic species, while alternate types of energy-source carbohydrate do not affect fungal dimorphism. In addition, pectin and high temperature can also trigger filamentous growth in some Ustilaginomycotina species. Second, I performed comparative transcriptomics to determine if a mechanism for yeast-to-hyphal dimorphic transition is conserved across multiple dimorphic species. Three species of Ustilaginomycotina (U. maydis, Tilletiopsis washingtonensis and Meira miltonrushii) plus one species from Ascomycota (Ophiostoma novo-ulmi) were included in the analyses. I found that the similarity of transcriptomic alteration is not dependent on phylogenetic relatedness. Genes in amino acid transport and metabolism, energy production and conversion and cytoskeleton are commonly altered during the dimorphic transition of all studied species. Moreover, I discovered several core genes which can play a conserved role in transducing signals for the dimorphic transition. Finally, I performed comparative analyses of 190 fungal genomes to determine genomic properties that are associated with types of fungal growth form. I found that small genome size is a characteristic for yeast-like fungi. Few indicator genes, such as genes encoding proteins in the NADPH oxidase complex and cytoskeletons, which are predominantly lost in yeast-like fungi in both Ascomycota and Basidiomycota. However, many other genes are associated with types of growth form in a lineage-specific manner. Findings from this dissertation will serve as fundamentals for future research in fungal cell biology, especially in fungal dimorphism. Additionally, results from this study suggest cautions when extrapolating results from model species onto non-model species.