Cryptococcus neoformans is a basidiomycetous yeast capable of causing fatal meningoencephalitis in immunocompromised individuals. Naturally found worldwide in association with pigeon droppings, C. neoformans produces melanin pigments that play a protective role in both natural and physiological settings. Significant variation in the level of melanin production has been observed among natural populations of C. neoformans, although the factors behind this phenotypic variation remain unclear. In my project, I quantified the level of melanin production in 54 strains of C. neoformans and characterized the change in melanin response to three common stressors encountered by C. neoformans in natural and clinical environments: thermal, oxidative and nitrosative stresses. Using this data, I assessed the contribution of genetic factors, environmental factors and genotype-environmental interactions to the observed variance in melanin. The analysis revealed that over 50% of the variance in melanin was attributable to genetic differences among the strains. I identified three single nucleotide polymorphisms (SNPs) in the LAC1 gene, which is essential for melanin synthesis, to be significantly associated with melanin production in thermal and oxidative stresses. One of the SNPs is predicted to cause an amino acid change (P182A/T) in a putative catalytic domain of the Lac1 protein. The genotype-environment interactions were also found to contribute to a significant proportion of the variance in phenotype. In the majority of tested stress conditions, environmental factors only contributed to a small amount of variance ranging from 5-15%. These results highlight the complex interplay of genetic, environmental and interaction factors that contribute to an observable phenotype. C. neoformans is a model fungal pathogen whose ability to produce melanin is a well-established virulence factor. The results from this project contribute to our understanding of the evolutionary trajectory and regulation of melanin production in C. neoformans. / Thesis / Master of Science (MSc) / The yeast, Cryptococcus neoformans, is naturally found worldwide in soil and pigeon droppings, and causes fatal infections in people with a weakened immune system. It produces dark melanin pigments that aid the yeast in neutralizing the immune response of the infected host. My project sought to characterize the variation in melanin levels found among natural populations of C. neoformans and to determine the factors that contribute to this variation. I identified genetic differences between the strains to be the biggest cause of variation in melanin levels. Changes in the LAC1 gene which is essential for melanin synthesis, were found to shape the melanin production significantly when the yeast is exposed to stressful conditions. Interactions between genes and the environment were also revealed to be a significant contribution to the observed differences in melanin levels. These results highlight the complex interplay of factors that shape the expression of observable traits.