Genetic analysis of pigment production in the fungus <i>Exophiala dermatitidis</i> mutant strains obtained via nontargeted UV mutagenesis

Exophiala dermatitidis is a polyextremotolerant black yeast species. E. dermatitidis produces 1,8-dihydroxynaphthalene (DHN) melanin via the polyketide synthase 1 (PKS1) pathway enabling it to survive harmful conditions. This study focused on random nontargeted mutagenesis to obtain albino (alb) and hyperpigmented (hyp) mutants. Notably, all 17 alb mutants possessed mutations in PKS1 whereas the 113 hyperpigmented (hyp) mutants harbored mutations impacting a range of functions. Cell morphology and phenotypic assays showed additional differences between the alb and hyp mutants. Strikingly, 3 of the albino mutants (alb1, alb2, and alb3) were conditional in that despite the presence of mutations in PKS1 they were able to produce melanin upon exposure to different carbon sources. These mutants otherwise shared similar cell morphology and growth patterns with the obligate albinos. No additional shared mutations were found among the conditional albinos. Temperature and UV irradiation assays demonstrated reduced growth of albino mutants at higher temperatures (i.e. 42 °C) and a greater sensitivity to higher doses of UV. Single nucleotide variant calling showed that some hyperpigmented mutants had a greater number of single nucleotide variants compared to albino strains. To date, this is the first study to generate and characterize conditional albino mutants in E. dermatitidis with the ability to recover melanin production.