The reference strain SC5314 contains a rare, dominant allele of the transcription factor Rob1 that modulates filamentation, biofilm formation, and oral commensalism.

is a commensal fungus that colonizes the human oral cavity and gastrointestinal tract but also causes mucosal as well as invasive disease. The expression of virulence traits in clinical isolates is heterogeneous and the genetic basis of this heterogeneity is of high interest. The reference strain SC5314 is highly invasive and expresses robust filamentation and biofilm formation relative to many other clinical isolates. Here, we show that SC5314 derivatives are heterozygous for the transcription factor Rob1 and contain an allele with a rare gain-of-function SNP that drives filamentation, biofilm formation, and virulence in a model of oropharyngeal candidiasis. These findings explain, in part, the outlier phenotype of the reference strain and highlight the role heterozygosity plays in the strain-to-strain variation of diploid fungal pathogens.