Morphogenetic transitions are prevalent in the fungal kingdom. For a leading human fungal pathogen, Candida albicans, the capacity to transition between yeast and filaments is key for virulence. For the model yeast Saccharomyces cerevisiae, filamentation enables nutrient acquisition. A recent functional genomic screen in S. cerevisiae identified Mfg1 as a regulator of morphogenesis that acts in complex with Flo8 and Mss11 to mediate transcriptional responses crucial for filamentation. In C. albicans, Mfg1 also interacts physically with Flo8 and Mss11 and is critical for filamentation in response to diverse cues, but the mechanisms through which it regulates morphogenesis remained elusive. Here, we explored the consequences of perturbation of Mfg1, Flo8, and Mss11 on C. albicans morphogenesis, and identified functional divergence of complex members. We observed that C. albicans Mss11 was dispensable for filamentation, and that overexpression of FLO8 caused constitutive filamentation even in the absence of Mfg1. Harnessing transcriptional profiling and chromatin immunoprecipitation coupled to microarray analysis, we identified divergence between transcriptional targets of Flo8 and Mfg1 in C. albicans. We also established that Flo8 and Mfg1 cooperatively bind to promoters of key regulators of filamentation, including TEC1, for which overexpression was sufficient to restore filamentation in the absence of Flo8 or Mfg1. To further explore the circuitry through which Mfg1 regulates morphogenesis, we employed a novel strategy to select for mutations that restore filamentation in the absence of Mfg1. Whole genome sequencing of filamentation-competent mutants revealed chromosome 6 amplification as a conserved adaptive mechanism. A key determinant of the chromosome 6 amplification is FLO8, as deletion of one allele blocked morphogenesis, and chromosome 6 was not amplified in evolved lineages for which FLO8 was re-located to a different chromosome. Thus, this work highlights rewiring of key morphogenetic regulators over evolutionary time and aneuploidy as an adaptive mechanism driving fungal morphogenesis.
Bibliographical noteFunding Information:
EJP, AOV and JLX are supported by Canadian Institutes of Health Research Frederick Banting and Charles Best Canada Graduate Scholarships. EJP and SHK are supported by Ontario Graduate Scholarships. AS is supported by LB692-Nebraska Tobacco Settlement Biomedical Research Development New Initiative Grant and Nebraska's Established Program to Stimulate Competitive Research (EPSCoR) First Award. A-CG was supported by a CIHR Foundation grant (FDN-143301). Proteomics work performed at the Network Biology Collaborative Centre at the Lunenfeld-Tanenbaum Research Institute was supported by Canada Foundation for Innovation funding, by the Ontarian Government and by Genome Canada and Ontario Genomics (OGI-139). A-CG is the Canada Research Chair (Tier 1) in Functional Proteomics. LEC is supported by the Canadian Institutes of Health Research (CIHR) Foundation Grant (FDN-154288) and the Natural Sciences and Engineering Council (NSERC) of Canada Discovery Grants (06261 and 462167). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. We thank Kristi Papamihali for help with strain construction, and all the members of the Cowen lab for helpful discussions.