Functional divergence of a global regulatory complex governing fungal filamentation

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.形态发生转变在真菌界中广泛存在。对于重要的人类致病真菌白色念珠菌（Candida albicans）而言，在酵母态与菌丝态之间发生形态转换的能力是其致病力的关键所在。而模式生物酿酒酵母（Saccharomyces cerevisiae）的菌丝形成能力，则有助于其获取营养物质。近期在酿酒酵母中开展的一项功能基因组筛选研究，将Mfg1鉴定为形态发生的调控因子，它可与Flo8及Mss11形成复合物，介导对菌丝形成至关重要的转录应答。在白色念珠菌中，Mfg1同样可与Flo8和Mss11发生物理相互作用，且在响应多种信号触发菌丝形成的过程中发挥关键作用，但其调控形态发生的具体分子机制此前仍未明确。本研究探究了对Mfg1、Flo8及Mss11进行基因扰动后对白色念珠菌形态发生造成的影响，并鉴定出该复合物各成员存在功能分化。我们发现，白色念珠菌的Mss11对于菌丝形成并非必需；而即便在Mfg1缺失的情况下，FLO8的过表达也可引发持续性的菌丝形成。通过结合转录组分析与染色质免疫共沉淀联合微阵列分析技术，我们鉴定出白色念珠菌中Flo8与Mfg1的转录靶标存在差异。同时我们证实，Flo8与Mfg1可协同结合菌丝形成关键调控因子的启动子区域，其中包括TEC1；对TEC1进行过表达，即可在Flo8或Mss11缺失的情况下恢复菌丝形成能力。为进一步解析Mfg1调控形态发生的调控网络，我们采用了一种全新的筛选策略，用以分离在Mfg1缺失时仍可恢复菌丝形成的突变株。对具备菌丝形成能力的突变株进行全基因组测序后发现，6号染色体扩增是一种保守的适应性机制。6号染色体扩增的关键决定因素为FLO8：若单等位基因敲除FLO8则会阻断形态发生，且在FLO8易位至其他染色体的进化株系中，并未观察到6号染色体的扩增。综上，本研究揭示了关键形态发生调控因子在进化过程中的重编程现象，并证实非整倍体是驱动真菌形态发生的一种适应性机制。