Elucidation of the determinant for orchestration of solo unisexual cycle in an important human fungal pathogen

Abstract

In fungi, the sex-determination program universally directs sexual development and syngamy (the fusion of gametes) that underlies pre-meiotic diploidization. However, the contribution of sex-determination to syngamy-independent sexual cycle, which requires autopolyploidization as an alternative approach to elevate ploidy before meiosis, remains unclear in fungi and other eukaryotes. The human fungal pathogen Cryptococcus neoformans, as a model organism for studying fungal sexual reproduction, can undergo syngamy-dependent bisexual and syngamy-independent solo unisexual reproduction, in which endoreplication is considered to enable pre-meiotic self-diploidization. Here, by characterizing a mutant lacking all the core sex-determination factors, we show that sex-determination plays a central role in bisexual syngamy but is not strictly required for unisexual development and self-diploidization. This implies an unknown circuit, rather than the sex-determination program, for specifically coordinating Cryptococcus unisexual cycle. We reveal that syngamy and self-diploidization are both governed by the Qsp1-directed paracrine system via two regulatory branches, Vea2 and Cqs2. Vea2 directs bisexual syngamy through the sex-determination program; conversely, Cqs2 is dispensable for bisexual syngamy but activates unisexual endoreplication. Through functional profiling of 41 transcription factors documented to regulate Cryptococcus sexual development, we reveal that only Cqs2 can drive and integrate all unisexual phases and ensure the production of meiospore progenies. Furthermore, ChIP-seq analysis together with genetic evaluation indicate that Cqs2 induces unisexual self-diploidization through its direct control of PUM1, whose expression is sufficient to drive autopolyploidization. Therefore, Cqs2 serves as the critical determinant that orchestrates Cryptococcus multistage unisexual cycle that does not strictly require the sexual-determination program.