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

Abstract

38In fungi, the sex-determination program universally directs sexual development and 39 syngamy (the fusion of gametes) that underlies pre-meiotic diploidization. However, 40 the contribution of sex-determination to syngamy-independent sexual cycle, which 41 requires autopolyploidization as an alternative approach to elevate ploidy before 42 meiosis, remains unclear in fungi and other eukaryotes. The human fungal pathogen 43 Cryptococcus neoformans, as a model organism for studying fungal sexual 44 reproduction, can undergo syngamy-dependent bisexual and syngamy-independent 45 solo unisexual reproduction, in which endoreplication is considered to enable pre-46 meiotic self-diploidization. Here, by characterizing a mutant lacking all the core sex-47 determination factors, we show that sex-determination plays a central role in bisexual 48 syngamy but is not strictly required for unisexual development and self-diploidization. 49 This implies an unknown circuit, rather than the sex-determination program, for 50 specifically coordinating Cryptococcus unisexual cycle. We reveal that syngamy and 51 self-diploidization are both governed by the Qsp1-directed paracrine system via two 52 regulatory branches, Vea2 and Cqs2. Vea2 directs bisexual syngamy through the sex-53 determination program; conversely, Cqs2 is dispensable for bisexual syngamy but 54 activates unisexual endoreplication. Through functional profiling of 41 transcription 55 factors documented to regulate Cryptococcus sexual development, we reveal that only 56 Cqs2 can drive and integrate all unisexual phases and ensure the production of 57 meiospore progenies. Furthermore, ChIP-seq analysis together with genetic evaluation 58 indicate that Cqs2 induces unisexual self-diploidization through its direct control of 59 PUM1, whose expression is sufficient to drive autopolyploidization. Therefore, Cqs2 60 serves as the critical determinant that orchestrates Cryptococcus multistage unisexual 61 cycle that does not strictly require the sexual-determination program. 62 63 64 65 66 67 68 69 70 71 Introduction 72A sex-determination system is used for the development of sexual characteristics in 73 evolutionally diverse eukaryotes, including fungi [1]. In fungi, the sex-determination 74 system universally directs syngamy (the fusion of gametes to enable pre-meiotic 75 diploidization) and sexual differentiation events during either heterothallic or 76 homothallic mating [2, 3]. However, syngamy does not always underlie sexual 77 development and the two processes can be unconnected in many eukaryotes [4]; the 78 contribution of sex-determination to the fitness related to syngamy-independent solo 79 sexual cycle has yet to be investigated in fungi or other eukaryotes. Cryptococcus 80 neoformans is a model organism for studying fungal sexual reproduction. This fungus 81 is a prevalent human fungal pathogen causing more than 200,000 new human infections 82 annually [5], and sex is hypothesized to contribute to its infections through meiosis-83 created lineage adv...