Department of Fundamental Microbiology, University of Lausanne, Lausanne, Switzerland.
PLoS Biol. 2021 Jan 6;19(1):e3001067. doi: 10.1371/journal.pbio.3001067. eCollection 2021 Jan.
To ensure genome stability, sexually reproducing organisms require that mating brings together exactly 2 haploid gametes and that meiosis occurs only in diploid zygotes. In the fission yeast Schizosaccharomyces pombe, fertilization triggers the Mei3-Pat1-Mei2 signaling cascade, which represses subsequent mating and initiates meiosis. Here, we establish a degron system to specifically degrade proteins postfusion and demonstrate that mating blocks not only safeguard zygote ploidy but also prevent lysis caused by aberrant fusion attempts. Using long-term imaging and flow-cytometry approaches, we identify previously unrecognized and independent roles for Mei3 and Mei2 in zygotes. We show that Mei3 promotes premeiotic S-phase independently of Mei2 and that cell cycle progression is both necessary and sufficient to reduce zygotic mating behaviors. Mei2 not only imposes the meiotic program and promotes the meiotic cycle, but also blocks mating behaviors independently of Mei3 and cell cycle progression. Thus, we find that fungi preserve zygote ploidy and survival by at least 2 mechanisms where the zygotic fate imposed by Mei2 and the cell cycle reentry triggered by Mei3 synergize to prevent zygotic mating.
为了确保基因组的稳定性，有性繁殖的生物需要确保交配时结合了恰好 2 个单倍体配子，并且减数分裂仅发生在二倍体合子中。在裂殖酵母 Schizosaccharomyces pombe 中，受精触发了 Mei3-Pat1-Mei2 信号级联反应，该反应抑制了随后的交配并启动了减数分裂。在这里，我们建立了一种 degron 系统，专门在融合后降解蛋白质，并证明交配不仅阻止了合子的二倍体性，还阻止了因异常融合尝试而导致的裂解。通过长期成像和流式细胞术方法，我们在合子中鉴定了 Mei3 和 Mei2 的以前未被识别的和独立的作用。我们表明，Mei3 独立于 Mei2 促进了减数分裂前 S 期，并且细胞周期进程既是减少合子交配行为所必需的，也是充分的。Mei2 不仅施加减数分裂程序并促进减数分裂周期，而且还独立于 Mei3 和细胞周期进程来阻止交配行为。因此，我们发现真菌通过至少 2 种机制来维持合子的二倍体性和生存能力，其中 Mei2 施加的合子命运和 Mei3 触发的细胞周期重新进入协同作用以防止合子交配。
