Understanding the mechanism of sexual development in Chlamydomonas reinhardtii

性发育是植物的主要生殖策略，直接影响着谷物和豆类的产量。在植物性发育过程中，花粉将雄性配子传递给卵细胞，受精后的雌性配子发育成胚胎和胚乳，胚乳成熟为休眠种子。了解受精后发育的机制是植物生物学家和农业部门的一个关键目标。对单细胞绿藻莱茵衣藻的开创性研究已经确定了配子细胞融合和随后合子发育的关键因素。衣藻的合子发育是由两个大小相等的性配子启动的，它们的融合结合了两个同源盒转录因子Gsm1和Gsp1。我们最近的研究表明，Gsm1/Gsp1异源二聚体启动约94%的早期合子特异性基因的转录，并触发受精后发育，包括合子壁组装和亲本特异性细胞器dna的消除，从而产生类似于植物种子的休眠合子孢子。Gsm1/Gsp1的植物同源性是苔藓小壶（Physcomitrella patens）单倍体向二倍体转变所必需和充分的，这表明衣藻与植物性发育相关分子的功能同源性。因此，衣藻代表了一个很好的模式系统，为遗传解剖保守的性发育调节机制。为了研究衣藻性发育的分子机制，我们的研究项目采用了三种不同的方法。首先，我们设计了一种自我交配的菌株，它很容易从克隆细胞群体中产生纯合子二倍体。利用自交菌株，我们确定了发育过渡和合子分化的关键因素。其次，我们研究了存在于决定性别的MTL+（交配型位点+）上的难以捉摸的因素，这些因素控制着叶绿体DNA的单代遗传（UPI）。我们发现了一个MTL+特异性基因，用于保护正cpDNA免受降解。第三，我们利用基于游动的报告系统探测氮(N)饥饿下激活的转录，并确定了感知和响应细胞N状态的关键因子。这些方法的结果为配子和受精卵两种细胞分化程序的基因调控机制提供了遗传学证据。在这个提案中，我提出了一个研究计划，包括五个短期项目的调查：1。PAF1复合体如何确保受精卵分化。2. RNAi系统在单倍体向二倍体转变中的作用。3. 叶绿体DNA单倍体消除的分子机制。4. N饥饿诱导转录的信号转导机制。

Sexual development is the primary reproductive strategy of plants, governing the processes directly affecting the yields of grains and beans. During plant sexual development, pollens deliver male gametes to egg cells, and fertilized female gametes develop into embryo and endosperm, which maturate into a dormant seed. Understanding the mechanisms of post-fertilization development is a key objective of plant biologists and agricultural sectors. Pioneering studies in the unicellular green alga, Chlamydomonas reinhardtii, have identified critical factors for gametic cell fusion and subsequent zygote development. Chlamydomonas zygote development is initiated by two sexual gametes of equal size, whose fusion combines two homeobox transcription factors, Gsm1 and Gsp1. Our recent study shows that the Gsm1/Gsp1 heterodimer initiates transcription of ~94% of the early zygote-specific genes and trigger post-fertilization development, including zygotic wall assembly and parent-specific elimination of organelle DNAs, consequently leading to produce dormant zygospores resembling the plant seeds. Plant homologs of Gsm1/Gsp1 are required and sufficient for the haploid-to-diploid transition in the moss, Physcomitrella patens, indicating functional homology of the molecules involved in sexual development between Chlamydomonas and plants. Thus, Chlamydomonas represents an excellent model system for genetic dissection of the conserved regulatory mechanisms of sexual development. To investigate the molecular mechanisms of sexual development in Chlamydomonas, our research program employed three different approaches. First, we engineered a self-mating strain that readily produces homozygous diploids from a clonal cell population. Using the self-mating strain, we identified critical factors for developmental transition and zygote differentiation. Second, we investigated elusive factors residing on the sex-determining MTL+ (Mating Type Locus plus), controlling the uniparental inheritance (UPI) of chloroplast DNA. We identified an MTL+ specific gene required for protecting the plus cpDNA from degradation. Third, we used a swimming-based reporter system probing the transcription activated under Nitrogen (N) starvation and identified crucial factors sensing and responding to the cellular N status. The results from these approaches provide genetic evidence for the gene regulatory mechanisms governing two cellular differentiation programs for gametes and zygotes. In this proposal, I propose a research program with five short-term projects investigating: 1. How PAF1 complex ensures the zygote differentiation. 2. The roles of the RNAi system in the haploid-to-diploid transition. 3. The molecular mechanism governing the uniparental elimination of chloroplast DNA. 4. The signal transduction mechanisms for the N starvation-induced transcription.