Understanding the mechanism of sexual development in Chlamydomonas reinhardtii

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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.