UBC Theses and Dissertations

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UBC Theses and Dissertations

Genetics and cell biology of Chlamydomonas zygote development Kariyawasam, Thamali


Embryogenesis describes the multicellular development from a single fertilized gamete into a stereotypical complex body-plan, consisting of multiple tissues and organs. Plants and animals, only two lineages of eukaryotes, have evolved embryogenesis, questioning their common features that enable the emergence of embryogenesis and its developmental mechanisms. I hypothesize that the zygote, representing the embryo-equivalent life cycle stage, served as fodder to invent molecular mechanisms crucial for the necessary sophistication of embryogenesis. This thesis queries the sexual development of the unicellular alga, Chlamydomonas reinhardtii, focused on a single cell-type, zygote, whose differentiation involves organelle fusion, selective elimination of organellar DNAs, withdrawal of the flagella, and assembly of the zygote-specific walls. To investigate and discover the molecular mechanisms required for Chlamydomonas zygote development, a zygote-defective mutant screening was performed using a genetically engineered self-mating strain. The screening collected 68 zygote-defective mutants (chapters 2 and 3), of which 19 zygotic and two gametic mutants were further investigated for their mutations and developmental defects. The results of this research i) identify the top-tier regulator, GSM1/GSP1 homeodomain protein heterodimer, serving as a primary trigger of the zygote development, and ii) suggest the second-tier regulatory system, Paf1 complex, relaying the GSM1/GSP1 trigger to execute the spore differentiation program. The gsp1 and gsm1 mutants (chapter 4) were further characterized to determine whether the GSM1/GSP1 dyad also controls the merger of genome-containing organelles following gametic fusion, one of the two pivotal events in sexual reproduction. This work documents that the GSM1/GSP1-heterodimeric complex serves as the primary switch to 1) ensure the unified zygotic identity by double-checking the compatibility between two gametes as a lock and key mechanism and 2) trigger the cellular transformation from motile gametes to non-motile multicellular aggregates. Lastly, my thesis contributes to the functional characterization of OTU2p, the sex-linked determinant of the selective CpDNA degradation in the C. reinhardtii zygotes. This work reports that i) OTU2p protects the translocons in the outer chloroplast envelope (TOCs) from proteasome-mediated degradation during nitrogen starvation, and ii) OTU2p prevents excessive recycling of the CpDNA upon phosphate starvation (chapter 5).

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