Dissecting centriole fate in C. elegans development

The presence of centrioles is critical for fundamental cell and developmental processes, including polarity, migration and division. Although centrioles are present in most proliferating cells, they are thought to vanish after cell cycle exit in some differentiated cells. However, the extent to which this is the case in an intact organism and whether cell fate dictates the fate of the centriole, is not known. Moreover, mechanisms governing centriole elimination are scarcely understood.In most metazoans, centrioles are eliminated in the oocyte and maintained in sperm, ensuring precise centriole number control after fertilization. In Drosophila, Polo removal concomitant with loss of pericentriolar material and microtubule-organising center (MTOC) activity was proposed to subsequently lead to centriole elimination. It was unknown whether this mechanism also applies to other species. Here, we dissect the role of Plk1 in the starfish Patiria miniata through live imaging of their oocytes. We find that blocking Plk1 kinase activity does not lead to precocious centriole elimination. In addition, in another starfish species, Asterias forbesi, we observed persistence of centrioles despite loss of MTOC activity. In summary, this work uncovered diversity in the mechanisms of centriole removal across species. The extent to which centriole elimination might be widespread in an intact organism is not known. The stereotypic cell lineage of C. elegans makes it uniquely well suited to investigate this question in a comprehensive manner. Conducting live imaging of centriolar markers using lattice light sheet microscopy, immunofluorescence analysis of fixed specimens stained with centriolar markers, as well as Correlative Light Electron Microscopy, I uncovered that centrioles disappear in most cells upon cell cycle exit during C. elegans embryogenesis. Importantly, by following specific cell lineages, I discovered that centriole fate is stereotyped throughout development. Interestingly, in addition to proliferative and intestinal cells, I found 7 non-proliferating cells that reproducibly retain centrioles in the L1 larval stage. Moreover, fusion of cells that maintained centrioles with cells that lost them could not induce widespread centriole elimination, indicating that a putative centriole elimination activity is not constantly on. Furthermore, through specific alterations of select progenitor cells and differentiated cells, we established that centriole fate follows cell fate. Overall, this work led to the first "centriole elimination map" in a complete organism, setting the stage for uncovering mechanistic insights into centriole elimination.In C. elegans paternally contributed centrioles were shown to be particularly stable over many cell cycles. The unusual long-term stability of paternally contributed centrioles is incompletely under-stood. Here, I show that paternally contributed centrioles remain stable after blocking centriole assembly in the embryo. Furthermore, I show that after the maternal-zygotic transition, new foci containing centriolar markers appear in a reproducible manner, suggesting attempted de novo centriole assembly, despite a block in the canonical centriole assembly pathway. Overall, this provides the foundation for an RNAi-based screen to identify components required for the stability of paternally contributed centrioles. Furthermore, this gives first hints that de novo centriole formation might be occuring in C. elegans.