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Assessing the Role of Germ Granules in Germline Development and Organismal Aging in C. elegans

Creative Commons 'BY' version 4.0 license

The germline is the totipotent and immortal lineage. In order to protect its developmental potential, the germline must prevent the expression of somatic factors that would lead germ cells to acquire a more differentiated fate. A shared feature of germ cells in different organisms is the presence of dense, non-membrane-bound organelles called germ granules. Germ granules in Caenorhabditis elegans, called P granules, are segregated to the germ lineage in embryos and at most stages of development are concentrated around the outer surface of nuclei. P granules are composed of RNA and RNA-binding proteins including the PGL and GLH proteins, and worms that lack these proteins are sterile, especially at high temperatures. Although much is known about the assembly, composition, and physical properties of P granules, their functions remain unknown. In this thesis, I used genetic analysis, immunostaining and microscopy, and transcript profiling to investigate the role of P granules in the C. elegans germline. To test the effects of eliminating P granules, worms were simultaneously depleted of the four most important P-granule proteins (PGL-1, PGL-3, GLH-1, and GLH-4). As expected from previous genetic analysis, this caused sterility. Unexpectedly, in a fraction of sterile worms, germ cells were observed to express markers of somatic fate. Transcript profiling of dissected P-granule-depleted gonads revealed up-regulation of many genes normally restricted to neuronal tissues and down-regulation of genes involved in germline processes. P granules do not appear to be necessary for larval germline development, and the transcriptomes of larval gonads with or without P granules are nearly identical. These findings support a model in which P granules maintain germ cell totipotency and immortality by preventing somatic development in the germline. This thesis also explored whether expression of a germ cell program, including P granules, in the somatic cells of worms extends their lifespan. This analysis was inspired by a previous study that claimed that long-lived daf-2 mutants use an ectopic germline program in their somatic cells to extend their lifespan. Contrary to what was previously reported, I did not find evidence of an ectopic germ cell program in daf-2 worms and showed that daf-2 mutants do not rely on P granules or a master germline chromatin regulator for their lifespan extension. I also found that worms that are known to express germline proteins in their somatic cells are not long-lived. Taken together, my findings demonstrate a role for P granules in preventing somatic development in the C. elegans germline and argue against the hypothesis that acquisition of a germ cell program in somatic cells increases lifespan.

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