The Drosophila Male Germline During DNA Damage and Aging

Abstract

Evolution is thought to drive the progression of populations, conferring advantages through alterations in structure and form. However, the corollary to this theory would dictate that the things that do not change, those which are evolutionarily conserved, would imply some importance to their existence. Though many examples of this type of conserved phenomena have been observed, the underlying purposes for their existence remain poorly explored or understood. Two ubiquitous features of germ cells in nearly all metazoans are their development as a cyst of interconnected cells and the relative sensitivity of the germline to DNA damage. We show that in the Drosophila male germline, cysts of interconnected spermatogonia always die in unison even when only a subset of the cells within display cytologically detectable DNA damage. Our experiments showed that this all-or-none germ cell death depends upon the connectivity between members of a cyst, and is likely based on mitochondrial signals originating from the damaged cells. Interestingly, the relative sensitivity of spermatogonia at any given stage of development correlated to the number of interconnected germ cells contained within the cyst, suggesting that degree of connectivity dictates the robustness with which spermatogonia induce germ cell death in response to insult. This created a model where we propose that perhaps one reason germ cell connectivity has been so strongly conserved is to confer a robust quality control mechanism to germ cells, ensuring the fidelity of genomes that are passed on to the next generation. Another general feature of nearly all eukaryotic genomes is the organization of the ribosomal RNA genes into highly-transcribed cistrons and large tandem arrays known as the rDNA. In yeast, instability of the rDNA due to its arrangement has been shown to play a central role in replicative aging, but little was known about its role in higher eukaryotes. We speculated that if similar instability would manifest during aging of multicellular organisms, it would be likely to occur in long-lived, mitotically-active tissue stem cells. We show that germline stem cells exhibit ectopic activation of normally-silent rDNA loci during aging, and germ cells experience a dramatic reduction of rRNA gene copy number on the actively transcribed array. Furthermore, these phenotypes present in the old parents (from abnormal rDNA activation to reduced gene copy number) are heritable and able to be observed in the subsequent generation. Thus our work suggests that there may be a conserved role for dynamicity of the rDNA and rDNA instability during aging. The work contained in this dissertation attempts to reconcile recurring themes observed in evolution by leveraging the powerful tools of the Drosophila model system. The results in the first part offer one possible explanation for why pre-meiotic germ cells are connected to one another, linking it to their well-documented sensitivity to DNA damage and speculating that the driving force behind it all is a desire to increase genomic quality control for gametes. The second half of this dissertation looks at a unique genomic element shared by most eukaryotes, the rDNA, and suggests that this remarkable conservation underlies a more fundamental role in replicative aging in multicellular organisms.