Aging and sperm signals alter DNA break formation and repair in the C. elegans germline

Female reproductive aging is associated with decreased oocyte quality and fertility. The nematode Caenorhabditis elegans is a powerful system for understanding the biology of aging and exhibits age-related reproductive defects that are analogous to those observed in many mammals, including dysregulation of DNA repair. C. elegans germline function is influenced simultaneously by both reproductive aging and signals triggered by limited supplies of sperm, which are depleted over chronological time. To delineate the causes of DNA repair defects in aged C. elegans germlines, we assessed both DNA double strand break (DSB) induction and repair during meiotic prophase I progression in aged germlines which were depleted of self-sperm, mated, or never exposed to sperm. We find that germline DSB induction is dramatically reduced only in hermaphrodites which have exhausted their endogenous sperm, suggesting that a signal due specifically to sperm depletion downregulates DSB formation. We also find that DSB repair is delayed in aged germlines regardless of whether hermaphrodites had either a reduction in sperm supply or an inability to endogenously produce sperm. These results demonstrate that in contrast to DSB induction, DSB repair defects are a feature of C. elegans reproductive aging independent of sperm presence. Finally, we demonstrate that the E2 ubiquitin-conjugating enzyme variant UEV-2 is required for efficient DSB repair specifically in young germlines, implicating UEV-2 in the regulation of DNA repair during reproductive aging. In summary, our study demonstrates that DNA repair defects are a feature of C. elegans reproductive aging and uncovers parallel mechanisms regulating efficient DSB formation in the germline. Author summary Aging leads to a decline in the quality of the female reproductive cells, known as oocytes. Oocytes subjected to reproductive aging experience an increase in both infertility and aneuploidies that cause miscarriages and birth defects. The nematode Caenorhabditis elegans is a classic model system used to determine the mechanisms of aging. Old C. elegans oocytes accrue many defects which may contribute to their reduced quality, including dysregulation of DNA repair. C. elegans fertility and germline function is also regulated oocyte-independently by sperm-dependent signals. To determine how aging and sperm may independently impact DNA repair in aging C. elegans oocytes, we control oocyte aging and sperm presence independently to evaluate their effects on DNA break formation and repair. We find that running out of sperm reduces the levels of DNA breaks which are produced, but the efficiency of DNA repair declines during aging independent of sperm effects. We also identify a protein which specifically promotes DNA repair in the oocytes of young animals, suggesting that this protein may regulate DNA repair in the germline during aging. Taken together, our research defines aging-specific and aging-independent mechanisms which regulate the genome integrity of oocytes.