Project Summary Age-related diseases are among the leading causes of morbidity, mortality, and healthcare expenditure both in the United States and the world at large. Age-associated declines in fertility are one of the earliest and most common symptoms of aging in both men and women, and almost one-fourth of couples between the ages of 18 and 45 suffer from impaired fertility or infertility. Although assisted reproductive technology can help couples with impaired fertility produce viable offspring, it can be very expensive and does not treat the root of the problem: germline aging. Existing literature on aging is relatively sparse, focusing on conventional model organisms with relatively short lifespans and how various treatments can slow their decline rather than prevent it entirely, or even reverse aging. The negligibly-senescent, highly-regenerative, hermaphroditic planarian S. mediterranea presents a unique model system to study germline aging because it is one of the few species to actively reverse an aging phenotype. I have observed that S. mediterranea experiences an age-related decline in its fertility that can be reversed by bisecting the planarians and allowing them to regenerate. The overall goal of my project is to determine how aging impairs fertility in S. mediterranea, and how regeneration overcomes this impairment. Based on preliminary data, I hypothesize that age-associated molecular damage accumulates in late gonadal precursor cells of the testes and (especially) ovaries, compromising the production of mature germ cells, and that during regeneration these impaired cells are replaced by functional equivalents to restore fertility. To evaluate this hypothesis, I will seek to address the following specific aims. Aim 1: I will assay changes in markers of gametogenesis to determine how germ and niche cell population are affected by aging and rejuvenation. Aim 2: I will use a panel of molecular biomarkers on isolated gonads to determine if age-associated biomarkers accumulate in the gonads over time, and if regeneration restores the gonads to a more youthful state. Aim 3: I will use selective mating experiments to determine if impairments in fertility are due to defects in the testes, ovaries, or both. From these experiments, I will learn more about the cellular, molecular, and physiological causes of germline aging in S. mediterranea, and how regeneration manages to reverse them. These results will also help us understand why other organisms are unable to prevent age-related gonadal degeneration, and potentially lay the groundwork for more robust fertility treatments that extend or even restore fertility in humans.