LAUSR

Abstract Seasonality in reproductive fitness is associated with a variation in testicular size and function in many animal species. These changes might occur because the demands of producing sperm and testosterone are only energetically feasible if reproductive success is likely. Testicular size reduction is also observed as one of the changes associated with aging. It is possible that age-onset and seasonal reduction of testis size require the activity of a common set of proteins and mechanistic pathways. SIRT1 protein is known to promote anti-ageing pathways and prevent apoptosis. Mice deleted for FSTL3, an inhibitor of TGFb ligands, develop significantly enlarged testes that do not regress with age and show increased testicular SIRT1 production. SOX9 is reported to decline with aging but is essential for the maintenance of seminiferous tubule architecture and maintenance of Sertoli cells. This supports a role for increased SIRT1 and SOX9 in maintaining testicular size and prolonging spermatogenesis. The common grey squirrel (Sciurus carolinensis) shows seasonal variation in testicular size and function making it an ideal model for investigating the mechanisms involved in seasonal testicular regulation. We investigated whether seasonal testicular regression has SIRT1 involvement and explored changes in proteins associated with proliferation (PCNA), maintenance of testicular function (SOX9) and apoptosis (Cleaved Caspase 3). Immunohistochemistry, TUNEL staining and western blots were performed on grey squirrel testis samples from different seasons. TUNEL showed significantly higher levels of apoptosis in winter (P=0.02), higher levels of Cleaved Caspase-3 were also seen in winter (P=0.04). The results demonstrate that there is no significant difference in levels of SIRT1 (P=0.05), suggesting a distinct mechanism for seasonal testis size regression to that associated with aging. Interestingly there was significantly higher levels of SOX9 in winter testis (P=0.03), indicating this might have a key role in seasonal regression, this being supported by the observation that loss of SOX9 leads to the irreversible testis regression seen in aging.