A lifespan atlas of the killifish testis defines a mid-age remodeling phase in vertebrate testicular aging

Background: Reproductive aging in vertebrates is commonly interpreted as a gradual decline; however, whether aging of the male gonad proceeds linearly or involves discrete transitions in tissue state remains unclear. The turquoise killifish ( Nothobranchius furzeri ), an exceptionally short-lived vertebrate, enables lifespan-wide resolution of both rapid germline establishment and subsequent aging within months. Results: Our staged histological atlas shows that the testis progresses from a simple gonadal primordium to complete spermatogenesis within three to four weeks post-hatching revealing an exceptionally compressed developmental program largely independent of somatic growth variation. Proliferative activity peaks in early adulthood, marking maximal spermatogenic output, but declines abruptly at mid-age rather than gradually. This inflection point coincides with coordinated repression of germline, mitotic, and meiotic programs and activation of extracellular matrix remodeling, angiogenic, inflammatory, and stromal pathways. These molecular shifts are accompanied by structural remodeling, including expansion of the interstitial compartment and accumulation of collagen-rich matrix. Late-life testes exhibit comparatively modest additional changes, suggesting stabilization of a remodeled, low-proliferative niche. Conclusions: Testicular aging in the turquoise killifish unfolds as a process separated by a discrete mid-life remodeling that links testicular decline to somatic niche remodeling. This defined transition provides a mechanistic entry point for dissecting vertebrate reproductive aging and establishes the killifish as a uniquely powerful model for identifying interventions that preserve germline function.