Comparative Oogenesis in Annual and Non-annual African Killifishes: Oocyte Staging in the Fastest-Aging Vertebrate, Nothobranchius furzeri

Annual killifishes have evolved specialized reproductive strategies that allow their embryos to survive the seasonal desiccation of ephemeral habitats, yet little is known about how these adaptations arise during oogenesis. We hypothesized that traits contributing to embryonic desiccation resistance may originate from modifications in the follicular wall and egg envelope during oocyte development. Here, we provide a comparative morphological and morphometric characterization of oogenesis in the annual African turquoise killifish Nothobranchius furzeri and the non-annual lyretail killifish Aphyosemion australe, from early oocyte development through Primary Growth (PG), Secondary Growth (SG), Maturation, and Ovulation. Despite their contrasting life histories, both species exhibited a highly conserved sequence of oocyte development. Cortical alveoli accumulated before oil droplets during PG, whereas SG was characterized by the progressive coalescence of yolk globules into a homogeneous fluid yolk mass and the peripheral displacement of oil droplets and cortical alveoli. Oocyte diameter did not differ significantly between species at any developmental stage, and the follicular epithelium followed a similar developmental pattern, suggesting that its basic morphological organization is conserved despite contrasting reproductive ecologies. In contrast, greater interspecific divergence was observed in the zona pellucida, which became significantly thicker in N. furzeri from early SG through Maturation and exhibited distinct patterns of chorionic ornamentation between species. These findings reveal a largely conserved program of oogenesis and follicular epithelial differentiation in annual and non-annual African killifishes, while highlighting greater morphological divergence in the egg envelope. This suggests that different components of the ovarian follicle may exhibit distinct degrees of evolutionary plasticity, with modifications of the zona pellucida potentially contributing to embryo-environment interactions and desiccation resistance in annual species.