Ultrastructure of Oogenesis in Actinarctus doryphorus (Heterotardigrada, Arthrotardigrada, Tanarctidae)

Ultrastructural studies of oogenesis in Heterotardigrada are limited. Here, we examined the ovarian organization of Actinarctus doryphorus using transmission electron microscopy and three-dimensional reconstruction. Four female specimens at different developmental stages, including a juvenile, were analyzed. The ovary is a single sac-like structure dorsally above the digestive tract, partially supported by longitudinal dorsal muscles. Early germ cells form anterior clusters interconnected by cytoplasmic bridges. Previtellogenic oocytes contain numerous mitochondria clustered around a prominent nuage, suggesting a conserved role in piRNA-mediated genome protection. Vitellogenic oocytes occupy the posterior half of the ovary, with the posterior-most cell developing as the egg and the remaining cells functioning as nurse cells. Nurse cells form a syncytium, and ER-rich accessory cells at the periphery likely contribute to yolk and material transfer. Large aggregations of lysosome-rich vesicles in peripheral ovarian cells suggest apoptotic degeneration of trophocytes and ER-rich cells after completing their function. These findings reveal that A. doryphorus exhibits polytrophic meroistic oogenesis with syncytial nurse cells, ER-rich accessory cells, and nuage-containing previtellogenic oocytes. This study provides the first comprehensive description of oogenesis in a heterotardigrade and highlights conserved mechanisms for cytoplasmic and genomic support during gametogenesis, suggesting avenues for exploring the evolutionary diversification of reproductive strategies in tardigrades.