Postembryonic Development and Morphology of the Reproductive System in the Acoel Hofstenia atroviridis

Reconstructing ancestral reproductive systems is essential for understanding the evolution of bilaterian body plans, yet the origin and development of reproductive organs remain poorly characterized. Here, we investigate postembryonic sexual development in the xenacoelomorph Hofstenia atroviridis, a basal acoel species, to gain insight into early bilaterian reproductive evolution. Individuals were reared from eggs to adulthood, and the ontogeny of reproductive organs was examined using histology and immunohistochemistry with muscle and neural markers. H. atroviridis is a protandrous simultaneous hermaphrodite, with sexual maturity correlated with body size rather than age. The male copulatory system comprises a seminal vesicle, granular vesicle, penis with a penile bulb, and a single copulatory stylet, accompanied by regionally specialized musculature and innervation. In contrast, the female reproductive system consists of paired, asaccate ovaries containing large, follicle cell-bound oocytes and lacks a discrete gonopore or copulatory organ. Fertilization occurs via traumatic insemination, and eggs are likely released through the mouth. Despite the organisms overall morphological simplicity, the male reproductive system exhibits pronounced structural differentiation. These findings suggest that sexual selection acting on a hermaphroditic ancestor may have contributed to the early diversification of bilaterian copulatory organs and establish H. atroviridis as a useful model for studying the evolutionary origins of animal reproductive systems.