Morphofunctional evaluation of gametogenesis in the endemic South Atlantic reef-builder Mussismilia harttii

Decline of coral reefs worldwide emphasizes the need for biological information to support species conservation and management strategies. Herein, we thoroughly explored the gametogenesis of the endemic and endangered Brazilian coral Mussismilia harttii. A distinct distribution of male (proximal) and female (distal) regions in the same mesentery confirms the hermaphroditism of M. harttii. Primordial germ cells (PGCs) originated from endoderm, expressed VASA/DDX4, and found the mesoglea as physical support and source of nutrition during gametogenesis. Based on the monthly follow-up study, we found three stages of gamete maturation. In spermatogenesis, the germ cell maturation inside the mesogleal compartment, which forms the stage I cysts, coincides with the co-inhabiting with stage III oocytes. The stage II cysts incorporate new migrating germ cells and hold germ cell differentiation as well. In stage III, spermatozoa are organized in rows and exhibit mature characteristics, such as round-to-oval heads and long flagella. Oogenesis, on the other hand, begins with the differentiation of PGCs into oogonia, which migrate to the mesoglea and give rise to primary oocytes. These oocytes undergo growth and differentiation processes, becoming secondary oocytes. In stage III, mature oocytes exhibit a high concentration of lipid granules, yolk bodies, and disassembly of the nuclear membrane. M. harttii populations have been dramatically reduced in Brazilian waters with high extinction risk. In this regard, our findings shed light on the current knowledge of endemic scleractinian coral reproduction, with potential application to develop biotechnologies, such as in vitro fertilization, aiming to safeguard this endangered coral species.