Germ cell cluster organization and oogenesis in Milnesium inceptum Morek, Suzuki, Schill, Georgiev, Yankova, Marley & Michalczyk, 2019 (Eutardigrada: Apochela)

Abstract This study examines the structure of female germ cell clusters and the process of oogenesis in the facultatively parthenogenetic tardigrade Milnesium inceptum Morek, Suzuki, Schill, Georgiev, Yankova, Marley & Michalczyk, 2019 (Eutardigrada, Apochela) using light microscopy, transmission electron microscopy (TEM), and scanning electron microscopy (SEM), as well as 3D reconstructions obtained via serial block-face scanning electron microscopy (SBEM). The meroistic ovary of Mil. inceptum is an unpaired, sac-like structure. It consists of a small germarium with oogonia and a large vitellarium containing germ cell clusters. Each cluster consists of five multinuclear cells interconnected by cytoplasmic bridges and numerous mononuclear cells attached to them. The oogenesis of this species is divided into three phases: previtellogenesis, vitellogenesis, and choriogenesis. The yolk is formed by authosynthesis within both oocytes and trophocytes. The egg capsule consists of two envelopes: a vitelline envelope and a three-layered chorion. The obtained results show that the germ cell cluster organization of Mil. inceptum differs from that described in tardigrades belonging to the order Parachela. These findings provide a deeper understanding of the reproductive biology of tardigrades.