Insights into ovary micromorphology and oogenesis in Drawida pellucida earthworm (Moniligastridae)

Moniligastrids are regarded as the most primitive group of earthworms; unlike Crassiclitellata, they lack a multilayered clitellum and produce yolky (mesolecithal) eggs, with some peculiarities in their reproductive systems not seen in other earthworms or Clitellata. While Moniligastridae have undergone classical morphological studies on reproductive organ localization and gross morphology, no reports detail ovary histology and ultrastructure. Here, using light and electron microscopy, histochemistry, and DNA barcoding, we analyzed Drawida pellucida from India to describe the ovary micromorphology and follow oogenesis. In the species studied, paired ovaries are feathery with multiple elongated lobes surrounding the intestine in segment XI. No ovarian chamber is present. Proximal lobe parts connect to the septum, housing oogonia and early meiotic cells (till diplotene), united into small germline cysts (≤10 cells). The cysts feature crassiclitellate-like architecture: a few cells interconnected by ring canals to a thin reticular cytophore. Distal regions contain growing oocytes appearing individual, lacking ring canals or cytophore. Notably, no nurse cells were observed, which is surprising as they occur in all studied clitellates to date. We propose to term these ovaries "Drawida" type due to substantial morphological and organizational differences from other earthworms. As oogenesis progresses, oocytes detach from ovarian lobes and gather in large, sac-shaped and irregularly thickened ovisacs, where they finalize yolk uptake. Oocytes are small (∼100 µm diameter), mesolecithal, rich in proteinaceous yolk and lipids. We also provide molecular markers for the studied species and compare the ovary/oogenesis with Crassiclitellata, aiding the debate on moniligastrid phylogeny.