Ovaries of Three Eukerria and Two Kerriona Earthworms (Annelida, Crassiclitellata, Ocnerodrilidae) Are Morphologically Different, but Their Microorganization and Oogenesis Are the Same

The ovaries of five barcoded earthworm species (nuclear 18S rRNA and 28S rRNA, and mitochondrial COI and 12S rRNA gene sequences are provided) from the family Ocnerodrilidae have been analyzed using various microscopic techniques. In both genera studied, ovaries are paired and located in the anterior part of segment XIII. Surprisingly, the ovarian morphology observed in Eukerria differs from that found in Kerriona. Ovaries of Eukerria are flattened, of rectangular or triangular shape, with one or two oocyte rows (egg strings) at the posterior part. Ovaries in Kerriona are fan-shaped, with numerous egg strings. Kerriona's ovaries are of the typical shape found in other representatives of the family Ocnerodrilidae and are similar to the ovaries characteristic of megascolecid earthworms. In contrast, Eukerria ovaries resemble those typical for Lumbricidae. This observation is inconsistent with other reports, which suggested that the ovaries in Ocnerodrilidae are fan-shaped and contain numerous egg strings. Despite the differences in morphology, the internal ovary organization and the course of oogenesis are the same in all studied species and resemble those known from other earthworms. The apical ovary part is composed of a solid mass of germline cells enveloped by thin somatic (follicular) cells. Germline cells are interconnected into syncytial cysts, in which each cell has a single intercellular bridge connecting it to the central cytoplasmic mass, the cytophore. Cysts unite oogonia and early meiotic cells. During diplotene, the future oocyte detaches from the cyst, forms microvilli, and gathers nutrients, whereas the remaining nurse cells do not grow and remain connected to the cytophore. Vitellogenic oocytes finally detach from the ovaries and float freely in the segment lumen. They are 80-90 µm in diameter and are regarded as oligolecithal. Nurse cells probably die within the ovaries; they have never been observed within the coelom.