The ovary structure and oogenesis in Hydrachnidia (Acariformes: Parasitengona).

内容摘要

Although our understanding of ovary structure in mites is limited, it has been shown that the female gonads in these arachnids are highly diverse compared with other chelicerates. Unlike other chelicerates, which typically have a panoistic ovary with a relatively consistent architecture, mites also possess a meroistic ovary. In the panoistic ovary, germline cells differentiate into oocytes, whereas in the meroistic ovary, the germline cells differentiate into oocytes and their accompanying nurse cells. Both types of ovaries have been observed in the two mite lineages, Acariformes and Parasitiformes. Hydrachnidia (Acariformes), also known as water mites, are one of the largest groups of freshwater arthropods. However, despite their common occurrence, they are less explored compared with freshwater insects and crustaceans. Data on the structure of the ovary in water mites are scarce and originate from the 20 c. The aim of the study is to analyze the ovary structure of adult females representing five families of water mites: Arrenuridae, Hydrachnidae, Hydrodromidae, Pionidae, and Limnesiidae, using light, fluorescence, and transmission electron microscopy. Serial block-face scanning electron microscopy technique was applied for 3D reconstruction of the germline cysts. Our results provide new data on the structure of the ovary in water mites, challenging earlier literature data on the panoistic ovary in this group. In all species examined, the presence of a meroistic ovary has been confirmed. The ovary contains a number of germline cysts composed of several germline cells diversified into one oocyte and a few nurse cells. Within the cysts, each cell (oocyte and nurse cells) is connected by a cytoplasmic bridge to a central cytoplasm. The oocytes start previtellogenesis and protrude on the ovary surface connected to the latter by long oocyte stalks. The nurse cells remain in the ovary wall; their nuclei are highly branched and polyploid. The nurse cells provide macromolecules and organelles to oocytes transferred by trophic cords in a microtubular-dependent manner. The results of our study indicate that the overall architecture of the ovary, the structure of the germline cysts, the pattern of germline cell differentiation, and the trophic support remain consistent across Hydrachnidia.