The Ovary Structure in Terrestrial Parasitengona Mites: The Case of Trombidiidae (Acariformes: Parasitengona)

Species of mites (Chelicerata: Arachnida) show a great variety of structures of the female gonads. In both evolutionary lines, Acariformes and Parasitiformes, the panoistic ovary, in which all germline cysts differentiate into oocytes, and the meroistic ovary, in which the oocytes grow supported by the nurse cells, have been documented. A less pronounced variation in the gonad structure could be expected at lower systematic levels, hence, we ask about the degree of differences within the family that is subordinate to Acariformes and represents the cohort Parasitengona. Based on the members of Trombidiidae (Acariformes: Trombidiformes, Parasitengona, Trombidioidea), we test the hypothesis that the general ovary type is constant at the family level. Our previous research on the female gonad in Allothrombium fuliginosum revealed that the meroistic ovary occurs in these mites. Here, we proceed with a detailed insight into the ovary structure in A. fuliginosum and examine the structure of the female gonad in other members of Trombidiidae, focusing on the following representatives of its nominotypical genus Trombidium: Trombidium brevimanum, Trombidium holosericeum, Trombidium heterotrichum, and Trombidium latum. For all species, studied with light, fluorescence, and transmission electron microscopy, we could confirm the presence of the meroistic ovary that is highly similar with respect to general architecture. The germline cysts show similarities in general morphology and the mode of germline cell differentiation; they consist of a few nurse cells and one oocyte. The connection between the nurse cells and oocytes is maintained by trophic cords that serve for the transport of organelles and macromolecules. Our results confirm the constancy of the structure of the female gonad at the intrageneric level and provide further support for the hypothesis on the lack of differences at the intrafamily level.