Oogenesis in ovipositing females of the microtrombidiid mitePlatytrombidium fasciatum(C.L. Koch) (Acariformes: Microtrombidiidae)

An overview of chelicerate ovaries, with special reference to mites - myths and facts

In arthropods of the subphylum Chelicerata a panoistic ovary, in which all germline cells differentiate into oocytes, prevails. Among the chelicerates, mites are believed to show a great variety of the structure of the female gonads. In general, the knowledge of the ovarian structure in mites is fragmentary and patchy. In both evolutionary lines, Acariformes and Parasitiformes, apart from the panoistic ovary, the meroistic ovary, in which the oocytes grow supported by their sibling cells, the nurse cells, occurs. The presence of the meroistic ovary is considered an apomorphic state. Previous studies revealed a various structure of the meroistic ovary in different mite taxa, and the differences came down, inter alia, to a different number and location of the nurse cells in relation to the oocytes. Here we provide a comprehensive review of the structure of the Chelicerata ovary, with special reference to the mite ovary. We also provide our preliminary results of the analysis of ovarian structure in two representatives of terrestrial Parasitengona (Acariformes), Allothrombium fuliginosum (Trombidiidae) and Erythraeus cinereus (Erythraeidae), performed using light, confocal and electron transmission microscopy. The analyses allowed for verification of data published before. In A. fuliginosum we showed the presence of the nurse cells in the ovarian wall, so the ovary should be classified as meroistic. In meroistic ovary of E. cinereus we found that each oocyte is connected to several mononucleated nurse cells. The verification of literature data and broadening the knowledge of the structure of the female gonad in mites, will result in estimating the usefulness of the ovary traits in phylogenetic analyses and will provide the basis for inference about the directions of evolutionary changes of female gonad at lower systematic levels.

Spectacular alterations in the female reproductive system during the ovarian cycle and adaptations for matrotrophy in chernetid pseudoscorpions (Pseudoscorpiones: Chernetidae)

Pseudoscorpions are small matrotrophic chelicerates. The embryos develop in a brood sac and feed on the nutritive fluid provided by the female. It was widely accepted that the nutritive fluid is synthesized in the ovary. Recent studies have shown that in Chelifer cancroides, a representative of Cheliferidae, considered one of the most derived pseudoscorpion families, the nutritive fluid is produced not only in the ovary but also in the oviducts. Since evolution of adaptations for matrotrophy in pseudoscorpions is poorly known, we aimed to verify our hypothesis that pseudoscorpions of the family Chernetidae, closely related to Cheliferidae, share the traits of adaptations to matrotrophy in the structure and function of the female reproductive system with C. cancroides. We analysed the structure of the ovary and oviducts in five representatives of chernetids with light, confocal, and transmission electron microscopy. The results confirmed our hypothesis and provided new data which broaden our knowledge of matrotrophic pseudoscorpions. We show that in chernetids, the ovary and oviducts undergo significant alterations including their size, multistep hypertrophy and polyploidization of the epithelial cells involved in secretion of the nutritive fluid, the complex secretory activity of the epithelial cells, massive degeneration of the epithelial cells that have completed secretion, and epithelium renewal.

Small body size of pseudoscorpions and a distinct architecture of the ovary: A step to miniaturization?

Chelicerata, the second largest subphylum of Arthropoda, includes invertebrates with a wide range of body size. Pseudoscorpions are among small or miniature chelicerates which exhibit several morphological, anatomical, and developmental features related to miniaturization, e.g., replacement of book lungs by tracheae, unpaired gonads, and matrotrophic development of the embryos outside the female body, in the brood sac. In this paper, we show the ovary structure of two pseudoscorpion species, Cheiridium museorum and Apocheiridium ferum (Cheiridiidae). Both cheiridiids are one of the smallest pseudoscorpions. The results of our observations conducted in light, transmission electron, and confocal microscopy demonstrate that the ovary of C. museorum and A. ferum, displays a significant structural difference that is unusual for chelicerates. The difference concerns the spatially restricted position of the germarium. We show that such ovary architecture results in a significantly reduced number of growing oocytes and in consequence a reduced number of deposited eggs. A centrally located germarium implies also a modified pattern of ovary development during oocyte growth due to long distance migration of the germline and the accompanying somatic cells. Herein, we postulate that such an ovary structure is related to the pseudoscorpion's small body size and it is a step towards miniaturization in the smaller pseudoscorpions species.

Female reproductive system and oogenesis in the mite Bakericheyla chanayi

The fine structure of the female reproductive system of a cheyletid mite Bakericheyla chanayi (Trombidiformes: Cheyletidae) is investigated for the first time. This system consists of an unpaired ovary, glandular oviduct, receptaculum seminis, long cuticle-lined vagina, and genital atrium terminating in the genital opening. A separate sperm access system has not been found. The receptaculum seminis opens into the distal oviduct region, where fertilization apparently takes place. The ovary contains clusters of oogonia (cystocytes), clustered early meiotic cells, a few growing previtellogenic oocytes, and 3 large nurse cells. The dorsal ovarian region is occupied by the clusters of bacteriocytes which harbor symbiotic bacteria. Oocytes undergo vitellogenesis in individual ovarian pouches, each connected to the corresponding nurse cell by an intercellular bridge. The fine structure of the bridge suggests transport between the interconnected cells in the course of vitellogenesis. The population of cystocytes was shown to be heterogenic. The electron-light cells enter meiosis and develop into the oocytes or nurse cells. The electron-dense cystocytes do not show meiotic transformation and probably give rise to the bacteriocytes. The early development of the nurse cells and oocytes is similar and accompanied by the blebbing of the nuclear envelope, appearance of nuage material and Balbiani bodies.

Physiological and molecular mechanisms underlying photoperiodism in the spider mite: comparisons with insects

Photoperiodism is an adaptive, seasonal timing system that enables organisms to coordinate their development and physiology to annual changes in the environment using day length (photoperiod) as a cue. This review summarizes our knowledge of the physiological mechanisms underlying photoperiodism in spider mites. In particular, the two-spotted spider mite Tetranychus urticae is focussed, which has long been used as a model species for studying photoperiodism. Photoperiodism is established by several physiological modules, such as the photoreceptor, photoperiodic time measurement system, counter system, and endocrine effector. It is now clear that retinal photoreception through the ocelli is indispensable for the function of photoperiodism, at least in T. urticae. Visual pigment, which comprised opsin protein and a vitamin A-based pigment, is involved in photoreception. The physiological basis of the photoperiodic time measurement system is still under debate, and we have controversial evidence for the hourglass-based time measurement and the oscillator-based time measurement. Less attention has been centred on the counter system in insects and mites. Mite reproduction is possibly regulated by the ecdysteroid, ponasterone A. Prior physiological knowledge has laid the foundation for the next steps essential for the elucidation of the molecular mechanisms driving photoperiodism.

Regulation of female reproduction in mites: a unifying model for the Acari

It is well established in the literature that circulating high levels of juvenile hormone (JH) are responsible for the initiation of vitellogenesis and female reproduction in most insects studied so far. Exceptions include some Diptera, Lepidoptera and Hymenoptera. The current view is that JH also regulates yolk protein (vitellogenin, Vg) synthesis and female reproduction in mites. However, there is no published evidence that mites have the common insect JHs at any stage of their development. Also, research on the effects of exogenous applications of JH and JH analogs on the reproduction of mites is contradictory. Significant information is available on the life history of mite reproduction, and new information has become available on mite storage proteins including Vg. Although initial studies suggested that ticks may respond to exogenously applied juvenile hormone or anti-JHs, current research shows that ticks cannot synthesize the common insect JHs and have no detectable levels of these hormones in their hemolymph during female reproduction. In ticks, it appears that ecdysteroids, and not JH, regulate expression of the Vg gene and the synthesis and release of Vg protein into the hemolymph. In fact within the Arthropoda, JH has been found only in insects. Methyl farnesoate and not JH regulates Vg synthesis in the Crustacea, the sister group to the insects. Based on this evidence, a new working hypothesis is proposed, i.e., that ecdysteroids and not the JHs regulate vitellogenesis in the Acari including both ticks and mites. To the present, the role of neuropeptides in the regulation of female reproduction in mites is not known.

Comparative midgut ultrastructure of unfed larvae and adult mites of Platytrombidium fasciatum (C.L. Koch, 1836) and Camerotrombidium pexatum (C.L. Koch, 1837) (Acariformes: Microtrombidiidae)

The midgut of unfed larvae and adult mites of Platytrombidium fasciatum (C.L. Koch, 1836) and Camerotrombidium pexatum (C.L. Koch, 1937) (Acariformes: Microtrombidiidae) was investigated by electron microscopy. The sac-like midgut occupies the entire body volume, ends blindly and is not divided into functionally differentiated diverticula or caeca. The midgut walls are composed of one type of digestive cell that greatly varies in shape and size. In larvae, the lumen of the midgut is poorly recognizable and its epithelium is loosely organized, although yolk granules are already utilized. In adults, the midgut forms compartments as a result of deep folds of the midgut walls, and the lumen is well distinguished. The epithelium is composed of flat, prismatic or club-like cells, which may contain nutritional vacuoles and residual bodies in various proportions that depend on digestive stages. In both larvae and adult mites, parts of cells may detach from the epithelium and float within the lumen. The cells contain a system of tubules and vesicles of a trans-Golgi network, whereas the apical surface forms microvilli as well as pinocytotic pits and vesicles. Lysosome-like bodies, lipid inclusions and some amount of glycogen particles are also present in the digestive cells. Spherites (concretions) are not found to be a constant component of the digestive cells and in adult mites occur for the most parts in the midgut lumen.