Reproduction, Gonad Structure, and Oogenesis in Tardigrades

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.

Microorganization of ovaries and oogenesis of Haplotaxis sp. (Clitellata: Haplotaxidae)

Ovaries of Haplotaxis sp. were studied in active and nonactive states, that is, in a sexually mature specimen and in specimens outside of the reproductive period. Two pairs of ovaries were found in segments XI and XII. Especially in the nonactive state, they were in close contact with copulatory glands. Each ovary was composed of germ cells interconnected with syncytial cysts, which were enveloped by a layer of somatic cells. Within cysts each germ cell had one ring canal connecting it to the common anuclear cytoplasmic mass called a cytophore. During oogenesis clustering germ cells differentiated into nurse cells and oocytes; thus, the oogenesis was recognized as meroistic. Vitellogenic oocytes were detached from the ovaries and continued yolk absorption within the body cavity. Because recent studies have shown the variety of ovaries and germ line cyst organization in clitellates and suggest their evolutionary conservatism at the family or subfamily level, the data presented here can be valid in understanding the phylogenetic relationships among Clitellata. In this context, ovaries found in Haplotaxis sp. resembled those of the "Tubifex" type. "Tubifex" ovaries are characteristic for numerous microdrile taxa (tubificines, limnodriloidines, propappids, lumbriculids, and leech-like branchiobdellids) and can be regarded as the primary character for these Clitellata in which germ-line cysts are formed during early oogenesis. As the family Haplotaxidae is currently considered to be paraphyletic and the species studied here belongs to Haplotaxidae sensu stricto, our results support the close relationship of Haplotaxidae sensu stricto to the clade consisting of Lumbriculidae, Branchiobdellida, and Hirudinida, in which lumbriculids are sister to the latter two.