Ovaries of Tubificinae (Clitellata, Naididae) resemble ovary cords found in Hirudinea (Clitellata)

Ovary micromorphology and oogenesis in a rhyacodriline oligochaete (Clitellata: Naididae, Rhyacodrilinae)

The main goal of the paper is to describe the ovary organization and oogenesis in Peristodrilus montanus, an aquatic oligochaete of the subfamily Rhyacodrilinae. The presented analysis will not only enrich the knowledge about how eggs are formed but, because of the suggested conservatism of ovary organization in clitellate annelids, can contribute to disentangling the complex phylogenetic relationships of the rhyacodrilines within Naididae. The paired, conically shaped ovaries are located in segment XI. They are composed of a dozen or so syncytial germ-line cysts, which are associated with somatic cells. Each germ cell in a cyst has one intercellular bridge that joins it to a central and anuclear cytoplasmic mass, the cytophore. This pattern of cyst organization is typical for all clitellates that have been studied to date. Initially, the germ cells in a cyst undergo a synchronous development, however, there is no synchrony between cysts, and therefore there is a developmental gradient (oogonia, pre-diplotene germ cells, germ cells in diplotene) of oogenesis along the long ovary axis. The cysts are composed of a maximum of 32 cells. Cysts with cells in diplotene detach from the ovaries and the extraovarian phase of oogenesis begins. The developmental synchrony is lost, one cell (an oocyte) per cyst starts to gather cell components and yolk and grows considerably. The remaining cells grow to some extent and function as nurse cells. Like in other microdriles, P. montanus oocytes are rich in yolk; other features of oogenesis are also similar to those that are known from other microdrile taxa. The system of ovary organization found in the studied species is broadly similar to the corresponding features known from Naidinae and Phreodrilidae and, to some extent, in Enchytraeidae. However, this system is different from the one that is known in Tubificinae, Limnodriloidinae and Branchiurinae. This article is protected by copyright. All rights reserved.

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.

Architecture and Life History of Female Germ-Line Cysts in Clitellate Annelids

Animal female and male germ-line cells often form syncytial units termed cysts, clusters, or clones. Within these cysts, the cells remain interconnected by specific cell junctions known as intercellular bridges or ring canals, which enable cytoplasm to be shared and macromolecules and organelles to be exchanged between cells. Numerous analyses have shown that the spatial organization of cysts and their functioning may differ between the sexes and taxa. The vast majority of our knowledge about the formation and functioning of germ-line cysts comes from studies of model species (mainly Drosophila melanogaster); the other systems of the cyst organization and functioning are much less known and are sometimes overlooked. Here, we present the current state of the knowledge of female germ-line cysts in clitellate annelids (Clitellata), which is a monophyletic taxon of segmented worms (Annelida). The organization of germ-line cysts in clitellates differs markedly from that of the fruit fly and vertebrates. In Clitellata, germ cells are not directly connected one to another, but, as a rule, each cell has one ring canal that connects it to an anuclear central cytoplasmic core, a cytophore. Thus, this pattern of cell distribution is similar to the germ-line cysts of Caenorhabditis elegans. The last decade of studies has revealed that although clitellate female germ-line cysts have a strong morphological plasticity, e.g., cysts may contain from 16 to as many as 2500 cells, the oogenesis always shows a meroistic mode, i.e., the interconnected cells take on different fates; a few (sometimes only one) become oocytes, whereas the rest play the role of supporting (nurse) cells and do not continue oogenesis.This is the first comprehensive summary of the current knowledge on the organization and functioning of female germ-line cysts in clitellate annelids.

Germ cell cluster organization and oogenesis in the tardigrade Dactylobiotus parthenogeneticus Bertolani, 1982 (Eutardigrada, Murrayidae)

Germ cell cluster organization and the process of oogenesis in Dactylobiotus parthenogeneticus have been described using transmission electron microscopy and light microscopy. The reproductive system of D. parthenogeneticus is composed of a single, sac-like, meroistic ovary and a single oviduct that opens into the cloaca. Two zones can be distinguished in the ovary: a small germarium that is filled with oogonia and a vitellarium that is filled with germ cell clusters. The germ cell cluster, which has the form of a modified rosette, consists of eight cells that are interconnected by stable cytoplasmic bridges. The cell that has the highest number of stable cytoplasmic bridges (four bridges) finally develops into the oocyte, while the remaining cells become trophocytes. Vitellogenesis of a mixed type occurs in D. parthenogeneticus. One part of the yolk material is produced inside the oocyte (autosynthesis), while the second part is synthesized in the trophocytes and transported to the oocyte through the cytoplasmic bridges. The eggs are covered with two envelopes: a thin vitelline envelope and a three-layered chorion. The surface of the chorion forms small conical processes, the shape of which is characteristic for the species that was examined. In our paper, we present the first report on the rosette type of germ cell clusters in Parachela.

The Ovary of Tubifex tubifex (Clitellata, Naididae, Tubificinae) Is Composed of One, Huge Germ-Line Cyst that Is Enriched with Cytoskeletal Components

Recent studies on the ovary organization and oogenesis in Tubificinae have revealed that their ovaries are small polarized structures that are composed of germ cells in subsequent stages of oogenesis that are associated with somatic cells. In syncytial cysts, as a rule, each germ cell is connected to the central cytoplasmic mass, the cytophore, via only one stable intercellular bridge (ring canal). In this paper we present detailed data about the composition of germ-line cysts in Tubifex tubifex with special emphasis on the occurrence and distribution of the cytoskeletal elements. Using fixed material and live cell imaging techniques, we found that the entire ovary of T. tubifex is composed of only one, huge multicellular germ-line cyst, which may contain up to 2,600 cells. Its architecture is broadly similar to the cysts that are found in other clitellate annelids, i.e. a common, anuclear cytoplasmic mass in the center of the cyst and germ cells that are connected to it via intercellular bridges. The cytophore in the T. tubifex cyst extends along the long axis of the ovary in the form of elongated and branched cytoplasmic strands. Rhodamine-coupled phalloidin staining revealed that the prominent strands of actin filaments occur inside the cytophore. Similar to the cytophore, F-actin strands are branched and they are especially well developed in the middle and outermost parts of the ovary. Microfilaments are also present in the ring canals that connect the germ cells with the cytophore in the narrow end of the ovary. Using TubulinTracker, we found that the microtubules form a prominent network of loosely and evenly distributed tubules inside the cytophore as well as in every germ cell. The well-developed cytoskeletal elements in T. tubifex ovary seem to ensure the integrity of such a huge germ-line cyst of complex (germ cells-ring canals-cytophore) organization. A comparison between the cysts that are described here and other well-known female germ-line cysts is also made.