The paired ovaries of E. albidus are like a bunch of grapes and are composed of clearly separated units, syncytial germ cysts (clusters), which are surrounded by a thin layer of somatic cells. Each cyst maintains the connection with the ovary by an extended stalk that is composed of somatic cells. The spatial architecture of the germ-line cysts found in E. albidus is the same as in other clitellate annelids that have been studied to date. As a rule, germ cells are located at the cyst periphery and each has only one ring canal that connects it to the common and centrally located cytoplasmic mass, the cytophore. Here we present data about the F-actin and microtubular cytoskeleton and some molecular components of the germ-line cysts. We show that the ring canals have an inner rim that is enriched with microfilaments and proteins that contain phosphotyrosine. The microtubules form a loose network in the cytoplasm of the oocyte and nurse cells; moreover, some of them pass through the ring canals to the cytophore. Numerous microtubules are also located in the somatic cells. The germ-line cysts in E. albidus ovaries consist of 16 cells, which is the lowest known number of interconnected germ cells within clitellate annelids. During oogenesis, the fate of interconnected germ cells differentiates and only one cell develops as the future egg, while the other 15 become nurse cells. This differentiation means ovary meroism. The nurse cells gather cell organelles and storage material that then pass through the ring canals and cytophore moving towards the growing oocyte. At the end of oogenesis, the vitellogenic oocyte surrounds the siblings' cells together with the cytophore and engulfs their remnants into the ooplasm. No morphological or molecular markers of the apoptosis of the nurse cells were found. Moreover, the nurse cells did not undergo polyploidisation. The measured DNA level was 4C, which indicates that these cells are not highly-specialised.

Animal germ cells tend to form clonal groups known as clusters or cysts. Germ cells within the cyst (cystocytes) are interconnected by intercellular bridges and thus constitute a syncytium. Our knowledge of the mechanisms that control the formation of germ-cell clusters comes from extensive studies carried on model organisms (Drosophila, Xenopus). Germ-cell clusters have also been described in worms (annelids, flat worms and nematodes), although their architecture differs significantly from that known in arthropods or vertebrates. Their peculiar feature is the presence of a central anucleate cytoplasmic core (cytophore, rachis) around which the cystocytes are clustered. Each cystocyte in such a cluster always has one intercellular bridge connecting it to the central cytoplasmic core. The way that such clusters are formed has remained a riddle for decades. By means of light, fluorescence and electron microscopy, we have analysed the formation and architecture of cystocyte clusters during early stages of spermatogenesis and oogenesis in a few species belonging to clitellate (oligochaetous) annelids. Our data indicate that the appearance of germ cells connected via a central cytophore is accompanied by a specific orientation of the mitotic spindles during cystocyte divisions. Spindle long axes are always oriented tangentially to the surface of the cytophore. In consequence, cystocytes divide perpendicularly to the plane of the existing intercellular bridge. Towards the final stages of cytokinesis, the contractile ring of the cleavage furrow merges with the rim of the intercellular bridge that connects the dividing cystocyte with the cytophore and forces partition of the existing bridge into two new bridges.

The ultrastructure of the ovaries and oogenesis was studied in three species of three genera of Tubificinae. The paired ovaries are small, conically shaped structures, connected to the intersegmental septum between segments X and XI by their narrow end. The ovaries are composed of syncytial cysts of germ cells interconnected by stable cytoplasmic bridges (ring canals) and surrounded by follicular cells. The architecture of the germ-line cysts is exactly the same as in all clitellate annelids studied to date, i.e. each cell in a cyst has only one ring canal connecting it to the central, anuclear cytoplasmic mass, the cytophore. The ovaries found in all of the species studied seem to be meroistic, i.e. the ultimate fate of germ cells within a cyst is different, and the majority of cells withdraw from meiosis and become nurse cells; the rest continue meiosis, gather macromolecules, cell organelles and storage material, and become oocytes. The ovaries are polarized; their narrow end contains mitotically dividing oogonia and germ cells entering the meiosis prophase; whereas within the middle and basal parts, nurse cells, a prominent cytophore and growing oocytes occur. During late previtellogenesis/early vitellogenesis, the oocytes detach from the cytophore and float in the coelom; they are usually enveloped by the peritoneal epithelium and associated with blood vessels. Generally, the organization of ovaries in all of the Tubificinae species studied resembles the polarized ovary cords found within the ovisacs of some Euhirudinea. The organization of ovaries and the course of oogenesis between the genera studied and other clitellate annelids are compared. Finally, it is suggested that germ-line cysts formation and the meroistic mode of oogenesis may be a primary character for all Clitellata.

In Hirudo medicinalis and Haemopis sanguisuga, two convoluted ovary cords are found within each ovary. Each ovary cord is a polarized structure composed of germ cells (oogonia, developing oocytes, nurse cells) and somatic cells (apical cell, follicular cells). One end of the ovary cord is club-shaped and comprises one huge apical cell, numerous oogonia, and small cysts (clusters) of interconnected germ cells. The main part of the cord contains fully developed cysts composed of numerous nurse cells connected via intercellular bridges with the cytophore, which in turn is connected by a cytoplasmic bridge with the growing oocyte. The opposite end of the cord degenerates. Cord integrity is ensured by Xattened follicular cells enveloping the cord; moreover, inside the cord, some follicular cells (internal follicular cells) are distributed among germ cells. As oogenesis progresses, the growing oocytes gradually protrude into the ovary lumen; as a result, fully developed oocytes arrested in meiotic metaphase I Xoat freely in the ovary lumen. This paper describes the successive stages of oogenesis of H. medicinalis in detail. Ovary organization in Hirudinea was classiWed within four diVerent types: nonpolarized ovary cords were found in glossiphoniids, egg follicles were described in piscicolids, ovarian bodies were found characteristic for erpobdellids, and polarized ovary cords in hirudiniforms. Ovaries with polarized structures equipped with apical cell (i.e. polarized ovary cords and ovarian bodies) (as found in arhynchobdellids) are considered as primary for Hirudinea while non-polarized ovary cords and the occurrence of egg follicles (rhynchobdellids) represent derived condition.

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Piscicola has a pair of elongated sac-shaped ovaries. Inside the ovaries are numerous small somatic cells and regularly spherical egg follicles. Each follicle is composed of three types of cells: many (average 30) germ cells (cystocytes) interconnected by intercellular bridges in clones (cysts), one intermediate cell, and three to five outer follicle cells (envelope cells). Each germ cell in a clone has one intercellular bridge connecting it to the central anucleate cytoplasmic mass, the cytophore. Each cluster of germ cells is completely embedded inside a single huge somatic follicle cell, the intermediate (interstitial) cell. The most spectacular feature of the intermediate cell is its development of a system of intracytoplasmic canals apparently formed of invaginations of its cell membrane. Initially the complex of germ cell cluster + intermediate cell is enclosed within an envelope composed of squamous cells. As oogenesis progresses the envelope cells gradually degenerate. All the germ cells that have terminated their mitotic divisions are of similar size and enter meiotic prophase, but one of the cystocytes promptly starts to grow faster and differentiates into the oocyte, whereas the remaining cystocytes withdraw from meiosis and become nurse cells (trophocytes). Numerous mitochondria, ER, and a vast amount of ribosomes are transferred from the trophocytes via the cytophore toward the oocyte. Eventually the oocyte ingests all the content of the cytophore, and the trophocytes degenerate. Little vitellogenesis takes place; the oocyte gathers nutrients in the form of small lipid droplets. At the end of oogenesis, an electron-dense fibrous vitelline envelope appears around the oocyte, among short microvilli. At the same time, electron-dense cortical granules occur in the oocyte cortical cytoplasm; at the end of oogenesis they are numerous, but after fertilization they disappear from the ooplasm. In the present article we point out many differences in the course of oogenesis in two related families of rhynchobdellids: piscicolids and glossiphoniids.

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Flowers of sexually deceptive taxa generally possess a set of morphological and physiological characters that mimic their insect pollinators. These characters often include a specific insect-like floral configuration, together with scent glands (osmophores) that produce fragrances which chemically resemble insect sex pheromones. Furthermore, these flowers tend not to produce pollinator food rewards. According to some authors, flowers of the Australian bladderwort Utricularia dunlopii (and species of the Utricularia capilliflora complex) resemble insects, and pollination perhaps occurs by pseudocopulation. The aims of this paper are to compare the structure and distribution of floral glandular trichomes in the Australian carnivorous plant U. dunlopii with those of closely related species assigned to the same section and to discuss their putative function. Floral tissues of U. dunlopii P. Taylor, Utricularia paulinae Lowrie, Utricularia dichotoma Labill. and Utricularia uniflora R.Br. (section Pleiochasia) were investigated using light microscopy, scanning electron microscopy, transmission electron microscopy and histochemistry. In U. dunlopii, two long, erect, filiform appendages arising from the upper lip of the corolla, together with three arising from the lower lip, bear numerous glandular trichomes that may function as osmophores. In other species, such as U. uniflora and U. paulinae, glandular papillae on the corolla palate may also function as osmophores. The floral anatomical and morphological organisation of U. dunlopii differs from that of the other investigated species, indicating that its insect pollinators are also likely to differ. Morphological and ultrastructural observations, while generally contributing to our understanding of the flower of U. dunlopii, do not refute the possibility that pollination here may occur by pseudocopulation. Further field-based investigations, however, are now necessary to test this hypothesis.