Germ cells cluster organization in polytrophic ovaries of neuroptera

Histological and ultrastructural analysis of polytrophic ovary structure in Neuroptera revealed an unusual organization of their germ cell clusters. In all species under study (representing 5 families), clusters with variable and unfixed numbers of cystocytes are formed. Moreover, spatial organization of cystocyte connections within the cluster is linear rather than typically branched; only a few branching sites being observed. The oocyte is located in the central, always linear, part of the cluster and therefore is directly connected via intercellular bridges with only two nurse cells. It is postulated here that the linear character of germ cell clusters in Neuroptera may result from asynchrony of cystocyte divisions. Mechanisms of germ cell cluster formation and differentiation are discussed.     

Germ-line cysts are formed during oogenesis in Erpobdella octoculata (Annelida,Clitellata, Erpobdellidae)

Abstract

Erpobdella octoculata (Clitellata, Hirudinea, Erpobdellidae) has paired ovarian sacs, each containing several rod-shaped structures termed ovarian bodies. Oogenesis takes place within the ovarian bodies. We show that in the apical part of the bodies the germ-line cells form syncytial cysts of cells interconnected by stable intercellular bridges. Germ-line cyst architecture is broadly similar to that of other clitellate annelids; that is, each germ cell has only one intercellular bridge connecting it to the anuclear cytoplasmic mass, the cytophore. Unlike germ-line cysts described in other leech species, the cytophore in cysts of E. octoculata is poorly developed, taking the form of thin cytoplasmic strands. Oogenesis in E. octoculata is meroistic because the germ cells forming the cysts (cystocytes) have diverse fates, i.e., nurse cells and oocytes appear. One large ramified cell (apical cell) occurs within the apical part of the ovarian body. We compare the ultrastructure of the apical cell found in E. octoculata with that of apical cells described recently in some hirudiniform leeches. The germ-line cysts as well as the oocytes are enveloped by somatic follicular cells. As in other leeches, the follicular cells surrounding the growing oocytes have cytoplasm perforated by intracellular canals. In view of the many similarities between E. octoculata ovarian bodies and the ovary cords described in glossiphoniids and especially in hirudiniform leeches, we suggest that the ovarian bodies found in E. octoculata are in fact modified ovary cords.     

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.     

Oogenesis in four species of Piscicola (Hirudinea, Rhynchobdellida)

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.     

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

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.     

Oogenesis in the honeybee Apis mellifera: cytological observations on the formation and differentiation of previtellogenic ovarian follicles

Summary Oogenesis is known to be important for embryonic pattern formation. For this reason we have studied the early differentiation of the honeybee ovariole histologically, ultrastructurally, and by staining F-actin with rhodaminyl-phalloidin. At the anterior tip of the ovariole, stem cells are lined up in a single file; they are organelle-poor but contain characteristic electrondense bodies with lysosomal properties. The presence of these bodies in cystocytes as well as prefollicle cells indicates that both cell types may be derived from the apical stem cells. During later stages of oogenesis, the follicle cells differentiate cytologically in different regions of the follicle. The organization of the intercellular bridges between cystocytes derived from a single cystoblast has been studied in detail. The polyfusomes in the intercellular bridges of cystocyte clusters stain with rhodaminyl-phalloidin and hence contain F-actin. Later, when the polyfusomes begin to desintegrate, F-actin rings form which line the rims of the intercellular bridges. Actin might be recruited from conspicuous F-actin stores which were detected in the germ-line cells. The F-actin rings are dissembled some time before the onset of vitellogenesis when the nurse chamber has grown to a length of about 200 m. At the basal side of the follicle cells (close to the basement membrane facing the haemocdele) parallel microfilament bundles encircle the ovariole. The microfilament bundles which are oriented mostly perpendicular to the long axis of the ovariole were first observed around the zone where the cystocyte divisions occur; after this phase the micro-filament bundles become organized differently in the follicle cells associated with the nurse cells and in the follicular epithelium of the oocyte. Correspondence to: H.O. Gutzeit     

Ultrastructure and cluster formation in ovaries of bark lice,Peripsocus phaeopterus (Stephens) and Stenopsocus stigmaticus (Imhof and Labram) (Insecta : Psocoptera)

Ultrastructure and previtellogenic growth of ovaries of Peripsocus phaeopterus (Stephens) and Stenopsocus stigmaticus (Imhof and Labram) (Insecta : Psocoptera) are described. The germ cell cluster formation was analyzed in an ovariole of a nymph using ultrathin serial sectioning. Fifteen germ cell clusters were found; 13 contained 4 cystocytes each, while 2 clusters, situated in the very tip, were composed of 2 cystocytes each. A fully developed cluster rises by 2 synchronized mitotic divisions, each followed by incomplete cytokinesis. Microtubules derived from the preceding mitoses form a transient midbody within the intercellular bridge. Later on, a fusome fills each bridge, while at fusomal rims parallel oriented microtubules are tightly associated. Some of these microtubules stretch to cell membranes nearby. The fusomes fuse into a polyfusome and a rosette is thus formed by which all intercellular bridges are drawn together. All cystocytes enter the prophase of meiosis up to pachynema. One of the 2 inner cells continues meiosis and develops as an oocyte, whereas all others transform into nurse cells. After rosette formation, the polyfusome-associated microtubules vanish and some time later, when the nurse cell-oocyte differentiation becomes apparent, the polyfusome itself becomes destroyed. The intercellular bridge, joining the first nurse cell with the 3rd moves away from the other 2. During previtellogenesis, 5 phases can be distinguished, 2 of which are interpreted as logarithmical growth phases with different slopes. The whole set of characters elaborated here for the polytrophic meroistic ovary of psocopterans is fully consistent with the characters of polytrophic meroistic ovaries of Holometabola, indicating a monophyletic origin.     

The arrest gene is required for germline cyst formation during Drosophila oogenesis

In Drosophila, oogenesis is initiated when a germline stem cell produces a differentiating daughter cell called the cystoblast. The cystoblast undergoes four rounds of synchronous divisions with incomplete cytokinesis to generate a syncytial cyst of 16 interconnected cystocytes, in which one cystocyte differentiates into an oocyte. Strong mutations of the arrest (aret) gene disrupt cyst formation and cause the production of clusters of ill-differentiated germline cells that retain cellular and molecular characteristics of cystoblasts. These mutant germ cells express high levels of BAM-C and SXL proteins in the cytoplasm but do not accumulate markers for advanced cystocytes or differentiating oocytes, such as the nuclear localization of SXL or the accumulation of osk mRNA, orb mRNA, and cytoplasmic dynein. However, the mutant germ cells do not contain spectrosomes, the cytoplasmic structure that objectifies the divisional asymmetry of the cystoblast. The aret mutant germ cells undergo active mitosis with complete cytokinesis. Their mitosis is accompanied by massive necrosis, so that the number of germ cells in a stem cell-derived cluster ranges from one to greater than 70. These defects of aret mutants reveal a novel function of aret as the first gene with a defined function in the cystoblast to cyst transition during early oogenesis.     

Reductions and new inventions dominate oogenesis of Strepsiptera (Insecta)

The endoparasitic life of strepsipterans (Insecta), especially neotenic females, reduces to a great extent external and internal organs. Light and electron microscopic investigation of ovaries of Elenchus tenuicornis (Kirby) confirms the following: (1) somatic tissues of ovaries are totally reduced, with the exception of some cells surrounding germ cell clusters; (2) a previtellogenic growth phase of oocytes is reduced; (3) nurse cells remain diploid and their membranes degenerate at the onset of vitellogenesis; (4) vitellogenesis is reduced, vitellin and fat vacuoles contribute only 50% to the final egg volume; and (5) chorionogenesis is reduced to a vitellin membrane. However, some features of normal development remain, allowing classification of the ovary type as polytrophic meroistic: (1) germ cells undergo synchronized, incomplete divisions, following the 2ⁿ rule, where all former intercellular bridges become localized in one cystocyte, while the other has none; and (2) only one cell is determined as the oocyte, all other cystocytes serve as nurse cells and the surrounding somatic cells transform into follicular cells. Novel events in oogenesis of strepsipterans include fission of clusters during the phase of cluster mitoses, and protection of oocyte nuclei, while nurse cell nuclei degenerate in the same cytoplasm.     

Ovary cord structure and oogenesis in Hirudo medicinalis and Haemopis sanguisuga (Clitellata, Annelida): remarks on different ovaries organization in Hirudinea

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 flattened 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 float freely in the ovary lumen. This paper describes the successive stages of oogenesis of H. medicinalis in detail. Ovary organization in Hirudinea was classified within four different types: non-polarized 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.     

Early oogenesis in the short-tailed fruit bat Carollia perspicillata: transient germ cell cysts and noncanonical intercellular bridges

The ovaries of early embryos (40 days post coitum/p.c.) of the bat Carollia perspicillata contain numerous germ-line cysts, which are composed of 10 to 12 sister germ cells (cystocytes). Variability in the number of cystocytes within the cyst and between the cysts (defying the Giardina rule) indicates that the mitotic divisions of the cystoblast are asynchronous in this bat species. Serial section analysis showed that the cystocytes are interconnected via intercellular bridges that are atypical, strongly elongated, short-lived, and rich in microtubule bundles and microfilaments. During slightly later stages of embryonic development (44-46 days p.c.), somatic cells penetrate the cyst, and their cytoplasmic projections separate individual oocytes. Separated oocytes surrounded by a single layer of somatic cells constitute the primordial ovarian follicles. The oocytes of C. perspicillata are similar to mouse oocytes and are asymmetric. In both species, this asymmetry is clearly recognizable in the localization of the Golgi complexes. The presence of germ-line cysts and intercellular bridges (although noncanonical) in the fetal ovaries of C. perspicillata suggest that the formation of germ-line cysts is an evolutionarily conserved phase in the development of the female gametes in a substantial part of the animal kingdom.     

Ovary organization and oogenesis in two species of Lumbriculida (Annelida,Clitellata)

The aim of the present study is to describe the organization of the ovary and mode of oogenesis at the ultrastructural level in two representatives of Lumbriculida – Lumbriculus variegatus and Stylodrilus heringianus. In both species studied, the ovaries are small and conically shaped structures that are attached to the intersegmental septum via a thin ligament. The ovaries are composed of germline cysts formed by germ cells interconnected by stable cytoplasmic bridges. As a rule, the cyst center is occupied by a poorly developed anuclear cytoplasmic mass, termed a cytophore, whereas the germ cells are located at the periphery of the cyst. Germline cysts are enveloped by somatic cells. The ovaries of the species studied are polarized, i.e., along the long axis of the ovary there is an evident gradient of germ cell development. The data obtained suggest ovary meroism, i.e., two categories of germ cells were found: oocytes, which continue meiosis, gather nutrients, grow and protrude into the body cavity, and nurse cells, which do not grow and are supposed to supply oocytes with cell organelles and macromolecules via the cytophore. The ovary structure and mode of oogenesis in the species studied were compared with those of other clitellate annelids. As a rule, in all clitellates studied to date, the ovaries are composed of germline cysts equipped with a cytophore and associated with somatic cells; however, the ovary morphology differs between taxa regarding several quantitative and qualitative features. The ovary organization and mode of oogenesis in L. variegatus and S. heringianus strongly resemble those found in Tubificinae and Branchiobdellida studied to date. Our results also support a sister-group relationship between Lumbriculida and a clade comprising ectoparasitic clitellates (i.e., Branchiobdellida, Acanthobdellida and Hirudinida) with Branchiobdellida as a plesiomorphic sister group to Acanthobdellida and Hirudinida.     

Oogenesis in the leech Glossiphonia heteroclita (Annelida, Hirudinea, Glossiphonidae). I. Ovary structure and previtellogenic growth of oocytes

Glossiphonia heteroclita has paired ovaries whose shape and dimensions change as oogenesis proceeds: during early previtellogenesis they are small and club-shaped, whereas during vitellogenesis they broaden and elongate considerably. During early oogenesis (previtellogenesis), each ovary is composed of an outer envelope (ovisac) that surrounds the ovary cavity and is filled with hemocoelomic fluid, in which a single and very convoluted ovary cord is bathed. The ovary cord consists of germline cells, including nurse cells and young oocytes surrounded by a layer of elongated follicle cells. Additionally, follicle cells with long cytoplasmic projections occur inside the ovary cord, where they separate germ cells from each other. The ovary cord contains thousands of nurse cells. Each nurse cell has one intercellular bridge, connecting it to a central anucleate cytoplasmic mass, the cytophore (rachis); it in turn is connected by one intercellular bridge with each growing oocyte. Numerous mitochondria, RER cisternae, ribosomes, and Golgi complexes are transported from the nurse cells, via the intercellular bridge and cytophore, to the growing oocytes. Oogenesis in G. heteroclita is synchronous with all oocytes in the ovary in the same stage of oogenesis. The youngest observed oocytes are slightly larger than nurse cells, and usually occupy the periphery of the ovary cord. As previtellogenesis proceeds, the oocytes gather a vast amount of cell organelles and become more voluminous. As a result, in late previtellogenesis the oocytes gradually protrude into the ovary cavity. Simultaneously with oocyte growth, the follicle cells differentiate into two subpopulations. The morphology of the follicle cells surrounding the nurse cells and penetrating the ovary cord does not change, whereas those enveloping the growing oocytes become more voluminous. Their plasma membrane invaginates deeply, forming numerous broad vesicles that eventually seem to form channels or conducts through which the hemocoelomic fluid can easily access the growing oocytes.     

Ultrastructural changes of the intercellular relationship in impaired human spermatogenesis

Summary In seven hypo- or aspermic patients, electron microscopic investigations of the intercellular connections of the seminiferous tubule were performed. The analysis of cell junctions of Sertoli cells and germ cells revealed irregularities of the Sertoli-cell junctions, hypoplasias of occluding junctions, hypo- and hyperplasias of the Sertoli-spermatid cell junctions and abnormal formation of Sertoli cell junctions with early spermatids, spermatocytes, and spermatogonia. Gap junction-like cell membrane specializations were very rare. Intercellular cytoplasmic bridges of germ cells were always present together with these cells. One hypoplastic bridge connecting two spermatogonia was found.The results allow a preliminary classification of impaired spermatogenesia. The changes of intercellular connections might disturb the blood-testis barrier as well as the intercellular communication in the seminiferous tubule. Evidence is available to support the suggestion that genetic causes play a considerable role in the etiology of the germ cell aplasia and the spermatogenic maturation arrest.     

Mayflies (ephemeroptera), the most “primitive” winged insects,have telotrophic meroistic ovaries

Germ line cell cluster formation in ovarioles of three different stages, each from a different mayfly species, was studied using ultra-thin serial sectioning. In the analysed ovariole of Cloeön sp., only one linear, zigzag germ line cell cluster was found, consisting of sibling cells connected by intercellular bridges which represent remnants of preceding synchronized mitotic cycles followed by incomplete cytokinesis. A polyfusome stretched through all sibling cells. At the tip of the ovariole, cytokinesis occurred without preceding division of nuclei; thus, intercellular bridges were lined up but the remaining cytoplasm between the bridges had no nuclei. The analysed Siphlonurus armatus vitellarium contained five oocytes at different stages of development. Each oocyte in the vitellarium was connected via a nutritive cord to the linear cluster of its sibling cells in the terminal trophic chamber. Each cluster had the same architecture as was found in Cloëon. The 3-dimensional arrangement and distribution of closed intercellular bridges strongly suggest that all five clusters are derived from a single primary clone. The position of oocytes within each cluster is random. However, each oocyte is embraced by follicular or prefollicular cells whilst all other sibling cells are enclosed by somatic inner sheath cells, clearly distinguishable from prefollicular cells. In the analysed ovariole of Ephemerella ignita, two small linear clusters were found in the tropharium beside two single cells, two isolated cytoplasmic bags with intercellular bridges but no nuclei, and some degenerating aggregates. One cluster was still connected to a growing oocyte via a nutritive cord. In all species the nurse cells remained small and no indications of polyploidization were found. We suggest that this ancient and previously unknown telotrophic meroistic ovary has evolved directly from panoistic ancestors.     

An ultrastructural study of the ovary cord organization and oogenesis in the amphibian leech Batracobdella algira (Annelida,Clitellata, Hirudinida)

This study reveals the ovary micromorphology and the course of oogenesis in the leech Batracobdella algira (Glossiphoniidae). Using light, fluorescence, and electron microscopies, the paired ovaries were analyzed. At the beginning of the breeding season, the ovaries were small, but as oogenesis progressed, they increased in size significantly, broadened, and elongated. A single convoluted ovary cord was located inside each ovary. The ovary cord was composed of numerous germ cells gathered into syncytial groups, which are called germ-line cysts. During oogenesis, the clustering germ cells differentiated into two functional categories, i.e., nurse cells and oocytes, and therefore, this oogenesis was recognized as being meroistic. As a rule, each clustering germ cell had one connection in the form of a broad cytoplasmic channel (intercellular bridge) that connected it to the cytophore. There was a synchrony in the development of the clustering germ cells in the whole ovary cord. In the immature leeches, the ovary cords contained undifferentiated germ cells exclusively, from which, previtellogenic oocytes and nurse cells differentiated as the breeding season progressed. Only the oocytes grew considerably, gathered nutritive material, and protruded at the ovary cord surface. The vitellogenic oocytes subsequently detached from the cord and filled tightly the ovary sac, while the nurse cells and the cytophore degenerated. Ripe eggs were finally deposited into the cocoons. A comparison of the ovary structure and oogenesis revealed that almost all of the features that are described in the studied species were similar to those that are known from other representatives of Glossiphoniidae, which indicates their evolutionary conservatism within this family.

     

Structure and development of ovaries in the weevil,Anthonomus pomorum (Coleoptera,Polyphaga). II. Germ cells of the trophic chamber

The analysis of the germ cell cluster formation in Anthonomus pomorum (Coleoptera, Polyphaga, Curculionidae) has revealed that both linear and branched clones of cystocytes occur in the pupa stage. In the branched clones a poorly developed polyfusome is formed and cystocytes with maximally 3 intercellular bridges were found. In the linear clones the polyfusomes are absent. Further divisions of cystocytes produce exclusively linearly arranged cells. Just after metamorphosis (Imago-A stage), the process of the germ cell membrane reduction starts. Only 2 groups of cells retain cell membranes: i.e the most anteriorly localized group of cystocytes and the posteriorly located presumptive oocytes. The former cells divide mitotically during the summer. As a result an anterior-posterior gradient of the syncytialization process arises in the Imago-B stage (females preparing for hibernation). In the sexually mature females (Imago-C) the trophic chamber consists of a huge syncytial area with numerous nurse cell nuclei embedded in a common cytoplasm, and posteriorly located young oocytes surrounded by prefollicular cells. In the light of recent hypothesis concerning the germ cell cluster formation and telotrophy anagenesis in Polyphaga the significance of the presented results is discussed.     

Requirement of RBP9, a Drosophila Hu homolog, for regulation of cystocyte differentiation and oocyte determination during oogenesis

The Drosophila RNA binding protein RBP9 and its Drosophila and human homologs, ELAV and the Hu family of proteins, respectively, are highly expressed in the nuclei of neuronal cells. However, biochemical studies suggest that the Hu proteins function in the regulation of mRNA stability, which occurs in the cytoplasm. In this paper, we show that RBP9 is expressed not only in the nuclei of neuronal cells but also in the cytoplasm of cystocytes during oogenesis. Despite the predominant expression of RBP9 in nerve cells, mutational analysis revealed a female sterility phenotype rather than neuronal defects for Rbp9 mutants. The female sterility phenotype of the Rbp9 mutants resulted from defects in oogenesis; the lack of Rbp9 activity caused the germarium region of the mutants to be filled with undifferentiated cystocytes. RBP9 appears to stimulate cystocyte differentiation by regulating the expression of bag-of-marbles (bam) mRNA, which encodes a developmental regulator of germ cells. RBP9 protein bound specifically to bam mRNA in vitro, which is required for cystocyte proliferation, and the number of cells that expressed BAM protein was increased 5- to 10-fold in the germarium regions of Rbp9 mutants. These results suggest that RBP9 protein binds to bam mRNA to down regulate BAM protein expression, which is essential for the initiation of cystocyte differentiation into functional egg chambers. In hypomorphic Rbp9 mutants, cystocytes differentiated into egg chambers; however, oocyte determination and positioning were perturbed. Therefore, the concentrated localization of RBP9 protein in the oocyte of the early egg chambers may be required for proper oocyte determination or positioning.     

Diptera--ovary structure and oogenesis in midges and flies

Comparative study of ovary development and oogenesis in the dipterans revealed significant differences between the Nematocera (lower dipterans, midges) and the Brachycera (true flies). The occurrence of these differences emphasizes well the phylogenetic division of the Diptera into these major subgroups. Basic discrepancies were found in the course of ovary development and in the mode of follicular cell differentiation. In contrast to more advanced flies, in midges the initial stages of germ cell differentiation, i.e. divisions of gonial cells, germ cell cluster formation and diversification of cystocytes within clusters take place exclusively in the larval and early pupal stages. Moreover, the formation of cystocyte clusters precedes that of ovarioles. Differences in the behaviour of some follicular cells found between the ovarian follicles of midges and advanced flies suggest that both major dipteran subgroups may differ in the scenario and/or the mechanisms of terminal signalling leading to the determination of the anteriormost part of the body.