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.     

Structure and development of the telotrophic ovariole in ensign scale insects (Hemiptera, Coccomorpha: Ortheziidae)

The paired ovaries of young larva of the 3rd instar of Orthezia urticae are filled with numerous germ cell clusters that can be regarded as ovariole anlagen. Germ cells (cystocytes) belonging to one cluster form a rosette, in the centre of which a polyfusome occurs. Staining with rhodamine-phalloidin has revealed that polyfusomes contain numerous microfilaments. The number of cystocytes per cluster is not stable and varies considerably. The ovaries of older larva become elongated with numerous young ovarioles protruding into the body cavity. The ovarioles are not subdivided into the tropharium and vitellarium. In this stage germ cells differentiate into oocytes and trophocytes (nurse cells). The ovaries of adult females are composed of about 20 (Newsteadia floccosa) or 30 (O. urticae) ovarioles. Their trophic chambers contain trophocytes and arrested oocytes. In the vitellarium, at the given moment, only one oocyte develops. It has been observed that after maturation of the first egg the arrested oocytes may develop.     

Germ-cell cluster formation in the telotrophic meroistic ovary of <Emphasis Type="Italic">Tribolium castaneum</Emphasis> (Coleoptera,Polyphaga, Tenebrionidae) and its implication on insect phylogeny

Tribolium castaneum has telotrophic meroistic ovarioles of the Polyphaga type. During larval stages, germ cells multiply in a first mitotic cycle forming many small, irregularly branched germ-cell clusters which colonize between the anterior and posterior somatic tissues in each ovariole. Because germ-cell multiplication is accompanied by cluster splitting, we assume a very low number of germ cells per ovariole at the beginning of ovariole development. In the late larval and early pupal stages, we found programmed cell death of germ-cell clusters that are located in anterior and middle regions of the ovarioles. Only those clusters survive that rest on posterior somatic tissue. The germ cells that are in direct contact with posterior somatic cells transform into morphologically distinct pro-oocytes. Intercellular bridges interconnecting pro-oocytes are located posteriorly and are filled with fusomes that regularly fuse to form polyfusomes. Intercellular bridges connecting pro-oocytes to pro-nurse cells are always positioned anteriorly and contain small fusomal plugs. During pupal stages, a second wave of metasynchronous mitoses is initiated by the pro-oocytes, leading to linear subclusters with few bifurcations. We assume that the pro-oocytes together with posterior somatic cells build the center of determination and differentiation of germ cells throughout the larval, pupal, and adult stages. The early developmental pattern of germ-cell multiplication is highly similar to the events known from the telotrophic ovary of the Sialis type. We conclude that among the common ancestors of Neuropterida and Coleoptera, a telotrophic meroistic ovary of the Sialis type evolved, which still exists in Sialidae, Raphidioptera, and a myxophagan Coleoptera family, the Hydroscaphidae. Consequently, the telotrophic ovary of the Polyphaga type evolved from the Sialis type. Electronic supplementary material Supplementary material is available in the online version of this article at and is accessible for authorized users.     

Germ cell cluster formation in insect ovaries

Three different ovariole types exist in insects: panoistic, polytrophic- and telotrophic-meroistic. Their ontogenetic development is comparable to all insect orders. Each ovariole is composed of somatic tissues and germ cells.Panoistic ovarioles can be developed: (1) by totally blocking germ cell cluster division (e.g. in “primitive” insect orders; and (2) after germ cell cluster formation by final cleavage of cystocytes, which develop as oocytes (e.g. in stoneflies or thrips).Polytrophic-meroistic ovaries, showing a set of identical characters, are found among hemirnetabolous and holometabolous insects, indicating a “basic type” of common origin. One characteristic feature is the differentiation of only one oocyte, which is derived from one central cell of the cluster, whereas all other siblings are transformed into nurse cells.Telotrophic ovaries differ from polytrophic ovaries by retention of all nurse cells in the anterior trophic chamber. In addition, oocyte-nurse cell determination can be shifted towards more oocytes in a cluster, and clusters or subclusters can fuse by cell membrane reduction among nurse cells. This type of ovary developed independently 3 times from polytrophic ancestors and once in mayflies directly from panoistic ancestors.     

Germ cell cluster formation and ovariole structure in viviparous and oviparous generations of the aphid Stomaphis quercus

The developing ovaries of S. quercus contain a limited number of oogonial cells which undergo a series of incomplete mitotic divisions resulting in the formation of clusters of cystocytes. Ovaries of viviparous generations contain 6 to 9 clusters, containing 32 cystocytes each, whereas ovaries of oviparous generations contain 5 clusters containing 45-60 cystocytes. During further development, clusters become surrounded by a single layer of follicular cells, and within each cluster the cystocytes differentiate into oocytes and trophocytes (nurse cells). Concurrently, cysts transform into ovarioles. The anterior part of the ovariole containing the trophocytes becomes the tropharium, whereas its posterior part containing oocytes transforms into the vitellarium. The vitellaria of viviparous females are composed of one or two oocytes, which develop until previtellogenesis. The nuclei of previtellogenic oocytes enter cycles of mitotic divisions which lead to the formation of the embryo. Ovarioles of oviparous females contain a single oocyte which develops through three stages: previtellogenesis, vitellogenesis and choriogenesis. The ovaries are accompanied by large cells termed bacteriocytes which harbor endosymbiotic microorganisms.     

Structure of ovaries of scale insects: ii. margarodidae (INSECTA,hemiptera, COCCINEA)

The paired, spindle-shaped ovaries of the second instar of the Polish cochineal, Porphyrophora polonica (L.) (Hemiptera: Coccinea) are filled with cystocytes that are arranged into rosettes. In the centre of each rosette, there is a polyfusome. During the third instar, cystocytes differentiate into oocytes and trophocytes (nurse cells) and ovarioles are formed. Ovaries of adult females are composed of about 300 ovarioles of the telotrophic type. Each of them is subdivided into a tropharium (trophic chamber) and vitellarium. The tropharium consists of trophocytes and arrested oocytes that may develop. The number of germ cells in the trophic chambers varies from 11 to 18 even between the ovarioles of the same ovary. The obtained results seem to confirm the concept of a monophyletic origin of the primitive scale insects (Archaeococcoidea).     

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.     

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.     

The role of polyfusomes in generating branched chains of cystocytes during Drosophila oogenesis

Three-dimensional models were constructed utilizing the information gained from electron micrographs of serial sections of two clones of cystocytes undergoing their terminal divisions. In each clone a polyfusome connected all eight cystocytes together. Each of the spindles was oriented so that one pole touched the polyfusomes, while the other pointed away from it. This positioning of spindles ensures that one cell of each dividing pair retains all previously formed canals, while the other receives none. The two cells that eventually come to contain the maximum number of canals and fusomal material are the ones that differentiate as pro-oocytes, while the others become nurse cells. The orientation of each spindle suggests that the polyfusome formed at one division determines the placement of the cytoskeletal fibers that anchor the spindles formed at the next division. There is a centripetal gathering together of new canals following each cycle of cystocyte division, which is thought to result from the subsequent contraction of the polyfusomal system. Females homozygous for the otu1 mutation are characterized by ovarian tumors, which result when germarial cystocytes undergo supernumerary divisions and fail to differentiate into either nurse cells or oocytes. An analysis of electron micrographs taken of serially sectioned, mutant germaria showed that most germ cells were single or belonged to clusters of two or three interconnected cells. Therefore otu1 cystocytes are unable to undergo a sustained series of arrested cleavages. These cystocytes contain fusomal material that shows ultrastructural differences from normal polyfusomes. We conclude: 1) that a normal polyfusomal system is a necessary prerequisite for the production of a branched chain of cystocytes and for their subsequent differentiation into pro-oocytes and nurse cells; and 2) that a product encoded by the otu+ gene is essential for the construction of a functional polyfusome.     

Structure of the female reproductive system of the lac insect,Kerria chinensis (Sternorrhyncha,Coccoidea: Kerridae)

The ovaries of female lac insects, Kerria chinensis Mahd (Sternorrhyncha: Coccoidea: Kerridae), at the last nymphal stage are composed of several balloon‐like clusters of cystocytes with different sizes. Each cluster consists of several clusters of cystocytes arranging in rosette forms. At the adult stage, the pair of ovaries consists of about 600 ovarioles of the telotrophic‐meroistic type. An unusual feature when considering most scale insects is that the lateral oviducts are highly branched, each with a number of short ovarioles. Each ovariole is subdivided into an anterior trophic chamber (tropharium) containing six or seven large trophocytes and a posterior vitellarium harbouring one oocyte which is connected with the trophic chamber via a nutritive cord. No terminal filament is present. Late‐stage adult females show synchronized development of the ovarioles, while in undernourished females, a small proportion of ovarioles proceed to maturity.     

The flea ovary: ultrastructure and analysis of cell clusters

Panoistic ovarioles are found in the order of fleas (Siphonaptera). Only in some species of the Hystrichopsylloidea do polytrophic meroistic ovaries occur. No stem cells and no dividing cystocytes are found in female imagines of Hystrichopsylla talpae. However, each germ cell cluster consists of 32 cells which are generated by five mitotic cycles during the pupal stage. One of the cells containing five intercellular bridges becomes the oocyte, the others serve as nurse cells. Thus, germ cell cluster formation follows the 2(n)-rule. However, no polyfusome is found and nurse cells do not form a rosette. Furthermore, nurse cells remain small and show the same ultrastructural characters as the oocytes, which became distinguishable from nurse cells only by their enhanced growth during pre-vitellogenesis. The first phase of pre-vitellogenesis is dominated by the production of an unknown cytoplasmatic component, consisting of spherical particles, clearly distinguishable from ribosomes by diameter and contrast. The next phase is characterized by a tremendous increase in the production of ribosomes. During this second phase another cytoplasmic component consisting of ball-like structures becomes prominent. During pre-vitellogenesis, germ cell nuclei undergo a pronounced structural change in which, finally, numerous extranucleolar particles predominate. Thus, H. talpae has a polytrophic meroistic ovary, but its oocyte genomes behave panoistically.     

The larval development of the telotrophic meroistic ovary in the bug Dysdercus intermedius (Heteroptera, Pyrrhocoridae)

Bug ovaries are of the telotrophic meroistic type. Nurse cells are restricted to the anterior tropharium and are in syncytial connection with the oocytes via the acellular trophic core region into which cytoplasmic projections of oocytes and nurse cells open. The origin of intercellular connections in bug ovaries is not well understood. In order to elucidate the cellular processes underlying the emergence of the syncytium, we analysed the development of the ovary of Dysdercus intermedius throughout the five larval instars. Up to the third instar, the germ cell population of an ovariole anlage forms a single, tight rosette. In the center of the rosette, phosphotyrosine containing proteins and f-actin accumulate. This center is filled with fusomal cytoplasm and closely interdigitating cell membranes known as the membrane labyrinth. With the molt to the fourth instar germ cells enhance their mitotic activity considerably. As a rule, germ cells divide asynchronously. Simultaneously, the membrane labyrinth expands and establishes a central column within the growing tropharium. In the fifth instar the membrane labyrinth retracts to an apical position, where it is maintained even in ovarioles of adult females. The former membrane labyrinth in middle and posterior regions of the tropharium is replaced by the central core to which nurse cells and oocytes are syncytially connected. Germ cells in the most anterior part of the tropharium, i.e. those in close proximity to the membrane labyrinth remain proliferative. The posterior-most germ cells enter meiosis and become oocytes. The majority of the ovarioles' germ cells, located in between these two populations, endopolyploidize and function as nurse cells. We conclude that the extensive multiplication of germ cells and their syncytial assembly during larval development is achieved by incomplete cytokineses followed by massive membrane production. Membranes are degraded as soon as the trophic core develops. For comparative reasons, we also undertook a cursory examination of early germ cell development in Dysdercus intermedius males. All results were compatible with the known basic patterns of early insect spermatogenesis. Germ cells run through mitotic and meiotic divisions in synchronous clusters emerging from incomplete cytokineses. During the division phase, the germ cells of an individual cluster are connected by a polyfusome rich in f-actin.     

Morphological aspects of cluster formation in the germarium of the sugarcane borer Diatraea saccharalis Fabricius (Lepidoptera: Pyralidae)

Diatraea saccharalis F. is one of the greatest pests of the sugar cane culture. This report aimed to characterize the germarium region of the sugarcane borer by light and transmission electron microscopy, emphasizing the morphological steps of the ovarian cluster formation. In the germarium of this insect, four zones could be morphologically identified during the cluster formation. In the most apical end of each ovariole--Zone I--the germ line stem cells undergo complete mitotic division, originating the cystoblasts. In the Zone II, each cystoblast produces a group of eight cells, the cystocytes, which are interconnected by the ring canals. Clusters containing all the cystocytes in the meiosis, characterizes the Zone III. Germ cells with ultrastructural features of apoptosis are also detected in this Zone. In the Zone IV the cystocytes differentiate, morphologically, into one oocyte and seven nurse cells. Interstitial somatic cells and pre-follicle cells exhibit, in their cytoplasm, heterogeneous vacuoles containing degenerated cellular fragments, characterized as apoptotic bodies. Our results pointed out to the morphological evidences related with important control mechanisms for new clusters/follicles production and for the cellular arrangement into the germarium, resulting from the programmed cell death. We believe that the morphological characterization of ovarian cluster formation in D. saccharalis provided valuable information for the understanding of the initial steps of oogenesis and contributed for the knowledge of the cellular mechanisms related with the oocyte production and with reproduction in insects.     

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.     

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.     

Structure and development of ovaries in the weevil, Anthonomus pomorum (Coleoptera, Polyphaga). I. Somatic tissues of the trophic chamber

In developing ovarioles of Anthonomus pomorum (Coleoptera, Polyphaga, Curculionidae) the trophic chambers (tropharia) are relatively large and consist of clusters (clones) of germ cells and various somatic tissues. Each ovariole is enclosed within an outer epithelial sheath (tunica externa). Throughout the pupal phase, the growth of this sheath is accelerated and precedes the development of the rest of the ovariole. As a result, the epithelial sheath proliferates anteriorly and forms an elongated "sleeve" that during the later stages of development becomes gradually filled by the growing tropharium. In the early pupal stage, a few terminal filament cells are observed in contact with the anterior end of the tropharium. These cells are separated from the rest of the trophic chamber by a transverse septum, which maintains continuity with the basal lamina. Beneath the basal lamina there is a layer of inner sheath cells, whereas inside the tropharium there are interstitial cells. These two types of cell differ morphologically in a mature ovary but they retain, until the end of the imago-B stage, a similar ultrastructure testifying to their common origin. At the posterior end of the tropharium, from the imago-B stage on, many young oocytes, surrounded by prefollicular cells, are observed. This is the so-called neck region of the tropharium. Extraction with Triton X-100 detergent showed that in a mature trophic chamber there are only individual microtubules arranged along the projections of interstitial cells. This indicates that the cytoskeleton elements (microfilaments and microtubules) participate only to a very limited extent in the spatial organisation of the tropharium in A. pomorum.     

Morphogenetic deficiencies in transplanted gonadal anlagen of the mutant l(3)pl (lethal-polyploid) in Drosophila hydei

Larval gonads of Drosophila hydei, homozygous for the lethal gene l(3)pl (lethal-polyploid), were cultured in normal hosts. Ovaries of the late third larval instar were implanted into metamorphosing larvae. These can attach to the gonoduct system of the host and transform into adult ovarian structures but the spectrum of their capacity to differentiate varies largely. In favourable cases mature oocytes can be formed which are fertile. More frequently mitotic disturbances in the follicle cells and cystocytes lead to the formation of abortive egg chambers and abnormally shaped oocytes. Testes of the middle third larval instar were cultured for 2 weeks in adult females. Primary spermatocytes are able to sustain meiotic divisions and form early spermatids, even though the occurrence of fractionated nuclei in post-meiotic germ cells indicates defective meiotic divisions. Post-meiotic differentiation is blocked in mutant spermatids which fail to elongate. The mutant gene l(3)pl thus, not only affects cell divisions, but also interacts in certain cytodifferentiation processes such as spermatid elongation and egg shaping. All cellular processes found so far to be abnormal in mutant tissues involve microtubular function. This suggests that the gene l(3)pl interacts with the microtubular system and several aspects of this interpretation are discussed.     

Formation of germ cell cluster in tubuliferan thrips (Thysanoptera)

The ovarian structure and oogenesis in the larval stages of 2 tubuliferan species, Bactrothrips brevitubus (Idolothripinae) and Holothrips yuasai (Phlaeothripinae) of the Thysanoptera were examined using ultrathin serial sections, with special reference to the cluster formation of germ cells. No cells identifiable as stem cells were found in the ovarian rudiments of the 1st and 2nd-instar larvae. The clusters of oogonial cells were observed frequently in the 1st-instar, but scarcely in the 2nd-instar larvae: all the oogonial clusters observed were composed of 2 cells. In the 2nd-instar larvae, the ovarian region posterior to the germarium, or the vitellarium, contained both solitary and clustered oocytes. The oocyte clusters were composed of less than 5 cells. The oocytes, located in the posterior region of the vitellarium, were all solitary and at the previtellogenic stages.A protuberance was found in some solitary germ cells. The structure may represent a remnant of the intercellular bridge, previously formed between the germ cells. The number of oocytes composing a cluster is small but does not always fit the 2ⁿ-rule. One possible explanation is the accelerated detachment process of oocytes from a cluster. The cluster formation of germ cells has been confirmed in the Tubulifera as well as in the Terebrantia, and this phenomenon can be recognized as a general feature of the panoistic ovaries of the Thysanoptera.     

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.     

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.