The telotrophic nature of ovarioles of polyphage coleoptera

Summary The ovarian type of 23 families of polyphage Coleoptera has been determined. The ovaries of all species belong to the telotrophic-meroistic type, indicated by nutritive cords connecting oocytes with the trophic tissue. These nutritive cords persist during previtellogenesis and the main part of vitellogenesis. The cytoplasm of the nutritive cords shows the same elements in similar frequencies as they were found in the nurse cell cytoplasm or in the ooplasm of oocytes during previtellogenesis. No indications are found supporting the idea that a reduction of nutritive cords early in ovarian development gives rise to a deviated panoistic ovary. The histological uniformity of nutritive cords among all investigated species as well as the fact that these species are scattered by chance among the polyphage Coleoptera seems to justify the conclusion that all Polyphaga s. str. have telotrophicmeroistic ovarioles. This investigation was supported in part by the Stiftung Volkswagenwerk     

The telotrophic-meroistic ovary of megaloptera. I. The ontogenetic development

Sialis flavilatera L. (Sialidae, Megaloptera) has telotrophic-meroistic ovarioles. The germ cells of the tropharium are organized into two distinct tissues, the central syncytium and the germ cell tapetum. The central syncytium consists of nurse cell nuclei embedded in a common cytoplasm which is rich in ribosomes and mitochondria. Cell membranes are totally absent. The germ cell tapetum surrounds the syncytium and consists of a monolayer of cells, each of which is connected with the central syncytium by an intercellular bridge. The oocytes differentiate from basal tapetum cells by previtellogenic growth. Their nutritive cords remain connected to the central syncytium by the intercellular bridge. Ovariole development starts soon after hatching with the immigration of germ cells into the ovariole-anlagen and is finished during pupal stages 23 months later. In apical regions of each tropharium, mitoses occur throughout larval life. The descendants enter the prophase of meiosis which lasts until pre-vitellogenesis; thus, a differential gradient of position and time is established. About 12 months after hatching, the central syncytium arises at the base of the tropharium from a membrane labyrinth in which intercellular bridges are entangled. Evidence is presented that endopolyploidization does not occur during germ cell differentiation. Finally, the results are compared with those found in Hemiptera and polyphage Coleoptera. The great diversities are interpreted as an indication for a polyphyletic origin of the telotrophic ovary.     

Differentiation of oocytes and trophocytes in insect ovaries with different ovariole structures]

Early stages of differentiation of the oocytes and nurse cells are comparatively studied in the polytrophic ovarioles in larvae, pupae and imago of the butterfly Laspeyresia pomonella and in the telotrophic ovarioles in larvae and imago of the bug Eurigaster integriceps. In L. pomonella, the oocytes and trophocytes, being the descendants of one oogonial cell, pass synchroniously through early stages of meiotic prophase up to the pachyten. After the pachyten chromosomes of the future trophocytes transform into diakinetic bivalents, whereas in the oocyte nucleus chromosomes retain their pachyten stage appearance. In the fifth instar larva of E. integriceps, two zones may be seen in the germarium of the telotrophic ovariole: the apical trophocyte zone and the distal oocyte zone. The oocytes develop up to the zygotene("bouquet") stage. As to the future trophocytes, they miss zygotene and reach directly diakinesis. Thus,the earlier divergence in the development ways of oocytes and trophocytes is observed in the telotrophic ovarioles, since the trophocyeres pass themeiotic stages more quickly then oocytes. The supposition is advanced that the quicker development of the nurse cells in the bug's ovarioles takes place due to missing the synaptonemal complex formation. The patterns of similarity and distinction between the telotrophic ovarioles in Coleoptera, on the one hand, and the polytrophic ovarioles of the butterfly L. pomonella and telotrophic ovarioles of the bug E. integricept, on the other hand, are discussed.     

Structure and ultrastructure of the ovary in the South American Veturius sinuatus (Eschscholtz) (Coleoptera,Passalidae)

The morphoanatomy of the ovary in Veturius sinuatus (Eschscholtz) was studied by light and transmission electron microscopy. Data from the female gonad of this species provide more extended and precise knowledge regarding the organization of the ovary in Passalidae. Ovaries are composed of a pair of long telotrophic meroistic ovarioles, with some differences compared to the bauplan of this ovary type in Polyphaga (Coleoptera). The terminal filament has an enlarged proximal region with irregularly shaped cells in apparent degeneration process embedded in a membranous system. Globular structures with amorphous content associated with interstitial cells are distributed throughout the tropharium. Trophocytes develop with the reduction of the plasma membrane between sibling nurse cells of each cluster. Previtellogenic oocytes have an irregular shape and various cytoplasmic prolongations. As oogenesis advances, a single prolongation in the anterior part of the oocyte extends to the tropharium. The ovary structure is comparable to that found in other American species of passalids, and further, the conformation of the terminal filament could be a plesiomorphic character of the family.     

Transport of molecules and organelles in meroistic ovarioles of insects

Abstract. Transport of macromolecules from the nurse cells to the oocyte has been well documented in meroistic ovarioles of insects. The transport mechanism, however, has remained elusive. Cytoskeletal elements presumably play a role in the transport process in some species, but comparative studies have revealed dramatic differences in the structural organization of microfilaments and microtubules, indicating that these elements may serve different functions. Based on electrophysiological data, it has been suggested that charged molecules move by intercellular electrophoresis. However, recent evidence suggests that this mechanism cannot account for the transport phenomena observed in Drosophila and in some other species. It is proposed that as an alternative (or additional) transport mechanism, an osmotically driven pressure flow from the nurse cells to the oocyte might operate in the ovarioles of some insects.     

The ovary ofRaphidia flavipes is telotrophic and of theSialis type (Insecta,Raphidioptera)

Summary The morphology and ultrastructure of the ovarioles ofRaphidia flavipes are described. The ovary ofRaphidia shows all the structural elements by which the telotrophic ovarioles of Megaloptera can be distinguished easily from those of polyphage Coleoptera or Hemiptera. This supports the view of sister-group relationship between the Raphididae and the Sialidae.Dedicated to Professor Dr. Dr. h.c. Bernhard Rensch on the occasion of his 80th birthdayThis investigation was supported in part by the Stiftung Volkswagenwerk     

The telotrophic ovary known from Neuropterida exists also in the myxophagan beetle <Emphasis Type="Italic">Hydroscapha natans</Emphasis>

The ovary structure of the myxophagan beetle, Hycdoscapha natans, was investigated by means of light and electron microscopy for the first time. Each of the two ovaries consists of three ovarioles, the functional units of insect oogenesis. The ovary type is telotrophic meroistic but differs strongly from the telotrophic ovary found among all polyphagous beetles investigated so far. All characters found here are typical of telotrophic ovaries of Sialidae and Raphidioptera. Both taxa belong to the Neuropterida. As in all telotrophic ovaries, all nurse cells are combined in an anterior chamber, the tropharium. The tropharium houses two subsets of germ cells: numerous nurse cell nuclei are combined in a central syncytium without any cell membranes in between, surrounded by a monolayer of single-germ cells, the tapetum cells. Each tapetum cell is connected to the central syncytium via an intercellular bridge. Tapetum cells of the posterior zone, which sufficiently contact prefollicular cells, are able to grow into the vitellarium and develop as oocytes. During previtellogenic and early vitellogenic growth, oocytes remain connected with the central syncytium of the tropharium via their anterior elongations, the nutritive cords. The morphological data are discussed in the light of those derived from ovaries of other Coleoptera and from the proposed sister group, the Neuropterida. The data strongly support a sister group relationship between Coleoptera and Neuropterida. Furthermore, several switches between polytrophic and telotrophic ovaries must have occurred during the radiation of ancient insect taxa.     

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.     

Morphology,ultrastructure, and germ cell cluster formation in ovarioles of aphids

The structure of aphid ovaries, including ovipare and virginopare morphs of five species, was investigated by light and electron microscopy. Aphids contain telotrophic meroistic ovarioles. The amount and distribution of cytoplasmic components of nurse cells, nutritive cords, and young oocytes are nearly identical to those known from scale insects and heteropterans. Each ovariole has a constant number of nurse cells and oocytes. In ovaries of ovipare morphs, the nurse cell nuclei enlarge by endomitosis (n = 2⁸n?2¹⁰n), whereas in virginopare morphs the nurse cell nuclei remain small (n = 2²n?2⁴n). Furthermore, in virginoparae the previtellogenic growth of oocytes is highly reduced, and vitellogenesis and chorionogenesis are blocked totally. Embryogenesis starts immediately after the shortened previtellogenic growth. In each ovariole, all germ cell descendants belong to one germ cell cluster that follows the 2ⁿ rule. The cluster normally contains 2⁵ = (32) cells, but other mostly smaller numbers also occur. In contrast to polytrophic meroistic ovarioles, more than one cell of each cluster will develop into an oocyte. In Drepanosiphum platanoides, 16 (2^n?1) nurse cells and 16 (2^n?1) oocytes exist in each cluster, whereas, in Metopolophium dirhodum, 8 (2^n?2) oocytes and 24 (2^n?1 + 2^n?2) nurse cells are normally found. In many ovarioles of Macrosiphum rosae, 21 nurse cells nourish 11 oocytes. Models of germ cell cluster formation in aphid ovaries are discussed.     

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.     

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.     

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.     

Ovaries and germline cysts and their evolution in Dermaptera (Insecta)

We studied the ovary structure and initial stages of oogenesis in 15 representatives of several dermapteran taxa, including the epizoic Arixeniina. In all examined species, the ovaries are meroistic–polytrophic. The ovaries of the basal taxa (‘Pygidicranidae’, ‘Diplatyidae’, and Labiduridae) are composed of elongated ovarioles, attached to short lateral oviducts. In these groups, ovarioles contain several (more than 30) ovarian follicles in a linear arrangement. In the Eudermaptera, the ovaries are composed of 1–6 (Spongiphoridae) or 20–40 (Forficulidae, Chelisochidae) short ovarioles (containing 2 ovarian follicles only) that open to strongly elongated lateral oviducts. In all investigated dermapterans, the ovarian follicles are composed of two germline cells only: an oocyte and a polyploid nurse cell that are covered by a simple follicular epithelium. Our studies indicate that despite a rather unique morphology of the ovarian follicles in the examined species, the processes leading to the formation of the oocyte and nurse cell units are significantly different in basal versus derived taxa.The ovaries of Arixenia esau are composed of 3 short ovarioles attached to a strongly dilated lateral oviduct, ‘the uterus’, containing developing embryos. Histological analysis suggests that the origin of the oocyte and nurse cell units in this species follows the pattern described in eudermapterans.The interpretation of our results in an evolutionary context supports the monophyly of the Dermaptera and Eudermaptera, and the inclusion of the Arixeniina and Hemimerina in the latter taxon.     

Programmed cell death of the ovarian nurse cells during oogenesis of the ladybird beetle Adalia bipunctata (Coleoptera: Coccinellidae)

Programmed cell death (PCD) is an evolutionary conserved and genetically regulated form of cell death, in which the cell plays an active role in its own demise. It is widely recognized that PCD can be morphologically classified into three major types: type I, known as apoptosis, type II, called autophagy, and type III, specified as cytoplasmic cell death. So far, PCD has been morphologically analyzed in certain model insect species of the meroistic polytrophic ovary-type, but has never been examined before in insects carrying meroistic telotrophic ovaries. In the present study, we attempted to thoroughly describe the three different types (I, II and III) of PCD occurring during oogenesis in the meroistic telotrophic ovary of the Coleoptera species Adalia bipunctata, at different developmental ages of the adult female insects. We reveal that in the ladybird beetle A. bipunctata, the ovarian tropharia undergo age-dependent forms of apoptotic, autophagic and cytoplasmic (paraptotic-like) cell death, which seem to operate in a rather synergistic fashion, in accordance with previous observations in Diptera and Lepidoptera species. Furthermore, we herein demonstrate the occurrence of morphogenetically abnormal ovarioles in A. bipunctata female insects. These atretic ovarioles collapse and die through a PCD-mediated process that is characterized by the combined activation of all three types of PCD. Conclusively, the distinct cell death programs (I, II and III) specifically engaged during oogenesis of A. bipunctata provide strong evidence for the structural and functional conserved nature of PCD during insect evolution among meroistic telotrophic and meroistic polytrophic ovary-type insects.     

Ontogenesis of the ovary in a moth midge,Tinearia alternata Say (Diptera: Psychodidae)

In a psychodid, Tinearia alternata, the initial differentiation of the polytrophic ovary occurs during the early larval stages. Early in development, each ovary anlage is a solid organ subdivided into three distinct zones: the cortex houses germ cells and somatic interstitial tissue, while two other somatic regions will give rise to the oviduct calyx and anterior part of the lateral oviduct. Germ cell cluster formation precedes the development of ovarioles. Each ovariole houses only one functional egg chamber. All ovarioles within paired ovaries are developmentally synchronized. In the larval ovaries, the newly formed egg chambers and then the ovarioles are intermingeled with and surrounded by the somatic interstitial tissue of the ovary cortex. The interstitial cells give rise to all the somatic elements of the ovarioles. In the pupal ovaries, the remaining interstitial tissue degenerates; thus, the ovarioles protrude into the body cavity. The ovaries in psychodids develop relatively large and swollen oviduct calyxes that are equivalent to receptaculum seminis (spermatheca). The morphological differentiation of germ cells within the egg chambers starts during late larval/early pupal stages. Nurse cell nuclei contain prominent nucleoli and polytene chromosomes. Oocyte growth results from accumulation of yolk and then, in the final stages of oogenesis, from an inflow of cytoplasm from the nurse cells. J. Morphol. 236:167–177, 1998. © 1998 Wiley-Liss, Inc.     

The ovaries of Mecoptera: basic similarities and one exception to the rule

Two entirely different types of ovaries (ovarioles) have been described in mecopterans. In the representatives of Meropeidae, Bittacidae, Panorpodidae and Panorpidae the ovarioles are of the polytrophic-meroistic type. Four regions: a terminal filament, germarium, vitellarium and ovariole stalk can be distinguished in the ovarioles. The germaria house numerous germ cell clusters. Each cluster arises as a result of 2 consecutive mitoses of a cystoblast and consists of 4 sibling cells. The oocyte always differentiates from one of the central cells of the cluster, whereas the remaining 3 cells develop into large, polyploid nurse cells. The vitellaria contain 7-12 growing egg chambers (= oocyte-nurse cell complexes). In contrast, the ovaries of the snow flea, Boreus hyemalis, are devoid of nurse cells and therefore panoistic (secondary panoistic). The ovarioles are composed of terminal filaments, vitellaria and ovariole stalks only; in adult females functional germaria are absent. Histochemical tests suggest that amplification of rDNA takes place in the oocyte nuclei. Resulting dense nucleolar masses undergo fragmentation into multiple polymorphic nucleoli. The classification of extant mecopterans as well as the phylogenetic relationships between Mecoptera and Siphonaptera are discussed in the context of presented data.     

Cell death in ovarioles causes permanent sterility in Frieseomelitta varia worker bees

Frieseomelitta varia worker bees do not lay eggs even when living in queenless colonies, a condition that favors ovary development and oviposition in the majority of highly social bees. The permanent sterility of these worker bees was initially attributed to a failure in ovary morphogenesis and differentiation. Using transmission electron microscopy we found that at the beginning of the pupal phase the ovaries of F. varia workers are formed by four ovarioles, each of them composed of 1) a terminal filament at the apex of the ovarioles, containing juxtaposed and irregularly shaped cells, 2) a germarium with clusters of cystocytes and prefollicular cells showing long cytoplasmic projections that envelop the cystocyte clusters, 3) fusiform interfollicular and basal stalk precursor cells, and 4) globular, irregularly contoured basal cells with large nuclei. However, during the pupal phase an accentuated and progressive process of cell death takes place in the ovarioles. The dying cells are characterized by large membrane bodies, electron-dense apoptotic bodies, vacuoles, vesiculation, secondary lysosomes, enlarged rough endoplasmic reticulum cisternae, swollen mitochondria, pycnotic nuclei, masses of chromatin adjacent to the convoluted nuclear envelope, and nucleoli showing signs of fragmentation. Cell death continues in ovarioles even after the emergence of the workers. Once they become nurse bees, the ovaries have become transformed into a cell mass in which structurally organized ovarioles can no longer be identified. In F. varia workers, ovariole cell death most certainly is part of the program of caste differentiation.     

Heteropteran ovaries: variations on the theme

Ovaries of heteropterans consist of telotrophic meroistic ovarioles that are composed of apically located tropharium and basal vitellarium, containing developing oocytes. The tropharium (trophic chamber) houses trophocytes (nurse cells) that are connected with the centrally located trophic core. The organization of the heteropteran tropharia is highly variable and differs in representatives of primitive versus advanced families. The differences concern the mitotic activity of the apical nurse cells, organization of the trophocytes (individual cells or "syncytial lobes"), their connection with the trophic core and the development of F-actin meshwork around the trophic core. In members of primitive taxa of the Heteroptera, tropharia are composed of individual, usually mononucleate trophocytes. On the contrary, tropharia in advanced heteropterans are built of large "cytoplasmic lobes" that contain several trophocyte nuclei. Mitotic divisions of the trophocytes in the apical part of the trophic chamber are observed in most bugs (except Dipsocoridae, Miridae and Cimicidae). Tropharia of Miridae represent an entirely different organization (they are built of one type of highly polyploid trophocytes). Anagenesis of heteropteran trophic chamber is discussed in the context of presented data.     

Egg envelopes of Baetis rhodani and Cloeon dipterum (Ephemeroptera, Baetidae): a comparative analysis between an oviparous and an ovoviviparous species

A comparative ultrastructural investigation on the eggshell (vitelline and chorionic envelopes) has been carried out in the nymphs of two mayfly species encompassed into the Baetidae, namely Baetis rhodani and Cloeon dipterum. During oocyte differentiation in the meroistic telotrophic ovarioles, gametes are connected to discrete nurse cells by trophic cords. In B. rhodani, each ovariole contains several eggs arranged in sequence, whereas in C. dipterum each contains a single egg. Follicle cells are competent for vitelline and chorionic envelope synthesis. Baetis rhodani is an oviparous species and the chorion is fairly thick, formed by an alveolate endochorion and a fibrillar exochorion delimited by a honey‐comb roof. Cloeon dipterum stands out among Ephemeroptera for being ovoviviparous. In B. rhodani, ovulation starts in the older nymphs with dark wing‐pads, whereas in C. dipterum choriogenesis ends in the imaginal stage. Here the chorion is very thin and laid eggs hatch almost immediately, allowing the larvulae to move out. The maturation of a single egg per ovariole is synchronized with the achievement of the adult stage. The acquisition of ovoviviparity has led to remarkable changes in the ovariole organization along with a simplification of the eggshell structure.