Posterior Egg Pole Research Articles

Egg structure and ultrastructure of Paterdecolyus yanbarensis (Insecta, Orthoptera, Anostostomatidae, Anabropsinae)

The egg structure of Paterdecolyus yanbarensis was examined using light, scanning electron and transmission electron microscopy. The egg surface shows a distinct honeycomb pattern formed by exochorionic ridges. Several micropyles are clustered on the ventral side of the egg. The egg membrane is composed of an exochorion penetrated with numerous aeropyles, an endochorion, and an extremely thin vitelline membrane. The endochorion is thickened at the posterior egg pole, probably associated with water absorption. A comparison of egg structure among Orthoptera revealed that the micropylar distribution pattern is conserved in Ensifera and Caelifera and might be regarded as a groundplan feature for each group; in Ensifera, multiple micropyles are clustered on the ventral side of the egg, whereas in Caelifera, micropyles are arranged circularly around the posterior pole of the egg.

Rescue of tropomyosin deficiency inDrosophilaand human cancer cells by synaptopodin reveals a role of tropomyosin α in RhoA stabilization

Tropomyosins are widespread actin-binding proteins that influence numerous cellular functions including actin dynamics, cell migration, tumour suppression, and Drosophila oocyte development. Synaptopodin is another actin-binding protein with a more restricted expression pattern in highly dynamic cell compartments such as kidney podocyte foot processes, where it promotes RhoA signalling by blocking the Smurf1-mediated ubiquitination of RhoA. Here, we show that synaptopodin has a shorter half-life but shares functional properties with the highly stable tropomyosin. Transgenic expression of synaptopodin restores oskar mRNA localization in Drosophila oocytes mutant for TmII, thereby rescuing germline differentiation and fertility. Synaptopodin restores stress fibres in tropomyosin-deficient human MDA-MB 231 breast cancer cells and TPMα-depleted fibroblasts. Gene silencing of TPMα but not TPMβ causes loss of stress fibres by promoting Smurf1-mediated ubiquitination and proteasomal degradation of RhoA. Functionally, overexpression of synaptopodin or RhoA(K6,7R) significantly reduces MDA-MB 231 cell migration. Our findings elucidate RhoA stabilization by structurally unrelated actin-binding proteins as a conserved mechanism for regulation of stress fibre dynamics and cell motility in a cell type-specific fashion.

Vasa mRNA accumulates at the posterior pole during blastoderm formation in the flour beetle Tribolium castaneum

The correct specification of germ cells during embryogenesis is a fundamental step in life that ensures the existence of the next generation. Although different species display various cellular modes of generating germ cells, the product of the vasa gene proves to be a reliable marker of primordial germ cells in metazoans [Extavour and Akam (2003) 130:5869-5884; Raz (2000) 1:1017]. Here, I report the isolation of the vasa ortholog from the red flour beetle Tribolium castaneum, named Tc-vasa, and describe its sequence, its genomic organisation and its expression pattern during early embryogenesis. Unlike in Drosophila where vasa messenger RNA (mRNA) is ubiquitously distributed in the egg, Tc-vasa mRNA gradually accumulates at the posterior egg pole during blastoderm formation. Shortly before gastrulation, Tc-vasa mRNA marks a group of intra-blastodermal cells at the posterior pole. In the germ rudiment, a ball-like group of vasa-positive cells adheres to the growth zone at the posterior end of the embryo. These vasa-positive cells likely represent the primordial germ cells that have not been described in Tribolium prior to gonad formation. At the beginning of germ growth, a small band of vasa-positive cells starts to migrate along the dorsal side of the growth zone. vasa transcription ceases during further germ band extension. In contrast to Drosophila, Tc-vasa transcripts cannot be detected in the germ cells within the gonadal anlage after segmentation is completed.

Facultative polar extrusion of yolk masses and of hatching at the posterior egg pole in tephritid fruit flies (Diptera)

The eggs of Anastrepha fracterculus, A. sororcula, A. obliqua, A. serpentina and of Ceratitis capitata (Diptera : Tephritidae) show considerable morphological differences. Differences were also observed in the depth the eggs are inserted into the host fruits. In vivo inspection of the embryos of Anastrepha species showed that they may extrude polar masses of yolk before dorsal closure. In A.fracterculus and A. sororcula, 4 classes of embryos were found : (1) yolk masses at both ends; (2) masses only at the anterior end; (3) just a posterior mass and (4) embryos without any free mass. Only embryos of classes 3 and 4 were found in A. obliqua, while in A. serpentina, all the embryos extruded masses by the posterior end (class 3). There are variations in the frequencies of the classes as well as in the size of the extruded masses. Just before eclosion, the larvae of the Anastrepha species ingest the anterior mass of yolk (when present), turn round inside the egg, suck the posterior extruded mass, and eclose near the posterior-third of the eggs. No masses were present in samples of Ceratitis capitata embryos and the larvae do not present these complex behaviors and eclose near the anterior pole of the egg. The few previous studies on the embryology of other tephritid species did not report similar phenomena. The differences found among the species in relation to the phenomenon of yolk mass extrusion correlate, apparently, to the taxonomic relationship among them, as established before by genetic and morphologic parameters. Additional analyses of other species and genera are required to confirm if yolk mass extrusion is distinctive for the genus Anastrepha.

Developmental and molecular characterization of mutations in the Drosophila-raf serine/threonine protein kinase.

Formation of the tail region of the Drosophila larva requires the activities of the terminal class genes. Genetic and molecular analyses of these genes suggests that localized activation of the receptor tyrosine kinase torso at the posterior egg pole triggers a signal transduction pathway. This pathway, mediated through the serine/threonine protein kinase D-raf and the protein tyrosine phosphatase corkscrew, controls the domains of expression of the transcription factors tailless and huckebein. In this paper, we report the molecular and developmental characterization of mutations in the D-raf gene. We show that mutations that alter conserved residues known to be necessary for kinase activity are associated with a null phenotype, demonstrating that D-raf kinase activity is required for its role in torso signaling. Another mutation, D-rafPB26, which prematurely truncates the kinase domain shows a weaker maternal effect phenotype that is strikingly similar to the corkscrew maternal effect phenotype, suggesting that a lower amount of kinase activity decreases the terminal signaling pathway. Finally, molecular and developmental characterization of two mutations that affect the late D-raf zygotic function(s) implies a novel role for D-raf in cell fate establishment in the eye. One of these mutations, D-rafC110, is associated with a single amino acid change within the putative D-raf regulatory region, while the other, D-rafHM-7, most likely reduces the wild-type amount of D-raf protein.

Parabiotic development of fused eggs from the Hymenopteron, Pimpla turionellae, and of eggs injected with energids

Explanted oocytes and eggs of different developmental stages from the Hymenopteron Pimpla were fused in pairs as "parabiotic tandems". Interactions within the tandem were analysed by time lapse films. Except for the exchange of nuclei, no joint development was observed, and each partner followed its own time pattern. The rapid cell cycles of the cleavage energids switched over to longer cycles according to the local developmental stage of the different egg regions, although all of the nuclei were still contained in a single plasmodium. In a second experimental series, nuclei in newly deposited eggs were X-rayed and replaced by cleavage energids from later stages injected into the wrong (=posterior) egg pole. Even injected blastoderm nuclei immediately took up mitotic activities and underwent rapid cell cycles characteristic of early cleavage. Normal embryos could be formed, although the nuclei had populated the egg in a reverse direction.

Ultrastructural features of egg development in oviparae of the vetch aphid, Megoura viciae buckton

In the ovaries of the oviparous morph of the aphid, Megoura viciae, resting oocytes are located in the basal region of each germarium. During previtellogenic egg development, electron-dense spheres appear in the ooplasm. During vitellogenesis a brush border develops at the oolemma, and numerous protein and lipid-like spheres accumulate in the egg cytoplasm. Follicle cells are of two morphologically distinct types, termed ‘type 1’ and ‘type 2’ follicle cells. Unlike the more numerous ‘type 1’ cell, ‘type 2’ cells do not become patent. The acellular tunica propria exterior to follicle cell apices remains intact throughout egg development. During late vitellogenesis symbiont invasion of eggs takes place via ‘receptor’ cells encircling the pedicel at the posterior egg pole. These cells shrink and/or degenerate to create intercellular spaces that facilitate symbiont transmission. The end of vitellogenesis is marked by vitelline membrane formation and secretion of the chorionic layers, at which time the next egg in the ovariole undergoes final stages of previtellogenic growth and enters vitellogenesis.

The Drosophila gene torso encodes a putative receptor tyrosine kinase

The maternal gene torso, required for determination of anterior and posterior terminal structures in the Drosophila embryo, was cloned using P-element tagging. Genetic evidence suggests that the action of the gene product is spatially restricted to the terminal regions; the torso messenger RNA, however, is evenly distributed. Structural similarities of the predicted torso protein with growth-factor receptor tyrosine kinases suggest that the spatial restriction of torso activity results from a localized activation of the torso protein at the anterior and posterior egg pole.

Drosophila nurse cells produce a posterior signal required for embryonic segmentation and polarity.

The segmental pattern of insect embryos depends on influences from morphogenetic centres near each of the egg poles. In Drosophila, maternal effect mutations are known that impair the normal function of each centre. Injection of wild-type cytoplasm into mutant eggs has revealed that morphogenetic signals localized at the anterior and posterior pole of eggs can be transplanted. We show here that these activities can also be detected during oogenesis. Posterior activity can be recovered at an early stage (stage 10, ref. 5) from the oocyte-nurse cell complex, but anterior activity can only be detected in the mature oocytes (stage 14). We conclude that the bicoid-dependent anterior signal, although produced by the nurse cells, does not become active before it is localized to the anterior egg pole, whereas posterior activity can be detected in the nurse cells before, and therefore independently of, its localization to the posterior egg pole.

Altered anteroposterior polarity of micropyle ring formation in the eggs of Pyrrhocoris apterus L. (Heteroptera : Pyrrhocoridae)

The anterior pole of the normal eggs of Pyrrhocoris apterus (Heteroptera : Pyrrhocoridae) carries a ring of aeromicropyles. In DMR (double micropyle ring) eggs, an additional aeromicropyle ring occurs on the posterior egg pole. The number of micropyles forming this supernumerary ring varies, being usually considerably lower than that observed in the normal (anterior) ring. More than 6% of eggs laid by the Apricot (Ap) mutant of P. apterus are of the DMR type. Reciprocal crosses between the wild-type and Ap homozygous bugs demonstrate a maternal effect of the Ap mutation on the DMR egg production. Since DMR eggs are segregated at a low frequency in the wild-type strains as well as in all analyzed mutations of the bug, the DMR trait is most probably not primarily coded by the Ap gene. Physiological mechanisms involved in the formation of DMR eggs and the correlations between altered egg shell morphology and embryogenesis are also discussed.

Manipulating the anteroposterior pattern of the Drosophila embryo

ABSTRACT The establishment of pattern and polarity in the insect egg has been studied intensively using a variety of experimental approaches. Drosophila, while the system of choice for genetic analysis of pattern formation has been rather neglected as an experimental organism and species with longer developmental time and larger eggs were preferred in classical studies. Among the dipteran insects, midges such as Chironomous and Smittia with their transparent chorion and synchronous development were found more rewarding. The classical methods of ligation, puncture, transplantation, destruction or removal of material, and centrifugation were applied to eggs of a variety of insect species. Although the degree of response to experimental manipulation was found to be widely different, there were similarities in the type of abnormal patterns produced by the various treatments which suggested more general conclusions:-the anteroposterior pattern is probably controlled by two centres of activity, localized at the anterior and posterior egg pole respectively, with a long-range effect on the entire egg axis (reviewed by Sander, 1976).

Water Absorption by Eggs and Serosal Specialization as Clues to Evolutionary Trends in Heteroptera

Water absorption by eggs, expressed by increase in fresh egg weight, was observed in Notonecta triguttata Say, Gerris lacustris latiabdominis Miyamoto, and G. gracilicornis gracilicornis Horvath. In these aquatic and semiaquatic bugs, whose eggs develop in water, serosal specialization at the posterior egg pole (columnar serosa) occurs. Close association was observed between onset of water absorption by the eggs and that of embryonic growth. Fresh weight of the developing eggs hardly changed in Hygia opaca Uhler, the eggs of which develop on damp ground, whereas it decreased steadily in Scotinophora lurida Burmeister and Coptosoma punctissimum Montandon, whose eggs develop on plant leaves. Serosal specialization at the posterior egg pole was never observed in these terrestrial bugs. Rearing of these heteropteran eggs under unusual cultural conditions and topical application of a microcauterizer at the columnar serosa in G. paludum insularis Motschulsky suggested that water absorption by the eggs was necessary to ensure enough oxygen to the embryos. Occurrence of serosal specialization and resulting water absorption by the eggs are considered from the viewpoint of heteropteran evolution.

The role of microtubules in the differentiation of ovarian follicles during vitellogenesis inDrosophila.

Microtubule distribution in vitellogenicDrosophila follicles has been analysed with a tubulin-specific monoclonal antibody. The functional role of microtubules in germ line cells and somatic follicle cells was tested by allowing vitellogenesis to proceed in vivo or in vitro in the presence of colchicine or other microtubule inhibitors. Resulting morphological abnormalities were studied in 1 μm sections of Epon-embedded follicles. In addition, the effect of the inhibitors on cytoplasmic streaming was analysed by time-lapse cinematography. The results can be summarized as follows: 1. Cytoplasmic streaming in the oocyte of stage 10-12 follicles was inhibited by colchicine, while streaming of nurse cell cytoplasm into the oocyte continued. 2. This differential effect on cytoplasmic streaming was also apparent in histological sections. Since in inhibitor-treated follicles the nurse cell cytoplasm streaming into the oocyte did not mix with the ooplasm, the oocyte became stratified with respect to cytoplasmic constituents (e.g. yolk spheres were restricted to lateral and posterior locations and nurse-cell-derived lipid droplets to anterior and central positions). 3. The oocyte nucleus is held in place by a dense microtubule network. After treatment with the inhibitors, the oocyte nucleus became translocated towards the posterior egg pole. 4. Actively migrating follicle cells (border cells and cells migrating centripetally between nurse cells and oocyte at stage 10B) stained brightly with anti-tubulin. In the pressence of colchicine, these groups of cells showed various abnormalities and no centripetal migration was observed. 5. If inhibitors were applied after this cell migration had taken place, these follicle cells became disorganized and often produced a highly irregularly shaped vitelline membrane. At stage 12 the follicle cells at the anterior egg pole (where normally the chorionic appendages and other specific structures of the egg shell are formed) rounded up, became highly irregular and did not produce any recognizable structures. 6. Exocytosis of yolk and components of the vitelline envelope from the columnar follicle cells surrounding the oocyte at stage 10 became abnormal in the presence of microtubule inhibitors. Preliminary experiments indicate that yolk accumulated in follicle cells while vitelline membrane products continued to be exocytosed. Apico-basal follicle cell polarity was apparently affected by the drug in so far as vitelline membrane material accumulated not only at the apical end (which is normal) but also laterally in the intercellular space between the follicle cells.

Ionic current patterns in insect ovaries: [formula omitted]. [formula omitted] and [formula omitted]. [formula omitted]. [formula omitted], Department of Zoology, University of Manitoba, Winnipeg, Manitoba, Canada R3T 2N2

In the ovaries of the oviparous morph of the aphid, Megoura viciae, resting oocytes are located in the basal region of each germarium. During previtellogenic egg development, electron-dense spheres appear in the ooplasm. During vitellogenesis a brush border develops at the oolemma, and numerous protein and lipid-like spheres accumulate in the egg cytoplasm. Follicle cells are of two morphologically distinct types, termed ‘type 1’ and ‘type 2’ follicle cells. Unlike the more numerous ‘type 1’ cell, ‘type 2’ cells do not become patent. The acellular tunica propria exterior to follicle cell apices remains intact throughout egg development. During late vitellogenesis symbiont invasion of eggs takes place via ‘receptor’ cells encircling the pedicel at the posterior egg pole. These cells shrink and/or degenerate to create intercellular spaces that facilitate symbiont transmission. The end of vitellogenesis is marked by vitelline membrane formation and secretion of the chorionic layers, at which time the next egg in the ovariole undergoes final stages of previtellogenic growth and enters vitellogenesis.

Photoreversible inhibition by ultraviolet light of germ line development in Smittia sp. (Chironomidae, Diptera)

Pole cell formation in embryos of the parthenogenetic midge, Smittia sp., can be delayed or inhibited by irradiation of the posterior egg pole with ultraviolet light (uv). This leaves the schedule of nuclear divisions and chromosome eliminations virtually unaffected. However, uv irradition delays the precocious migration to the posterior pole of one nucleus, which normally becomes included in the first pole cell. This effect is photoreversible, i.e., mitigated by application of blue light after uv. Photoreversibility indicates that a nucleic acid component is involved as an effective target. During normal development of Smittia a number of chromosomes are eliminated during mitosis V, not only from somatic nuclei but also in the germ line. In the latter, this mitosis takes place during the first gonial division in the larva. After uv irradiation, the first pole cell nucleus has undergone supernumerary mitoses before pole cell formation and, as a result, is driven into mitosis V precociously as the pole cell divides. This is frequently associated with chromosome elimination from pole cells, which in turn is correlated with subsequent disappearance of already formed pole cells. Adults derived from embryos without pole cells do not form ovaries. Pole cell formation, pole cell preservation, and ovary development are separately inhibited by uv, and inhibition of each step is photoreversible. The results are discussed in the context of germ cell determination, protection against chromosome elimination, and the role of chromosomes limited to the germ line.

The movement pattern during cleavage in artificially activated eggs ofPimpla turionellae (Hym.)

The early embryonic development ofPimpla is characterized by a complicated temporal and spatial pattern of ooplasmic movements detected in time-lapse films made during cleavage. The modified movements observed after the architecture of oviposited eggs had been altered artificially by centrifugation indicated that there are different dynamic systems for ooplasmic streaming, contractions, and nuclear migration. The discovery that unlaid, explanted oocytes ofPimpla can be activated by mechanical deformation provided a new way of studying alterations of egg architecture, nucleocytoplasmic interactions, and the control of morphogenetic processes during cleavage and blastoderm formation. In this article, development and ooplasmic movements in explanted oocytes with and without articifial activation are described and compared with those observed in eggs after normal oviposition. Four categories of explanted "eggs" can be distinguished: 1. Inexplanted eggs which are not activated by mechanical deformation, no movement of egg plasm can be observed, and nuclear multiplication never takes place. Thus the completion of meiosis as well as the ooplasmic movements must be triggered by deformation of the egg in the ovipositor. 2. Inartificially activated eggs with diphasic blastoderm formation, the following deviations from normal development are registered. The mixing motion at the anterior end of the egg, the transfer flow, and the forward component of the fountain flow are all absent. Instead, a homogenizing movement is observed in the ooplasm of the anterior region of the egg. The energids in this region then migrate directly to the periphery, and in due time form the blastoderm (first phase of blastoderm formation). In the posterior 2/3 of the egg, blastoderm formation is slightly retarded. The so-called mixing motion, the unipolar flow and the caudal part of the fountain flow take place as in normal development, and the energids become distributed throughout a central plasm column before they migrate radially to initiate a second phase of blastoderm formation. There are marked ooplasmic contractions at the egg poles. 3. Forartificially activated eggs with successive blastoderm formation we recorded the same deviations from normal development as in the cephalic region of eggs of category 2. Blastoderm formation also occurs in due time. In the caudal region of the egg, a "dilated" unipolar flow is found. The fountain flow is reduced and greatly delayed. Energids migrating from the anterior egg region into the posterior may be carried to the posterior egg pole in a central plasm by the fountain flow. A peripheral, ring-shaped contraction moving in a posterior direction indicates the zone where the preblastoderm gradually forms. A marked antero-posterior time gradient is evident in blastoderm formation. Development of these eggs is greatly retarded up to hatching of the larvae. 4. Ineggs without blastoderm formation after activating treatment, no energids could be found apart from the meiotic nuclei. Nevertheless, the ooplasmic movement pattern and the histological aspect of these eggs sometimes resembled those of eggs oviposited by the female. Also, formation of pseudo-pole cells could be observed. These observations demonstrate that pseudocleavage takes place in such eggs. The streaming system is apparently able to achieve the pattern of ooplasmic movements independently of nuclear multiplication. Our observations demonstrate the autonomy of the streaming systems and of energid migration. The third dynamic system, ooplasmic contractions, occurs in artificially activated eggs combined with the streaming system and/or nuclear multiplication. It may possibly act independently in the very early contractions at the egg poles; these may be comparable to events at the elevation of a fertilization membrane. The discussion concerns exogeneous and endogeneous factors which may affect the pattern of movements, and the functions of mixing motion and unipolar flows in restricting the early nuclear migration to the central plasm. Also discussed are the significance of the anterior and posterior initial regions ("Initialbereiche") and ofsuccessive blastoderm formation with respect to the relation between long-germ and short-germ egg types.