Comparative aspects of animal oogenesis

A role for retrotransposon LINE-1 in fetal oocyte attrition in mice

Fetal oocyte attrition (FOA) is a conserved but poorly understood process of elimination of more than two-thirds of meiotic prophase I (MPI) oocytes before birth. We now implicate retrotransposons LINE-1 (L1), activated during epigenetic reprogramming of the embryonic germline, in FOA in mice. We show that wild-type fetal oocytes possess differential nuclear levels of L1ORF1p, an L1-encoded protein essential for L1 ribonucleoprotein particle (L1RNP) formation and L1 retrotransposition. We demonstrate that experimental elevation of L1 expression correlates with increased MPI defects, FOA, oocyte aneuploidy, and embryonic lethality. Conversely, reverse transcriptase (RT) inhibitor AZT has a profound effect on the FOA dynamics and meiotic recombination, and it implicates an RT-dependent trigger in oocyte elimination in early MPI. We propose that FOA serves to select oocytes with limited L1 activity that are therefore best suited for the next generation.

In mouse oocytes the mitochondrion-originated germinal body-like structures accumulate mouse Vasa homologue (MVH) protein

Mouse Vasa homologue (MVH) antibodies were applied to mouse Graafian oocytes to clarify if mitochondrion-originated germinal body-like structures, described previously by conventional electron microscopy, were associated with the germ plasm. It was found that both the mitochondrion-like structures with cristae and the germinal body-like structures that lacked any signs of cristae were labelled specifically by the anti-MVH antibody. Moreover, some granules were MVH-positive ultrastructural hybrids of the mitochondria and germinal body-like structures, the presence of which clearly supported the idea of a mitochondrial origin for the germinal body-like structures. This finding is the first evidence that mitochondrion-originated germinal body-like granules represent mouse germ plasm.

Transovarial transmission of a plant virus is mediated by vitellogenin of its insect vector

Most plant viruses are transmitted by hemipteroid insects. Some viruses can be transmitted from female parent to offspring usually through eggs, but the mechanism of this transovarial transmission remains unclear. Rice stripe virus (RSV), a Tenuivirus, transmitted mainly by the small brown planthopper (Laodelphax striatellus), is also spread to the offspring through the eggs. Here, we used the RSV–planthopper system as a model to investigate the mechanism of transovarial transmission and demonstrated the central role of vitellogenin (Vg) of L. striatellus in the process of virus transmission into the eggs. Our data showed Vg can bind to pc3 in vivo and in vitro and colocalize in the germarium. RSV filamentous ribonucleoprotein particles (RNPs) only accumulated in the terminal filaments and pedicel areas prior to Vg expression and was not present in the germarium until Vg was expressed, where RSV RNPs and Vg had colocalized. Observations by immunoelectron microscopy (IEM) also indicated that these two proteins colocalized in nurse cells. Knockdown of Vg expression due to RNA interference resulted in inhibition of the invasion of ovarioles by RSV. Together, the data obtained indicated that RSV RNPs may enter the nurse cell of the germarium via endocytosis through binding with Vg. Finally, the virus enters the oocytes through nutritive cords, using the same route as for Vg transport. Our results show that the Vg of L. striatellus played a critical role in transovarial transmission of RSV and shows how viruses can use existing transovarial transportation systems in insect vectors for their own purposes.

Numerous parasites including viruses, bacteria, and microsporidia can be maternally transmitted, with the parasite passing from mother to offspring, usually through eggs. However, the process of the parasites spreading into eggs from primarily infected tissues and the factors that mediate this process in live hosts or vectors are unknown due to the lack of useful tools. Here, we used several techniques to investigate the molecular mechanisms of transovarial transmission of Rice stripe virus (RSV), a plant virus belonging to the genus Tenuivirus, by its insect vector (Laodelphax striatellus). We found that the nucleocapsid protein of RSV bound to insect's vitellogenin (Vg) in vitro and in vivo. We also found that RSV invaded the egg tubes of the ovariole until Vg is highly expressed, then colocalized with Vg in the germarium. When Vg expression was knocked down due to RNA interference, the invasion of ovarioles by RSV decreased largely. Our study provides new insights into the transovarial transmission of an important viral pathogen that uses existing transovarial transportation systems in insect vectors to invade eggs.

Embryonic development of Rhynchophorus palmarum (Coleoptera: Curculionidae): dynamics of energy source utilization

Energy homeostasis is an essential process during oogenesis, nutrients are required for suitable embryonic development, and recently, studies have investigated metabolic activity during this process. This work aims the investigation of dynamics of energy source utilization of Rhynchophorus palmarum during embryogenesis. For this, we first evaluated the mobilization kinetics of the lipids and glycogen. Thereafter, the synthesis of RNA, protein, and the involvement of enzyme of the glycolytic and pentose-phosphate pathways. Results showed that lipid content decreased in contrast with the lipase activity. The total glycogen amounts it was partly consumed and the glucose content increased, but then values remained stable until hatching. Total RNA content increased, and no significant changes in total protein content were observed. A study of the glycolytic pathway data showed activity of hexokinase and pyruvate kinase at the beginning of embryogenesis. Furthermore, glucose-6-phosphate formed is driven into the pentose-phosphate pathway viewed the high activity of glucose-6-phosphate dehydrogenase. Finally, these results showed that mobilization of different energy sources together with different enzymatic activities has an important role in embryonic development of R. palmarum.

Seeking the origin of female germline stem cells in the mammalian ovary

The function of female germline stem cells (FGSCs, also called oogonial stem cells) in the adult mammalian ovary is currently debated in the scientific community. As the evidence to support or discard the possible crucial role of this new class of germ cells in mammals has been extensively discussed, in this review, we wonder which could be their origin. We will assume that FGSCs are present in the post-natal ovaries and speculate as to what origin and characteristics such cells could have. We believe that the definition of these features might shed light on future experimental approaches that could clarify the ongoing debate.

Nuclear genes with sex bias in Ruditapes philippinarum (Bivalvia, veneridae): Mitochondrial inheritance and sex determination in DUI species

Mitochondria are inherited maternally in most metazoans, but in bivalves with Doubly Uniparental Inheritance (DUI) a mitochondrial lineage is transmitted through eggs (F-type), and another through sperm (M-type). In DUI species, a sex-ratio distortion of the progeny was observed: some females produce a female-biased offspring (female-biased family), others a male-biased progeny (male-biased family), and others a 50:50 sex-ratio. A peculiar segregation pattern of M-type mitochondria in DUI organisms appears to be correlated with the sex bias of these families. According to a proposed model for the inheritance of M-type mitochondria in DUI, the transmission of sperm mitochondria is controlled by three nuclear genes, named W, X, and Z. An additional S gene with different dosage effect would be involved in sex determination. In this study, we analyzed structure and localization of three transcripts (psa, birc, and anubl1) with specific sex and family biases in the Manila clam Ruditapes philippinarum. In situ hybridization confirmed the localization of these transcripts in gametogenic cells. In other animals, homologs of these genes are involved in reproduction and ubiquitination. We hypothesized that these genes may have a role in sex determination and could also be responsible for the maintenance/degradation of spermatozoon mitochondria during embryo development of the DUI species R. philippinarum, so that we propose them as candidate factors of the W/X/Z/S system.

Gonadal expression of Foxo1, but not Foxo3, is conserved in diverse Mammalian species

The Foxos are key effectors of the PI3K/Akt signaling pathway and regulate diverse physiologic processes. Two of these factors, Foxo1 and Foxo3, serve specific roles in reproduction in the mouse. Foxo3 is required for suppression of primordial follicle activation in females, while Foxo1 regulates spermatogonial stem cell maintenance in males. In the mouse ovary, Foxo1 is highly expressed in somatic cells (but not in oocytes), suggesting an important functional role for Foxo1 in these cells. Given that invertebrate model species such as Caenorhabditis elegans and Drosophila melanogaster harbor a single ancestral Foxo homolog, these observations suggest that gene duplication conferred a selective advantage by permitting the Foxos to adopt distinct roles in oogenesis and spermatogenesis. Our objective was to determine if the remarkably specific expression patterns of Foxo1 and Foxo3 in mouse gonads (and, by inference, Foxo function) are conserved in diverse mammalian species. Western blotting was used to validate isoform-specific antibodies in rodents, companion animals, farm animals, nonhuman primates, and humans. Following validation of each antibody, immunohistochemistry was performed to ascertain Foxo1 and Foxo3 gonadal expression patterns. While Foxo1 expression in spermatogonia and granulosa cells was conserved in each species evaluated, Foxo3 expression in oocytes was not. Our findings suggest that Foxo3 is not uniquely required for primordial follicle maintenance in nonrodent species and that other Foxos, particularly Foxo1, may contribute to oocyte maintenance in a functionally redundant manner.

Mouse primordial germ cells produce cysts that partially fragment prior to meiosis

Mammalian germ cells divide mitotically and form nests of associated cells just prior to entering meiosis. At least some nests contain germline cysts that arise by synchronous, incomplete mitotic divisions, but others may form by aggregation. To systematically investigate early murine germ cell development, we lineage marked the progeny of individual, newly arrived primordial germ cells in the E10.5 gonad. All the marked germ cells initially develop into clones containing two, four or eight cells, indicating cyst formation. Surprisingly, growing cysts in both sexes partially fragment into smaller cysts prior to completion and associate with cysts from unrelated progenitors. At the time divisions cease, female clones comprise five cysts on average that eventually give rise to about six primordial follicles. Male cyst cells break apart and probably become spermatogonial stem cells. Thus, cysts are invariant units of mouse germ cell development and cyst fragmentation provides insight into the amplification of spermatogonial stem cells and the origin of primordial follicles.

Coordination of spermatogenic processes in the testis: lessons from cystic spermatogenesis

A common observation in the vertebrate testis is that new germ cell clones enter spermatogenesis proper before previously formed clones have completed their development. The extent to which the developmental advance of any given germ cell clone in any phase of spermatogenesis is dependent on that of neighboring clones and/or on the coordinating influence of associated Sertoli cells in the immediate vicinity or of others further away remains unclear. This review presents an overall synthesis of findings in an ancient vertebrate, the spiny dogfish shark and shows that, even at this phyletic level, the developmental advance of a given germ cell clone is the outcome of various processes emanating from its spatiotemporal relationship with (1) its own complement of Sertoli cells in the anatomically distinct spermatocyst and (2) Sertoli cells associated with other germ cell clones that lie upstream or downstream in the spermatogenic progression and that secrete, among others, androgen and estrogen destined for target sites upstream. Analysis of the protracted spermatogenic cycle shows the coordination in space and time of spermatogenic and steroidogenic events. Furthermore, the natural withdrawal of pituitary gonadotropin support in the dogfish causes a distinct and highly ordered gradient of apoptosis among the spermatogonial generations; this in turn is a major contributing factor to the cyclic nature of sperm production observed in this lower vertebrate. Because of the simplicity of their testicular organization, their cystic spermatogenesis and their phylogenetic position, cartilaginous fishes constitute a valid vertebrate reference system for comparative analysis with higher vertebrates.

A stockpile of ova in the grass frog Rana temporaria is established once for the life span. Do ovaries in amphibians and in mammals follow the same evolutionary strategy?

Most anuran amphibians produce high numbers of eggs during several consecutive breeding seasons. The question is still open whether oocytes are formed anew as a result of oogonial proliferation after each spawning or the definitive pool of oocytes is established during the juvenile period and is sufficient for the whole reproductive life span of a female. Our quantitative studies show that primary oogonia in adult female frogs can proliferate, but they fail to differentiate further and do not enter meiosis, and thereby there is no supplementation of new generations of oocytes after each spawning. Ovaries of one-year-old grass frogs contain (median) 53,447 diplotene oocytes, in two-years-old frogs this number decreased to 33,583 and eventually reached 25,679 in virgin mature females. More than 50% decrease in the total oocyte number was accompanied by massive degeneration (atresia) of oocytes. The final number of oocytes in a female forms a stock for 11-12 breeding seasons and exceeds the number of eggs produced during the potential reproductive life span of this species. The phylogenetic context of oocyte recruitment modes in the major clades of vertebrates is discussed in respect to their ability to replenish the stock (a renewable stock in ovaries named "open" vs. a non-renewable stock in ovaries named "closed").

Differentiation and function of the ovarian somatic cells in the pseudoscorpion, Chelifer cancroides (Linnaeus, 1761) (Chelicerata: Arachnida: Pseudoscorpionida)

Pseudoscorpion females carry fertilized eggs and embryos in specialized brood sacs, where embryos are fed with a nutritive fluid produced and secreted by somatic ovarian cells. We used various microscopic techniques to analyze the organization of the somatic cells in the ovary of a pseudoscorpion, Chelifer cancroides. In young specimens, the ovary is a cylindrical mass of internally located germline cells (oogonia and early previtellogenic oocytes) and two types of somatic cells: the epithelial cells of the ovarian wall and the internal interstitial cells. In subsequent stages of the ovary development, the oocytes grow and protrude from the ovary into the hemocoel (opisthosomal cavity). At the same time the interstitial cells differentiate into the follicular cells that directly cover the oocyte surface, whereas some epithelial cells of the ovarian wall form the oocyte stalks - tubular structures that connect the oocytes with the ovarian tube. The follicular cells do not seem to participate in oogenesis. In contrast, the cells of the stalk presumably have a dual function. During ovulation the stalk cells appear to contribute to the formation of the external egg envelope (chorion), while in the post-ovulatory phase of ovary function they cooperate with the other cells of the ovarian wall in the production of the nutritive fluid for the developing embryos.

Small RNAs in germline development

One of the most important and evolutionarily conserved strategies to control gene expression in higher metazoa is posttranscriptional regulation via small regulatory RNAs such as microRNAs (miRNAs), endogenous small interfering RNAs (endo-siRNAs), and piwi-interacting RNAs (piRNAs). Primordial germ cells, which are defined by their totipotent potential and noted for their dependence on posttranscriptional regulation by RNA-binding proteins, rely on these small regulatory RNAs for virtually every aspect of their development, including specification, migration, and differentiation into competent gametes. Here, we review current knowledge of the roles miRNAs, endo-siRNAs, and piRNAs play at all stages of germline development in various organisms, focusing on studies in the mouse.

Embryonic poly(A)-binding protein (EPAB) is required for oocyte maturation and female fertility in mice

Gene expression during oocyte maturation and early embryogenesis up to zygotic genome activation requires translational activation of maternally-derived mRNAs. EPAB [embryonic poly(A)-binding protein] is the predominant poly(A)-binding protein during this period in Xenopus, mouse and human. In Xenopus oocytes, ePAB stabilizes maternal mRNAs and promotes their translation. To assess the role of EPAB in mammalian reproduction, we generated Epab-knockout mice. Although Epab(-/-) males and Epab(+/-) of both sexes were fertile, Epab(-/-) female mice were infertile, and could not generate embryos or mature oocytes in vivo or in vitro. Epab(-/-) oocytes failed to achieve translational activation of maternally-stored mRNAs upon stimulation of oocyte maturation, including Ccnb1 (cyclin B1) and Dazl (deleted in azoospermia-like) mRNAs. Microinjection of Epab mRNA into Epab(-/-) germinal vesicle stage oocytes did not rescue maturation, suggesting that EPAB is also required for earlier stages of oogenesis. In addition, late antral follicles in the ovaries of Epab(-/-) mice exhibited impaired cumulus expansion, and a 8-fold decrease in ovulation, associated with a significant down-regulation of mRNAs encoding the EGF (epidermal growth factor)-like growth factors Areg (amphiregulin), Ereg (epiregulin) and Btc (betacellulin), and their downstream regulators, Ptgs2 (prostaglandin synthase 2), Has2 (hyaluronan synthase 2) and Tnfaip6 (tumour necrosis factor α-induced protein 6). The findings from the present study indicate that EPAB is necessary for oogenesis, folliculogenesis and female fertility in mice.

Developmental diversity of amphibians

The current model amphibian, Xenopus laevis, develops rapidly in water to a tadpole which metamorphoses into a frog. Many amphibians deviate from the X. laevis developmental pattern. Among other adaptations, their embryos develop in foam nests on land or in pouches on their mother's back or on a leaf guarded by a parent. The diversity of developmental patterns includes multinucleated oogenesis, lack of RNA localization, huge non-pigmented eggs, and asynchronous, irregular early cleavages. Variations in patterns of gastrulation highlight the modularity of this critical developmental period. Many species have eliminated the larva or tadpole and directly develop to the adult. The wealth of developmental diversity among amphibians coupled with the wealth of mechanistic information from X. laevis permit comparisons that provide deeper insights into developmental processes.

A very simple mode of follicular cell diversification in Euborellia fulviceps (Dermaptera, Anisolabididae) involves actively migrating cells

The ovaries of Euborellia fulviceps are composed of five elongated ovarioles of meroistic-polytrophic type. The individual ovariole has three discernible regions: the terminal filament, germarium, and vitellarium. The terminal filament is a stalk of flattened, disc-shaped somatic cells. In the germarium, germline cells in subsequent stages of differentiation are located, and the vitellarium comprises numerous ovarian follicles arranged linearly. The individual ovarian follicles within the vitellarium are separated by prominent interfollicular stalks. The follicles are composed by two germline cells only: an oocyte and a single, polyploid nurse cell, which are surrounded by a monolayer of somatic follicular cells (FCs). During subsequent stages of oogenesis, initially uniform follicular epithelium begins to diversify into morphologically and physiologically distinct subpopulations. In E. fulviceps, the FC diversification mode is rather simple and leads to the formation of only three different FC subpopulations: (1) cuboidal FCs covering the oocyte, (2) stretched FCs surrounding the nurse cell and (3) FCs actively migrating between oocyte and a nurse cell. We found that FCs from the latter subpopulation send long and thin filopodium-like and microtubule-rich processes penetrating between the oocyte and nurse cell membranes. This suggests that, in E. fulviceps, cells from at least one FCs subpopulation show the ability to change position within an ovarian follicle by means of active migration.

Cell lineage analysis of the mammalian female germline

Fundamental aspects of embryonic and post-natal development, including maintenance of the mammalian female germline, are largely unknown. Here we employ a retrospective, phylogenetic-based method for reconstructing cell lineage trees utilizing somatic mutations accumulated in microsatellites, to study female germline dynamics in mice. Reconstructed cell lineage trees can be used to estimate lineage relationships between different cell types, as well as cell depth (number of cell divisions since the zygote). We show that, in the reconstructed mouse cell lineage trees, oocytes form clusters that are separate from hematopoietic and mesenchymal stem cells, both in young and old mice, indicating that these populations belong to distinct lineages. Furthermore, while cumulus cells sampled from different ovarian follicles are distinctly clustered on the reconstructed trees, oocytes from the left and right ovaries are not, suggesting a mixing of their progenitor pools. We also observed an increase in oocyte depth with mouse age, which can be explained either by depth-guided selection of oocytes for ovulation or by post-natal renewal. Overall, our study sheds light on substantial novel aspects of female germline preservation and development.

Many aspects of mammalian female germline development during embryogenesis and throughout adulthood are either unknown or under debate. In this study we applied a novel method for the reconstruction of cell lineage trees utilizing microsatellite mutations, accumulated during mouse life, in oocytes and other cells, sampled from young and old mice. Analysis of the reconstructed cell lineage trees shows that oocytes are clustered separately from bone-marrow derived cells, that oocytes from different ovaries share common progenitors, and that oocyte depth (number of cell divisions since the zygote) increases significantly with mouse age.

Doubly uniparental inheritance of mitochondria as a model system for studying germ line formation

BACKGROUND

Doubly Uniparental Inheritance (DUI) of mitochondria occurs when both mothers and fathers are capable of transmitting mitochondria to their offspring, in contrast to the typical Strictly Maternal Inheritance (SMI). DUI was found in some bivalve molluscs, in which two mitochondrial genomes are inherited, one through eggs, the other through sperm. During male embryo development, spermatozoon mitochondria aggregate in proximity of the first cleavage furrow and end up in the primordial germ cells, while they are dispersed in female embryos.

METHODOLOGY/PRINCIPAL FINDINGS

We used MitoTracker, microtubule staining and transmission electron microscopy to examine the mechanisms of this unusual distribution of sperm mitochondria in the DUI species Ruditapes philippinarum. Our results suggest that in male embryos the midbody deriving from the mitotic spindle of the first division concurs in positioning the aggregate of sperm mitochondria. Furthermore, an immunocytochemical analysis showed that the germ line determinant Vasa segregates close to the first cleavage furrow.

CONCLUSIONS/SIGNIFICANCE

In DUI male embryos, spermatozoon mitochondria aggregate in a stable area on the animal-vegetal axis: in organisms with spiral segmentation this zone is not involved in cleavage, so the aggregation is maintained. Moreover, sperm mitochondria reach the same embryonic area in which also germ plasm is transferred. In 2-blastomere embryos, the segregation of sperm mitochondria in the same region with Vasa suggests their contribution in male germ line formation. In DUI male embryos, M-type mitochondria must be recognized by egg factors to be actively transferred in the germ line, where they become dominant replacing the Balbiani body mitochondria. The typical features of germ line assembly point to a common biological mechanism shared by DUI and SMI organisms. Although the molecular dynamics of the segregation of sperm mitochondria in DUI species are unknown, they could be a variation of the mechanism regulating the mitochondrial bottleneck in all metazoans.

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.

Small RNAs in early mammalian development: from gametes to gastrulation

Small non-coding RNAs, including microRNAs (miRNAs), endogenous small interfering RNAs (endo-siRNAs) and Piwi-interacting RNAs (piRNAs), play essential roles in mammalian development. The function and timing of expression of these three classes of small RNAs differ greatly. piRNAs are expressed and play a crucial role during male gametogenesis, whereas endo-siRNAs are essential for oocyte meiosis. By contrast, miRNAs are ubiquitously expressed in somatic tissues and function throughout post-implantation development. Surprisingly, however, miRNAs are non-essential during pre-implantation embryonic development and their function is suppressed during oocyte meiosis. Here, we review the roles of small non-coding RNAs during the early stages of mammalian development, from gamete maturation through to gastrulation.

Evolution and development of polarized germ cell cysts: new insights from a polychaete worm, Ophryotrocha labronica

Polarized oogenic cysts are clonal syncytia of germ cells in which some of the sister cells (cystocytes) differentiate not as oocytes, but instead as nurse cells: polyploid cells that support oocyte development. The intricate machinery required to establish and maintain divergent cell fates within a syncytium, and the importance of associated oocyte patterning for subsequent embryonic development, have made polarized cysts valuable subjects of study in developmental and cell biology. Nurse cell/oocyte specification is best understood in insects, particularly Drosophila melanogaster. However, polarized cysts have evolved independently in several other animal phyla. We describe the differentiation of female cystocytes in an annelid worm, the polychaete Ophryotrocha labronica. These worms are remarkable for their elegantly simple cysts, which comprise a single oocyte and nurse cell, making them an appealing complement to insects as subjects of study. To elucidate the process of cystocyte differentiation in O. labronica, we have constructed digital 3D models from electron micrographs of serially sectioned ovarian tissue. These models show that 2-cell cysts arise by fragmentation of larger "parental" cysts, rather than as independent units. The parental cysts vary in size and organization, are produced by asynchronous, indeterminate mitotic divisions of progenitor cystoblasts, and lack fusome-like organizing organelles. All of these characteristics represent key cytological differences from "typical" cyst development in insects like D. melanogaster. In light of such differences and the plasticity of female cyst structure among other animals, we suggest that it is time to reassess common views on the conservation of oogenic cysts and the importance of cysts in animal oogenesis generally.

MPK-1 ERK controls membrane organization in C. elegans oogenesis via a sex-determination module

Tissues that generate specialized cell types in a production line must coordinate developmental mechanisms with physiological demand, although how this occurs is largely unknown. In the Caenorhabditis elegans hermaphrodite, the developmental sex-determination cascade specifies gamete sex in the distal germline, while physiological sperm signaling activates MPK-1/ERK in the proximal germline to control plasma membrane biogenesis and organization during oogenesis. We discovered repeated utilization of a self-contained negative regulatory module, consisting of NOS-3 translational repressor, FEM-CUL-2 (E3 ubiquitin ligase), and TRA-1 (Gli transcriptional repressor), which acts both in sex determination and in physiological demand control of oogenesis, coordinating these processes. In the distal germline, where MPK-1 is not activated, TRA-1 represses the male fate as NOS-3 functions in translational repression leading to inactivation of the FEM-CUL-2 ubiquitin ligase. In the proximal germline, sperm-dependent physiological MPK-1 activation results in phosphorylation-based inactivation of NOS-3, FEM-CUL-2-mediated degradation of TRA-1 and the promotion of membrane organization during oogenesis.

The developing ovary of the South American plains vizcacha, Lagostomus maximus (Mammalia, Rodentia): massive proliferation with no sign of apoptosis-mediated germ cell attrition

Apoptosis-dependent massive germ cell death is considered a constitutive trait of the developing mammalian ovary that eliminates 65–85% of the germinal tissue depending on the species. After birth and during adult lifetime, apoptotic activity moves from the germ cell proper to the somatic compartment, decimating germ cells through follicular atresia until the oocyte reserve is exhausted. In contrast, the South American rodent Lagostomus maximus shows suppressed apoptosis-dependent follicular atresia in the adult ovary, with continuous folliculogenesis and massive polyovulation, which finally exhausts the oocyte pool. The absence of follicular atresia in adult L. maximus might arise from a failure to move apoptosis from the germinal stratum to the somatic compartment after birth or being a constitutive trait of the ovarian tissue with no massive germ cell degeneration in the developing ovary. We tested these possibilities by analysing oogenesis, expression of germ cell-specific VASA protein, apoptotic proteins BCL2 and BAX, and DNA fragmentation by TUNEL assay in the developing ovary of L. maximus. Immunolabelling for VASA revealed a massive and widespread colonisation of the ovary and proliferation of germ cells organised in nests that disappeared at late development when folliculogenesis began. No sign of germ cell attrition was found at any time point. BCL2 remained positive throughout oogenesis, whereas BAX was slightly detected in early development. TUNEL assay was conspicuously negative throughout the development. These results advocate for an unrestricted proliferation of germ cells, without apoptosis-driven elimination, as a constitutive trait of L. maximus ovary as opposed to what is normally found in the developing mammalian ovary.

Loss of the extraproteasomal ubiquitin receptor Rings lost impairs ring canal growth in Drosophila oogenesis

Rings lost fulfills a novel function in Drosophila development for a ubiquitin receptor as an essential mediator of ring canal growth during oogenesis.

In Drosophila melanogaster oogenesis, there are 16 germline cells that form a cyst and stay connected to each other by ring canals. Ring canals allow the cytoplasmic transport of proteins, messenger ribonucleic acids, and yolk components from the nurse cells into the oocyte. In this paper, we describe the protein Rings lost (Rngo) and show that it is required for ring canal growth in germline cysts. rngo is an essential gene, and germline clones of a rngo-null allele show defects in ovary development, including mislocalization of ring canal proteins and fusion of germline cells. Rngo appears to be a ubiquitin receptor that possesses a ubiquitin-like domain, a ubiquitin-associated domain, and a retroviral-like aspartate protease (RVP) domain. Rngo binds to ubiquitin and to the 26S proteasome and colocalizes with both in germline cells, and its RVP domain is required for dimerization of Rngo and for its function in vivo. Our results thus show, for the first time, a function for a ubiquitin receptor in Drosophila development.

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.

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.

Sperm and oocyte communication mechanisms controlling C. elegans fertility

During sexual reproduction in many species, sperm and oocyte secrete diffusible signaling molecules to help orchestrate the biological symphony of fertilization. In the Caenorhabditis elegans gonad, bidirectional signaling between sperm and oocyte is important for guiding sperm to the fertilization site and inducing oocyte maturation. The molecular mechanisms that regulate sperm guidance and oocyte maturation are being delineated. Unexpectedly, these mechanisms are providing insight into human diseases, such as amyotrophic lateral sclerosis, spinal muscular atrophy, and cancer. Here we review sperm and oocyte communication in C. elegans and discuss relationships to human disorders.

Altered sex hormone concentrations and gonadal mRNA expression levels of activin signaling factors in hatchling alligators from a contaminated Florida lake

Activins and estrogens participate in regulating the breakdown of ovarian germ cell nests and follicle assembly in mammals. In 1994, our group reported elevated frequencies of abnormal, multioocytic ovarian follicles in 6 month old, environmental contaminant-exposed female alligators after gonadotropin challenge. Here, we investigated if maternal contribution of endocrine disrupting contaminants to the egg subsequently alters estrogen/inhibin/activin signaling in hatchling female offspring, putatively predisposing an increased frequency of multioocytic follicle formation. We quantified basal and exogenous gonadotropin-stimulated concentrations of circulating plasma steroid hormones and ovarian activin signaling factor mRNA abundance in hatchling alligators from the same contaminated (Lake Apopka) and reference (Lake Woodruff) Florida lakes, as examined in 1994. Basal circulating plasma estradiol and testosterone concentrations were greater in alligators from the contaminated environment, whereas activin/inhibin betaA subunit and follistatin mRNA abundances were lower than values measured in ovaries from reference lake animals. Challenged, contaminant-exposed animals showed a more robust increase in plasma estradiol concentration following an acute follicle stimulating hormone (FSH) challenge compared with reference site alligators. Aromatase and follistatin mRNA levels increased in response to an extended FSH challenge in the reference site animals, but not in the contaminant-exposed animals. In hatchling alligators, ovarian follicles have not yet formed; therefore, these endocrine differences are likely to affect subsequent ovarian development, including ovarian follicle assembly.

Is the mitochondrial cloud the selection machinery for preferentially transmitting wild-type mtDNA between generations? Rewinding Müller's ratchet efficiently

In animal mitochondrial DNA inheritance, it remains largely unclear where the mitochondrial genetic bottleneck localizes and how it works in rewinding Müller's ratchet. In a variety of different animals germ plasm mRNAs typically aggregate along with numerous mitochondria to form the mitochondrial cloud (MC) during oogenesis. The MC has been found to serve as messenger transport organizer for germ plasm mRNAs. Germ plasm RNAs in MC will specifically distribute to the primordial germ cells of the future embryo. It has been proposed that the MC might be the site where selected mitochondria accumulate for specific transmission to grandchildren but this idea received relatively little attention and the criterion by which mitochondria are selected remains unknown. Our recent results in zebrafish provided further evidence for selective mitochondria accumulation in the MC by showing that mitochondria with high-inner membrane potential tend to be recruited preferentially into the MC, and these mitochondria are transported along with germ plasm to the cortex of the vegetal pole. By analyzing the composition, behavior and functions of the MC, and in reviewing related literature, we found strong support for the proposition that the MC corresponds to the position and function of the mitochondrial genetic bottleneck.

Gonadal mRNA expression levels of TGFbeta superfamily signaling factors correspond with post-hatching morphological development in American alligators

Paracrine factor signaling regulates many aspects of vertebrate gonadal development. We investigated key ovarian and testicular morphological markers of the American alligator (Alligator mississippiensis) during the first 5 months post-hatching and correlated gonadal development with mRNA expression levels of a suite of regulatory factors. In both sexes, we observed significant morphology changes, including ovarian follicle assembly and meiotic progression of testicular germ cells. Concomitant with these changes were sexually dimorphic and ontogenetically variable mRNA expressions. In ovaries, FOXL2, aromatase, and follistatin mRNA expression was greater than in testes at all ages. At one week after hatching, we observed ovarian medullary remodeling in association with elevated activin/inhibin beta A subunit, follistatin, and aromatase mRNA expressions. Three and 5 months following hatching and concomitant with follicle assembly, ovaries showed increased mRNA expression levels of GDF9 and the mitotic factor PCNA. In testes, the activin/inhibin alpha and beta B subunit transcript levels were greater than in ovaries at all ages. Elevated testicular expression of GDF9 mRNA levels at 5 months after hatching aligned with increased spermatogenic activity. We propose that the mRNA expression levels and concomitant morphological changes observed here affect the establishment of alligator reproductive health and later fertility.

A new vision of the origin and the oocyte development in the Ostariophysi applied to Gymnotus sylvius (Teleostei: Gymnotiformes)

Based on new knowledge coming from marine perciform species, the origin of oocytes and their development in the Ostariophysi, Gymnotus sylvius is described. In both Gymnotus sylvius and marine perciform fish, oogonia are found in the germinal epithelium that forms the surface of the ovarian lamellae. At the commencement of folliculogenesis, proliferation of oogonia and their entrance into meiosis gives rise to germ cell nests that extend into the stroma from the germinal epithelium. Both cell nests and the germinal epithelium are supported by the same basement membrane that separates them from the stroma. At the time of meiotic arrest, oocytes in a cell nest become separated one from the other as processes of prefollicle cells, these being derived from epithelial cells in the germinal epithelium, gradually encompass and individualize them while also synthesizing a basement membrane around themselves during folliculogenesis. The oocyte enters primary growth while still within the cell nest. At the completion of folliculogenesis, the oocyte and follicle cells, composing the follicle, are encompassed by a basement membrane. The follicle remains connected to the germinal epithelium as the both share a portion of common basement membrane. Cells originating from the stroma encompass the ovarian follicle, except where there is a shared basement membrane, to form the theca. The follicle, basement membrane and theca form the follicular complex. Oocyte development occurs inside the follicular complex. Development is divided into the stages primary and secondary growth, oocyte maturation and ovulation. Cortical alveoli appear in the ooplasm just prior to the beginning of secondary growth, the vitellogenic stage that begins with yolk deposition and proceeds until the oocyte is full-grown and the ooplasm is filled with yolk globules. Maturation is characterized by the germinal vesicle or nuclear migration, germinal vesicle breakdown or nuclear envelop fragmentation and the resumption of meiosis. At the ovulation the egg is released from the follicular complex into the ovarian lumen. When compared to marine Perciformes that lay pelagic eggs, oocyte development in Gymnotus sylvius has fewer steps within the stages of development, the two most remarkable being the absence of oil droplet formation during primary and secondary growth, (and the consequent absence of the oil droplets fusion during maturation), and the hydrolysis of yolf preceding ovulation.

The ovary of the bignose fanskate Sympterygia acuta Garman, 1877 (Chondrichthyes, Rajidae) in the Bahía Blanca estuary, Argentina: morphology and reproductive features

Chondrichthyes have become an important economic resource in recent years. In spite of this importance, there is little knowledge about their reproductive biology, especially of species from the south-western Atlantic. In this work, we study the morphology and histology of the ovary of Sympterygia acuta. The results show that oogonia were present only in the immature females (stage I of maturity scale), whereas specimens in stages II and III of maturity displayed oocytes in all developmental steps. The most important histological features that vary throughout oocyte development are the presence or absence of yolk, the number of types and layers of follicular cells and the degree of development of the thecae. Follicular cells are, at least, of two different types. Finally, a new point of view for the determination of maturity stages in the field based on the size of follicles is discussed.

The DAZL and PABP families: RNA-binding proteins with interrelated roles in translational control in oocytes

Gametogenesis is a highly complex process that requires the exquisite temporal, spatial and amplitudinal regulation of gene expression at multiple levels. Translational regulation is important in a wide variety of cell types but may be even more prevalent in germ cells, where periods of transcriptional quiescence necessitate the use of post-transcriptional mechanisms to effect changes in gene expression. Consistent with this, studies in multiple animal models have revealed an essential role for mRNA translation in the establishment and maintenance of reproductive competence. While studies in humans are less advanced, emerging evidence suggests that translational regulation plays a similarly important role in human germ cells and fertility. This review highlights specific mechanisms of translational regulation that play critical roles in oogenesis by activating subsets of mRNAs. These mRNAs are activated in a strictly determined temporal manner via elements located within their 3'UTR, which serve as binding sites for trans-acting factors. While we concentrate on oogenesis, these regulatory events also play important roles during spermatogenesis. In particular, we focus on the deleted in azoospermia-like (DAZL) family of proteins, recently implicated in the translational control of specific mRNAs in germ cells; their relationship with the general translation initiation factor poly(A)-binding protein (PABP) and the process of cytoplasmic mRNA polyadenylation.

Postembryonic epigenesis of Vasa-positive germ cells from aggregated hemoblasts in the colonial ascidian, Botryllus primigenus

We investigated whether Vasa was a germline-specific marker in the colonial ascidian Botryllus primigenus, and whether it was inducible epigenetically in the adult life span. We cloned a Botryllus Vasa homologue (BpVas). The deduced open reading frame encoded 687 amino acid residues. It was expressed specifically by germline cells such as the loose cell mass, oogonia and juvenile oocytes in the ovary, and the primordial testis (compact cell mass), spermatogonia and juvenile spermatocytes in the testis. The loose cell mass, the most primitive germline cells, showed an ultrastructure of undifferentiated cells known as hemoblasts. The hemoblasts did not contain electron-dense materials or a mitochondrial assembly in the cytoplasm. These organelles appeared later in the oogonia and oocytes. When the loose cell mass and developing germ cells were eliminated by extirpating all zooids and buds from the colonies, BpVas transcripts disappeared completely from the vascularized colonies. After 14 days, when the colonies regenerated by vascular budding, BpVas-positive cells reappeared in some cases, and in 30 day colonies, BpVas-positive germ cells were observed in all the regenerated colonies. These results show that in B. primigenus, germ cells are inducible de novo from the Vasa-negative cells even at postembryonic stages.

Apoptosis maintains oocyte quality in aging Caenorhabditis elegans females

In women, oocytes arrest development at the end of prophase of meiosis I and remain quiescent for years. Over time, the quality and quantity of these oocytes decreases, resulting in fewer pregnancies and an increased occurrence of birth defects. We used the nematode Caenorhabditis elegans to study how oocyte quality is regulated during aging. To assay quality, we determine the fraction of oocytes that produce viable eggs after fertilization. Our results show that oocyte quality declines in aging nematodes, as in humans. This decline affects oocytes arrested in late prophase, waiting for a signal to mature, and also oocytes that develop later in life. Furthermore, mutations that block all cell deaths result in a severe, early decline in oocyte quality, and this effect increases with age. However, mutations that block only somatic cell deaths or DNA-damage-induced deaths do not lower oocyte quality. Two lines of evidence imply that most developmentally programmed germ cell deaths promote the proper allocation of resources among oocytes, rather than eliminate oocytes with damaged chromosomes. First, oocyte quality is lowered by mutations that do not prevent germ cell deaths but do block the engulfment and recycling of cell corpses. Second, the decrease in quality caused by apoptosis mutants is mirrored by a decrease in the size of many mature oocytes. We conclude that competition for resources is a serious problem in aging germ lines, and that apoptosis helps alleviate this problem.