Germline Syncytia Research Articles

Caenorhabditis elegans septins contribute to the development and structure of the oogenic germline.

Oocytes must be exceptionally large cells in order to support embryonic development. Throughout animal phylogeny, a specialized cell called a syncytium, wherein many nuclei share a continuous cytoplasm, achieves oogenesis. The syncytial nature of germline architecture is key to its function and depends on conserved components of the cortical cytoskeleton. Septins form non-polar cytoskeletal polymers that associate with membranes. In the syncytial germline of the nematode Caenorhabditis elegans, septins are highly enriched on the cortex and generally required for fertility, but the role of septins in the germline is poorly understood. We report that the C. elegans septins, UNC-59 and UNC-61, are important for germline extension during development, the maintenance of its syncytial architecture, and production of oocytes. While much of our findings substantiate the idea that the two C. elegans septins act together, we also found evidence that they have distinct functions. Loss of UNC-61 perturbed germline extension during germline development, while the loss of UNC-59 function severely affected germline architecture in adult hermaphrodites. Consultation of clustering results from a large-scale high-throughput screen suggested that septins are involved in germ cell proliferation and/or differentiation. In sum, our findings implicate a conserved cytoskeletal component in the complex architecture of a germline syncytium.

Binucleate germ cells in Caenorhabditis elegans are removed by physiological apoptosis.

Cell death plays a major role during C. elegans oogenesis, where over half of the oogenic germ cells die in a process termed physiological apoptosis. How germ cells are selected for physiological apoptosis, or instead become oocytes, is not understood. Most oocytes produce viable embryos when apoptosis is blocked, suggesting that physiological apoptosis does not function to cull defective germ cells. Instead, cells targeted for apoptosis may function as nurse cells; the germline is syncytial, and all germ cells appear to contribute cytoplasm to developing oocytes. C. elegans has been a leading model for the genetics and molecular biology of apoptosis and phagocytosis, but comparatively few studies have examined the cell biology of apoptotic cells. We used live imaging to identify and examine pre-apoptotic germ cells in the adult gonad. After initiating apoptosis, germ cells selectively export their mitochondria into the shared pool of syncytial cytoplasm; this transport appears to use the microtubule motor kinesin. The apoptotic cells then shrink as they expel most of their remaining cytoplasm, and close off from the syncytium. Shortly thereafter the apoptotic cells restructure their microtubule and actin cytoskeletons, possibly to maintain cell integrity; the microtubules form a novel, cortical array of stabilized microtubules, and actin and cofilin organize into giant cofilin-actin rods. We discovered that some apoptotic germ cells are binucleate; the binucleate germ cells can develop into binucleate oocytes in apoptosis-defective strains, and appear capable of producing triploid offspring. Our results suggest that the nuclear layer of the germline syncytium becomes folded during mitosis and growth, and that binucleate cells arise as the layer unfolds or everts; all of the binucleate cells are subsequently removed by apoptosis. These results show that physiological apoptosis targets at least two distinct populations of germ cells, and that the apoptosis machinery efficiently recognizes cells with two nuclei.

CYK-4 functions independently of its centralspindlin partner ZEN-4 to cellularize oocytes in germline syncytia.

Throughout metazoans, germ cells undergo incomplete cytokinesis to form syncytia connected by intercellular bridges. Gamete formation ultimately requires bridge closure, yet how bridges are reactivated to close is not known. The most conserved bridge component is centralspindlin, a complex of the Rho family GTPase-activating protein (GAP) CYK-4/MgcRacGAP and the microtubule motor ZEN-4/kinesin-6. Here, we show that oocyte production by the syncytial Caenorhabditis elegans germline requires CYK-4 but not ZEN-4, which contrasts with cytokinesis, where both are essential. Longitudinal imaging after conditional inactivation revealed that CYK-4 activity is important for oocyte cellularization, but not for the cytokinesis-like events that generate syncytial compartments. CYK-4's lipid-binding C1 domain and the GTPase-binding interface of its GAP domain were both required to target CYK-4 to intercellular bridges and to cellularize oocytes. These results suggest that the conserved C1-GAP region of CYK-4 constitutes a targeting module required for closure of intercellular bridges in germline syncytia.

Orchestrating early embryonic divisions

In Caenorhabditis elegans, the syncytial germ line generates oocytes that undergo ovulation into the spermatheca, where they are fertilized. The fertilized embryos then undergo two meiotic divisions, followed by mitotic divisions before its deposition outside of the uterus. This series of events is depicted in the image, from germ line syncytium (top) to embryos (bottom). This article is protected by copyright. All rights reserved

Semi-permeable Diffusion Barriers Enhance Patterning Robustness in the C. elegans Germline

Positional information derived from local morphogen concentration plays an important role in patterning. A key question is how morphogen diffusion and gene expression regulation shape positional information into an appropriate profile with suitably low noise. We address this question using a model system--the C. elegans germline--whose regulatory network has been well characterized genetically but whose spatiotemporal dynamics are poorly understood. We show that diffusion within the germline syncytium is a critical control of stem cell differentiation and that semi-permeable diffusion barriers present at key locations make it possible--in combination with a feedback loop in the germline regulatory network--for mitotic zone size to be robust against spatial noise in Notch signaling. Spatial averaging within compartments defined by diffusion barriers is an advantageous patterning strategy, which attenuates noise while still allowing for sharp transitions between compartments. This strategy could apply to other organs.

Formation and ultrastructural organization of gonads in Oikopleura gracilis Lohmann, 1896 (Urochordata: Appendicularia)

This study deals with the formation and ultrastructural organization of the gonads in a common species of appendicularian, Oikopleura gracilis, from Peter the Great Bay. Light microscopy observations show that the gonads develop from a transparent primordium that is located in the basolateral part of the gonad cavity; the primordium increases in size in the process of development and differentiates into the testis and ovary. The testis is covered by a single layer of ultrastructurally uniform follicular epithelium and contains a population of proliferating male gonocytes. The ovary contains two types of germ line nuclei, which are large polyploid nuclei that belong to the auxiliary cells and small meiotic nuclei of the oocytes. The two nuclei types, together with a common cytoplasm, form a syncytium of the ovary, or the coenocyst. As in the dioecious Oikopleura dioica, the coenocyst of O. gracilis produces naked oocytes that are devoid of a type III follicular membrane. The coenocyst is covered by a single-layered follicular epithelium, in which two cell types can be distinguished ultrastructurally. Thus, the synchronous maturation of sex products in O. gracilis is achieved by the formation of the germ-line syncytium in the testis and the coenocyst in the ovary, which generates a large number of simultaneously ripening oocytes that are competent for fertilization.

Comparative organization of follicle, accessory cells and spawning anlagen in dynamic semelparous clutch manipulators, the urochordate Oikopleuridae

The urochordate appendicularians play a key trophic role in marine ecosystems and are the second largest component of zooplankton after copepods. Part of their success is due to their ability to undergo rapid population blooms in response to changes in primary productivity. Nonetheless, the reproductive biology of this important group remains poorly understood. In the present study, we investigated the organization of male and female germ and accessory somatic cells in the Oikopleuridae. We found that the structure of the ovary had been previously misconstrued as consisting of germ and accessory 'cells' interspersed together, whereas, in fact, the germline exists as a giant transparent syncytium. Somatic follicle cells, integral to regulation of the temporal progression of gametogenesis, could be classified into three types in females and two in males, and we characterized functional gap junctions between follicle cells and the germline syncytium in both sexes. The number of follicle cells per oocyte produced was much reduced in comparison with many commonly studied model organisms. We further identified a novel anlagen that permits spawning of the animal via rupture of the gonad wall, which is obligatory for the release of oocytes, but optional for the release of sperm that usually occurs via the spermiduct. The organization of the female germline in the Oikopleuridae shares some features of meroistic oogenesis with the arthropod Drosophila, but the process of synchronous oogenesis in these semelparous organisms remains quite distinctive with respect to that previously characterized in the animal kingdom and certainly within the chordate phylum.

Multiple apoptotic caspase cascades are required in nonapoptotic roles for Drosophila spermatid individualization.

Spermatozoa are generated and mature within a germline syncytium. Differentiation of haploid syncytial spermatids into single motile sperm requires the encapsulation of each spermatid by an independent plasma membrane and the elimination of most sperm cytoplasm, a process known as individualization. Apoptosis is mediated by caspase family proteases. Many apoptotic cell deaths in Drosophila utilize the REAPER/HID/GRIM family proapoptotic proteins. These proteins promote cell death, at least in part, by disrupting interactions between the caspase inhibitor DIAP1 and the apical caspase DRONC, which is continually activated in many viable cells through interactions with ARK, the Drosophila homolog of the mammalian death-activating adaptor APAF-1. This leads to unrestrained activity of DRONC and other DIAP1-inhibitable caspases activated by DRONC. Here we demonstrate that ARK- and HID-dependent activation of DRONC occurs at sites of spermatid individualization and that all three proteins are required for this process. dFADD, the Drosophila homolog of mammalian FADD, an adaptor that mediates recruitment of apical caspases to ligand-bound death receptors, and its target caspase DREDD are also required. A third apoptotic caspase, DRICE, is activated throughout the length of individualizing spermatids in a process that requires the product of the driceless locus, which also participates in individualization. Our results demonstrate that multiple caspases and caspase regulators, likely acting at distinct points in time and space, are required for spermatid individualization, a nonapoptotic process.

A Model of Elegance

GenBank, http://www.ncbi.nlm.nih.gov/Web/GenbankGenome Sequencing Center, University of Washington School of Medicine, http://genome.wustl.edu/gsc/gschmpg.htmlThe Sanger Center, http://www.sanger.ac.ukXREFdb, http://www.ncbi.nlm.nih.gov/Bassett/modelorgs