Generation and Characterization of Germline-Specific Autophagy and Mitochondrial Reactive Oxygen Species Reporters in Drosophila

Oogenesis is a fundamental process that forms the egg and, is crucial for the transmission of genetic information to the next generation. Drosophila oogenesis has been used extensively as a genetically tractable model to study organogenesis, niche-germline stem cell communication, and more recently reproductive aging including germline stem cell (GSC) aging. Autophagy, a lysosome-mediated degradation process, is implicated in gametogenesis and aging. However, there is a lack of genetic tools to study autophagy in the context of gametogenesis and GSC aging. Here we describe the generation of three transgenic lines mcherry-Atg8a, GFP-Ref(2)P and mito-roGFP2-Orp1 that are specifically expressed in the germline compartment including GSCs during Drosophila oogenesis. These transgenes are expressed from the nanos promoter and present a better alternative to UASp mediated overexpression of transgenes. These fluorescent reporters can be used to monitor and quantify autophagy, and the production of reactive oxygen species during oogenesis. These reporters provide a valuable tool that can be utilized in designing genetic screens to identify novel regulators of autophagy and redox homeostasis during oogenesis.

The testicular soma of Tsc22d3 knockout mice supports spermatogenesis and germline transmission from spermatogonial stem cell lines upon transplantation

Spermatogonial stem cells (SSCs) are adult stem cells that are slowly cycling and self-renewing. The pool of SSCs generates very large numbers of male gametes throughout the life of the individual. SSCs can be cultured in vitro for long periods of time, and established SSC lines can be manipulated genetically. Upon transplantation into the testes of infertile mice, long-term cultured mouse SSCs can differentiate into fertile spermatozoa, which can give rise to live offspring. Here, we show that the testicular soma of mice with a conditional knockout (conKO) in the X-linked gene Tsc22d3 supports spermatogenesis and germline transmission from cultured mouse SSCs upon transplantation. Infertile males were produced by crossing homozygous Tsc22d3 floxed females with homozygous ROSA26-Cre males. We obtained 96 live offspring from six long-term cultured SSC lines with the aid of intracytoplasmic sperm injection. We advocate the further optimization of Tsc22d3-conKO males as recipients for testis transplantation of SSC lines.

Implications and Current Limitations of Oogenesis from Female Germline or Oogonial Stem Cells in Adult Mammalian Ovaries

A now large body of evidence supports the existence of mitotically active germ cells in postnatal ovaries of diverse mammalian species, including humans. This opens the possibility that adult stem cells naturally committed to a germline fate could be leveraged for the production of female gametes outside of the body. The functional properties of these cells, referred to as female germline or oogonial stem cells (OSCs), in ovaries of women have recently been tested in various ways, including a very recent investigation of the differentiation capacity of human OSCs at a single cell level. The exciting insights gained from these experiments, coupled with other data derived from intraovarian transplantation and genetic tracing analyses in animal models that have established the capacity of OSCs to generate healthy eggs, embryos and offspring, should drive constructive discussions in this relatively new field to further exploring the value of these cells to the study, and potential management, of human female fertility. Here, we provide a brief history of the discovery and characterization of OSCs in mammals, as well as of the in-vivo significance of postnatal oogenesis to adult ovarian function. We then highlight several key observations made recently on the biology of OSCs, and integrate this information into a broader discussion of the potential value and limitations of these adult stem cells to achieving a greater understanding of human female gametogenesis in vivo and in vitro.

Toward Identifying Subnetworks from FBF Binding Landscapes in Caenorhabditis Spermatogenic or Oogenic Germlines

Metazoan PUF (Pumilio and FBF) RNA-binding proteins regulate various biological processes, but a common theme across phylogeny is stem cell regulation. In Caenorhabditis elegans, FBF (fem-3 Binding Factor) maintains germline stem cells regardless of which gamete is made, but FBF also functions in the process of spermatogenesis. We have begun to "disentangle" these biological roles by asking which FBF targets are gamete-independent, as expected for stem cells, and which are gamete-specific. Specifically, we compared FBF iCLIP binding profiles in adults making sperm to those making oocytes. Normally, XX adults make oocytes. To generate XX adults making sperm, we used a fem-3(gf) mutant requiring growth at 25°; for comparison, wild-type oogenic hermaphrodites were also raised at 25°. Our FBF iCLIP data revealed FBF binding sites in 1522 RNAs from oogenic adults and 1704 RNAs from spermatogenic adults. More than half of these FBF targets were independent of germline gender. We next clustered RNAs by FBF-RNA complex frequencies and found four distinct blocks. Block I RNAs were enriched in spermatogenic germlines, and included validated target fog-3, while Block II and III RNAs were common to both genders, and Block IV RNAs were enriched in oogenic germlines. Block II (510 RNAs) included almost all validated FBF targets and was enriched for cell cycle regulators. Block III (21 RNAs) was enriched for RNA-binding proteins, including previously validated FBF targets gld-1 and htp-1 We suggest that Block I RNAs belong to the FBF network for spermatogenesis, and that Blocks II and III are associated with stem cell functions.

Biology and manipulation technologies of male germline stem cells in mammals

BACKGROUND

Spermatogonial stem cells (SSCs) are the origin of sperm and defined by their functions of "colonization in the testis" and "spermatogenesis". In vitro manipulation techniques of SSCs contribute to a wide variety of fields including reproductive medicine and molecular breeding. This review presents the recent progress of the biology and manipulation technologies of SSCs.

METHODS

Research articles regarding SSC biology and technologies were collected and summarized.

MAIN FINDINGS

Dr. Ralph Brinster developed the spermatogonial transplantation technique that enables SSC detection by functional markers. Using this technique, cultured SSCs, termed germline stem (GS) cells, were established from the mouse. GS cells provide the opportunity to produce genome-edited animals without using zygotes. In vitro spermatogenesis allows production of haploid germ cells from GS cells without spermatogonial transplantation. The recent advancement of pluripotent stem cell culture techniques has also achieved production of functional GS-like cells in addition to male/female germ cells.

CONCLUSION

Although in vitro manipulation techniques of GS cells have been developed for the mouse, it appears to be difficult to apply these techniques to other species. Understanding and control of interspecies barriers are required to extend this technology to nonrodent mammals.

JNK signaling triggers spermatogonial dedifferentiation during chronic stress to maintain the germline stem cell pool in the Drosophila testis

Exhaustion of stem cells is a hallmark of aging. In the Drosophila testis, dedifferentiated germline stem cells (GSCs) derived from spermatogonia increase during lifespan, leading to the model that dedifferentiation counteracts the decline of GSCs in aged males. To test this, we blocked dedifferentiation by mis-expressing the differentiation factor bag of marbles (bam) in spermatogonia while lineage-labeling these cells. Strikingly, blocking bam-lineage dedifferentiation under normal conditions in virgin males has no impact on the GSC pool. However, in mated males or challenging conditions, inhibiting bam-lineage dedifferentiation markedly reduces the number of GSCs and their ability to proliferate and differentiate. We find that bam-lineage derived GSCs have significantly higher proliferation rates than sibling GSCs in the same testis. We determined that Jun N-terminal kinase (JNK) activity is autonomously required for bam-lineage dedifferentiation. Overall, we show that dedifferentiation provides a mechanism to maintain the germline and ensure fertility under chronically stressful conditions.

Enhancing Nanos expression via the bacterial TomO protein is a conserved strategy used by the symbiont Wolbachia to fuel germ stem cell maintenance in infected Drosophila females

The toxic manipulator of oogenesis (TomO) protein has been identified in the wMel strain of Wolbachia that symbioses with the vinegar fly Drosophila melanogaster, as a protein that affects host reproduction. TomO protects germ stem cells (GSCs) from degeneration, which otherwise occurs in ovaries of host females that are mutant for the gene Sex-lethal (Sxl). We isolated the TomO homologs from wPip, a Wolbachia strain from the mosquito Culex quinquefasciatus. One of the homologs, TomO_wPip 1, exerted the GSC rescue activity in fly Sxl mutants when lacking its hydrophobic stretches. The GSC-rescuing action of the TomO_wPip 1 variant was ascribable to its abilities to associate with Nanos (nos) mRNA and to enhance Nos protein expression. The analysis of structure-activity relationships with TomO homologs and TomO deletion variants revealed distinct modules in the protein that are each dedicated to different functions, i.e., subcellular localization, nos mRNA binding or Nos expression enhancement. We propose that modular reshuffling is the basis for structural and functional diversification of TomO protein members.

A sisRNA/miRNA Axis Prevents Loss of Germline Stem Cells during Starvation in Drosophila

Animal reproduction responds to nutritional status. During starvation, Drosophila and Caenorhabditis elegans enter a period of reproductive diapause with increase apoptosis, while maintaining a stable pool of germline stem cells (GSCs). How GSCs are protected is not understood. Here, we show that a sisRNA/miRNA axis maintains ovarian GSCs during starvation in Drosophila. Starvation induces the expression of an ovary-enriched sisRNA sisR-2, which negatively regulates GSC maintenance via a fatty acid metabolism gene dFAR1. sisR-2 promotes the expression of bantam, which in turn inhibits the activity of sisR-2, forming a negative feedback loop. Therefore, bantam acts as a buffer to counteract sisR-2 activity to prevent GSC loss during starvation. We propose that the sisR-2/bantam axis confers robustness to GSCs in Drosophila.

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sisR-2 regulates the number of GSCs

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sisR-2 regulates GSC maintenance by repressing dFAR1

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bantam regulates GSC maintenance by repressing sisR-2 activity

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sisR-2/bantam axis protects GSCs from starvation

Germline Stem Cell Heterogeneity Supports Homeostasis in Drosophila

Adult and embryonic stem cells exhibit fluctuating gene expression; however, the biological significance of stem cell heterogeneity is not well understood. We show that, in Drosophila, female germline stem cells (GSCs) exhibit heterogeneous expression of a GSC differentiation-promoting factor Regena (Rga). The Drosophila homolog of human SON, dsn, is required to maintain GSC heterogeneity by suppressing sustained high levels of Rga. Reducing the expression of Rga in dsn mutants restores GSC heterogeneity and self-renewal. Thus, GSC heterogeneity is linked to GSC homeostasis.

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Female germline stem cells have heterogeneous Rga expression

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dsn suppresses rga transcription and maintains heterogeneity

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dsn maintains germline stem cells

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Lowering rga restores heterogeneity and suppresses dsn phenotypes

Cyclin B3 Deficiency Impairs Germline Stem Cell Maintenance and Its Overexpression Delays Cystoblast Differentiation in Drosophila Ovary

It is well known that cyclinB3 (cycB3) plays a key role in the control of cell cycle progression. However, whether cycB3 is involved in stem cell fate determination remains unknown. The Drosophila ovary provides an exclusive model for studying the intrinsic and extrinsic factors that modulate the fate of germline stem cells (GSCs). Here, using this model, we show that DrosophilacycB3 plays a new role in controlling the fate of germline stem cells (GSC). Results from cycB3 genetic analyses demonstrate that cycB3 is intrinsically required for GSC maintenance. Results from green fluorescent protein (GFP)-transgene reporter assays show that cycB3 is not involved in Dad-mediated regulation of Bmp signaling, or required for dpp-induced bam transcriptional silencing. Double mutants of bam and cycB3 phenocopied bam single mutants, suggesting that cycB3 functions in a bam-dependent manner in GSCs. Deficiency of cycB3 fails to cause apoptosis in GSCs or influence cystoblast (CB) differentiation into oocytes. Furthermore, overexpression of cycB3 dramatically increases the CB number in Drosophila ovaries, suggesting that an excess of cycB3 function delays CB differentiation. Given that the cycB3 gene is evolutionarily conserved, from insects to humans, cycB3 may also be involved in controlling the fate of GSCs in humans.

Spaghetti, a homolog of human RPAP3 (RNA polymerase II-associated protein 3), determines the fate of germline stem cells in Drosophila ovary

The Drosophila ovary provides an attractive model for studying the extrinsic or intrinsic factors that regulate the fate of germline stem cells (GSCs). Using this model, we identified a new role for Drosophila spaghetti (spag), encoding a homolog of human RNA polymerase II-associated protein 3 (RPAP3), in regulating the fate of ovarian GSCs. Results from spag knockdown and genetic mosaic studies suggest that spag functions as an intrinsic factor for GSCs maintenance. Loss of Spag by, either spag RNAi or null mutation failed to trigger apoptosis in ovarian GSCs. Overexpression of spag led to negligible increases in the number of GSC/Cystoblast (CB) cells, suggesting that an excess of Spag is not sufficient to accelerate the proliferation of GSCs or delay CBs' differentiation. Our study provides evidence supporting that spag is involved in adult stem cells maintenance. In addition, the RNAi screen results showed that knockdown of Hsp90, one of known Spag interacting partners, led to loss of ovarian GSCs in Drosophila. Heterozygous mutations in hsp90 (hsp90/+) dramatically accelerated the GSC loss in spag RNAi ovaries, suggesting that the Spag-contained complex possibly plays an essential role in controlling the GSCs fate.

Polycomb Group Gene E(z) Is Required for Spermatogonial Dedifferentiation in Drosophila Adult Testis

Dedifferentiation is an important process to replenish lost stem cells during aging or regeneration after injury to maintain tissue homeostasis. Here, we report that Enhancer of Zeste [E(z)], a component of the Polycomb repression complex 2 (PRC2), is required to maintain a stable pool of germline stem cells (GSCs) within the niche microenvironment. During aging, germ cells with reduced E(z) activity cannot meet that requirement, but the defect arises from neither increased GSC death nor premature differentiation. Instead, we found evidence that the decrease of GSCs upon the inactivation of E(z) in the germline could be attributed to defective dedifferentiation. During recovery from genetically manipulated GSC depletion, E(z) knockdown germ cells also fail to replenish lost GSCs. Taken together, our data suggest that E(z) acts intrinsically in germ cells to activate dedifferentiation and thus replenish lost GSCs during both aging and tissue regeneration.

Redox Homeostasis Plays Important Roles in the Maintenance of the Drosophila Testis Germline Stem Cells

Oxidative stress influences stem cell behavior by promoting the differentiation, proliferation, or apoptosis of stem cells. Thus, characterizing the effects of reactive oxygen species (ROS) on stem cell behavior provides insights into the significance of redox homeostasis in stem cell-associated diseases and efficient stem cell expansion for cellular therapies. We utilized the Drosophila testis as an in vivo model to examine the effects of ROS on germline stem cell (GSC) maintenance. High levels of ROS induced by alteration in Keap1/Nrf2 activity decreased GSC number by promoting precocious GSC differentiation. Notably, high ROS enhanced the transcription of the EGFR ligand spitz and the expression of phospho-Erk1/2, suggesting that high ROS-mediated GSC differentiation is through EGFR signaling. By contrast, testes with low ROS caused by Keap1 inhibition or antioxidant treatment showed an overgrowth of GSC-like cells. These findings suggest that redox homeostasis regulated by Keap1/Nrf2 signaling plays important roles in GSC maintenance.

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Germline stem cell homeostasis in the Drosophila testis is susceptible to ROS levels

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Oxidative stress decreases germline stem cell number by promoting differentiation

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EGFR signaling is involved in precocious GSC differentiation caused by high ROS levels

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Low levels of ROS can promote a growth of germline stem cells

Quantitative analysis of male germline stem cell differentiation reveals a role for the p53-mTORC1 pathway in spermatogonial maintenance

p53 protects cells from DNA damage by inducing cell-cycle arrest upon encountering genomic stress. Among other pathways, p53 elicits such an effect by inhibiting mammalian target of rapamycin complex 1 (mTORC1), the master regulator of cell proliferation and growth. Although recent studies have indicated roles for both p53 and mTORC1 in stem cell maintenance, it remains unclear whether the p53-mTORC1 pathway is conserved to mediate this process under normal physiological conditions. Spermatogenesis is a classic stem cell-dependent process in which undifferentiated spermatogonia undergo self-renewal and differentiation to maintain the lifelong production of spermatozoa. To better understand this process, we have developed a novel flow cytometry (FACS)-based approach that isolates spermatogonia at consecutive differentiation stages. By using this as a tool, we show that genetic loss of p53 augments mTORC1 activity during early spermatogonial differentiation. Functionally, loss of p53 drives spermatogonia out of the undifferentiated state and causes a consistent expansion of early differentiating spermatogonia until the stage of preleptotene (premeiotic) spermatocyte. The frequency of early meiotic spermatocytes is, however, dramatically decreased. Thus, these data suggest that p53-mTORC1 pathway plays a critical role in maintaining the homeostasis of early spermatogonial differentiation. Moreover, our FACS approach could be a valuable tool in understanding spermatogonial differentiation.

Expression of Cnnm1 and Its Association with Stemness, Cell Cycle, and Differentiation in Spermatogenic Cells in Mouse Testis

Cyclin M1 (CNNM1) functions as a copper storage protein in neuronal cells. We report that Cnnm1 is expressed in mouse testis and brain and has a coding sequence of 1761 bp that encodes a 586 amino acid protein with a molecular weight of 66 kDa. Cnnm1 is expressed in the testes of mice from neonatal to adult stages with relatively higher levels in neonates. CNNM1 expression appeared to be restricted to c-KIT- and OCT3/4-positive cells in the testis, indicating that they are early spermatogonial cells. Spermatogonial stem cells in primary culture expressed Cnnm1, and their differentiation into embryoid body-like clusters in vitro resulted in the loss of Cnnm1 expression. Silencing of Cnnm1 in GC1-spg cells resulted in a significant reduction in the number of cells in G1 phase with concomitant increase in the numbers of cells in both S and G2/M phases. Further, retinoic acid downregulated the expression of Cnnm1 in GC1-spg cells. We conclude that CNNM1 is associated with stemness and self-renewal, and its downregulation triggers differentiation in spermatogonial cells in mouse.

Molecular Evolution of Drosophila Germline Stem Cell and Neural Stem Cell Regulating Genes

Here, we study the molecular evolution of a near complete set of genes that had functional evidence in the regulation of the Drosophila germline and neural stem cell. Some of these genes have previously been shown to be rapidly evolving by positive selection raising the possibility that stem cell genes as a group have elevated signatures of positive selection. Using recent Drosophila comparative genome sequences and population genomic sequences of Drosophila melanogaster, we have investigated both long- and short-term evolution occurring across these two different stem cell systems, and compared them with a carefully chosen random set of genes to represent the background rate of evolution. Our results showed an excess of genes with evidence of a recent selective sweep in both germline and neural stem cells in D. melanogaster. However compared with their control genes, both stem cell systems had no significant excess of genes with long-term recurrent positive selection in D. melanogaster, or across orthologous sequences from the melanogaster group. The evidence of long-term positive selection was limited to a subset of genes with specific functions in both the germline and neural stem cell system.

Nuclear lamina builds tissues from the stem cell niche

Recent studies show that nuclear lamins, the type V intermediate filament proteins, are required for proper building of at least some organs. As the major structural components of the nuclear lamina found underneath the inner nuclear membranes, lamins are ubiquitously expressed in all animal cells. How the broadly expressed lamins support the building of specific tissues is not understood. By studying Drosophila testis, we have uncovered a mechanism by which lamin-B functions in the cyst stem cell (CySC) and its differentiated cyst cell, the cell types known to form the niche/microenvironment for the germline stem cells (GSC) and the developing germ line, to ensure testis organogenesis (1). In this extra view, we discuss some remaining questions and the implications of our findings in the understanding of how the ubiquitous nuclear lamina regulates tissue building in a context-dependent manner.

Isolation of Undifferentiated Female Germline Cells from Adult Drosophila Ovaries

This unit describes a method for isolating undifferentiated, stem cell-like germline cells from adult Drosophila ovaries. Here, we demonstrate that this population of cells can be effectively purified from hand-dissected ovaries in considerably large quantities. Tumor ovaries with expanded populations of undifferentiated germline cells are first removed from fly abdomens and dissociated into a cell suspension with the aid of protease treatment. The target cells, which express Vasa-green fluorescent protein (GFP) fusion protein under the control of the germline-specific vasa promoter, are specifically selected from the suspension via fluorescence-activated cell sorting (FACS). These protocols can be adapted to isolate other cell types from fly ovaries, such as somatic follicle cells or escort cells, by driving GFP expression in the respective target cells.

Use of DEAD-box polypeptide-4 (Ddx4) gene promoter-driven fluorescent reporter mice to identify mitotically active germ cells in post-natal mouse ovaries

Several laboratories have independently isolated mitotically active germ cells, termed female germline stem cells or oogonial stem cells (OSCs), from adult mouse ovaries. However, a recent study using Ddx4-Cre;Rosa26 reporter mice concluded that such germ cells do not exist. Given the disparity in conclusions drawn in this study compared with others, we felt it was important to re-assess the utility of Ddx4-Cre;Rosa26 reporter mice for identification of OSCs in adult mouse ovaries. Transgenic Ddx4-Cre mice were crossed with Rosa26(tdTm/tdTm) mice to drive restricted tomato red (tdTm) gene expression in cells in which the Ddx4 gene promoter has been activated. Crude dispersion of ovaries from recombined offspring generated cell fractions containing tdTm-positive immature oocytes, which are incapable of proliferation and thus probably represent the uncharacterized reporter-positive ovarian cells identified in the paper Zhang et al. (2012) as being mitotically inactive. Dispersed ovaries further subjected to fluorescence-activated cell sorting yielded a large population of non-germline tdTm-positive cells, indicative of promoter 'leakiness' in the Ddx4-Cre mouse line. Nonetheless, a small percentage of these tdTm-positive cells exhibited externalized (extracellular, ec) expression of Ddx4 protein (ecDdx4-positive), expressed markers of primitive germ cells but not of oocytes, and actively proliferated in culture, all of which are characteristic features of OSCs. Thus, crude dispersion of ovaries collected from Ddx4 gene promoter-driven reporter mice is not, by itself, a reliable approach to identify OSCs, whereas the same ovarian dispersates further subjected to cell sorting strategies yield purified OSCs that can be expanded in culture.

Functional and molecular features of the Id4+ germline stem cell population in mouse testes

Chan et al. generated transgenic mice in which spermatogonial stem cells expressed an Id4-Gfp transgene. Id4-Gfp⁺ cells exist primarily as a subset of the type A_single pool and eventually comprise ∼2% of the undifferentiated spermatogonial population in adulthood. RNA sequencing analysis revealed genes whose expression is unique for the Id4-Gfp⁺/stem cell and Id4-Gfp⁻/progenitor fractions. These findings provide evidence that stem cells exist as a rare subset of the A_single pool and reveal transcriptome features distinguishing stem cell and progenitor states within the mammalian male germline.

The maintenance of cycling cell lineages relies on undifferentiated subpopulations consisting of stem and progenitor pools. Features that delineate these cell types are undefined for many lineages, including spermatogenesis, which is supported by an undifferentiated spermatogonial population. Here, we generated a transgenic mouse line in which spermatogonial stem cells are marked by expression of an inhibitor of differentiation 4 (Id4)-green fluorescent protein (Gfp) transgene. We found that Id4-Gfp⁺ cells exist primarily as a subset of the type A_single pool, and their frequency is greatest in neonatal development and then decreases in proportion during establishment of the spermatogenic lineage, eventually comprising ∼2% of the undifferentiated spermatogonial population in adulthood. RNA sequencing analysis revealed that expression of 11 and 25 genes is unique for the Id4-Gfp⁺/stem cell and Id4-Gfp⁻/progenitor fractions, respectively. Collectively, these findings provide the first definitive evidence that stem cells exist as a rare subset of the A_single pool and reveal transcriptome features distinguishing stem cell and progenitor states within the mammalian male germline.

The coevolutionary period of Wolbachia pipientis infecting Drosophila ananassae and its impact on the evolution of the host germline stem cell regulating genes

The endosymbiotic bacteria Wolbachia pipientis is known to infect a wide range of arthropod species yet less is known about the coevolutionary history it has with its hosts. Evidence of highly identical W. pipientis strains in evolutionary divergent hosts suggests horizontal transfer between hosts. For example, Drosophila ananassae is infected with a W. pipientis strain that is nearly identical in sequence to a strain that infects both D. simulans and D. suzukii, suggesting recent horizontal transfer among these three species. However, it is unknown whether the W. pipientis strain had recently invaded all three species or a more complex infectious dynamic underlies the horizontal transfers. Here, we have examined the coevolutionary history of D. ananassae and its resident W. pipientis to infer its period of infection. Phylogenetic analysis of D. ananassae mitochondrial DNA and W. pipientis DNA sequence diversity revealed the current W. pipientis infection is not recent. In addition, we examined the population genetics and molecular evolution of several germline stem cell (GSC) regulating genes of D. ananassae. These studies reveal significant evidence of recent and long-term positive selection at stonewall in D. ananassae, whereas pumillio showed patterns of variation consistent with only recent positive selection. Previous studies had found evidence for adaptive evolution of two key germline differentiation genes, bag of marbles (bam) and benign gonial cell neoplasm (bgcn), in D. melanogaster and D. simulans and proposed that the adaptive evolution at these two genes was driven by arms race between the host GSC and W. pipientis. However, we did not find any statistical departures from a neutral model of evolution for bam and bgcn in D. ananassae despite our new evidence that this species has been infected with W. pipientis for a period longer than the most recent infection in D. melanogaster. In the end, analyzing the GSC regulating genes individually showed two of the seven genes to have evidence of selection. However, combining the data set and fitting a specific population genetic model significant proportion of the nonsynonymous sites across the GSC regulating genes were driven to fixation by positive selection. Clearly the GSC system is under rapid evolution and potentially multiple drivers are causing the rapid evolution.

A-single spermatogonia heterogeneity and cell cycles synchronize with rat seminiferous epithelium stages VIII-IX

In mammalian testes, "A-single" spermatogonia function as stem cells that sustain sperm production for fertilizing eggs. Yet, it is not understood how cellular niches regulate the developmental fate of A-single spermatogonia. Here, immunolabeling studies in rat testes define a novel population of ERBB3(+) germ cells as approximately 5% of total SNAP91(+) A-single spermatogonia along a spermatogenic wave. As a function of time, ERBB3(+) A-single spermatogonia are detected during a 1- to 2-day period each 12.9-day sperm cycle, representing 35%-40% of SNAP91(+) A-single spermatogonia in stages VIII-IX of the seminiferous epithelium. Local concentrations of ERBB3(+) A-single spermatogonia are maintained under the mean density measured for neighboring SNAP91(+) A-single spermatogonia, potentially indicative of niche saturation. ERBB3(+) spermatogonia also synchronize their cell cycles with epithelium stages VIII-IX, where they form physical associations with preleptotene spermatocytes transiting the blood-testis barrier and Sertoli cells undergoing sperm release. Thus, A-single spermatogonia heterogeneity within this short-lived and reoccurring microenvironment invokes novel theories on how cellular niches integrate with testicular physiology to orchestrate sperm development in mammals.

Stem cells supporting other stem cells

Adult stem cell therapies are increasingly prevalent for the treatment of damaged or diseased tissues, but most of the improvements observed to date are attributed to the ability of stem cells to produce paracrine factors that have a trophic effect on existing tissue cells, improving their functional capacity. It is now clear that this ability to produce trophic factors is a normal and necessary function for some stem cell populations. In vivo adult stem cells are thought to self-renew due to local signals from the microenvironment where they live, the niche. Several niches have now been identified which harbor multiple stem cell populations. In three of these niches - the Drosophila testis, the bulge of the mammalian hair follicle, and the mammalian bone marrow - one type of stem cell has been found to produce factors that contribute to the maintenance of a second stem cell population in the shared niche. In this review, I will examine the architecture of these three niches and discuss the molecular signals involved. Together, these examples establish a new paradigm for stem cell behavior, that stem cells can promote the maintenance of other stem cells.

Poly(ADP-ribose) glycohydrolase and poly(ADP-ribose)-interacting protein Hrp38 regulate pattern formation during Drosophila eye development

Drosophila Hrp38, a homolog of human hnRNP A1, has been shown to regulate splicing, but its function can be modified by poly(ADP-ribosyl)ation. Notwithstanding such findings, our understanding of the roles of poly(ADP-ribosyl)ated Hrp38 on development is limited. Here, we have demonstrated that Hrp38 is essential for fly eye development based on a rough-eye phenotype with disorganized ommatidia observed in adult escapers of the hrp38 mutant. We also observed that poly(ADP-ribose) glycohydrolase (Parg) loss-of-function, which caused increased Hrp38 poly(ADP-ribosyl)ation, also resulted in the rough-eye phenotype with disrupted ommatidial lattice and reduced number of photoreceptor cells. In addition, ectopic expression of DE-cadherin, which is required for retinal morphogenesis, fully rescued the rough-eye phenotype of the hrp38 mutant. Similarly, Parg mutant eye clones had decreased expression level of DE-cadherin with orientation defects, which is reminiscent of DE-cadherin mutant eye phenotype. Therefore, our results suggest that Hrp38 poly(ADP-ribosyl)ation controls eye pattern formation via regulation of DE-cadherin expression, a finding which has implications for understanding the pathogenic mechanisms of Hrp38-related Fragile X syndrome and PARP1-related retinal degeneration diseases.

An evolutionary perspective on adult female germline stem cell function from flies to humans

The concept that oogenesis continues into reproductive life has been well established in nonmammalian species. Recent studies of mice and women indicate that oocyte formation is also not, as traditionally believed, restricted to the fetal or perinatal periods. Analogous to de novo oocyte formation in flies and fish, newly formed oocytes in adult mammalian ovaries arise from germline stem cells (GSCs) or, more specifically, oogonial stem cells (OSCs). Studies of mice have confirmed that isolated OSCs, once delivered back into adult ovaries, are capable of generating fully functional eggs that fertilize to produce healthy embryos and offspring. Parallel studies of OSCs recently purified from ovaries of reproductive-age women indicate that these cells closely resemble their mouse ovary-derived counterparts, although the fertilization competency of oocytes generated by human OSCs awaits clarification. Despite the ability of OSCs to produce new oocytes during adulthood, oogenesis will still ultimately cease with age, contributing to ovarian failure. The causal mechanisms behind these events in mammals are unknown, but studies of flies have revealed that GSC niche dysfunction plays a critical role in age-related oogenic failure. Such insights derived from evaluation of nonmammalian species, in which postnatal oogenesis has been studied in depth, may aid in development of new strategies to alleviate ovarian failure and infertility in mammals.