Genetic regulation of sex determination and maintenance in zebrafish (Danio rerio)

The killifish germline regulates longevity and somatic repair in a sex-specific manner

Classical evolutionary theories propose tradeoffs among reproduction, damage repair and lifespan. However, the specific role of the germline in shaping vertebrate aging remains largely unknown. In this study, we used the turquoise killifish (Nothobranchius furzeri) to genetically arrest germline development at discrete stages and examine how different modes of infertility impact life history. We first constructed a comprehensive single-cell gonadal atlas, providing cell-type-specific markers for downstream phenotypic analysis. We show here that germline depletion-but not arresting germline differentiation-enhances damage repair in female killifish. Conversely, germline-depleted males instead showed an extension in lifespan and rejuvenated metabolic functions. Through further transcriptomic analysis, we highlight enrichment of pro-longevity pathways and genes in germline-depleted male killifish and demonstrate functional conservation of how these factors may regulate longevity in germline-depleted Caenorhabditis elegans. Our results, therefore, demonstrate that different germline manipulation paradigms can yield pronounced sexually dimorphic phenotypes, implying alternative responses to classical evolutionary tradeoffs.

The killifish germline regulates longevity and somatic repair in a sex-specific manner

Classical evolutionary theories propose tradeoffs between reproduction, damage repair, and lifespan. However, the specific role of the germline in shaping vertebrate aging remains largely unknown. Here, we use the turquoise killifish ( N. furzeri ) to genetically arrest germline development at discrete stages, and examine how different modes of infertility impact life-history. We first construct a comprehensive single-cell gonadal atlas, providing cell-type-specific markers for downstream phenotypic analysis. Next, we show that germline depletion - but not arresting germline differentiation - enhances damage repair in female killifish. Conversely, germline-depleted males instead showed an extension in lifespan and rejuvenated metabolic functions. Through further transcriptomic analysis, we highlight enrichment of pro-longevity pathways and genes in germline-depleted male killifish and demonstrate functional conservation of how these factors may regulate longevity in germline-depleted C. elegans . Our results therefore demonstrate that different germline manipulation paradigms can yield pronounced sexually dimorphic phenotypes, implying alternative responses to classical evolutionary tradeoffs.

foxg1a is required for hair cell development and regeneration in the zebrafish lateral line

Mechanosensory hair cells located in the inner ear mediate the sensations of hearing and balance. If damaged, mammalian inner ear hair cells are unable to regenerate, resulting in permanent sensory deficits. Aquatic vertebrates like zebrafish (Danio rerio) have a specialized class of mechanosensory hair cells found in the lateral line system, allowing them to sense changes in water current. Unlike mammalian inner ear hair cells, lateral line hair cells can robustly regenerate following damage. In mammalian models, the transcription factor Foxg1 functions to promote normal development of the inner ear. Foxg1a is expressed in lateral line sensory organs in zebrafish larvae, but its function during lateral line development and regeneration has not been investigated. We find that loss of Foxg1a function results in reduced hair cell development and regeneration, as well as decreased cellular proliferation in the lateral line system. These data suggest that Foxg1 may be a valuable target for investigation of clinical hair cell regeneration.

Summary statement

Our work demonstrates a role for Foxg1a in developing and regenerating new sensory cells through proliferation.

Direct male development in chromosomally ZZ zebrafish

The genetics of sex determination varies across taxa, sometimes even within a species. Major domesticated strains of zebrafish (Danio rerio), including AB and TU, lack a strong genetic sex determining locus, but strains more recently derived from nature, like Nadia (NA), possess a ZZ male/ZW female chromosomal sex-determination system. AB fish pass through a juvenile ovary stage, forming oocytes that survive in fish that become females but die in fish that become males. To understand mechanisms of gonad development in NA zebrafish, we studied histology and single cell transcriptomics in developing ZZ and ZW fish. ZW fish developed oocytes by 22 days post-fertilization (dpf) but ZZ fish directly formed testes, avoiding a juvenile ovary phase. Gonads of some ZW and WW fish, however, developed oocytes that died as the gonad became a testis, mimicking AB fish, suggesting that the gynogenetically derived AB strain is chromosomally WW. Single-cell RNA-seq of 19dpf gonads showed similar cell types in ZZ and ZW fish, including germ cells, precursors of gonadal support cells, steroidogenic cells, interstitial/stromal cells, and immune cells, consistent with a bipotential juvenile gonad. In contrast, scRNA-seq of 30dpf gonads revealed that cells in ZZ gonads had transcriptomes characteristic of testicular Sertoli, Leydig, and germ cells while ZW gonads had granulosa cells, theca cells, and developing oocytes. Hematopoietic and vascular cells were similar in both sex genotypes. These results show that juvenile NA zebrafish initially develop a bipotential gonad; that a factor on the NA W chromosome, or fewer than two Z chromosomes, is essential to initiate oocyte development; and without the W factor, or with two Z doses, NA gonads develop directly into testes without passing through the juvenile ovary stage. Sex determination in AB and TU strains mimics NA ZW and WW zebrafish, suggesting loss of the Z chromosome during domestication. Genetic analysis of the NA strain will facilitate our understanding of the evolution of sex determination mechanisms.

Cyclin F can alter the turnover of TDP-43

Previously, we demonstrated that the SCFcyclin F complex directly mediates the poly-ubiquitylation of TDP-43, raising the question of whether cyclin F can be used to enhance the turnover of TDP-43. A hurdle to the use of cyclin F, however, is that the overexpression of cyclin F can lead to the initiation of cell death pathways. Accordingly, the aim of this study was to identify and evaluate a less toxic variant of cyclin F. To do so, we first confirmed and validated our previous findings that cyclin F binds to TDP-43 in an atypical manner. Additionally, we demonstrated that mutating the canonical substrate region in cyclin F (to generate cyclin FMRL/AAA) led to reduced binding affinity to known canonical substrates without impacting the interaction between cyclin F and TDP-43. Notably, both wild-type and cyclin FMRL/AAA effectively reduced the abundance of TDP-43 in cultured cells whilst cyclin FMRL/AAA also demonstrated reduced cell death compared to the wild-type control. The decrease in toxicity also led to a reduction in morphological defects in zebrafish embryos. These results suggest that cyclin F can be modified to enhance its targeting of TDP-43, which in turn reduces the toxicity associated with the overexpression of cyclin F. This study provides greater insights into the interaction that occurs between cyclin F and TDP-43 in cells and in vivo.

Intergenerational plasticity aligns with temperature-dependent selection on offspring metabolic rates

Metabolic rates are linked to key life-history traits that are thought to set the pace of life and affect fitness, yet the role that parents may have in shaping the metabolism of their offspring to enhance survival remains unclear. Here, we investigated the effect of temperature (24°C or 30°C) and feeding frequency experienced by parent zebrafish (Danio rerio) on offspring phenotypes and early survival at different developmental temperatures (24°C or 30°C). We found that embryo size was larger, but survival lower, in offspring from the parental low food treatment. Parents exposed to the warmer temperature and lower food treatment also produced offspring with lower standard metabolic rates-aligning with selection on embryo metabolic rates. Lower metabolic rates were correlated with reduced developmental and growth rates, suggesting selection for a slow pace of life. Our results show that intergenerational phenotypic plasticity on offspring size and metabolic rate can be adaptive when parent and offspring temperatures are matched: the direction of selection on embryo size and metabolism aligned with intergenerational plasticity towards lower metabolism at higher temperatures, particularly in offspring from low-condition parents. These findings provide evidence for adaptive parental effects, but only when parental and offspring environments match. This article is part of the theme issue 'The evolutionary significance of variation in metabolic rates'.

Estrogen Signaling Inhibits the Expression of anti-Müllerian hormone (amh) and gonadal-soma-derived factor (gsdf) during the Critical Time of Sexual Fate Determination in Zebrafish

The mechanism of fish gonadal sex differentiation is complex and regulated by multiple factors. It has been widely known that proper steroidogenesis in Leydig cells and sex-related genes in Sertoli cells play important roles in gonadal sex differentiation. In teleosts, the precise interaction of these signals during the sexual fate determination remains elusive, especially their effect on the bi-potential gonad during the critical stage of sexual fate determination. Recently, all-testis phenotypes have been observed in the cyp17a1-deficient zebrafish and common carp, as well as in cyp19a1a-deficient zebrafish. By mating cyp17a1-deficient fish with transgenic zebrafish Tg(piwil1:EGFP-nanos3UTR), germ cells in the gonads were labelled with enhanced green fluorescent protein (EGFP). We classified the cyp17a1-deficient zebrafish and their control siblings into primordial germ cell (PGC)-rich and -less groups according to the fluorescence area of the EGFP labelling. Intriguingly, the EGFP-labelled bi-potential gonads in cyp17a1+/+ fish from the PGC-rich group were significantly larger than those of the cyp17a1-/- fish at 23 days post-fertilization (dpf). Based on the transcriptome analysis, we observed that the cyp17a1-deficient fish of the PGC-rich group displayed a significantly upregulated expression of amh and gsdf compared to that of control fish. Likewise, the upregulated expressions of amh and gsdf were observed in cyp19a1a-deficient fish as examined at 23 dpf. This upregulation of amh and gsdf could be repressed by treatment with an exogenous supplement of estradiol. Moreover, tamoxifen, an effective antagonist of both estrogen receptor α and β (ERα and Erβ), upregulates the expression of amh and gsdf in wild-type (WT) fish. Using the cyp17a1- and cyp19a1a-deficient zebrafish, we provide evidence to show that the upregulated expression of amh and gsdf due to the compromised estrogen signaling probably determines their sexual fate towards testis differentiation. Collectively, our data suggest that estrogen signaling inhibits the expression of amh and gsdf during the critical time of sexual fate determination, which may broaden the scope of sex steroid hormones in regulating gonadal sex differentiation in fish.

Kremen1 regulates the regenerative capacity of support cells and mechanosensory hair cells in the zebrafish lateral line

Mechanosensory hair cells in the inner ear mediate the sensations of hearing and balance, and in the specialized lateral line sensory system of aquatic vertebrates, the sensation of water movement. In mammals, hair cells lack the ability to regenerate following damage, resulting in sensory deficits. In contrast, non-mammalian vertebrates, such as zebrafish, can renew hair cells throughout their lifespan. Wnt signaling is required for development of inner ear and lateral line hair cells and regulates regeneration. Kremen1 inhibits Wnt signaling and hair cell formation, though its role in regeneration is unknown. We used a zebrafish kremen1 mutant line to show overactive Wnt signaling results in supernumerary support cells and hair cell regeneration without increased proliferation, in contrast with the previously described role of Wnt signaling during hair cell regeneration. This work allows us to understand the biology of mechanosensory hair cells and how regeneration might be promoted following damage.

Gamma irradiation-induced offspring masculinization is associated with epigenetic changes in female zebrafish

Sex ratio variation is a key topic in ecology, because of its direct effects on population dynamics and thus, on animal conservation strategies. Among factors affecting sex ratio, types of sex determination systems have a central role, since some species could have a sex determined by genetic factors, environmental factors or a mix of those two. Yet, most studies on the factors affecting sex determination have focused on temperature or endocrine-disrupting chemicals (EDCs), and much less is known regarding other factors. Exposure to gamma irradiation was found to trigger offspring masculinization in zebrafish. Here we aimed at deciphering the potential mechanisms involved, by focusing on stress (i.e. cortisol) and epigenetic regulation of key genes involved in sex differentiation in fish. Cortisol levels in exposed and control (F0) zebrafish females' gonads were similar. However, irradiation increased the DNA methylation level of foxl2a and cyp19a1a in females of the F0 and F1 generation, respectively, while no effects were detected in testis. Overall, our results suggest that parental exposure could alter offspring sex ratio, at least in part by inducing methylation changes in ovaries.

Direct Male Development in Chromosomally ZZ Zebrafish

The genetics of sex determination varies across taxa, sometimes even within a species. Major domesticated strains of zebrafish ( Danio rerio ), including AB and TU, lack a strong genetic sex determining locus, but strains more recently derived from nature, like Nadia (NA), possess a ZZ male/ZW female chromosomal sex-determination system. AB strain fish pass through a juvenile ovary stage, forming oocytes that survive in fish that become females but die in fish that become males. To understand mechanisms of gonad development in NA zebrafish, we studied histology and single cell transcriptomics in developing ZZ and ZW fish. ZW fish developed oocytes by 22 days post-fertilization (dpf) but ZZ fish directly formed testes, avoiding a juvenile ovary phase. Gonads of some ZW and WW fish, however, developed oocytes that died as the gonad became a testis, mimicking AB fish, suggesting that the gynogenetically derived AB strain is chromosomally WW. Single-cell RNA-seq of 19dpf gonads showed similar cell types in ZZ and ZW fish, including germ cells, precursors of gonadal support cells, steroidogenic cells, interstitial/stromal cells, and immune cells, consistent with a bipotential juvenile gonad. In contrast, scRNA-seq of 30dpf gonads revealed that cells in ZZ gonads had transcriptomes characteristic of testicular Sertoli, Leydig, and germ cells while ZW gonads had granulosa cells, theca cells, and developing oocytes. Hematopoietic and vascular cells were similar in both sex genotypes. These results show that juvenile NA zebrafish initially develop a bipotential gonad; that a factor on the NA W chromosome or fewer than two Z chromosomes is essential to initiate oocyte development; and without the W factor or with two Z doses, NA gonads develop directly into testes without passing through the juvenile ovary stage. Sex determination in AB and TU strains mimics NA ZW and WW zebrafish, suggesting loss of the Z chromosome during domestication. Genetic analysis of the NA strain will facilitate our understanding of the evolution of sex determination mechanisms.

Production of all-male non-transgenic zebrafish by conditional primordial germ cell ablation

Many fish species exhibit remarkable sexual dimorphism, with males possessing numerous advantageous traits for commercial production by aquaculture such as faster growth rate, more efficient food energy utilization for muscle development, and better breeding performance. Several studies have shown that a decrease in the number of primordial germ cells (PGCs) during early development leads predominantly to male progeny. In this study, we developed a method to obtain all-male zebrafish (Danio rerio) by targeted PGC ablation using the nitroreductase/metronidazole (NTR/Mtz) system. Embryos generated by female heterozygous Tg(nanos3:nfsB-mCherry-nanos3 3'UTR) and male wild-types (WTs) were treated with vehicle or Mtz. Compared to vehicle-treated controls, 5.0 and 10.0 mM Mtz treatment for 24 h significantly reduced the number of PGCs and yielded an exclusively male phenotype in adulthood. The gonads of offspring treated with 5.0 mM Mtz exhibited relatively normal morphology and histological characteristics. Furthermore, these males were able to chase females, spawn, and produce viable offspring, while about 20.0% of males treated with 10.0 mM Mtz were unable to produce viable offspring. The 5.0 mM Mtz treatment protocol may thus be suitable for large-scale production of fertile male offspring. Moreover, about half of these males were WT as evidenced by the absence of nfsB gene expression. It may thus be possible to breed an all-male WT fish population by Mtz-mediated PGC ablation.

Macrophage activation drives ovarian failure and masculinization in zebrafish

BMP15 is a conserved regulator of ovarian development and maintenance in vertebrates. In humans, premature ovarian insufficiency is caused by autoimmunity and genetic factors, including mutation of BMP15. The cellular mechanisms underlying ovarian failure caused by BMP15 mutation and immune contributions are not understood. Using zebrafish, we established a causal link between macrophage activation and ovarian failure, which, in zebrafish, causes sex reversal. We define a germline-soma signaling axis that activates macrophages and drives ovarian failure and female-to-male sex reversal. Germline loss of zebrafish Bmp15 impairs oogenesis and initiates this cascade. Single-cell RNA sequencing and genetic analyses implicate ovarian somatic cells that express conserved macrophage-activating ligands as mediators of ovarian failure and sex reversal. Genetic ablation of macrophages or elimination of Csf1Rb ligands, Il34 or Csf1a, delays or blocks premature oocyte loss and sex reversal. The axis identified here provides insight into the cells and pathways governing oocyte and ovary maintenance and potential therapeutic targets to preserve female fertility.

Defining the cellular complexity of the zebrafish bipotential gonad

Zebrafish are routinely used to model reproductive development, function, and disease, yet we still lack a clear understanding of the fundamental steps that occur during early bipotential gonad development, including when endothelial cells, pericytes, and macrophage arrive at the bipotential gonad to support gonad growth and differentiation. Here, we use a combination of transgenic reporters and single-cell sequencing analyses to define the arrival of different critical cell types to the larval zebrafish gonad. We determined that blood initially reaches the gonad via a vessel formed from the swim bladder artery, which we have termed the gonadal artery. We find that vascular and lymphatic development occurs concurrently in the bipotential zebrafish gonad and our data suggest that similar to what has been observed in developing zebrafish embryos, lymphatic endothelial cells in the gonad may be derived from vascular endothelial cells. We mined preexisting sequencing datasets to determine whether ovarian pericytes had unique gene expression signatures. We identified 215 genes that were uniquely expressed in ovarian pericytes, but not expressed in larval pericytes. Similar to what has been shown in the mouse ovary, our data suggest that pdgfrb+ pericytes may support the migration of endothelial tip cells during ovarian angiogenesis. Using a macrophage-driven photoconvertible protein, we found that macrophage established a nascent resident population as early as 12 dpf and can be observed removing cellular material during gonadal differentiation. This foundational information demonstrates that the early bipotential gonad contains complex cellular interactions, which likely shape the health and function of the mature gonad.

Exploring the Effects of Rearing Densities on Epigenetic Modifications in the Zebrafish Gonads

Rearing density directly impacts fish welfare, which, in turn, affects productivity in aquaculture. Previous studies have indicated that high-density rearing during sexual development in fish can induce stress, resulting in a tendency towards male-biased sex ratios in the populations. In recent years, research has defined the relevance of the interactions between the environment and epigenetics playing a key role in the final phenotype. However, the underlying epigenetic mechanisms of individuals exposed to confinement remain elucidated. By using zebrafish (Danio rerio), the DNA methylation promotor region and the gene expression patterns of six genes, namely dnmt1, cyp19a1a, dmrt1, cyp11c1, hsd17b1, and hsd11b2, involved in the DNA maintenance methylation, reproduction, and stress were assessed. Zebrafish larvae were subjected to two high-density conditions (9 and 66 fish/L) during two periods of overlapping sex differentiation of this species (7 to 18 and 18 to 45 days post-fertilization, dpf). Results showed a significant masculinization in the populations of fish subjected to high densities from 18 to 45 dpf. In adulthood, the dnmt1 gene was differentially hypomethylated in ovaries and its expression was significantly downregulated in the testes of fish exposed to high-density. Further, the cyp19a1a gene showed downregulation of gene expression in the ovaries of fish subjected to elevated density, as previously observed in other studies. We proposed dnmt1 as a potential testicular epimarker and the expression of ovarian cyp19a1a as a potential biomarker for predicting stress originated from high densities during the early stages of development. These findings highlight the importance of rearing densities by long-lasting effects in adulthood conveying cautions for stocking protocols in fish hatcheries.

Wnt4a Is Indispensable for Genital Duct Elongation but Not for Gonadal Sex Differentiation in the Medaka, Oryzias latipes

In most vertebrates, the oviducts and sperm ducts are derived from the Müllerian ducts and Wolffian ducts, respectively. However, in teleosts, the genital ducts are formed by the posterior extension of gonads in both sexes. Whether the genital ducts of teleosts are newly evolved organs or variants of Müllerian ducts is an important question for understanding evolutionary mechanisms of morphogenesis. One of the genes essential for Müllerian duct formation in mice is Wnt4, which is expressed in the mesenchyme and induces invagination of the coelomic epithelium and its posterior elongation. Here, we addressed the above question by examining genital duct development in mutants of two Wnt4 genes in the medaka (wnt4a is orthologous to mouse Wnt4, and wnt4b is paralogous). The wnt4b mutants had a short body but were fertile with normal genital ducts. In contrast, both male and female wnt4a mutants had their posterior elongation of the gonads stopped within or just outside the coelom. The mutants retained the posterior parts of ovarian cavities or sperm duct primordia, which are potential target tissues of Wnt4a. The gonads of female scl mutants (unable to synthesize sex steroids) lacked these tissues and did not develop genital ducts. Medaka wnt4a was expressed in the mesenchyme ventral to the genital ducts in both sexes. Taken together, the data strongly suggest that the mouse Müllerian ducts and the medaka genital ducts share homologous developmental processes. Additionally, the wnt4a or wnt4b single mutants and the double mutants did not show sex-reversal, implying that both genes are dispensable for gonadal sex differentiation in the medaka.

Defining the cellular complexity of the zebrafish bipotential gonad

Zebrafish are routinely used to model reproductive development, function, and disease, yet we still lack a clear understanding of the fundamental steps that occur during early bipotential gonad development, including when endothelial cells, pericytes, and macrophage cells arrive at the bipotential gonad to support gonad growth and differentiation. Here, we use a combination of transgenic reporters and single-cell sequencing analyses to define the arrival of different critical cell types to the larval zebrafish gonad. We determined that blood initially reaches the gonad via a vessel formed from the swim bladder artery, which we have termed the gonadal artery. We find that vascular and lymphatic development occurs concurrently in the bipotential zebrafish gonad and our data suggest that similar to what has been observed in developing zebrafish embryos, lymphatic endothelial cells in the gonad may be derived from vascular endothelial cells. We mined preexisting sequencing data sets to determine whether ovarian pericytes had unique gene expression signatures. We identified 215 genes that were uniquely expressed in ovarian pericytes that were not expressed in larval pericytes. Similar to what has been shown in the mouse ovary, our data suggest that pdgfrb+ pericytes may support the migration of endothelial tip cells during ovarian angiogenesis. Using a macrophage-driven photoconvertible protein, we found that macrophage established a nascent resident population as early as 12 dpf and can be observed removing cellular material during gonadal differentiation. This foundational information demonstrates that the early bipotential gonad contains complex cellular interactions, which likely shape the health and function of the mature, differentiated gonad.

The RNA-binding protein Adad1 is necessary for germ cell maintenance and meiosis in zebrafish

The double stranded RNA binding protein Adad1 (adenosine deaminase domain containing 1) is a member of the adenosine deaminase acting on RNAs (Adar) protein family with germ cell-specific expression. In mice, Adad1 is necessary for sperm differentiation, however its function outside of mammals has not been investigated. Here, through an N-ethyl-N-nitrosourea (ENU) based forward genetic screen, we identified an adad1 mutant zebrafish line that develops as sterile males. Further histological examination revealed complete lack of germ cells in adult mutant fish, however germ cells populated the gonad, proliferated, and entered meiosis in larval and juvenile fish. Although meiosis was initiated in adad1 mutant testes, the spermatocytes failed to progress beyond the zygotene stage. Thus, Adad1 is essential for meiosis and germline maintenance in zebrafish. We tested if spermatogonial stem cells were affected using nanos2 RNA FISH and a label retaining cell (LRC) assay, and found that the mutant testes had fewer LRCs and nanos2-expressing cells compared to wild-type siblings, suggesting that failure to maintain the spermatogonial stem cells resulted in germ cell loss by adulthood. To identify potential molecular processes regulated by Adad1, we sequenced bulk mRNA from mutants and wild-type testes and found mis-regulation of genes involved in RNA stability and modification, pointing to a potential broader role in post-transcriptional regulation. Our findings suggest that the RNA regulatory protein Adad1 is required for fertility through regulation of spermatogonial stem cell maintenance in zebrafish.

Target-selective vertebrate motor axon regeneration depends on interaction with glial cells at a peripheral nerve plexus

A critical step for functional recovery from peripheral nerve injury is for regenerating axons to connect with their pre-injury targets. Reestablishing pre-injury target specificity is particularly challenging for limb-innervating axons as they encounter a plexus, a network where peripheral nerves converge, axons from different nerves intermingle, and then re-sort into target-specific bundles. Here, we examine this process at a plexus located at the base of the zebrafish pectoral fin, equivalent to tetrapod forelimbs. Using live cell imaging and sparse axon labeling, we find that regenerating motor axons from 3 nerves coalesce into the plexus. There, they intermingle and sort into distinct branches, and then navigate to their original muscle domains with high fidelity that restores functionality. We demonstrate that this regeneration process includes selective retraction of mistargeted axons, suggesting active correction mechanisms. Moreover, we find that Schwann cells are enriched and associate with axons at the plexus, and that Schwann cell ablation during regeneration causes profound axonal mistargeting. Our data provide the first real-time account of regenerating vertebrate motor axons navigating a nerve plexus and reveal a previously unappreciated role for Schwann cells to promote axon sorting at a plexus during regeneration.

The use of in silico extreme pathway (ExPa) analysis to identify conserved reproductive transcriptional-regulatory networks in humans, mice, and zebrafish

Vertebrate sex determination and differentiation are coordinated by the activations and maintenance of reproductive transcriptional-regulatory networks (TRNs). There is considerable interest in studying the conserved design principles and functions of reproductive TRNs given that their intricate regulation is susceptible to disruption by gene mutations or exposures to exogenous endocrine disrupting chemicals (or EDCs). In this manuscript, the Boolean rules describing reproductive TRNs in humans, mice, and zebrafish, were represented as a pseudo-stoichiometric matrix model. This model mathematically described the interactions of 35 transcription factors with 21 sex determination and differentiation genes across the three species. The in silico approach of Extreme Pathway (ExPa) analysis was used to predict the extent of TRN gene activations subject to the species-specific transcriptomics data, from across various developmental life-stages. A goal of this work was to identify conserved and functional reproductive TRNs across the three species. ExPa analyses predicted the sex differentiation genes, DHH, DMRT1, and AR, to be highly active in male humans, mice, and zebrafish. Whereas FOXL2 was the most active gene in female humans and mice; and CYP19A1A in female zebrafish. These results agree with the expectation that regardless of a lack of sex determination genes in zebrafish, the TRNs responsible for canalizing male vs. female sexual differentiation are conserved with mammalian taxa. ExPa analysis therefore provides a framework with which to study the TRNs that influence the development of sexual phenotypes. And the in silico predicted conservation of sex differentiation TRNs between mammals and zebrafish identifies the piscine species as an effective in vivo model to study mammalian reproductive systems under normal or perturbed pathologies.

Sex blind: bridging the gap between drug exposure and sex-related gene expression in Danio rerio using next-generation sequencing (NGS) data and a literature review to find the missing links in pharmaceutical and environmental toxicology studies

The sex of both humans and Danio rerio has previously been shown to affect the way individuals respond to drug exposure. Genes which allow identification of sex in juvenile zebrafish show potential to reveal these confounding variables between sex in toxicological and preclinical trials but the link between these is so far missing. These sex-specific, early expressed genes where expression is not altered by drug exposure must be carefully selected for this purpose. We aimed to discover genes which can be used in pharmaceutical trials and environmental toxicology studies to uncover sex-related variations in gene expression with drug application using the model organism Danio rerio. Previously published early sex determining genes from King et al. were evaluated as well as additional genes selected from our zebrafish Next-generation sequencing (NGS) data which are known from previously published works not to be susceptible to changes in expression with drug exposure. NGS revealed a further ten female-specific genes (vtg1, cyp17a1, cyp19a1a, igf3, ftz-f1, gdf9, foxl2a, Nr0b1, ipo4, lhcgr) and five male related candidate genes (FKBP5, apobb1, hbaa1, dmrt1, spata6) which are also expressed in juvenile zebrafish, 28 days post fertilisation (dpf). Following this, a literature review was performed to classify which of these early-expressed sex specific genes are already known to be affected by drug exposure in order to determine candidate genes to be used in pharmaceutical trials or environmental toxicology testing studies. Discovery of these early sex-determining genes in Danio rerio will allow identification of sex-related responses to drug testing to improve sex-specific healthcare and the medical treatment of human patients.

Sex Reversal Induced by Dietary Supplementation with 17α-Methyltestosterone during the Critical Period of Sex Differentiation in Oriental River Prawn (Macrobrachium nipponense)

The steroid 17α-methyltestosterone (MT) inhibits ovarian function and is often used to induce sex reversal artificially in vertebrates. In the present study, different concentrations of MT were added as dietary supplementation, and the effects on sex ratio, growth, and gonadal development were examined. After 40 days, the sex ratio (male:female) in each group increased at different degrees with 50 (1.36:1), 100 (1.57:1), and 200 (2.61:1) mg/kg MT, and neo-males with testis-ovary coexistence were observed in the 200 mg/kg MT group. Furthermore, 50 and 100 mg/kg MT could induce female reversion in neo-males. Histologically, the development of the testes in experimental groups was slower, but the ovaries of the experimental and control groups had similar developmental rates. The expression levels of DMRT11E, Foxl2, and SoxE1 in males at 200 mg/kg MT were 8.65-, 3.75-, and 3.45-fold greater than those of the control group. In crustaceans, sex reversal through vertebrate sex hormones can be observed. Neo-males (sex-reversed female prawns) were maintained by exogenous androgen, and over-reliance led to slow testis growth, small body size, and low growth rate, but sperm was still produced. In female prawns, MT inhibited ovary development and promoted growth.

On the evolution of variation in sexual reproduction through the prism of eukaryotic microbes

Almost all eukaryotes undergo sexual reproduction to generate diversity and select for fitness in their population pools. Interestingly, the systems by which sex is defined are highly diverse and can even differ between evolutionarily closely related species. While the most commonly known form of sex determination involves males and females in animals, eukaryotic microbes can have as many as thousands of different mating types for the same species. Furthermore, some species have found alternatives to sexual reproduction and prefer to grow clonally and yet undergo infrequent facultative sexual reproduction. These organisms are mainly invertebrates and microbes, but several examples are also present among vertebrates suggesting that alternative modes of sexual reproduction evolved multiple times throughout evolution. In this review, we summarize the sex-determination modes and variants of sexual reproduction found across the eukaryotic tree of life and suggest that eukaryotic microbes provide unique opportunities to study these processes in detail. We propose that understanding variations in modes of sexual reproduction can serve as a foundation to study the evolution of sex and why and how it evolved in the first place.

Development of Ovaries and Sex Change in Fish: Bringing Potential into Action

BACKGROUND

Encompassing about half of the 60,000 species of vertebrates, fish display the greatest diversity of sex determination mechanisms among metazoans. As such that phylum offers a unique playground to study the impressive variety of gonadal morphogenetic strategies, ranging from gonochorism, with either genetic or environmental sex determination, to unisexuality, with either simultaneous or consecutive hermaphroditism.

SUMMARY

From the two main types of gonads, the ovaries embrace the important role to produce the larger and non-motile gametes, which is the basis for the development of a future organism. The production of the egg cells is complex and involves the formation of follicular cells, which are necessary for the maturation of the oocytes and the production of feminine hormones. In this vein, our review focuses on the development of ovaries in fish with special emphasis on the germ cells, including those that transition from one sex to the other as part of their life cycle and those that are capable of transitioning to the opposite sex depending on environmental cues.

KEY MESSAGES

Clearly, establishing an individual as either a female or a male is not accomplished by the sole development of two types of gonads. In most cases, that dichotomy, be it final or transient, is accompanied by coordinated transformations across the entire organism, leading to changes in the physiological sex as a whole. These coordinated transformations require both molecular and neuroendocrine networks, but also anatomical and behavioural adjustments. Remarkably, fish managed to tame the ins and outs of sex reversal mechanisms to take the most advantages of changing sex as adaptive strategies in some situations.

Genome-wide identification, evolution and expression analysis of bone morphogenetic protein (BMP) gene family in chinese soft-shell turtle (Pelodiscus sinensis)

Introduction: Bone morphogenetic proteins (BMPs) play a crucial role in bone formation and differentiation. Recent RNA-Seq results suggest that BMPs may be involved in the sex differentiation of P. sinensis, yet more relevant studies about BMPs in P. sinensis are lacking. Methods: Herein, we identified BMP gene family members, analyzed the phylogeny, collinear relationship, scaffold localization, gene structures, protein structures, transcription factors and dimorphic expression by using bioinformatic methods based on genomic and transcriptomic data of P. sinensis. Meanwhile, qRT-PCR was used to verify the RNA-Seq results and initially explore the function of the BMPs in the sex differentiation of P. sinensis. Results: A total of 11 BMP genes were identified, 10 of which were localized to their respective genomic scaffolds. Phylogenetic analysis revealed that BMP genes were divided into eight subfamilies and shared similar motifs ("WII", "FPL", "TNHA", "CCVP", and "CGC") and domain (TGF-β superfamily). The results of the sexually dimorphic expression profile and qRT-PCR showed that Bmp2, Bmp3, Bmp15l, Bmp5, Bmp6 and Bmp8a were significantly upregulated in ovaries, while Bmp2lb, Bmp7, Bmp2bl and Bmp10 were remarkable upregulated in testes, suggesting that these genes may play a role in sex differentiation of P. sinensis. Discussion: Collectively, our comprehensive results enrich the basic date for studying the evolution and functions of BMP genes in P. sinensis.

Modeling the effects of genetic- and diet-induced obesity on melanoma progression in zebrafish

Obesity is a rising concern and associated with an increase in numerous cancers, often in a sex-specific manner. Preclinical models are needed to deconvolute the intersection between obesity, sex and melanoma. Here, we generated a zebrafish system that can be used as a platform for studying these factors. We studied how germline overexpression of Agrp along with a high-fat diet affects melanomas dependent on BRAFV600E and loss of p53. This revealed an increase in tumor incidence and area in male, but not female, obese fish, consistent with the clinical literature. We then determined whether this was further affected by additional somatic mutations in the clinically relevant genes rb1 or ptena/b. We found that the male obesogenic effect on melanoma was present with tumors generated with BRAF;p53;Rb1 but not BRAF;p53;Pten. These data indicate that both germline (Agrp) and somatic (BRAF, Rb1) mutations contribute to obesity-related effects in melanoma. Given the rapid genetic tools available in the zebrafish, this provides a high-throughput system to dissect the interactions of genetics, diet, sex and host factors in obesity-related cancers.

A glia cell dependent mechanism at a peripheral nerve plexus critical for target-selective axon regeneration

A critical step for functional recovery from peripheral nerve injury is for regenerating axons to connect with their pre-injury targets. Reestablishing pre-injury target specificity is particularly challenging for limb-innervating axons as they encounter a plexus, a network where peripheral nerves converge, axons from different nerves intermingle, and then re-sort into target-specific bundles. Here, we examine this process at a plexus located at the base of the zebrafish pectoral fin, equivalent to tetrapod forelimbs. Using live cell imaging and sparse axon labeling, we find that regenerating motor axons from three nerves coalesce into the plexus. There, they intermingle and sort into distinct branches, and then navigate to their original muscle domains with high fidelity that restores functionality. We demonstrate that this regeneration process includes selective retraction of mistargeted axons, suggesting active correction mechanisms. Moreover, we find that Schwann cells are enriched and associate with axons at the plexus, and that Schwann cell ablation during regeneration causes profound axonal mistargeting. Our data provide the first real time account of regenerating vertebrate motor axons navigating a nerve plexus and reveal a previously unappreciated role for Schwann cells to promote axon sorting at a plexus during regeneration.

Metzincin metalloproteases in PGC migration and gonadal sex conversion

Development of a functional gonad includes migration of primordial germ cells (PGCs), differentiations of somatic and germ cells, formation of primary follicles or spermatogenic cysts with somatic gonadal cells, development and maturation of gametes, and subsequent releasing of mature germ cells. These processes require extensive cellular and tissue remodeling, as well as broad alterations of the surrounding extracellular matrix (ECM). Metalloproteases, including MMPs (matrix metalloproteases), ADAMs (a disintegrin and metalloproteinases), and ADAMTS (a disintegrin and metalloproteinase with thrombospondin motifs), are suggested to have critical roles in the remodeling of the ECM during gonad development. However, few research articles and reviews are available on the functions and mechanisms of metalloproteases in remodeling gonadal ECM, gonadal development, or gonadal differentiation. Moreover, most studies focused on the roles of transcription and growth factors in early gonad development and primary sex determination, leaving a significant knowledge gap on how differentially expressed metalloproteases exert effects on the ECM, cell migration, development, and survival of germ cells during the development and differentiation of ovaries or testes. We will review gonad development with focus on the evidence of metalloprotease involvements, and with an emphasis on zebrafish as a model for studying gonadal sex differentiation and metalloprotease functions.

baz1b loss-of-function in zebrafish produces phenotypic alterations consistent with the domestication syndrome

BAZ1B is a ubiquitously expressed nuclear protein with roles in chromatin remodeling, DNA replication and repair, and transcription. Reduced BAZ1B expression disrupts neuronal and neural crest development. Variation in the activity of BAZ1B has been proposed to underly morphological and behavioral aspects of domestication through disruption of neural crest development. Knockdown of baz1b in Xenopus embryos and Baz1b loss-of-function (LoF) in mice leads to craniofacial defects consistent with this hypothesis. We generated baz1b LoF zebrafish using CRISPR/Cas9 gene editing to test the hypothesis that baz1b regulates behavioral phenotypes associated with domestication in addition to craniofacial features. Zebrafish with baz1b LoF show mild underdevelopment at larval stages and distinctive craniofacial features later in life. Mutant zebrafish show reduced anxiety-associated phenotypes and an altered ontogeny of social behaviors. Thus, in zebrafish, developmental deficits in baz1b recapitulate both morphological and behavioral phenotypes associated with the domestication syndrome in other species.

A Simple Method for Inducing Masculinization of Zebrafish Stocks Using 17α-Methyltestosterone

Severely skewed sex ratios in zebrafish stocks can pose significant hurdles for line propagation and sperm cryopreservation. To overcome female-biased sex ratios in stocks derived from imported sperm samples, the Zebrafish International Resource Center has implemented routine supplementation of larval food with 17α-methyltestosterone to skew gonadal sex differentiation toward masculinization. Resulting stocks averaged 80% males.

Environmentally-induced sex reversal in fish with chromosomal vs. polygenic sex determination

Sex ratio depends on sex determination mechanisms and is a key demographic parameter determining population viability and resilience to natural and anthropogenic stressors. There is increasing evidence that the environment can alter sex ratio even in genetically sex-determined species (GSD), as elevated temperature can cause female-to-male sex reversal (neomales). Alarmingly, neomales are being discovered in natural populations of several fish, amphibian and reptile species worldwide. Understanding the basis of neomale development is important for conservation biology. Among GSD species, it is unknown whether those with chromosomal sex determination (CSD), the most common system, will better resist the influence of high temperature than those with polygenic sex determination (PSD). Here, we compared the effects of elevated temperature in two wild zebrafish strains, Nadia (NA) and Ekkwill (EKW), which have CSD with a ZZ/ZW system, against the AB laboratory strain, which has PSD. First, we uncovered novel sex genotypes and the results showed that, at control temperature, the masculinization rate roughly doubled with the addition of each Z chromosome, while some ZW and WW fish of the wild strains became neomales. Surprisingly, we found that at elevated temperatures WW fish were just as likely as ZW fish to become neomales and that all strains were equally susceptible to masculinization. These results demonstrate that the Z chromosome is not essential for male development and that the dose of W buffers masculinization at the control temperature but not at elevated temperature. Furthermore, at the elevated temperature the testes of neomales, but not of normal males, contained more spermatozoa than at the control temperature. Our results show in an unprecedented way that, in a global warming scenario, CSD species may not necessarily be better protected against the masculinizing effect of elevated temperature than PSD species, and reveal genotype-by-temperature interactions in male sex determination and spermatogenesis.

Comprehensive Transcriptome Analysis Reveals Sex-Specific Alternative Splicing Events in Zebrafish Gonads

Alternative splicing is an important way of regulating gene functions in eukaryotes. Several key genes involved in sex determination and gonadal differentiation, such as nr5a1 and ddx4, have sex-biased transcripts between males and females, suggesting a potential regulatory role of alternative splicing in gonads. Currently, the sex-specific alternative splicing events and genes have not been comprehensively studied at the genome-wide level in zebrafish. In this study, through global splicing analysis on three independent sets of RNA-seq data from matched zebrafish testes and ovaries, we identified 120 differentially spliced genes shared by the three datasets, most of which haven’t been reported before. Functional enrichment analysis showed that the GO terms of mRNA processing, mRNA metabolism and microtubule-based process were strongly enriched. The testis- and ovary-biased alternative splicing genes were identified, and part of them (tp53bp1, tpx2, mapre1a, kif2c, and ncoa5) were further validated by RT-PCR. Sequence characteristics analysis suggested that the lengths, GC contents, and splice site strengths of the alternative exons or introns may have different influences in different types of alternative splicing events. Interestingly, we identified an unexpected high proportion (over 70%) of non-frameshift exon-skipping events, suggesting that in these cases the two protein isoforms derived from alternative splicing may both have functions. Furthermore, as a representative example, we found that the alternative splicing of ncoa5 causes the loss of a conserved RRM domain in the short transcript predominantly produced in testes. Our study discovers novel sex-specific alternative splicing events and genes with high reliabilities in zebrafish testes and ovaries, which would provide attractive targets for follow-up studies to reveal the biological significances of alternative splicing events and genes in sex determination and gonadal differentiation.

Becoming female: Ovarian differentiation from an evolutionary perspective

Differentiation of the bipotential gonadal primordium into ovaries and testes is a common process among vertebrate species. While vertebrate ovaries eventually share the same functions of producing oocytes and estrogens, ovarian differentiation relies on different morphogenetic, cellular, and molecular cues depending on species. The aim of this review is to highlight the conserved and divergent features of ovarian differentiation through an evolutionary perspective. From teleosts to mammals, each clade or species has a different story to tell. For this purpose, this review focuses on three specific aspects of ovarian differentiation: ovarian morphogenesis, the evolution of the role of estrogens on ovarian differentiation and the molecular pathways involved in granulosa cell determination and maintenance.

Transcriptome analysis revealed gene expression feminization of testis after exogenous tetrodotoxin administration in pufferfish Takifugu flavidus

Tetrodotoxin (TTX) is a deadly neurotoxin and usually accumulates in large amounts in the ovaries but is non-toxic or low toxic in the testis of pufferfish. The molecular mechanism underlying sexual dimorphism accumulation of TTX in ovary and testis, and the relationship between TTX accumulation with sex related genes expression remain largely unknown. The present study investigated the effects of exogenous TTX treatment on Takifugu flavidus. The results demonstrated that exogenous TTX administration significantly incresed level of TTX concentration in kidney, cholecyst, skin, liver, heart, muscle, ovary and testis of the treatment group (TG) than that of the control group (CG). Transcriptome sequencing and analysis were performed to study differential expression profiles of mRNA and piRNA after TTX administration of the ovary and testis. The results showed that compared with female control group (FCG) and male control group (MCG), TTX administration resulted in 80 and 23 piRNAs, 126 and 223 genes up and down regulated expression in female TTX-treated group (FTG), meanwhile, 286 and 223 piRNAs, 2 and 443 genes up and down regulated expression in male TTX-treated group (MTG). The female dominant genes cyp19a1, gdf9 and foxl2 were found to be up-regulated in MTG. The cyp19a1, whose corresponding target piRNA uniq_554482 was identified as down-regulated in the MTG, indicating the gene expression feminization in testis after exogenous TTX administration. The KEGG enrichment analysis revealed that differentially expressed genes (DEGs) and piRNAs (DEpiRNAs) in MTG vs MCG group were more enriched in metabolism pathways, indicating that the testis produced more metabolic pathways in response to exogenous TTX, which might be a reason for the sexual dimorphism of TTX distribution in gonads. In addition, TdT-mediated dUTP-biotin nick end labeling staining showed that significant apoptosis was detected in the MTG testis, and the role of the cell apoptotic pathways was further confirmed. Overall, our research revealed that the response of the ovary and testis to TTX administration was largely different, the ovary is more tolerant whereas the testis is more sensitive to TTX. These data will deepen our understanding on the accumulation of TTX sexual dimorphism in Takifugu.

Transgenerational effects of zinc in zebrafish following early life stage exposure

Although toxic effects of zinc (Zn) have been well established in the different developmental stages in fish, long-lasting effects of Zn exposure during embryonic development have not been explored. Exposure to an environmentally relevant Zn concentration of 10 μM (650 μg/L) during the first five days after fertilization did not affect survival, body weight, malformations or overall hatching success of F0 and F1 larvae. Zn exposure did, however, result in delayed hatching in both the F0 and F1 generations and caused significant changes in homeostasis of Zn and selenium (Se) in F0 and F1 fish. This was especially pronounced when F1 embryos from Zn-exposed parents were treated with 30 μM (2000 μg/L) Zn. In the F0 generation, skewed sex ratio towards males and changes in homeostasis of Zn, Se and manganese (Mn) in the brain, gill, liver and gonad of adult fish were also observed. These changes were associated with altered expression of Zn- and Mn-regulatory genes and sex differentiation genes in F0 and F1 fish. The present study suggests that fish may carry memory from embryo-larval Zn exposure into adulthood and further to the next generation. The present study shows that ecotoxicological risk of an exposure to Zn during embryo-larval development may persist long after recovery and may also manifest in the F1 generation.

Feminization of channel catfish with 17β-oestradiol involves methylation and expression of a specific set of genes independent of the sex determination region

Exogenous oestrogen 17β-oestradiol (E2) has been shown to effectively induce feminization in teleosts. However, the molecular mechanisms underlying the process remain unclear. Here, we determined global DNA methylation and gene expression profiles of channel catfish (Ictalurus punctatus) during early sex differentiation after E2 treatment. Overall, the levels of global DNA methylation after E2 treatment were not significantly different from those of controls. However, a specific set of genes were differentially methylated, which included many sex differentiation-related pathways, such as MARK signalling, adrenergic signalling, Wnt signalling, GnRH signalling, ErbB signalling, and ECM-receptor interactions. Many genes involved in these pathways were also differentially expressed after E2 treatment. Specifically, E2 treatments resulted in upregulation of female-related genes and downregulation of male-related genes in genetic males during sex reversal. However, E2-induced sex reversal did not cause sex-specific changes in methylation profiles or gene expression within the sex determination region (SDR) on chromosome 4, suggesting that E2-induced sex reversal was a downstream process independent of the sex determination process that was regulated by sex-specific methylation within the SDR.

Single-cell transcriptome reveals insights into the development and function of the zebrafish ovary

Zebrafish are an established research organism that has made many contributions to our understanding of vertebrate tissue and organ development, yet there are still significant gaps in our understanding of the genes that regulate gonad development, sex, and reproduction. Unlike the development of many organs, such as the brain and heart that form during the first few days of development, zebrafish gonads do not begin to form until the larval stage (≥5 dpf). Thus, forward genetic screens have identified very few genes required for gonad development. In addition, bulk RNA sequencing studies which identify genes expressed in the gonads do not have the resolution necessary to define minor cell populations that may play significant roles in development and function of these organs. To overcome these limitations, we have used single-cell RNA sequencing to determine the transcriptomes of cells isolated from juvenile zebrafish ovaries. This resulted in the profiles of 10,658 germ cells and 14,431 somatic cells. Our germ cell data represents all developmental stages from germline stem cells to early meiotic oocytes. Our somatic cell data represents all known somatic cell types, including follicle cells, theca cells and ovarian stromal cells. Further analysis revealed an unexpected number of cell subpopulations within these broadly defined cell types. To further define their functional significance, we determined the location of these cell subpopulations within the ovary. Finally, we used gene knockout experiments to determine the roles of foxl2l and wnt9b for oocyte development and sex determination and/or differentiation, respectively. Our results reveal novel insights into zebrafish ovarian development and function and the transcriptome profiles will provide a valuable resource for future studies.

Clemizole and Trazodone are Effective Antiseizure Treatments in a Zebrafish Model of STXBP1 Disorder

CRISPR-Cas9-generated zebrafish carrying a 12 base-pair deletion in stxbpb1b, a paralog sharing 79% amino acid sequence identity with human, exhibit spontaneous electrographic seizures during larval stages of development. Zebrafish stxbp1b mutants provide an efficient preclinical platform to test antiseizure therapeutics. The present study was designed to test antiseizure medications approved for clinical use and two recently identified repurposed drugs with antiseizure activity. Larval homozygous stxbp1b zebrafish (4 days post-fertilization) were agarose-embedded and monitored for electrographic seizure activity using a local field recording electrode placed in midbrain. Frequency of ictal-like events was evaluated at baseline and following 45 min of continuous drug exposure (1 mM, bath application). Analysis was performed on coded files by an experimenter blinded to drug treatment and genotype. Phenytoin, valproate, ethosuximide, levetiracetam, and diazepam had no effect on ictal-like event frequency in stxbp1b mutant zebrafish. Clemizole and trazodone decreased ictal-like event frequency in stxbp1b mutant zebrafish by 80% and 83%, respectively. These results suggest that repurposed drugs with serotonin receptor binding affinities could be effective antiseizure treatments. Clemizole and trazodone were previously identified in a larval zebrafish model for Dravet syndrome. Based primarily on these preclinical zebrafish studies, compassionate-use and double-blind clinical trials with both drugs have progressed. The present study extends this approach to a preclinical zebrafish model representing STXBP1-related disorders, and suggests that future clinical studies may be warranted.

Compensatory engulfment and Müller glia reactivity in the absence of microglia

Microglia are known for important phagocytic functions in the vertebrate retina. Reports also suggest that Müller glia have phagocytic capacity, though the relative levels and contexts in which this occurs remain to be thoroughly examined. Here, we investigate Müller glial engulfment of dying cells in the developing zebrafish retina in the presence and absence of microglia, using a genetic mutant in which microglia do not develop. We show that in normal conditions clearance of dying cells is dominated by microglia; however, Müller glia do have a limited clearance role. In retinas lacking intact microglial populations, we found a striking increase in the engulfment load assumed by the Müller glia, which displayed prominent cellular compartments containing apoptotic cells, several of which localized with the early phagosome/endosome marker Rab5. Consistent with increased engulfment, lysosomal staining was also increased in Müller glia in the absence of microglia. Increased engulfment load led to evidence of Müller glia reactivity including upregulation of gfap but did not trigger cell cycle re-entry by differentiated Müller glia. Our work provides important insight into the phagocytic capacity of Müller glia and the ability for compensatory functions and downstream effects. Therefore, effects of microglial deficiency or depletion on other glial cell types should be well-considered in experimental manipulations, in neurodegenerative disease, and in therapeutic approaches that target microglia. Our findings further justify future work to understand differential mechanisms and contexts of phagocytosis by glial cells in the central nervous system, and the significance of these mechanisms in health and disease.

Bloom syndrome helicase contributes to germ line development and longevity in zebrafish

RecQ helicases—also known as the “guardians of the genome”—play crucial roles in genome integrity maintenance through their involvement in various DNA metabolic pathways. Aside from being conserved from bacteria to vertebrates, their importance is also reflected in the fact that in humans impaired function of multiple RecQ helicase orthologs are known to cause severe sets of problems, including Bloom, Werner, or Rothmund-Thomson syndromes. Our aim was to create and characterize a zebrafish (Danio rerio) disease model for Bloom syndrome, a recessive autosomal disorder. In humans, this syndrome is characterized by short stature, skin rashes, reduced fertility, increased risk of carcinogenesis, and shortened life expectancy brought on by genomic instability. We show that zebrafish blm mutants recapitulate major hallmarks of the human disease, such as shortened lifespan and reduced fertility. Moreover, similarly to other factors involved in DNA repair, some functions of zebrafish Blm bear additional importance in germ line development, and consequently in sex differentiation. Unlike fanc genes and rad51, however, blm appears to affect its function independent of tp53. Therefore, our model will be a valuable tool for further understanding the developmental and molecular attributes of this rare disease, along with providing novel insights into the role of genome maintenance proteins in somatic DNA repair and fertility.

Sex-Related Effects on Cardiac Development and Disease

Cardiovascular diseases (CVD) are the leading cause of morbidity and mortality. Interestingly, male and female patients with CVD exhibit distinct epidemiological and pathophysiological characteristics, implying a potentially important role for primary and secondary sex determination factors in heart development, aging, disease and therapeutic responses. Here, we provide a concise review of the field and discuss current gaps in knowledge as a step towards elucidating the “sex determination–heart axis”. We specifically focus on cardiovascular manifestations of abnormal sex determination in humans, such as in Turner and Klinefelter syndromes, as well as on the differences in cardiac regenerative potential between species with plastic and non-plastic sexual phenotypes. Sex-biased cardiac repair mechanisms are also discussed with a focus on the role of the steroid hormone 17β-estradiol. Understanding the “sex determination–heart axis” may offer new therapeutic possibilities for enhanced cardiac regeneration and/or repair post-injury.

Genetic and Neurological Deficiencies in the Visual System of mct8 Mutant Zebrafish

Thyroid hormones (THs; T3 and T4) enter cells using specific transporters and regulate development and metabolism. Mutation in the TH transporter monocarboxylate transporter 8 (MCT8, SLC16A2) is associated with brain hypothyroidism and neurological impairment. We established mct8 mutant (mct8-/-) zebrafish as a model for MCT8 deficiency, which causes endocrinological, neurological, and behavioral alterations. Here, we profiled the transcriptome of mct8-/- larvae. Among hundreds of differentially expressed genes, the expression of a cluster of vision-related genes was distinct. Specifically, the expression of the opsin 1 medium wave sensitive 2 (opn1mw2) decreased in two mct8 mutants: mct8-/- and mct8^-25bp-/- larvae, and under pharmacological inhibition of TH production. Optokinetic reflex (OKR) assays showed a reduction in the number of conjugated eye movements, and live imaging of genetically encoded Ca²⁺ indicator revealed altered neuronal activity in the pretectum area of mct8^-25bp-/- larvae. These results imply that MCT8 and THs regulate the development of the visual system and suggest a mechanism to the deficiencies observed in the visual system of MCT8-deficiency patients.

Generation and Characterization of a Zebrafish IL-2Rγc SCID Model

The IL-2 family of cytokines act via receptor complexes that share the interleukin-2 receptor gamma common (IL-2Rγc) chain to play key roles in lymphopoiesis. Inactivating IL-2Rγc mutations results in severe combined immunodeficiency (SCID) in humans and other species. This study sought to generate an equivalent zebrafish SCID model. The zebrafish il2rga gene was targeted for genome editing using TALENs and presumed loss-of-function alleles analyzed with respect to immune cell development and impacts on intestinal microbiota and tumor immunity. Knockout of zebrafish Il-2rγc.a resulted in a SCID phenotype, including a significant reduction in T cells, with NK cells also impacted. This resulted in dysregulated intestinal microbiota and defective immunity to tumor xenotransplants. Collectively, this establishes a useful zebrafish SCID model.

Sexual determination in zebrafish

Zebrafish have emerged as a major model organism to study vertebrate reproduction due to their high fecundity and external development of eggs and embryos. The mechanisms through which zebrafish determine their sex have come under extensive investigation, as they lack a definite sex-determining chromosome and appear to have a highly complex method of sex determination. Single-gene mutagenesis has been employed to isolate the function of genes that determine zebrafish sex and regulate sex-specific differentiation, and to explore the interactions of genes that promote female or male sexual fate. In this review, we focus on recent advances in understanding of the mechanisms, including genetic and environmental factors, governing zebrafish sex development with comparisons to gene functions in other species to highlight conserved and potentially species-specific mechanisms for specifying and maintaining sexual fate.

Attractin Participates in Schizophrenia by Affecting Testosterone Levels

Attractin (ATRN) is a widely expressed glycoprotein that is involved in energy homeostasis, neurodevelopment, and immune response. It is encoded by a gene spanning 180 kb on chromosome 20p13, a region previously implicated in schizophrenia by linkage studies. To address a possible role of ATRN in disorders of the central nervous system, we created an atrn knockout zebrafish line and performed behavioral tests. Adult atrn^–/– zebrafish exhibited more pronounced attack behavior relative to wild-type control zebrafish in a tracking analysis. Biochemical analysis revealed elevated testosterone levels in atrn^–/– zebrafish. At the gene expression level, we noted an upregulation of cyp51 and hsd17b7, key proteins in testosterone synthesis in the brains of both adult and larvae of atrn^–/– zebrafish. In order to further elucidate the relationship between testosterone and behavioral syndromes, we then compared testosterone levels of 9,008 psychiatric patients and 247 healthy controls from the same catchment area. Of all subjects examined, male subjects with schizophrenia exhibited lower testosterone levels compared with controls. In contrast, female subjects with a diagnosis of schizophrenia or bipolar disorder featured higher testosterone levels than did same sex controls. Purposeful sampling of extreme groups showed reduced ATRN expression in a subset of these subjects. Finally, we identified 14 subjects with ATRN mutations. All of whom displayed abnormal testosterone levels. In summary, the interplay of ATRN and testosterone may help to explain sexual dimorphisms in selected behavioral phenotypes.

Metalloproteases in gonad formation and ovulation

Changes in expression or activation of various metalloproteases including matrix metalloproteases (Mmp), a disintegrin and metalloprotease (Adam) and a disintegrin and metalloprotease with thrombospondin motif (Adamts), and their endogenous inhibitors (tissue inhibitors of metalloproteases, Timp), have been shown to be critical for ovulation in various species from studies in past decades. Some of these metalloproteases such as Adamts1, Adamts9, Mmp2, and Mmp9 have also been shown to be regulated by luteinizing hormone (LH) and/or progestin, which are essential triggers for ovulation in all vertebrate species. Most of these metalloproteases also express broadly in various tissues and cells including germ cells and somatic gonad cells. Thus, metalloproteases likely play roles in gonad formation processes comprising primordial germ cell (PGC) migration, development of germ and somatic cells, and sex determination. However, our knowledge on the functions and mechanisms of metalloproteases in these processes in vertebrates is still lacking. This review will summarize our current knowledge on the metalloproteases in ovulation and gonad formation with emphasis on PGC migration and germ cell development.

Agrin/Lrp4 signal constrains MuSK-dependent neuromuscular synapse development in appendicular muscle

The receptor tyrosine kinase MuSK, its co-receptor Lrp4 and the Agrin ligand constitute a signaling pathway that is crucial in axial muscle for neuromuscular synapse development, yet whether this pathway functions similarly in appendicular muscle is unclear. Here, using the larval zebrafish pectoral fin, equivalent to tetrapod forelimbs, we show that, similar to axial muscle, developing appendicular muscles form aneural acetylcholine receptor (AChR) clusters prior to innervation. As motor axons arrive, neural AChR clusters form, eventually leading to functional synapses in a MuSK-dependent manner. We find that loss of Agrin or Lrp4 function, which abolishes synaptic AChR clusters in axial muscle, results in enlarged presynaptic nerve regions and progressively expanding appendicular AChR clusters, mimicking the consequences of motoneuron ablation. Moreover, musk depletion in lrp4 mutants partially restores synaptic AChR patterning. Combined, our results provide compelling evidence that, in addition to the canonical pathway in which Agrin/Lrp4 stimulates MuSK activity, Agrin/Lrp4 signaling in appendicular muscle constrains MuSK-dependent neuromuscular synapse organization. Thus, we reveal a previously unappreciated role for Agrin/Lrp4 signaling, thereby highlighting distinct differences between axial and appendicular synapse development.

Meiotic Chromosome Dynamics in Zebrafish

Recent studies in zebrafish have revealed key features of meiotic chromosome dynamics, including clustering of telomeres in the bouquet configuration, biogenesis of chromosome axis structures, and the assembly and disassembly of the synaptonemal complex that aligns homologs end-to-end. The telomere bouquet stage is especially pronounced in zebrafish meiosis and sub-telomeric regions play key roles in mediating pairing and homologous recombination. In this review, we discuss the temporal progression of these events in meiosis prophase I and highlight the roles of proteins associated with meiotic chromosome architecture in homologous recombination. Finally, we discuss the interplay between meiotic mutants and gonadal sex differentiation and future research directions to study meiosis in living cells, including cell culture.

Molecular Therapy for Choroideremia: Pre-clinical and Clinical Progress to Date

Choroideremia is an inherited retinal disease characterised by a degeneration of the light-sensing photoreceptors, supporting retinal pigment epithelium and underlying choroid. Patients present with the same symptoms as those with classic rod-cone dystrophy: (1) night blindness early in life; (2) progressive peripheral visual field loss, and (3) central vision decline with a slow progression to legal blindness. Choroideremia is monogenic and caused by mutations in CHM. Eight clinical trials (three phase 1/2, four phase 2, and one phase 3) have started (four of which are already finished) to evaluate the therapeutic efficacy of gene supplementation mediated by subretinal delivery of an adeno-associated virus serotype 2 (AAV2/2) vector expressing CHM. Furthermore, one phase 1 clinical trial has been initiated to evaluate the efficiency of a novel AAV variant to deliver CHM to the outer retina following intravitreal delivery. Lastly, a non-viral-mediated CHM replacement strategy is currently under development, which could lead to a future clinical trial. Here, we summarise the rationale behind these various studies, as well as any results published to date. The diversity of these trials currently places choroideremia at the forefront of the retinal gene therapy field. As a consequence, the trial outcomes, regardless of the results, have the potential to change the landscape of gene supplementation for inherited retinal diseases.

Role of Alternative Splicing in Sex Determination in Vertebrates

During the process of sex determination, a germ-cell-containing undifferentiated gonad is converted into either a male or a female reproductive organ. Both the composition of sex chromosomes and the environment determine sex in vertebrates. It is assumed that transcription level regulation drives this cascade of mechanisms; however, transcription factors can alter gene expression beyond transcription initiation by controlling pre-mRNA splicing and thereby mRNA isoform production. Using the key time window in sex determination and gonad development in mice, it has been reported that new non-transcriptional events, such as alternative splicing, could play a key role in sex determination in mammals. We know the role of key regulatory factors, like WT1(+/-KTS) or FGFR2(b/c) in pre-mRNA splicing and sex determination, indicating that important steps in the vertebrate sex determination process probably operate at a post-transcriptional level. Here, we discuss the role of pre-mRNA splicing regulators in sex determination in vertebrates, focusing on the new RNA-seq data reported from mice fetal gonadal transcriptome.