Histological Analysis of Gonads in Zebrafish

Reproductive adaptation of Astyanax mexicanus under nutrient limitation

Reproduction is a fundamental biological process for the survival and continuity of species. Examining changes in reproductive strategies offers valuable insights into how animals have adapted their life histories to different environments. Since reproduction is one of the most energy-intensive processes in female animals, nutrient scarcity is expected to interfere with the ability to invest in gametes. Lately, a new model to study adaptation to nutrient limitation has emerged; the Mexican tetra Astyanax mexicanus. This fish species exists as two different morphs, a surface river morph and a cave-dwelling morph. The cave-dwelling morph has adapted to the dark, lower biodiversity, and nutrient-limited cave environment and consequently evolved an impressive starvation resistance. However, how reproductive strategies have adapted to nutrient limitations in this species remains poorly understood. Here, we compared breeding activities and maternal contributions between laboratory-raised surface fish and cavefish. We found that cavefish produce different clutch sizes of eggs with larger yolk compared to surface fish, indicating a greater maternal nutrient deposition in cavefish embryos. To systematically characterize yolk compositions, we used untargeted proteomics and lipidomics approaches to analyze protein and lipid profiles in 2-cell stage embryos and found an increased proportion of sphingolipids in cavefish compared to surface fish. Additionally, we generated transcriptomic profiles of surface fish and cavefish ovaries using a combination of single cell and bulk RNA sequencing to examine differences in maternal contribution. We found that genes essential for hormone regulation were upregulated in cavefish follicular somatic cells compared to surface fish. To evaluate whether these differences contribute to their reproductive abilities under natural-occurring stress, we induced breeding in starved female fish. Remarkably, cavefish maintained their ability to breed under starvation, whereas surface fish largely lost this ability. We identified insulin-like growth factor 1a receptor (igf1ra) as a potential candidate gene mediating the downregulation of ovarian development genes, potentially contributing to the starvation-resistant fertility of cavefish. Taken together, we investigated the female reproductive strategies in Astyanax mexicanus, which will provide fundamental insights into the adaptations of animals to environments with extreme nutrient deficit.

Reproductive Adaptation of Astyanax mexicanus Under Nutrient Limitation

Reproduction is a fundamental biological process for the survival and continuity of species. Examining changes in reproductive strategies offers valuable insights into how animals have adapted their life histories to different environments. Since reproduction is one of the most energy-intensive processes in female animals, nutrient scarcity is expected to interfere with the ability to invest in gametes. Lately, a new model to study adaptation to nutrient limitation has emerged; the Mexican tetra Astyanax mexicanus . This fish species exists as two different morphs, a surface river morph and a cave-dwelling morph. The cave-dwelling morph has adapted to the dark, biodiversity, and nutrient-limited cave environment and consequently evolved an impressive starvation resistance. However, how reproductive strategies have adapted to nutrient limitations in this species remains poorly understood. Here, we compared breeding activities and maternal contributions between laboratory-raised surface fish and cavefish. We found that cavefish produce different clutch sizes of eggs with larger yolk compared to surface fish, indicating a greater maternal nutrient deposition in cavefish embryos. To systematically characterize yolk compositions, we used untargeted proteomics and lipidomics approaches to analyze protein and lipid profiles in 2-cell stage embryos and found an increased proportion of sphingolipids in cavefish compared to surface fish. Additionally, we generated transcriptomic profiles of surface fish and cavefish ovaries using a combination of single cell and bulk RNA sequencing to examine differences in maternal contribution. We found that genes essential for hormone regulation were upregulated in cavefish follicular somatic cells compared to surface fish. To evaluate whether these differences contribute to their reproductive abilities under natural-occurring stress, we induced breeding in starved female fish. Remarkably, cavefish maintained their ability to breed under starvation, whereas surface fish largely lost this ability. We identified insulin-like growth factor 1a receptor ( igf1ra ) as a potential candidate gene mediating the downregulation of ovarian development genes, potentially contributing to the starvation-resistant fertility of cavefish. Taken together, we investigated the female reproductive strategies in Astyanax mexicanus , which will provide fundamental insights into the adaptations of animals to environments with extreme nutrient deficit.

Germ cell progression through zebrafish spermatogenesis declines with age

Vertebrate spermatogonial stem cells maintain sperm production over the lifetime of an animal, but fertility declines with age. Although morphological studies have informed our understanding of typical spermatogenesis, the molecular and cellular mechanisms underlying the maintenance and decline of spermatogenesis are not yet understood. We used single-cell RNA sequencing to generate a developmental atlas of the aging zebrafish testis. All testes contained spermatogonia, but we observed a progressive decline in spermatogenesis that correlated with age. Testes from some older males only contained spermatogonia and a reduced population of spermatocytes. Spermatogonia in older males were transcriptionally distinct from spermatogonia in testes capable of robust spermatogenesis. Immune cells including macrophages and lymphocytes drastically increased in abundance in testes that could not complete spermatogenesis. Our developmental atlas reveals the cellular changes as the testis ages and defines a molecular roadmap for the regulation of spermatogenesis.

Vertebrate endocrine disruptors induce sex-reversal in blue mussels

Mollusks are the second most diverse animal phylum, yet little is known about their endocrinology or how they respond to endocrine disrupting compound (EDC) pollution. Characteristic effects of endocrine disruption are reproductive impairment, skewed sex ratios, development of opposite sex characteristics, and population decline. However, whether classical vertebrate EDCs, such as steroid hormone-like chemicals and inhibitors of steroidogenesis, exert effects on mollusks is controversial. In the blue mussel, Mytilus edulis, EDC exposure is correlated with feminized sex ratios in wild and laboratory mussels, but sex reversal has not been confirmed. Here, we describe a non-destructive qPCR assay to identify the sex of M. edulis allowing identification of males and females prior to experimentation. We exposed male mussels to 17α-ethinylestradiol and female mussels to ketoconazole, EDCs that mimic vertebrate steroid hormones or inhibit their biosynthesis. Both chemicals changed the sex of individual mussels, interfered with gonadal development, and disrupted gene expression of the sex differentiation pathway. Impacts from ketoconazole treatment, including changes in steroid levels, confirmed a role for steroidogenesis and steroid-like hormones in mollusk endocrinology. The present study expands the possibilities for laboratory and field monitoring of mollusk species and provides key insights into endocrine disruption and sexual differentiation in bivalves.

Use of All-Male cyp17a1-Deficient Zebrafish (Danio rerio) for Evaluation of Environmental Estrogens

Natural and synthetic environmental estrogens (EEs) are widespread and have received extensive attention. Our previous studies demonstrated that depletion of the cytochrome P450 17a1 gene (cyp17a1) leads to all-testis differentiation phenotype in zebrafish and common carp. In the present study, cyp17a1-deficient zebrafish with defective estrogen biosynthesis were used for the evaluation of EEs, as assessed by monitoring vitellogenin (vtg) expression. A rapid and sensitive assessment procedure was established with the 3-day administration of estradiol (E2), followed by examination of the transcriptional expression of vtgs in our cyp17a1-deficient fish. Compared with the control fish, a higher E2-mediated vtg upregulation observed in cyp17a1-deficient zebrafish exposed to 0.1 μg/L E2 is known to be estrogen receptor-dependent and likely due to impaired in vivo estrogen biosynthesis. The more responsive vtg expression in cyp17a1-deficient zebrafish was observed when exposed to 200 and 2000 μg/L bisphenol A (BPA) and perfluoro-1-octanesulfonate (PFOS). The estrogenic potentials of E2, BPA, and PFOS were compared and assessed by the feminization effect on ovarian differentiation in cyp17a1-deficient zebrafish from 18 to 50 days postfertilization, based on which a higher sensitivity of E2 in ovarian differentiation than BPA and PFOS was concluded. Collectively, through the higher sensitivity to EEs and the capacity to distinguish chemicals with different estrogenic potentials exhibited by the all-male cyp17a1-deficient zebrafish with impaired estrogen biosynthesis, we demonstrated that they can be used as an excellent in vivo model for the evaluation of EEs. Environ Toxicol Chem 2024;43:1062-1074. © 2024 SETAC.

Smoothened mediates medaka spermatogonia proliferation via Gli1-Rgcc-Cdk1 axis†

The proliferation of spermatogonia directly affects spermatogenesis and male fertility, but its underlying molecular mechanisms are poorly understood. In this study, Smoothened (Smo), the central transducer of Hedgehog signaling pathway, was characterized in medaka (Oryzias latipes), and its role and underlying mechanisms in the proliferation of spermatogonia were investigated. Smo was highly expressed in spermatogonia. In ex vivo testicular organ culture and a spermatogonial cell line (SG3) derived from medaka mature testis, Smo activation promoted spermatogonia proliferation, while its inhibition induced apoptosis. The expression of glioma-associated oncogene homolog 1 (gli1) and regulator of cell cycle (rgcc) was significantly upregulated in SG3 after Smo activation. Furthermore, Gli1 transcriptionally upregulated the expression of rgcc, and Rgcc overexpression rescued cell apoptosis caused by Smo or Gli1 inhibition. Co-immunoprecipitation assay indicated that Rgcc could interact with cyclin-dependent kinase 1 (Cdk1) to regulate the cell cycle of spermatogonia. Collectively, our study firstly reveals that Smo mediates the proliferation of spermatogonia through Gli1-Rgcc-Cdk1 axis. In addition, Smo and Gli1 are necessary of the survival of spermatogonia. This study deepens our understanding of spermatogonia proliferation and survival at the molecular level, and provides insights into male fertility control and reproductive disease treatment.

Germ cells do not progress through spermatogenesis in the infertile zebrafish testis

Vertebrate spermatogonial stem cells maintain sperm production over the lifetime of an animal but fertility declines with age. While morphological studies have greatly informed our understanding of typical spermatogenesis, the molecular and cellular mechanisms underlying spermatogenesis are not yet understood, particularly with respect to the onset of fertility. We used single-cell RNA sequencing to generate a developmental atlas of the zebrafish testis. Using 5 timepoints across the adult life of a zebrafish, we described cellular profiles in the testis during and after fertility. While all germ cell stages of spermatogenesis are detected in testes from fertile adult zebrafish, testes from older infertile males only contained spermatogonia and a reduced population of spermatocytes. These remaining germ cells are transcriptionally distinct from fertile spermatogonia. Immune cells including macrophages and lymphocytes drastically increase in abundance in infertile testes. Our developmental atlas reveals the cellular changes as the testis ages and defines a molecular roadmap for the regulation of male fertility.

Identification of fish spermatogenic cells through high-throughput immunofluorescence against testis with an antibody set

Image-based identification and quantification of different types of spermatogenic cells is of great importance, not only for reproductive studies but also for genetic breeding. Here, we have developed antibodies against spermatogenesis-related proteins in zebrafish (Danio rerio), including Ddx4, Piwil1, Sycp3, and Pcna, and a high-throughput method for immunofluorescence analysis of zebrafish testicular sections. By immunofluorescence analysis of zebrafish testes, our results demonstrate that the expression of Ddx4 decreases progressively during spermatogenesis, Piwil1 is strongly expressed in type A spermatogonia and moderately expressed in type B spermatogonia, and Sycp3 has distinct expression patterns in different subtypes of spermatocytes. Additionally, we observed polar expression of Sycp3 and Pcna in primary spermatocytes at the leptotene stage. By a triple staining of Ddx4, Sycp3, and Pcna, different types/subtypes of spermatogenic cells were easily characterized. We further demonstrated the practicality of our antibodies in other fish species, including Chinese rare minnow (Gobiocypris rarus), common carp (Cyprinus carpio), blunt snout bream (Megalobrama amblycephala), rice field eel (Monopterus albus) and grass carp (Ctenopharyngodon idella). Finally, we proposed an integrated criterion for identifying different types/subtypes of spermatogenic cells in zebrafish and other fishes using this high-throughput immunofluorescence approach based on these antibodies. Therefore, our study provides a simple, practical, and efficient tool for the study of spermatogenesis in fish species.

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 days post-fertilization). Thus, forward genetic screens have identified very few genes required for gonad development. In addition, bulk RNA-sequencing studies that identify genes expressed in the gonads do not have the resolution necessary to define minor cell populations that may play significant roles in the 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.

Rad21l1 cohesin subunit is dispensable for spermatogenesis but not oogenesis in zebrafish

During meiosis I, ring-shaped cohesin complexes play important roles in aiding the proper segregation of homologous chromosomes. RAD21L is a meiosis-specific vertebrate cohesin that is required for spermatogenesis in mice but is dispensable for oogenesis in young animals. The role of this cohesin in other vertebrate models has not been explored. Here, we tested if the zebrafish homolog Rad21l1 is required for meiotic chromosome dynamics during spermatogenesis and oogenesis. We found that Rad21l1 localizes to unsynapsed chromosome axes. It is also found between the axes of the mature tripartite synaptonemal complex (SC) in both sexes. We knocked out rad21l1 and found that nearly all rad21l1-/- mutants develop as fertile males, suggesting that the mutation causes a defect in juvenile oogenesis, since insufficient oocyte production triggers female to male sex reversal in zebrafish. Sex reversal was partially suppressed by mutation of the checkpoint gene tp53, suggesting that the rad21l1 mutation activates Tp53-mediated apoptosis or arrest in females. This response, however, is not linked to a defect in repairing Spo11-induced double-strand breaks since deletion of spo11 does not suppress the sex reversal phenotype. Compared to tp53 single mutant controls, rad21l1-/- tp53-/- double mutant females produce poor quality eggs that often die or develop into malformed embryos. Overall, these results indicate that the absence of rad21l1-/- females is due to a checkpoint-mediated response and highlight a role for a meiotic-specific cohesin subunit in oogenesis but not spermatogenesis.