A light- and electron-microscopic study on the migration of primordial germ cells in the teleost, Oryzias latipes

Gsdf is not indispensable for male differentiation in the medaka species Oryzias hubbsi

Sex determination mechanisms differ widely among vertebrates, particularly in fish species, where diverse sex chromosomes and sex-determining genes have evolved. However, the sex-differentiation pathways activated by these sex-determining genes appear to be conserved. Gonadal soma-derived growth factor (Gsdf) is one of the genes conserved across teleost fish, especially in medaka fishes of the genus Oryzias, and is implicated in testis differentiation and germ cell proliferation. However, its role in sex differentiation remains unclear. In this study, we investigated Gsdf function in Oryzias hubbsi, a species with a ZW sex-determination system. We confirmed its male-dominant expression, as in other species. However, histological analyses revealed no male-to-female sex reversal in Gsdf-knockout fish, contrary to findings in other medaka species. Genetic sex determination remained intact without Gsdf function, indicating a Gsdf-independent sex-differentiation pathway in O. hubbsi. Instead, Gsdf loss led to germ cell overproliferation in both sexes and accelerated onset of meiosis in testes, suggesting a role in germ cell proliferation. Notably, the feminizing effect of germ cells observed in O. latipes was absent, suggesting diverse germ cell-somatic cell relationships in Oryzias gonad development. Our study highlights species-specific variations in the molecular pathways governing sex determination and differentiation, emphasizing the need for further exploration to elucidate the complexities of sexual development.

Molecular biological, physiological, cytological, and epigenetic mechanisms of environmental sex differentiation in teleosts: A systematic review

Human activities have been exerting widespread stress and environmental risks in aquatic ecosystems. Environmental stress, including temperature rise, acidification, hypoxia, light pollution, and crowding, had a considerable negative impact on the life histology of aquatic animals, especially on sex differentiation (SDi) and the resulting sex ratios. Understanding how the sex of fish responds to stressful environments is of great importance for understanding the origin and maintenance of sex, the dynamics of the natural population in the changing world, and the precise application of sex control in aquaculture. This review conducted an exhaustive search of the available literature on the influence of environmental stress (ES) on SDi. Evidence has shown that all types of ES can affect SDi and universally result in an increase in males or masculinization, which has been reported in 100 fish species and 121 cases. Then, this comprehensive review aimed to summarize the molecular biology, physiology, cytology, and epigenetic mechanisms through which ES contributes to male development or masculinization. The relationship between ES and fish SDi from multiple aspects was analyzed, and it was found that environmental sex differentiation (ESDi) is the result of the combined effects of genetic and epigenetic factors, self-physiological regulation, and response to environmental signals, which involves a sophisticated network of various hormones and numerous genes at multiple levels and multiple gradations in bipotential gonads. In both normal male differentiation and ES-induced masculinization, the stress pathway and epigenetic regulation play important roles; however, how they co-regulate SDi is unclear. Evidence suggests that the universal emergence or increase in males in aquatic animals is an adaptation to moderate ES. ES-induced sex reversal should be fully investigated in more fish species and extensively in the wild. The potential aquaculture applications and difficulties associated with ESDi have also been addressed. Finally, the knowledge gaps in the ESDi are presented, which will guide the priorities of future research.

Paternal source of germ plasm determinants in the viviparous teleost, Gambusia holbrooki; dads do matter

The identity of germ cells, the progenitors of life, is thought to be acquired by two modes; either by maternal signals (preformed) or induced de novo from pluripotent cells (epigenesis) in the developing embryos. However, paternal roles seem enshrouded or completely overlooked in this fundamental biological process. Hence, we investigated the presence of germplasm transcripts in the sperm of Gambusia holbrooki, a live-bearing fish, demonstrating their presence and suggesting paternal contributions. Interestingly, not all germplasm markers were present (nanos1 and tdrd6) in the sperm, but some were conspicuous (dazl, dnd-α, piwi II, and vasa), indicating that the latter is required for establishing germ cell identity in the progeny, with a possible parent-specific role. Furthermore, there were also spatial differences in the distribution of these determinants, suggesting additional roles in sperm physiology and/or fertility. Our results support the hypothesis that dads also play a vital role in establishing the germ cell identity, especially in G. holbrooki, which shares elements of both preformation and induction modes of germline determination. This, coupled with its life history traits, makes G. holbrooki an excellent system for dissecting evolutionary relationships between the two germline determination modes, their underpinning mechanisms and ultimately the perpetuity of life.

Gonad Ontogeny and Sex Differentiation in a Poeciliid, Gambusia holbrooki: Transition from a Bi- to a Mono-Lobed Organ

Despite their uniqueness, the ontogeny and differentiation of the single-lobed gonads in the poeciliids are very poorly understood. To address this, we employed both cellular and molecular approaches to systematically map the development of the testes and ovary in Gambusia holbrooki from pre-parturition to adulthood, encompassing well over 19 developmental stages. The results show that putative gonads form prior to the completion of somitogenesis in this species, a comparatively early occurrence among teleosts. Remarkably, the species recapitulates the typical bi-lobed origin of the gonads during early development that later undergoes steric metamorphosis to form a single-lobed organ. Thereafter, the germ cells undergo mitotic proliferation in a sex-dependent manner before the acquisition of the sexual phenotype. The differentiation of the ovary preceded that of the testes, which occurred before parturition, where the genetic females developed meiotic primary oocytes stage I, indicating ovarian differentiation. However, genetic males showed gonial stem cells in nests with slow mitotic proliferation at the same developmental stage. Indeed, the first signs of male differentiation were obvious only post-parturition. The expression pattern of the gonadosoma markers foxl2, cyp19a1a, amh and dmrt1 in pre- and post-natal developmental stages were consistent with morphological changes in early gonad; they were activated during embryogenesis, followed by the onset of gonad formation, and a sex-dimorphic expression pattern concurrent with sex differentiation of the ovary (foxl2, cyp19a1a) and testes (amh and dmrt1). In conclusion, this study documents for the first time the underlying events of gonad formation in G. holbrooki and shows that this occurs relatively earlier than those previously described for ovi- and viviparous fish species, which may contribute to its reproductive and invasive prowess.

Oxidative Stress Causes Masculinization of Genetically Female Medaka Without Elevating Cortisol

Medaka (Oryzias latipes) is a teleost fish with an XX/XY sex determination system. Sex reversal from female-to-male (masculinization of XX fish) can be induced through cortisol elevation from exposure to environmental stress such as high temperature during sexual differentiation. However, the effects of oxidative stress, generated via metabolic reactions and biological defense mechanisms, on the sexual differentiation of medaka are unclear. Here, we investigated the effect of oxidative stress on medaka sexual differentiation using hydrogen peroxide (H₂O₂), which induces oxidative stress in vertebrates. H₂O₂ treatment from 0 to 5 days post-hatching induced masculinization of wild-type XX medaka, but not of gonadal soma-derived growth factor (gsdf) or peroxisome proliferator-activated receptor alpha-a (pparaa) knockout XX fish. Co-treatment with an oxidative stress inhibitor caused masculinization recovery but co-treatment with a cortisol synthesis inhibitor did not. H₂O₂ treatment significantly upregulated gsdf and pparaa expression in XX medaka. However, H₂O₂ did not elevate cortisol levels in medaka larvae during sexual differentiation. These results strongly indicate that oxidative stress induces masculinization of XX medaka without causing elevation of cortisol.

Gonadal sex differentiation and early ovarian/testicular development in cultured Pacific bluefin tuna, Thunnus orientalis (Temminck et Schlegel)

Pacific bluefin tuna (PBT), Thunnus orientalis, is one of the most important species for aquaculture in Japan. Recently, the reduction in muscle fat content associated with sexual maturation in farmed PBT has become a serious problem. To develop technologies for inducing sterility, detailed and reliable data on gonadal development in PBT are needed. Here, we demonstrated the process of gonadal sex differentiation, and of early ovarian and testicular development during the immature stages in PBT. Gonadal sex differentiation was first characterized by the formation of the ovarian cavity in female and of the efferent ducts in male 57 days post hatching (dph). The gonads then differentiated into ovaries or testes according to the genotypic sex until 83 dph. During this period, primordial germ cells, oogonia, and type-A spermatogonia were solitarily distributed in the gonads, and the number of germ cells did not differ between sexes. After gonadal sex differentiation, gonads of PBTs developed in a sexually dimorphic manner: proliferation and differentiation of germ cells occurred earlier in the ovaries than in the testes. The oogonia in ovaries formed cysts at 185 dph, but the type-A spermatogonia were solitarily distributed in testes at this stage, and cysts of type-A spermatogonia were first observed at 247 dph. Moreover, the oogonia entered meiosis and differentiated into chromatin-nucleolus stage oocytes until 247 dph, and subsequently into peri-nucleolus stage oocytes until 285 dph, whereas the type-A spermatogonia differentiated into type-B spermatogonia, spermatocytes, spermatids, and spermatozoa from 446 dph onwards. We believe the results of this study provide the necessary basis for future studies on sterile PBT production.

Sex determination, gonadal sex differentiation, and plasticity in vertebrate species

A diverse array of sex determination (SD) mechanisms, encompassing environmental to genetic, have been found to exist among vertebrates, covering a spectrum from fixed SD mechanisms (mammals) to functional sex change in fishes (sequential hermaphroditic fishes). A major landmark in vertebrate SD was the discovery of the SRY gene in 1990. Since that time, many attempts to clone an SRY ortholog from nonmammalian vertebrates remained unsuccessful, until 2002, when DMY/dmrt1by was discovered as the SD gene of a small fish, medaka. Surprisingly, however, DMY/dmrt1by was found in only 2 species among more than 20 species of medaka, suggesting a large diversity of SD genes among vertebrates. Considerable progress has been made over the last 3 decades, such that it is now possible to formulate reasonable paradigms of how SD and gonadal sex differentiation may work in some model vertebrate species. This review outlines our current understanding of vertebrate SD and gonadal sex differentiation, with a focus on the molecular and cellular mechanisms involved. An impressive number of genes and factors have been discovered that play important roles in testicular and ovarian differentiation. An antagonism between the male and female pathway genes exists in gonads during both sex differentiation and, surprisingly, even as adults, suggesting that, in addition to sex-changing fishes, gonochoristic vertebrates including mice maintain some degree of gonadal sexual plasticity into adulthood. Importantly, a review of various SD mechanisms among vertebrates suggests that this is the ideal biological event that can make us understand the evolutionary conundrums underlying speciation and species diversity.

Presence of the matrix metalloproteinases during the migration of the primordial germ cells in zebrafish gonadal ridge

In vertebrates, the primordial germ cells (PGCs) differentiate from extragonadal regions, migrating to gonadal ridge during the embryonic development. However, recent studies in mammals indicate that the PGCs originate from the epiblast and subsequently migrate into the yolk sac. Cell and molecular bases involved in routes during the migration of these cells are still not well understood. Thus, in an attempt to evaluate the participation of matrix metalloproteinases (MMPs) during the gonadal primordium formation in Danio rerio (zebrafish), the route of migration of PGCs was analyzed. In zebrafish, during the migration of the PGCs to the forming gonad, they bind by cytoplasmic processes to the extracellular matrix and migrate through amoeboid movements until they reach the gonadal ridge. During the epiboly, MMPs were not detected. However, after organogenesis, three MMP types were expressed in the somatic cells that were located ahead of the PGCs in the migration route. This expression was maintained throughout the mesentery and was not detected in the PGCs. Upon reaching the gonadal ridge, the PGCs and somatic cells express MMPs and epithelium begins to be formed. After the formation of the basement membrane, the germinal epithelium is delineated by the somatic cells, which remodeling the extracellular matrix. So, a PGC organization occurs through the tissue, forming the gonadal primordium. Concomitantly, granulocytes expressing different MMPs are present. This data in exposing the role of MMPs during the PGC migration to the forming gonad, may point a new way in understanding the reproductive biology of the vertebrates in general.

Exposure to 4-nonylphenol induces a shift in the gene expression of gsdf and testis-ova formation and sex reversal in Japanese medaka (Oryzias latipes)

The branched isomer mixture 4-nonylphenol (4-NP) has been used worldwide as a surfactant, and can have endocrine-disrupting effects on aquatic organisms. For instance, 4-NP induces the formation of testis-ova (i.e., testicular and ovarian tissue in the same gonad) or male to female sex reversal of various teleost fishes. Recently, our group revealed that altered gsdf gene expression is associated with disruption of gonadal differentiation in Japanese medaka (Oryzias latipes) embryos exposed to methyltestosterone or bisphenol A, suggesting that gsdf might be useful as a biomarker for predicting the impact of endocrine-disrupting chemicals (EDCs) on gonadal differentiation. Here, we used 4-NP to examine further whether gsdf expression at the embryo stage is useful for predicting EDC impact on gonadal sex differentiation. When fertilized medaka eggs were exposed to 32 or 100 μg/L 4-NP, testis-ova in genetic males and sex reversal from genetic male to phenotypic female were observed. At stage 38 (just before hatching), 4-NP exposure at 1-100 μg/L did not affect gsdf expression in XX embryos compared with the nontreated control; however, in XY embryos, the gsdf expression in the 100 μg/L-exposed group was significantly lower than that in the controls. The 4-NP concentration at which gsdf expression was suppressed was equal to that at which testis-ova and sex reversal were induced. These results indicate that expression of the gsdf gene at the embryonic stage in medaka is a useful biomarker for predicting the impact of EDCs on sexual differentiation.

Germ plasm-related structures in marine medaka gametogenesis; novel sites of Vasa localization and the unique mechanism of germ plasm granule arising

Germ plasm, a cytoplasmic factor of germline cell differentiation, is suggested to be a perspective tool for in vitro meiotic differentiation. To discriminate between the: (1) germ plasm-related structures (GPRS) involved in meiosis triggering; and (2) GPRS involved in the germ plasm storage phase, we investigated gametogenesis in the marine medaka Oryzias melastigma. The GPRS of the mitosis-to-meiosis period are similar in males and females. In both sexes, five events typically occur: (1) turning of the primary Vasa-positive germ plasm granules into the Vasa-positive intermitochondrial cement (IMC); (2) aggregation of some mitochondria by IMC followed by arising of mitochondrial clusters; (3) intramitochondrial localization of IMC-originated Vasa; followed by (4) mitochondrial cluster degradation; and (5) intranuclear localization of Vasa followed by this protein entering the nuclei (gonial cells) and synaptonemal complexes (zygotene-pachytene meiotic cells). In post-zygotene/pachytene gametogenesis, the GPRS are sex specific; the Vasa-positive chromatoid bodies are found during spermatogenesis, but oogenesis is characterized by secondary arising of Vasa-positive germ plasm granules followed by secondary formation and degradation of mitochondrial clusters. A complex type of germ plasm generation, 'the follicle cell assigned germ plasm formation', was found in late oogenesis. The mechanisms discovered are recommended to be taken into account for possible reconstruction of those under in vitro conditions.

Increase of cortisol levels after temperature stress activates dmrt1a causing female-to-male sex reversal and reduced germ cell number in medaka

In vertebrates, there is accumulating evidence that environmental factors as triggers for sex determination and genetic sex determination are not two opposing alternatives but that a continuum of mechanisms bridge those extremes. One prominent example is the model fish species Oryzias latipes which has a stable XX/XY genetic sex determination system, but still responds to environmental cues, where high temperatures lead to female-to-male sex reversal. However, the mechanisms behind are still unknown. We show that high temperatures increase primordial germ cells (PGC) numbers before they reach the genital ridge, which, in turn, regulates the germ cell proliferation. Complete ablation of PGCs led to XX males with germ cell less testis, whereas experimentally increased PGC numbers did not reverse XY genotypes to female. For the underlying molecular mechanism, we provide support for the explanation that activation of the dmrt1a gene by cortisol during early development of XX embryos enables this autosomal gene to take over the role of the male determining Y-chromosomal dmrt1bY.

Heat shock factor 1 protects germ cell proliferation during early ovarian differentiation in medaka

The heat shock response is important for the viability of all living organisms. It involves the induction of heat shock proteins whose expression is mainly regulated by heat shock factor 1 (HSF1). Medaka (Oryzias latipes) is a teleost fish with an XX/XY sex determination system. High water temperature (HT) inhibits the female-type proliferation of germ cells and induces the masculinisation of XX medaka in some cases during gonadal sex differentiation. Here, we investigated the roles of HSF1 on the proliferation of germ cells using HSF1 knockout medaka. Loss of HSF1 function under HT completely inhibited the female-type proliferation of germ cells, induced the expression of the anti-Mullerian hormone receptor type 2 (amhr2) and apoptosis-related genes, and suppressed that of the dead end (dnd) and heat shock protein-related genes. Moreover, the loss of HSF1 and AMHR2 function under HT recovered female-type proliferation in germ cells, while loss of HSF1 function under HT induced gonadal somatic cell apoptosis during early sex differentiation. These results strongly suggest that HSF1 under the HT protects the female-type proliferation of germ cells by inhibiting amhr2 expression in gonadal somatic cells. These findings provide new insights into the molecular mechanisms underlying environmental sex determination.

Ovarian aromatase loss-of-function mutant medaka undergo ovary degeneration and partial female-to-male sex reversal after puberty

Although estrogens have been generally considered to play a critical role in ovarian differentiation in non-mammalian vertebrates, the specific functions of estrogens during ovarian differentiation remain unclear. We isolated two mutants with premature stops in the ovarian aromatase (cyp19a1) gene from an N-ethyl-N-nitrosourea-based gene-driven mutagenesis library of the medaka, Oryzias latipes. In XX mutants, gonads first differentiated into normal ovaries containing many ovarian follicles that failed to accumulate yolk. Subsequently, ovarian tissues underwent extensive degeneration, followed by the appearance of testicular tissues on the dorsal side of ovaries. In the newly formed testicular tissue, strong expression of gsdf was detected in sox9a2-positive somatic cells surrounding germline stem cells suggesting that gsdf plays an important role in testicular differentiation during estrogen-depleted female-to-male sex reversal. We conclude that endogenous estrogens synthesized after fertilization are not essential for early ovarian differentiation but are critical for the maintenance of adult ovaries.

Mutation in cpsf6/CFIm68 (Cleavage and Polyadenylation Specificity Factor Subunit 6) causes short 3'UTRs and disturbs gene expression in developing embryos, as revealed by an analysis of primordial germ cell migration using the medaka mutant naruto

Our previous studies analyzing medaka mutants defective in primordial germ cell (PGC) migration identified cxcr4b and cxcr7, which are both receptors of the chemokine sdf1/cxcl12, as key regulators of PGC migration. Among PGC migration mutants, naruto (nar) is unique in that the mutant phenotype includes gross morphological abnormalities of embryos, suggesting that the mutation affects a broader range of processes. A fine genetic linkage mapping and genome sequencing showed the nar gene encodes Cleavage and Polyadenylation Specificity Factor subunit 6 (CPSF6/CFIm68). CPSF6 is a component of the Cleavage Factor Im complex (CFIm) which plays a key role in pre-mRNA 3'-cleavage and polyadenylation. 3'RACE of sdf1a/b and cxcr7 transcripts in the mutant embryos indicated shorter 3'UTRs with poly A additions occurring at more upstream positions than wild-type embryos, suggesting CPSF6 functions to prevent premature 3'UTR cleavage. In addition, expression of the coding region sequences of sdf1a/b in nar mutants was more anteriorly extended in somites than wild-type embryos, accounting for the abnormally extended distribution of PGCs in nar mutants. An expected consequence of shortening 3'UTR is the escape from the degradation mechanism mediated by microRNAs interacting with distal 3'UTR sequence. The abnormal expression pattern of sdf1a coding sequence may be at least partially accounted for by this mechanism. Given the pleiotropic effects of nar mutation, further analysis using the nar mutant will reveal processes in which CPSF6 plays essential regulatory roles in poly A site selection and involvement of 3'UTRs in posttranscriptional gene regulation in various genes in vivo.

Gonadogenesis analysis and sex differentiation in cultured turbot (Scophthalmus maximus)

As a flatfish, the turbot (Scophthalmus maximus) is one of the most important farmed fish species with great commercial value, which has a strong sexual dimorphism on growth rate and sexual maturity. In this study, using histology, the basic information on proliferation and migration of germ cells and early gonadal development during sex differentiation in turbot were described in detail. There were six to nine individual primordial germ cells (PGCs) with large nuclei until 15 days post-hatching (dph). The PGCs located under the mesonephric ducts undergo migration along the dorsal mesentery toward the region of the genital ridge from 0 to 15 dph. During migration, the number of PGCs was constant, and the expression of vasa had no significant changes. At 20 dph, the aggregation of somatic cells at genital ridge indicated the formation of primary gonad. Furthermore, the number of PGCs was increased to 60 and the expression of vasa was upregulated for the first time. The undifferentiated gonads with no morphological indications of sex differentiation grew larger with the increase in germ cells and somatic cells number/size from 20 to 35 dph. During 36-52 dph, cytological gonadal differentiation was observed. In presumptive testes of type I gonadal tissue (with a lance shape), the number of germ cells increased steadily and the germ cells had the same characteristics as before. Meanwhile, in presumptive ovaries of type II gonadal tissue (with a club-like shape), the germ cells proliferated and induced in two different populations of germ cells. One type had the morphological characteristics as undifferentiated germ cells, while the other type of germ cells underwent mitosis exhibiting smaller size and mottled nuclei. At 60 dph, ovarian cavity was present in the gonad of type II, which would develop into ovaries. However, spermatogonial cysts were not noticed in the gonad of type I until 90 dph, which indicated the formation of the testes.

Androgen induces gonadal soma-derived factor, Gsdf, in XX gonads correlated to sex-reversal but not Dmrt1 directly, in the teleost fish, northern medaka (Oryzias sakaizumii)

In the inbred HNI-II strain of Oryzias sakaizumii, Dmy and Gsdf are expressed in XY gonads from Stages 35 and 36, respectively, similarly to the inbred Hd-rR strain of Oryzias latipes. However, Dmrt1 respectively becomes detectable at Stage 36 and 5 days post hatching (dph) in the two strains. In XX HNI-II embryos, 17α-methyltestosterone (MT) induces Gsdf mRNA from Stage 36, accompanied by complete sex-reversal in all treated individuals (MT, 10 ng/mL), while Dmrt1 mRNA was first detectable at 5 dph. In XX d-rR, MT induced Gsdf mRNA expression and sex-reversal in only some of the treated individuals. Together, these results suggest the testis differentiation cascade in XY individuals differs between the HNI-II and Hd-rR strains. In addition, it is suggested that androgen-induced XX sex-reversal proceeds via an androgen-Gsdf-Dmrt1 cascade and that Gsdf plays an important role in sex-reversal in medaka.

Perchlorate Exposure Reduces Primordial Germ Cell Number in Female Threespine Stickleback

Perchlorate is a common aquatic contaminant that has long been known to affect thyroid function in vertebrates, including humans. More recently perchlorate has been shown to affect primordial sexual differentiation in the aquatic model fishes zebrafish and threespine stickleback, but the mechanism has been unclear. Stickleback exposed to perchlorate from fertilization have increased androgen levels in the embryo and disrupted reproductive morphologies as adults, suggesting that perchlorate could disrupt the earliest stages of primordial sexual differentiation when primordial germ cells (PGCs) begin to form the gonad. Female stickleback have three to four times the number of PGCs as males during the first weeks of development. We hypothesized that perchlorate exposure affects primordial sexual differentiation by reducing the number of germ cells in the gonad during an important window of stickleback sex determination at 14-18 days post fertilization (dpf). We tested this hypothesis by quantifying the number of PGCs at 16 dpf in control and 100 mg/L perchlorate-treated male and female stickleback. Perchlorate exposure from the time of fertilization resulted in significantly reduced PGC number only in genotypic females, suggesting that the masculinizing effects of perchlorate observed in adult stickleback may result from early changes to the number of PGCs at a time critical for sex determination. To our knowledge, this is the first evidence of a connection between an endocrine disruptor and reduction in PGC number prior to the first meiosis during sex determination. These findings suggest that a mode of action of perchlorate on adult reproductive phenotypes in vertebrates, including humans, such as altered fecundity and sex reversal or intersex gonads, may stem from early changes to germ cell development.

The basement membrane and the sex establishment in the juvenile hermaphroditism during gonadal differentiation of the Gymnocorymbus ternetzi (Teleostei: Characiformes: Characidae)

Although there are several studies on morphogenesis in Teleostei, until now there is no research describing the role of the basement membrane in the establishment of the germinal epithelium during gonadal differentiation in Characiformes. In attempt to study these events that result in the formation of ovarian and testicular structures, gonads of Gymnocorymbus ternetzi were prepared for light microscopy. During gonadal development in G. ternetzi, all individuals first developed ovarian tissue. The undifferentiated gonad was formed by somatic cells (SC) and primordial germ cells (PGCs). After successive mitosis, the PGCs became oogonia, which entered into meiosis originating oocytes. An interstitial tissue developed. In half of the individuals, presumptive female, prefollicle cells synthesized a basement membrane around oocyte forming a follicle. Along the ventral region of the ovary, the tissue invaginated to form the ovigerous lamellae, bordered by the germinal epithelium. Stroma developed and the follicle complexes were formed. The gonadal aromatase was detected in interstitial cells in the early steps of the gonadal differentiation in both sexes. In another half of the individuals, presumptive male, there was no synthesis of basement membrane. The interstitium was invaded by numerous granulocytes. Pre-Leydig cells proliferated. Apoptotic oocytes were observed and afterward degenerated. Spermatogonia appeared near the degenerating oocytes and associated to SCs, forming testicular tubules. Germinal epithelium developed and the basement membrane was synthesized. Concomitantly, there was decrease of the gonadal aromatase and increase in the 3β-HSD enzyme expression. Thus, the testis was organized on an ovary previously developed, constituting an indirect gonochoristic differentiation.

Primordial germ cell migration in the yellowtail kingfish (Seriola lalandi) and identification of stromal cell-derived factor 1

Primordial germ cells (PGCs) are progenitors of the germ cell lineage, giving rise to either spermatogonia or oogonia after the completion of gonadal differentiation. Currently, there is little information on the mechanism of PGCs migration leading to the formation of the primordial gonad in perciform fish. Yellowtail kingfish (Seriola lalandi) (YTK) (order Perciforms) inhabit tropical and temperate waters in the southern hemisphere. Fundamental details into the molecular basis of larval development in this species can be easily studied in Australia, as they are commercially cultured and readily available. In this study, histological analysis of YTK larvae revealed critical time points for the migration of PGCs to the genital ridge, resulting in the subsequent development of the primordial gonad. In YTK larvae at 3, 5, 7 and 10 days post hatch (DPH), PGCs were not yet enclosed by somatic cells, indicating the primordial gonad had not yet started to form. While at 15, 18 and 20 DPH PGCs had already settled at the genital ridge and started to become enclosed by somatic cells indicating the primordial gonad had started to develop. A higher number of PGCs were observed in the larvae at 15 and 18 DPH indicating PGCs proliferation, which corresponds with them becoming enclosed by the somatic cells. Directional migration of PGCs toward the genital ridge is a critical event in the subsequent development of a gonad. In zebrafish, mouse and chicken, stromal-cell derived factor (SDF1) signalling is one of the key molecules for PGC migration. We subsequently isolated from YTK the SDF1 (Slal-SDF1) gene, which encodes for a 98-residue precursor protein with a signal peptide at the N-terminus. There is spatial conservation between fish species of four cysteine residues at positions C9, C11, C34 and C49, expected to form disulphide bonds and stabilize the SDF structure. In YTK, Slal-SDF1 gene expression analyses shows that this gene is expressed in larvae from 1 to 22 DPH and demonstrates distinct spatial localisation in the larvae at 7 DPH. These results provide a platform for further studies into the molecular machinery of PGC migration in yellowtail kingfish, as well as other perciform fish species.

Differentiation and morphogenesis of the ovary and expression of gonadal development-related genes in the protogynous hermaphroditic ricefield eel Monopterus albus

The ovarian differentiation, morphogenesis and expression of some putative gonadal development-related genes were analysed in the ricefield eel Monopterus albus, a protogynous hermaphroditic teleost with a single elongate ovary. At c. 1 day post-hatching (dph), the gonadal ridge was colonized with primordial germ cells (PGCs) at the periphery and transformed into the gonadal primordium, which appeared to contain two germinal epithelia. At c. 7 dph, four ovarian cavities appeared in the gonadal tissue with two in each germinal epithelial compartment, and the indifferent gonad might have begun to differentiate into the ovary. The oocytes at the leptotene stage in meiosis I appeared at c. 14 dph, and oocytes at the diplotene stage at c. 30 dph. As development proceeded, the connective tissue separating the two germinal epithelia disappeared, and two of the four ovarian cavities collapsed into one. At 60 dph, the gonad had already taken the shape as observed in the adults. One outer and two inner ovarian cavities could be easily recognized, with slightly basophilic primary growth oocytes usually residing close to the outer ovarian cavity. The expression of cyp19a1a and erb in the early gonad was detected at 6 dph. The abundant expression of foxl2 coincided with the up-regulation of cyp19a1a at 8 dph onwards. The expression of dmrt1 isoforms was not detectable until 8 dph for dmrt1a and dmrt1b and until 33 dph for dmrt1d. The earlier appearance of cyp19a1a compared to dmrt1 transcripts in the indifferent gonad may contribute to the initial differentiation of the gonad towards the ovary in M. albus.

Expression and localization of gonadotropic hormone subunits (Gpa, Fshb, and Lhb) in the pituitary during gonadal differentiation in medaka

To clarify the appearance of and chronological changes in two different gonadotropic hormone (Gth) cells, we examined the dynamics of Gth cells in detail during gonadal differentiation and development in the d-rR strain of medaka (Oryzias latipes). Expression of the sex-determining gene Dmy was evident in gonadal somatic cells at 5 days post-fertilization (dpf). Glycoprotein-α (Gpa)-positive cells first appeared in the pituitary at 4 dpf, regardless of genetic sex, while follicle-stimulating hormone-β (Fshb)-positive cells was detected in XX and XY embryos at 5 and 6 dpf, respectively. In contrast, luteinizing hormone-β (Lhb)-positive cells were observed in both sexes of medaka after 70 days post-hatching (dph). The density of Fshb-positive cells in the pituitary was significantly and transiently higher in XX than in XY fry at 0 dph, and thereafter no significant differences were detected before sexual maturation. In this study, temporal expression of Fshb was observed, indicating that Fsh cells become differentiated before hatching and that sexual dimorphism in Fsh cells occurs transiently after sex determination in medaka.

Loss of follicle-stimulating hormone receptor function causes masculinization and suppression of ovarian development in genetically female medaka

FSH, a glycoprotein hormone, is circulated from the pituitary and functions by binding to a specific FSH receptor (FSHR). FSHR is a G protein-coupled, seven-transmembrane receptor linked to the adenylyl cyclase or other pathways and is expressed in gonadal somatic cells. In some nonmammalian species, fshr expression is much higher in the ovary than in the testis during gonadal sex differentiation, suggesting that FSHR is involved in ovarian development in nonmammalian vertebrates. However, little is known of FSHR knockout phenotypes in these species. Here we screened for fshr mutations by a medaka (Oryzias latipes) target-induced local lesion in the genomes and identified one nonsense mutation located in the BXXBB motif, which is involved in G protein activation. Next, we used an in vitro reporter gene assay to demonstrate that this mutation prevents FSHR function. We then analyzed the phenotypes of fshr mutant medaka. The fshr mutant male medaka displayed normal testes and were fertile, whereas the mutant female fish displayed small ovaries and were infertile because vitellogenesis was inhibited. The mutant females also have suppressed expression of ovary-type aromatase (cyp19a1a), a steroidogenic enzyme responsible for the conversion of androgens to estrogens, resulting in decreased 17β-estradiol levels. Moreover, loss of FSHR function caused female-to-male sex reversal in some cases. In addition, the transgenic overexpression of fshr in fshr mutants rescued FSHR function. These findings strongly suggest that in the medaka, FSH regulates the ovarian development and the maintenance mainly by the elevation of estrogen levels. We present the first FSHR knockout phenotype in a nonmammalian species.

Male gonadal differentiation and the paedomorphic evolution of the testis in Teleostei

Testis differentiation from representatives of the Otophysi (Cyprinus carpio), Percomorpha (Amatitlania nigrofasciata), and Atherinomorpha (Poecilia reticulata) was comparatively described. In the undifferentiated gonad of C. carpio, the primordial germ cells (PGCs) are scattered throughout the gonads while in A. nigrofasciata and P. reticulata the PGCs are restricted to the ventral periphery. In the dorsal region of the developing gonads, with the exception of C. carpio, somatic cell rearrangements result in the differentiation of the sperm duct. Pre-Sertoli cells wrap around single spermatogonia forming cysts that proliferate forming acinar-clusters. In C. carpio and A. nigrofasciata, the cysts in each acinar-cluster move away from each other, creating a central lumen. In C. carpio, the acinar-clusters then fuse to each other forming tubules that become lined by the germinal epithelium. Subsequently, the tubules anastomose dorsally and create the sperm duct. In A. nigrofasciata, the acinar-clusters elongate, forming lobules that individually connect to the sperm duct. These are lined by the germinal epithelium. In P. reticulata, the spermatogonial cysts remain in the acinar-cluster organization. Subsequently, developing ducts connect each cluster to the sperm duct and lobules subsequently develop. In the differentiated testis of C. carpio and A. nigrofasciata, spermatogonia are distributed along the lengths of the anastomosing tubules or lobules, respectively. However, in P. reticulata, the spermatogonia remain restricted to the terminal end of the lobules. Considering testis ontogeny, the spermatogonial acinar-cluster is the adult characteristic of more derived taxa that approximate the early gonad developmental stages of the basal taxa.

Nanosecond pulsed electric field suppresses development of eyes and germ cells through blocking synthesis of retinoic acid in Medaka (Oryzias latipes)

Application of nanosecond pulsed electric fields (nsPEFs) has attracted rising attention in various scientific fields including medical, pharmacological, and biological sciences, although its effects and molecular mechanisms leading to the effects remain poorly understood. Here, we show that a single, high-intensity (10-30 kV/cm), 60-ns PEF exposure affects gene expression and impairs development of eyes and germ cells in medaka (Oryzias latipes). Exposure of early blastula stage embryos to nsPEF down-regulated the expression of several transcription factors which are essential for eye development, causing abnormal eye formation. Moreover, the majority of the exposed genetic female embryos showed a fewer number of germ cells similar to that of the control (unexposed) genetic male at 9 days post-fertilization (dpf). However, all-trans retinoic acid (atRA) treatment following the exposure rescued proliferation of germ cells and resumption of normal eye development, suggesting that the phenotypes induced by nsPEF are caused by a decrease of retinoic acid levels. These results confirm that nsPEFs induce novel effects during embryogenesis in medaka.

Histological observations of early gonadal development to form asymmetrically in the dwarf gourami Colisa lalia

The asymmetrical and latitudinal formation of the gonads during early development in the dwarf gourami Colisa lalia was histologically observed using fish reared in laboratory. Hatching larvae obtained 33 hours after spawning possessed primordial germ cells (PGCs), which aggregated to the dorsal surface of the gut. In prolarvae on day 7, the gonadal anlagen were still situated on the dorsal surface of the gut, however, in those on day 10, the left gonadal anläge began to shift leftward, although the coelom did not develop sufficiently. In prolarvae on day 20, the right gonadal anlage also began to move leftward. During this period, the gut developed considerably in the right side of the coelom. On day 25, the developing gonads were located in the interspace of the developing spiral valve intestine. Sex differentiation of gonads appeared to begin by day 25. From those observations, we concluded that asymmetricity of gonads in C. lalia is caused by a limited space of the coelom opened on the left side of the body during the early phase of the formation of gonadal anlagen, which may be accurately determined by the position in which the gut develops.

oct4-EGFP reporter gene expression marks the stem cells in embryonic development and in adult gonads of transgenic medaka

Maintenance of pluripotency in stem cells is tightly regulated among vertebrates. One of the key genes in this process is oct4, also referred to as pou5f1 in mammals and pou2 in teleosts. Pou5f1 evolved by duplication of pou2 early in the tetrapod lineage, but only monotremes and marsupials retained both genes. Either pou2 or pou5f1 was lost from the genomes of the other tetrapods that have been analyzed to date. Consequently, these two homologous genes are often designated oct4 in functional studies. In most vertebrates oct4 is expressed in pluripotent cells of the early embryo until the blastula stage, and later persist in germline stem cells until adulthood. The isolation and analysis of stem cells from embryo or adult individuals is hampered by the need for reliable markers that can identify and define the cell populations. Here, we report the faithful expression of EGFP under the control of endogenous pou2/oct4 promoters in transgenic medaka (Oryzias latipes). In vivo imaging in oct4-EGFP transgenic medaka reveals the temporal and spatial expression of pou2 in embryos and adults alike. We describe the temporal and spatial patterns of endogenous pou2 and oct4-EGFP expression in medaka with respect to germline and adult stem cells, and discuss applications of oct4-EGFP transgenic medaka in reproductive and stem cell biology.

Characterization of the carbonic anhydrases 15b expressed in PGCs during early zebrafish development

The main function of carbonic anhydrases is to regulate acid-base balance. In the present study, the zebrafish CA15b sequence was identified from the National Center for Biotechnology Information database (accession No. NM_213182). The 1716 base pair full-length cDNA of CA15b was obtained by 3' and 5'-rapid amplification of cDNA ends analysis. It was expressed (reverse transcription polymerase chain reaction and Western blot analysis) in the ovary, heart, brain, and muscle, but not in testis or liver. Based on in situ hybridization, CA15b mRNA was transcribed in the ooplasm of stage I to stage II oocytes, in the cortex of stage III oocytes, and along the periphery of stage IV oocytes. Furthermore, this protein was localized (immunohistochemistry) in the plasma membrane of oocytes. Based on whole-mount in situ hybridization, CA15b mRNA was present in every blastomere of embryos from one-cell to blastula stages. Strong signals of the transcripts were present along cleavage furrows of two- and eight-cell stage embryos, which subsequently condensed into four clusters of cells during the blastula stage. During subsequent stages, the four groups of CA15b-expressing cells appeared to move toward the dorsal side of the embryos, clustered into two groups on either side of the midline, and remained visible as they migrated toward the region of the gonad in embryos at 24 hours postfertilization. Expression patterns of CA15b were similar to those of vasa, a marker of primordial germ cells. Thus, we hypothesized that CA15b might be necessary for development of primordial germ cells and female germ cells in zebrafish.

Expression of vasa and nanos3 during primordial germ cell formation and migration in Atlantic cod (Gadus morhua L.)

Primordial germ cells (PGCs), progenitors of gametes, are specified very early in embryonic development and undergo an active migration to the site where the future gonads will form. While the developmental pattern of PGCs during embryogenesis has been documented in few model teleost fishes, there is currently no information available for any representative of Superorder Paracanthopterygii. This includes Atlantic cod (Gadus morhua), which is a historically important food fish in both fisheries and aquaculture industries. In the present study, we cloned and characterized vasa and nanos3 and used them as germ cell markers in Atlantic cod. Sequencing results showed prospective vasa and nanos3 mRNA contained the domains used to describe their respective protein family. Furthermore, phylogenetic analysis using the amino acid sequence placed Atlantic cod Vasa distinct from representatives of three other taxonomic Superorders. Atlantic cod Nanos3 was placed with other homologues from the Nanos3 subfamily. Expression of both genes was detected from the first cleavage division; both were specifically expressed in Atlantic cod PGCs from the 32-cell stage. While nanos3 expression ceased during early somitogenesis, vasa was strongly expressed throughout embryonic development. Using vasa as a marker, we described the Atlantic cod PGC migration pattern. We demonstrated that Atlantic cod PGCs migrate ventral to the trunk mesoderm. With the exception of Pacific herring (Clupea pallasii), PGCs in other described teleost fishes migrate lateral to the trunk. The results from this study are the first step toward understanding germ line formation in Atlantic cod.

Hyperproliferation of mitotically active germ cells due to defective anti-Müllerian hormone signaling mediates sex reversal in medaka

The function of AMH (Anti-Müllerian hormone), a phylogenetically ancient member of the TGFβ family of proteins, in lower vertebrates is largely unknown. Previously, we have shown that the gene encoding the type II anti-Müllerian hormone receptor, amhrII, is responsible for excessive germ cell proliferation and male-to-female sex reversal in the medaka hotei mutant. In this study, functional analyses in cultured cells and of other amhrII mutant alleles indicate that lack of AMH signaling causes the hotei phenotype. BrdU incorporation experiments identified the existence of both quiescent and mitotically active germ cells among the self-renewing, type I population of germ cells in the developing gonad. AMH signaling acts in supporting cells to promote the proliferation of mitotically active germ cells but does not trigger quiescent germ cells to proliferate in the developing gonad. Furthermore, we show that the male-to-female sex reversal phenotype in hotei mutants is not a direct consequence of AMH signaling in supporting cells, but is instead mediated by germ cells. Our data demonstrate that interfollicular AMH signaling regulates proliferation at a specific stage of germ cell development, and that this regulation is crucial for the proper manifestation of gonadal sex directed by sex determination genes.

Expression of 3β-hydroxysteroid dehydrogenase (hsd3b), star and ad4bp/sf-1 during gonadal development in medaka (Oryzias latipes)

In most vertebrates, sex steroids play a critical role in gonadal development, maturation of germ cells, and development of secondary sexual characteristics. Sex steroids are synthesized in steroid-producing cells (SPCs) in the testis known as Leydig cells, as well as in thecal and granulosa cells in the ovary. In SPCs, cholesterol is sequentially catalyzed by a set of steroidogenic factors and enzymes in order to produce sex steroids. Therefore, integrated expression of the genes involved in steroidogenesis is critical for the proper production of sex steroids. In the present study, regulatory mechanisms of steroidogenic factors and enzymes were examined. We focused on hsd3b, star and ad4bp/sf-1 as well as the description of temporal and spatial expression of these genes during gonadal development in medaka (Oryzias latipes). During testicular development, hsd3b, star and ad4bp/sf-1 were co-expressed in the interstitial somatic cells subsequent to the formation of the seminiferous tubule precursor, suggesting that ad4bp/sf-1 regulated the transcription of both hsd3b and star. During ovarian development, the expression pattern of hsd3b coincided with that of cyp11a1, but not with that of aromatase. Although ad4bp/sf-1 was mainly expressed in presumptive follicular cells, it was also detected in hsd3b positive interstitial cells in the developing ovary. Contrary to our expectations, the onset of star expression occurred during a later stage of ovarian development than the expression of other steroidogenic enzymes. Thus, the regulation mechanism of star transcription appears to differ from that of the other steroidogenic enzymes in the developing ovary, but not in the developing testis.

Gonadal morphogenesis and sex differentiation in the viviparous fish Chapalichthys encaustus (Teleostei, Cyprinodontiformes, Goodeidae)

This study describes the structural and ultrastructural characteristics of gonadal sex differentiation and expression of Vasa, a germline marker, in different developmental stages of embryos and newborn fry of the barred splitfin Chapalichthys encaustus, a viviparous freshwater teleost endemic to Mexico. In stage 2 embryos, the gonadal crest was established; gonadal primordia were located on the coelomic epithelium, formed by scarce germ and somatic cells. At stage 3, the undifferentiated gonad appeared suspended from the mesentery of the developing swimbladder and contained a larger number of germ and somatic cells. At stages 4 and 5, the gonads had groups of meiotic and non-meiotic germ cells surrounded by somatic cells; meiosis was evident from the presence of synaptonemal complexes. These stages constituted a transition towards differentiation. At stage 6 and at birth, the gonad was morphologically differentiated into an ovary or a testis. Ovarian differentiation was revealed by the presence of follicles containing meiotic oocytes, and testicular differentiation by the development of testicular lobules containing spermatogonia in mitotic arrest, surrounded by Sertoli cells. Nuage, electron-dense material associated with mitochondria, was observed in germ cells at all gonadal stages. The Vasa protein was detected in all of the previously described stages within the germ-cell cytoplasm. This is the first report on morphological characteristics and expression of the Vasa gene during sexual differentiation in viviparous species of the Goodeidae family. Chapalichthys encaustus may serve as a model to study processes of sexual differentiation in viviparous fishes and teleosts.

Cloning and sequence analysis of a vasa homolog in the European sea bass (Dicentrarchus labrax): tissue distribution and mRNA expression levels during early development and sex differentiation

Vasa is a protein expressed mainly in germ cells and conserved across taxa. However, sex-related differences and environmental influences on vasa expression have not been documented. This study characterized the cDNA of a vasa homolog in the European sea bass, Dicentrarchuslabrax (sb-vasa), a gonochoristic fish with temperature influences on gonadogenesis. The 1911 bp open reading frame predicted a 637-amino acid protein with the eight conserved domains typical of Vasa proteins. Comparisons of the deduced amino acid sequence with those of other vertebrates and invertebrates revealed the highest homology (68-85%) with those of other teleosts. An updated tree with the full-length sequences for Vasa proteins in 66 species belonging to six different phyla was constructed, establishing the evolutionary relationships of Vasa amino acid sequences. European sea bass vasa was highly expressed in gonads with little or no expression in other tissues. Real time RT-PCR quantification of the temporal expression of sb-vasa from early development throughout sex differentiation showed that mRNA levels were high in unfertilized eggs, decreased during larval development and increased again during the period of germ cell proliferation. Rearing of fish at high temperature resulted in further increased sb-vasa levels, most likely reflecting temperature effects on both somatic and gonadal growth. Differences in expression were also found well before sex differentiation and persisted until the end of the first year, with higher levels present in females. These differences in expression demonstrate the implication of vasa during the initial stages of fish sex differentiation and gametogenesis and suggest that, through its helicase activity, it might be implicated in the translational regulation of mRNAs involved in the specification and differentiation of gonadal-specific cell types.

High temperature causes masculinization of genetically female medaka by elevation of cortisol

In poikilothermic vertebrates, sex determination is sometimes influenced by environmental factors such as temperature. However, little is known about the molecular mechanisms underlying environmental sex determination. The medaka (Oryzias latipes) is a teleost fish with an XX/XY sex determination system. Recently, it was reported that XX medaka can be sex-reversed into phenotypic males by high water temperature (HT; 32-34 degrees C) treatment during the sex differentiation period. Here we report that cortisol caused female-to-male sex reversal and that metyrapone (an inhibitor of cortisol synthesis) inhibited HT-induced masculinization of XX medaka. HT treatment caused elevation of whole-body levels of cortisol, while metyrapone suppressed the elevation by HT treatment during sexual differentiation. Moreover, cortisol and 33 degrees C treatments inhibited female-type proliferation of germ cells as well as expression of follicle-stimulating hormone receptor (fshr) mRNA in XX medaka during sexual differentiation. These results strongly suggest that HT induces masculinization of XX medaka by elevation of cortisol level, which, in turn, causes suppression of germ cell proliferation and of fshr mRNA expression.

Fish germ cells

Fish, like many other animals, have two major cell lineages, namely the germline and soma. The germ-soma separation is one of the earliest events of embryonic development. Germ cells can be specifically labeled and isolated for culture and transplantation, providing tools for reproduction of endangered species in close relatives, such as surrogate production of trout in salmon. Haploid cell cultures, such as medaka haploid embryonic stem cells have recently been obtained, which are capable of mimicking sperm to produce fertile offspring, upon nuclear being directly transferred into normal eggs. Such fish originated from a mosaic oocyte that had a haploid meiotic nucleus and a transplanted haploid mitotic cell culture nucleus. The first semi-cloned fish is Holly. Here we review the current status and future directions of understanding and manipulating fish germ cells in basic research and reproductive technology.

Gonadal sex differentiation and expression of Sox9a2, Dmrt1, and Foxl2 in Oryzias luzonensis

Oryzias luzonensis is closely related to the medaka, O. latipes. The sex of both species is determined by an XX-XY system. However, the testis determining gene (DMY/Dmrt1bY) found in O. latipes does not exist in O. luzonensis. Instead, a different gene is thought to act as a testis determining gene. In this study, we focused the gonadal sex differentiation process in O. luzonensis under different testis determining gene. First, we observed the gonadal development of O. luzonensis histologically. We then analyzed the expression of Sox9a2/Sox9b, Dmrt1, and Foxl2 during early development. Our results suggest that the sexual differentiation of germ cells in O. luzonensis is initiated later than in O. latipes. However, the timing of the sexual differentiation of the supporting cell linage is similar between the species.

Gonad morphogenesis in vertebrates: divergent means to a convergent end

A critical element of successful sexual reproduction is the generation of sexually dimorphic adult reproductive organs, the testis and ovary, which produce functional gametes. Examination of different vertebrate species shows that the adult gonad is remarkably similar in its morphology across different phylogenetic classes. Surprisingly, however, the cellular and molecular programs employed to create similar organs are not evolutionarily conserved. We highlight the mechanisms used by different vertebrate model systems to generate the somatic architecture necessary to support gametogenesis. In addition, we examine the different vertebrate patterns of germ cell migration from their site of origin to colonize the gonad and highlight their roles in sex-specific morphogenesis. We also discuss the plasticity of the adult gonad and consider how different genetic and environmental conditions can induce transitions between testis and ovary morphology.

Female-specific increase in primordial germ cells marks sex differentiation in threespine stickleback (Gasterosteus aculeatus)

Gonadal sex differentiation is increasingly recognized as a remarkably plastic process driven by species-specific genetic or environmental determinants. Among aquatic vertebrates, gonadal sex differentiation is a frequent endpoint in studies of endocrine disruption with little appreciation of underlying developmental mechanisms. Work in model organisms has highlighted the diversity of master sex-determining genes rather than uncovering any broad similarities prompting the highly conserved developmental decision of testes versus ovaries. Here we use molecular genetic markers of chromosomal sex combined with traditional histology to examine the transition of the bipotential gonads to ovaries or testes in threespine stickleback (Gasterosteus aculeatus). Serially-sectioned threespine stickleback fry were analyzed for qualitative and quantitative indications of sexual differentiation, including changes in gonadal morphology, number of germ cells and the incidence of gonadal apoptosis. We show that threespine stickleback sampled from anadromous and lacustrine populations are differentiated gonochorists. The earliest sex-specific event is a premeiotic increase in primordial germ cell number followed by a female-specific spike in apoptosis in the undifferentiated gonad of genetic females. The data suggest that an increase in PGC number may direct the undifferentiated gonad toward ovarian differentiation.

Mullerian inhibiting substance is required for germ cell proliferation during early gonadal differentiation in medaka (Oryzias latipes)

Müllerian inhibiting substance (MIS) is a glycoprotein belonging to the TGF-beta superfamily. In mammals, MIS is responsible for the regression of Müllerian ducts in the male fetus. However, the role of MIS in gonadal sex differentiation of teleost fish, which have no Müllerian ducts, has yet to be clarified. In the present study, we examined the expression pattern of mis and mis type 2 receptor (misr2) mRNAs and the function of MIS signaling in early gonadal differentiation in medaka (teleost, Oryzias latipes). In situ hybridization showed that both mis and misr2 mRNAs were expressed in the somatic cells surrounding the germ cells of both sexes during early sex differentiation. Loss-of-function of either MIS or MIS type II receptor (MISRII) in medaka resulted in suppression of germ cell proliferation during sex differentiation. These results were supported by cell proliferation assay using 5-bromo-2'-deoxyuridine labeling analysis. Treatment of tissue fragments containing germ cells with recombinant eel MIS significantly induced germ cell proliferation in both sexes compared with the untreated control. On the other hand, culture of tissue fragments from the MIS- or MISRII-defective embryos inhibited proliferation of germ cells in both sexes. Moreover, treatment with recombinant eel MIS in the MIS-defective embryos dose-dependently increased germ cell number in both sexes, whereas in the MISRII-defective embryos, it did not permit proliferation of germ cells. These results suggest that in medaka, MIS indirectly stimulates germ cell proliferation through MISRII, expressed in the somatic cells immediately after they reach the gonadal primordium.