Androgen dependent development of a modified anal fin, gonopodium, as a model to understand the mechanism of secondary sexual character expression in vertebrates

Gene Duplication of Androgen Receptor As An Evolutionary Driving Force Underlying the Diversity of Sexual Characteristics in Teleost Fishes

Sexual dimorphism allows species to meet their fitness optima based on the physiological availability of each sex. Although intralocus sexual conflict appears to be a genetic constraint for the evolution of sex-specific traits, sex-linked genes and the regulation of sex steroid hormones contribute to resolving this conflict by allowing sex-specific developments. Androgens and their receptor, androgen receptor (Ar), regulate male-biased phenotypes. In teleost fish, ar ohnologs have emerged as a result of teleost-specific whole genome duplication (TSGD). Recent studies have highlighted the evolutionary differentiation of ar ohnologs responsible for the development of sexual characteristics, which sheds light on the need for comparative studies on androgen regulation among different species. In this review, we discuss the importance of ar signaling as a regulator of male-specific traits in teleost species because teleost species are suitable experimental models for comparative studies owing to their great diversity in male-biased morphological and physiological traits. To date, both in vivo and in vitro studies on teleost ar ohnologs have shown a substantial influence of ars as a regulator of male-specific reproductive traits such as fin elongation, courtship behavior, and nuptial coloration. In addition to these sexual characteristics, ar substantially influences immunity, inducing a sex-biased immune response. This review aims to provide a comprehensive understanding of the current state of teleost ar studies and emphasizes the potential of teleost fishes, given their availability, to find molecular evidence about what gives rise to the spectacular diversity among fish species.

Developmental regulation of the male urogenital papilla in the male marine teleost black rockfish, Sebastes schlegelii (Hilgendorf, 1880)†

The male external genitalia of the black rockfish (Sebastes schlegelii Hilgendorf, 1880) is a fleshy protrusion known as the urogenital papilla (UGP), which functions to deliver sperm into the female reproductive tract for internal fertilization. It is not known which genes regulate the development of the UGP. The aim of this study was to identify key genes that regulate the development of the UGP in black rockfish and to determine the distribution of androgen receptor gene (ar) in the UGP. A total of 26 adult males and 560 juvenile fish were used in the experiment, in which we divided all normally developing juveniles into normal development and androgen groups. We added methyltestosterone solution (100 μg/l) to the androgen group-treated fish tank, soaked for 2 h per day for 38 days, and sampled 5~10 samples each time every 5 days during the culture process. Gene expression changes related to UGP were analyzed with tissue specificity between control and androgen groups during sex differentiation, adult male maturation, and the copulation stage (September to December) using real-time quantitative polymerase chain reaction. The expression of ar was also localized by two-color in situ hybridization in the UGP region of juvenile fish. Androgen treatment enhanced ar expression levels and the ar signal was stronger in the UGP region of both adult breeding fish and androgen-treated juvenile fish. This study provides insights into the regulation of the external genitalia of black rockfish and presents vital information for the artificial breeding of viviparous fish.

17β-Trenbolone activates androgen receptor, upregulates transforming growth factor beta/bone morphogenetic protein and Wnt signaling pathways, and induces masculinization of caudal and anal fins in female guppies (Poecilia reticulata)

Sexually mature female guppies (Poecilia reticulata) were exposed to environmentally relevant concentrations (20, 200, and 2000 ng/L) of 17β-trenbolone for four weeks. As evidenced by the increased caudal fin index and anal fins developing into gonopodium-like structures, exposed females displayed masculinized secondary sexual characteristics. Differential gene expression and subsequent pathway analysis of mRNA sequencing data revealed that the transcription of transforming growth factor beta/bone morphogenetic protein signaling pathway and Wnt signaling pathway were upregulated following 17β-trenbolone exposure. Enzyme-linked immunosorbent assays showed that the bone morphogenetic protein 7 protein content was elevated after 17β-trenbolone exposure. Finally, real-time PCR revealed that 17β-trenbolone treatment significantly increased androgen receptor mRNA levels, and molecular docking showed potent interaction between 17β-trenbolone and guppy androgen receptor. Furthermore, 17β-trenbolone-induced masculinization of caudal and anal fins in female guppies, concomitant to the upregulated expression of differentially expressed genes involved in the above-mentioned two signaling pathways, was significantly inhibited by flutamide (androgen receptor antagonist). These findings demonstrated that 17β-trenbolone masculinized fins of female guppies by activating the androgen receptor. This study revealed that 17β-trenbolone could upregulate signaling pathways related to fin growth and differentiation, and eventually cause caudal and anal fin masculinization in female guppies.

Evolutionary differentiation of androgen receptor is responsible for sexual characteristic development in a teleost fish

Teleost fishes exhibit complex sexual characteristics in response to androgens, such as fin enlargement and courtship display. However, the molecular mechanisms underlying their evolutionary acquisition remain largely unknown. To address this question, we analyse medaka (Oryzias latipes) mutants deficient in teleost-specific androgen receptor ohnologs (ara and arb). We discovered that neither ar ohnolog was required for spermatogenesis, whilst they appear to be functionally redundant for the courtship display in males. However, both were required for reproductive success: ara for tooth enlargement and the reproductive behaviour eliciting female receptivity, arb for male-specific fin morphogenesis and sexual motivation. We further showed that differences between the two ar ohnologs in their transcription, cellular localisation of their encoded proteins, and their downstream genetic programmes could be responsible for the phenotypic diversity between the ara and arb mutants. These findings suggest that the ar ohnologs have diverged in two ways: first, through the loss of their roles in spermatogenesis and second, through gene duplication followed by functional differentiation that has likely resolved the pleiotropic roles derived from their ancestral gene. Thus, our results provide insights into how genome duplication impacts the massive diversification of sexual characteristics in the teleost lineage.

Genetic dissection of steroid-hormone modulated social behavior: Novel paralogous genes are a boon for discovery

Research across species has led to important discoveries on the functions of steroid hormones in the regulation of behavior. However, like in many fields, advancements in transgenic and mutagenic technology allowed mice to become the premier genetic model for conducting many experiments to understand how steroids control social behavior. Since there has been a general lack of parallel methodological developments in other species, many of the findings cannot be generalized. This is especially the case for teleost fish, in which a whole-genome duplication produced novel paralogs for key steroid hormone signaling genes. In this review, we summarize technical advancements over the history of the field of neuroendocrinology that have led to important insights in our understanding of the control of social behavior by steroids. We demonstrate that early mouse genetic models to understand these mechanisms suffered from several issues that were remedied by more precise transgenic technological advancements. We then highlight the importance of CRISPR/Cas9 gene editing tools that will in time bridge the gap between mice and non-traditional model species for understanding principles of steroid hormone action in the modulation of social behavior. We specifically highlight the role of teleost fish in bridging this gap because they are 1) highly genetically tractable and 2) provide a novel advantage in achieving precise genetic control. The field of neuroendocrinology is entering a new "gene editing revolution" that will lead to novel discoveries about the roles of steroid hormones in the regulation and evolutionary trajectories of social behavior.

Masculinization of Adult Gambusia holbrooki: A Case of Recapitulation of Protogyny in a Gonochorist?

Simple Summary

Although gonochoristic fish express one sex or the other in adulthood, some adults display traits similar to those of their opposite sex, suggesting plasticity similar to hermaphrodites. To fully test this potential in the eastern mosquitofish, a gonochorist, two adult stages of females were fed 17α-Methyltestosterone incorporated feed (0–200 mg/kg diet) for 50 days. The hormone (particularly at 50 mg/kg diet) stimulated the formation of complex male copulatory structures and testicular tissue as well as upregulated expression of Anti-Müllerian Hormone gene and altered the behavior from females to males. Collectively, we infer that Gambusia holbrooki retains potential for sex reversal at adulthood, similar to what occurs naturally in hermaphroditic fish and can provide an ideal system to investigate these mechanisms in a regulated fashion.

Abstract

17α-Methyltestosterone (MT) is a synthetic steroid that has been widely used to masculinize many fish species when administered early during larval development, however, reports on its efficacy on adults is limited. To this end, this study investigated the efficacy of MT in the masculinization of the eastern mosquitofish (G. holbrooki) at two adult stages (maiden and repeat gravid females). The treated females were fed control or respective MT incorporated feed (0–200 mg/kg diet) for 50 days. Effects of the hormone on secondary sexual characteristics, internal gonad morphology, expression of the Anti-Müllerian Hormone (amh) gene and sexual behavior of the treated females were investigated. The results showed that MT at the dose of 50 mg/kg feed stimulated secondary sexual character development, upregulated expression of amh, formation of testicular tissue and a shift in the behavior similar to those of normal males, prominently so in treated maiden gravid females. Post-treatment, long-term observations indicated that only two masculinized females reverted back to being females and gave birth to young. Induction of masculinizing effects in most individuals suggests that the sexual phenotype of this species appears to be highly plastic with potential to sex reverse at adulthood. This in combination with its small size and short reproductive cycle could provide an ideal system to explore the mechanisms of sequential hermaphroditism in fish and contribute to genetic control of this pest fish.

Control of testes mass by androgen receptor paralogs in a cichlid

Steroid hormones play numerous important and diverse roles in the differentiation and development of vertebrate primary and secondary reproductive characteristics. However, the exact role of androgen receptors-which bind circulating androgens-in this regulatory pathway is unclear. Teleost fishes further complicate this question by having two paralogs of the androgen receptor (ARα and ARβ) resulting from a duplication of their ancestral genome. We investigated the functional role of these two ARs on adult testes mass, by eliminating receptor function of one or both paralogs using CRISPR/Cas9 genome edited Astatotilapia burtoni, an African cichlid fish. Fish with two or more functional AR alleles were more likely to be male compared to fish with one or fewer, suggesting that the two paralogs may play redundant roles in the A. burtoni sex determination system. We replicated previous work showing that fish lacking functional ARβ possess testes smaller than wild-type fish, while fish lacking ARα possess testes larger than wild-type fish. However, we found novel evidence supporting a complex relationship between the two AR paralogs in the regulation of testes mass. For instance, the effects of ARα mutation on testes mass are eliminated in homozygous ARβ mutants but the reverse is not true. These results suggest a dynamic relationship between these two AR paralogs where ARβ functions may be permissive to ARα functions in the control of testes mass. This mechanism may contribute to the robust physiological plasticity displayed by A. burtoni and other social teleost fishes.

Effects of dexamethasone on the morphology, gene expression and hepatic histology in adult female mosquitofish (Gambusia affinis)

Glucocorticoids (GCs), including natural hormones as well as synthetic chemicals, can pose influences on physiological performance, development and reproduction of fish. Dexamethasone (DEX) is a synthetic glucocorticoid widely used as pharmaceutical and usually exists in effluents with varying degrees of concentrations. In this study, adult female mosquitofish (Gambusia affinis) were treated by DEX at concentrations of 0, 0.5, 5 and 50 μg/L for 60 days. Morphological parameters of anal fin and skeleton, mRNA expression abundance, and histological alterations of liver were investigated to assess effects of DEX on mosquitofish. The results showed that DEX increased number of sections of ray 3 in anal fin and decreased 16L, 15D and 16D in skeletal parameters, which indicates DEX could potentially lead to weak masculinization. Furthermore, transcriptional expression levels of ARα, ARβ, ERβ, VTGC and CYP19A genes were notably down-regulated by DEX, which will contribute to weak masculinization in females. In addition, the damage to liver tissue was also induced by DEX. Taken together, this research demonstrated that aquatic environments contaminated by DEX have negative effects on mosquitofish at a population level.

Endocrine disrupting effects of binary mixtures of 17β-estradiol and testosterone in adult female western mosquitofish (Gambusia affinis)

Androgens and estrogens often co-exist in aquatic environments and pose potential risks to fish populations. However, little is known about the endocrine disrupting effects of the mixture of androgens and estrogens in fish. In this study, transcriptional level of target genes related to the hypothalamic-pituitary-gonadal-liver (HPGL) axis, sex hormone level, VTG protein concentration, histology and secondary sex characteristic were assessed in the ovaries and livers of adult female western mosquitofish (Gambusia affinis) exposed to 17β-estradiol (E2), testosterone (T), and mixtures of E2 and T for 91 days. The results showed that the transcriptional expression of cytochrome P450, family 19, subfamily A, polypeptide 1a (Cyp19a1a) was suppressed in the 200 ng/L T treatment and the 50 ng/L E2 + 200 ng/L T treatment in the ovaries. Steroidogenic acute regulatory protein (Star) and Cyp11a1 showed a similar expression pattern in the T treatment to its corresponding T + E2 mixtures. In the ovaries, the concentrations of 17β-estradiol and testosterone were decreased in most treatments compared with the solvent control. VTG protein was induced in all steroid treatment. However, exposure to T or E2 + T mixture did not cause the abnormal cells of the ovaries and livers and an extension of the anal fins in female G. affinis. This study demonstrates that chronic exposure to E2, T and their mixtures affects the transcripts of genes in the HPGL axis, steroid hormone level and VTG protein concentration in the ovaries and livers, but fails to cause the histopathological effect of the ovaries and livers and alter the morphology of the anal fins in G. affinis.

Effects of 17α-ethinylestradiol on sex ratio, gonadal histology and perianal hyperpigmentation of Cnesterodon decemmaculatus (Pisces, Poeciliidae) during a full-lifecycle exposure

The effects of 17α-ethinylestradiol (EE₂) on sex ratio, gonopodium morphology, and gonadal histology of C. decemmaculatus were assessed by a full-lifecycle exposure experiment. Newborn fish were waterborne exposed to 30, 100, and 300 ng EE₂/L for 90 d, using 50 fish per treatment. Additionally, in December of 2016, a field survey was conducted on a C. decemmaculatus population inhabiting the Girado Creek downstream of the Chascomus city wastewater effluent discharge. After 90 d of exposure, EE₂ was able to histologically skew the sex ratio toward females and inhibit the full gonopodium development since the lowest tested concentration (LOEC = 30 ng/L). At higher concentrations, EE₂ was toxic, inducing mortality in a concentration-dependent fashion (90 d-LC₅₀ = 109.9 ng/L) and altering the gonadal histoarchitecture, causing neither testes nor ovaries discernible histologically (LOEC = 100 ng/L). In addition, a novel response, perianal hyperpigmentation, was discovered been induced by the EE₂ exposure in a concentration-dependent fashion (90 d-EC₅₀ = 39.3 ng/L). A higher proportion of females and perianal hyperpigmentation were observed in wild fish collected from the Girado Creek. The major reached conclusions are: i) EE₂ induce different effects on the sexual traits of C. decemmaculatus when exposed from early-life or adult stages. ii) The most sensitive effects observed in the laboratory occur in a creek receiving wastewater effluent. iii) The perianal hyperpigmentation comes-up as a promising biomarker of exposure to estrogenic compounds.

The intraspecific diversity of tooth morphology in the large-spotted catshark Scyliorhinus stellaris: insights into the ontogenetic cues driving sexual dimorphism

Teeth in sharks are shed and replaced throughout their lifetime. Morphological dental changes through ontogeny have been identified in several species and have been correlated with shifts in diet and the acquisition of sexual maturity. However, these changes were rarely quantified in detail along multiple ontogenetic stages, which makes it difficult to infer the developmental processes responsible for the observed plasticity. In this work, we use micro-computed tomography and 3D geometric morphometrics to describe and analyze the tooth size and shape diversity across three ontogenetic stages (hatchling, juvenile, and sexually mature) in the large-spotted catshark Scyliorhinus stellaris (Linnaeus, 1758). We first describe the intra-individual variation of tooth form for each sex at each ontogenetic stage. We provide a tooth morphospace for palatoquadrate and Meckelian teeth and identify dental features, such as relative size and number of cusps, involved in the range of variation of the observed morphologies. We then use these shape data to draw developmental trajectories between ontogenetic stages and for each tooth position within the jaw to characterize ontogenetic patterns of sexual dimorphism. We highlight the emergence of gynandric heterodonty between the juvenile and mature ontogenetic stages, with mature females having tooth morphologies more similar to juveniles' than mature males that display regression in the number of accessory cusps. From these data, we speculate on the developmental processes that could account for such developmental plasticity in S. stellaris.

Androgen signaling in male fishes: Examples of anti-androgenic chemicals that cause reproductive disorders

Similar to other vertebrates, androgens regulate spermatogenesis in fishes. In teleosts, the main androgen is 11-Ketotestosterone (11-KT), which is oxidized testosterone (T) at the C₁₁ position. Compared to T, 11-KT is a nonaromatizable steroid, and does not convert to 17β-estradiol. However, circulatory levels of both T and 11-KT undergo seasonal variations along with testicular development. Physiological functions of androgens are mediated via androgen receptor (Ar). So far, nuclear Ar and membrane Ar have been identified in fishes. In the present study, we reviewed androgen biosynthesis in fishes, compared molecular structure of nuclear Ar in models of mammals and fishes, and investigated the mechanisms of action of environmental contaminants that differentially disrupt androgen signaling in fish reproduction. In the latter case, the adverse effects of vinclozolin (VZ) and bis(2-ethylhexyl) phthalate (DEHP) are compared. Both VZ and DEHP are capable of decreasing sperm quality in males. Vinclozolin causes an increase in 11-KT production associated with increases in kisspeptin (kiss-1) and salmon gonadotropin-releasing hormone (gnrh3) mRNA levels as well as circulatory levels of luteinizing hormone (Lh). In contrast, DEHP inhibits 11-KT production associated with a decrease in circulatory Lh levels. However, DEHP-inhibited 11-KT production is not associated with changes in kiss-1 and gnrh3 mRNA levels. Studies also show that VZ alters ar mRNA levels, while DEHP is without effect. These suggest that VZ and DEHP act differentially to cause androgen-dependent reproductive disorder in male fishes. Molecular analyses of the nuclear AR show that both DNA and ligand binding domains (DBD and LBD, respectively) are highly conserved within models of mammals and fishes. A phylogeny tree of the AR shows distinct clusters between mammals and fishes. In fishes, subtypes of Arα and Arβ are also separated in distinct clusters. Thus, further studies need to generate ar knockout fish model to better elucidate androgen regulation of reproduction in fishes via Ar.

Male-specific asymmetric curvature of anal fin in a viviparous teleost, Xenotoca eiseni

Xenotoca eiseni is a viviparous teleost from the family Goodeidae. Internal fertilization occurs in this species; therefore, the male fish must transfer their sperm to the females. To this end, the males use their anal fins as external genitalia during mating. However, in goodeid species, there are a few reports of anal fin modifications functioning as genitalia. In the present study, I describe a male-specific lateral curvature with fin robe thickening on the two most posterior rays of the anal fin of X. eiseni. Morphological and behavioral analyses suggested that the lateral curvature provides a directional preference for coupling of male X. eiseni. The modification appears to be a male secondary sex characteristic. Other goodeid species Xenotoca melanosoma and Chapalichthys pardalis also possess the lateral curvature, but without robe thickening. Furthermore, Zoogoneticus quitzeoensis exhibited only curvature structure either smaller than those of other species or absent. Therefore, the anal fin curvature may not necessarily be used for internal fertilization in all species investigated in this study. However, it is still possible that it provides novel advantages during mating in certain goodeid species.

Environmentally relevant concentrations of bifenthrin affect the expression of estrogen and glucocorticoid receptors in brains of female western mosquitofish

In recent decades, pyrethroid pesticides have been deemed a safer alternative to previously used pesticides. While some evidence supports this assumption in mammals and birds, exposure to certain pyrethroids can affect concentrations of hormones vital to reproduction in fish. Thus, we hypothesized that pyrethroid exposure impacts fish reproductive behavior and the expression of genes associated with reproduction. We tested our hypothesis by examining effects of the widely used pyrethroid pesticide, bifenthrin, on the reproductive behaviors of the broadly distributed livebearing western mosquitofish, Gambusia affinis. We exposed sexually mature female fish to one of five environmentally relevant concentrations of bifenthrin and conducted behavioral assays to assess reproductive, social, and space use behaviors before and after exposure. We did not detect changes in behaviors measured in response to bifenthrin. However, exposure was associated with increased expression of an estrogen receptor gene (ER-α) and glucocorticoid receptor (GR) in brain tissue at bifenthrin concentrations at concentrations of 5.90 and 24.82 ng/L, and 5.90 and 12.21 ng/L, respectively. Our study supports the perspective that the use of multiple endpoints through integrative approaches is essential for understanding the cumulative impact of pollutants. Integrating physiological, morphological, and behavioral investigations of nonlethal concentrations of pollutants like bifenthrin may heighten our potential to predict their impact on individuals, populations, and communities.

Characterization and expression of androgen receptors in olive flounder

Androgens are critical hormones that regulate sex differentiation, sexual maturation, and spermatogenesis in vertebrates, which is mainly mediated by androgen receptors (ARs). Reports on transcript variants of ar (AR encoding gene) in human are almost always associated with cancers and androgen insensitivity syndrome. However, the knowledge of ar variants in teleosts is scarce. In this study, arβ and two transcript variants of arα (arα1 and arα2) in olive flounder (Paralichthys olivaceus) were cloned and analyzed. Their expression patterns were investigated in 16 adult female and male tissues by RT-PCR, respectively. arα1 was expressed in the majority of tissues excluding male liver, medulla oblongata and female cerebellum, with higher levels in male gonad, kidney, head kidney, intestine, stomach, spleen, heart and gill than in female. arα2 had similar expression patterns as arα1, with lower levels in general. arβ was also widely expressed in various tissues excluding male spleen, female spleen and gill, with higher levels in male gonad, kidney, head kidney, intestine and lower levels in hypothalamus than in female. Compared with arβ, much lower expression levels of arα1 and arα2 were detected in different brain areas. The real-time quantitative PCR (qPCR) results showed that the total arα expression level was relatively higher during olive flounder gonadal differentiation and before the onset of testis differentiation, whereas arβ was expressed significantly higher during male gonadal differentiation period than female gonadal differentiation period. The in vitro transient transfection assays showed that ARα1, ARα2 and ARβ could all suppress the activity of cyp19a (p450arom aromatase gene) promoter, and the inhibitory effect of ARα1 was dose dependent. Our results imply that arα1, arα2 and arβ are sex-related genes and they might play important roles in gonadal differentiation in flounder.

Functional distinctions associated with the diversity of sex steroid hormone receptors ESR and AR

Sex steroid hormones including estrogens and androgens play fundamental roles in regulating reproductive activities and they act through estrogen and androgen receptors (ESR and AR). These steroid receptors have evolved from a common ancestor in association with several gene duplications. In most vertebrates, this has resulted in two ESR subtypes (ESR1 and ESR2) and one AR, whereas in teleost fish there are at least three ESRs (ESR1, ESR2a and ESR2b) and two ARs (ARα and ARβ) due to a lineage-specific whole genome duplication. Functional distinctions have been suggested among these receptors, but to date their roles have only been characterized in a limited number of species. Sexual differentiation and the development of reproductive organs are indispensable for all animal species and in vertebrates these events depend on the action of sex steroid hormones. Here we review the recent progress in understanding of the functions of the ESRs and ARs in the development and expression of sexually dimorphic characteristics associated with steroid hormone signaling in vertebrates, with representative fish, amphibians, reptiles, birds and mammals.

Physiological responses and gene expression changes in the western mosquitofish (Gambusia affinis) exposed to progesterone at environmentally relevant concentrations

Progesterone (P4) is a natural and synthetic steroid, widely distributed in the aquatic environments. It can lead to adverse effects on the endocrine system in aquatic organisms. This study investigated the toxicological effects of exposure to environmentally relevant concentrations (4, 44, and 410ng/L) of progesterone for 42 d on adult female mosquitofish, Gambusia affinis. We performed morphological and histological analyses on gonads, anal fins, liver, and gills after the exposure of mosquito fish to P4. The expression levels of genes (vtg, er, and ar isoforms) related to fish reproduction and detoxification (cyp1a) in the liver were quantified by quantitative real-time polymerase chain reaction. The results showed that the progesterone exposure induced slight masculinization in female mosquitofish, influenced the oocyte maturation as revealed by histology of the ovaries, and caused severe damages to the liver and gills of adult female mosquitofish. It also suppressed the mRNAs expression of vtg, er, cyp1a, and significantly enhanced the expression of ar mRNA in the liver. This study reveals the molecular and physiological effects of progesterone at environmentally relevant concentrations, which might further be translated to alterations in the reproduction of mosquitofish.

Contribution of gene flow to the evolution of recombination suppression in sex chromosomes

Polymorphism of alleles that benefit one sex but harm the other (sexually antagonistic alleles) generates selective pressures for reduced recombination between themselves and sex-determination loci. Such polymorphism can be maintained within a population when selection coefficients are sufficiently balanced between males and females. However, if regulatory mutations restrict gene expression only to one sex, these alleles become neutral in the other sex and easily fixed within a population, removing the selective pressures for recombination suppression in sex chromosomes. When there is spatial variation in selection regimes, however, alleles that are deleterious in one sex and neutral in the other can be maintained in other neighboring populations and gene flow may continuously supply deleterious alleles. We hypothesized that this maintenance of genetic variation may promote the establishment of recombination suppression in sex chromosomes even in cases where selection is limited to one sex. Using individual-based simulations, we show that spatial variation in male-limited selection and gene flow can promote the establishment of Y-autosome fusions, a special case of recombination suppression in sex chromosomes. This can be explained by the fact that fused Y-chromosomes that capture alleles that are beneficial for local males have a higher mean fitness compared to unfused Y chromosomes in the presence of deleterious gene flow. We also simulated the case of sex-concordant selection and found that gene flow of alleles that are deleterious in both sexes did not substantially increase the establishment rates of Y-autosome fusions across the parameter space examined. This can be accounted for by the fact that foreign alleles that are deleterious in both sexes can be efficiently removed from the population compared to alleles that are neutral in females. These results indicate that how gene flow affects the establishment rates of Y-autosome fusions depends largely on selection regimes. Spatial variation in sex-specific selection and gene flow should be appreciated as a factor affecting sex chromosome evolution.

Intersex and liver alterations induced by long-term sublethal exposure to 17α-ethinylestradiol in adult male Cnesterodon decemmaculatus (Pisces: Poeciliidae)

The aim of the present study was to assess the responses of the gonopodium morphology and the gonadal and liver histology of adult male Cnesterodon decemmaculatus to sublethal long-term exposure concentrations of 17α-ethinylestradiol (EE2). Two experiments were conducted exposing the fish to waterborne concentrations of EE2 ranging from 20 ng/L to 200 ng/L for 8 wk, 12 wk, and 16 wk. Intersex gonads were observed after 8 wk and 16 wk in fish exposed to 200 ng EE2/L and 100 ng EE2/L, respectively. Oocytes' development from testis germ cells and replacement of the efferent duct periodic acid-Schiff-positive secretion surrounding spermatozeugmata by parenchymal tissue and duct structure alterations were the major observed changes in the gonads. In contrast, no response was observed in the gonopodium morphology. Liver histology was also altered, showing increasing steatosis, single-cell necrosis to generalized necrosis, and disruption of acinar organization from 100 ng EE2/L to 200 ng EE2/L. In summary, the present results showed that although EE2 was not able to alter the morphology of a developed gonopodium, it was capable of inducing development of testicular oocytes in adult male C. decemmaculatus at environmentally relevant concentrations. Thus, externally normal but intersex C. decemmaculatus males would be expected in the wastewater-receiving streams that the species inhabits. According to the literature, the present study would be the first indicating estrogen-induced intersex in adult male poeciliid. Environ Toxicol Chem 2017;36:1738-1745. © 2016 SETAC.

Cytochrome P450 1A mRNA in the guppy Phalloceros caudimaculatus and response to beta-naphthoflavone and environmental samples

The cytochrome P450 1A (CYP1A) mRNA is induced by environmental contaminants such as PAHs, PCBs and dioxins. The present study cloned the CYP1A transcript from the guppy Phalloceros caudimaculatus, which represents a potential fish for toxicological studies in South America. The newly identified CYP1A encodes a protein with 521 amino acids that shared 96-70% identity with other fishes. The characterization of organ- and time-dependent induction of CYP1A using RT-qPCR was evaluated after waterborne exposure to beta-naphthoflavone (BNF; 1μM). The minimum exposure time that elicited significant CYP1A induction was 1h for liver, gill, gut, brain, anal fin and fingerlings; 2h for dorsal fin; and 4h for kidney and tail fin. CYP1A tended to reach peak induction in the first few hours (4h-8h) of experiment in most organs, although levels remained induced until the end of the experiment (96h). Validation of CYP1A use in environmental sample was performed by exposing P. caudimaculatus to elutriate made from sediment of three streams located in adjacent areas of the Patos Lagoon Estuary (RS, Brazil). CYP1A in liver, gills and anal fin was induced by elutriate made from urban (S1) and industrial (S2) sites; and not induced by a reference site located 22 Km from potential contaminant sources, suggesting that environmental contamination plays a role in this induction. The results suggest that fins could be used for CYP1A biomarker analysis and employed in non-lethal biopsy methods for environmental monitoring. The responsiveness of the newly identified CYP1A to BNF and elutriate indicates that the guppy P. caudimaculatus could be used for environmental toxicology investigations in South American environments.

Ancestral androgenic differentiation pathways are repurposed during the evolution of adult sexual plasticity

Although early exposure to androgens is necessary to permanently organize male phenotype in many vertebrates, animals that exhibit adult sexual plasticity require mechanisms that prevent early fixation of genital morphology and allow for genital morphogenesis during adult transformation. In Lythrypnus dalli, a teleost fish that exhibits bi-directional sex change, adults display dimorphic genitalia morphology despite the absence of sex differences in the potent fish androgen 11-ketotestosterone. Based on conserved patterns of vertebrate development, two steroid-based mechanisms may regulate the early development and adult maintenance of dimorphic genitalia; local androgen receptor (AR) and steroidogenic enzyme expression. Consistent with the ancestral pattern of AR expression during the multipotential phase of differentiation, juvenile differentiation into either sex involved high mesenchymal AR expression. In adults, AR expression was high throughout the male genitalia, but low or absent in females. Consistent with the hypothesis that adult sexual plasticity repurposes pathways from primary differentiation, we show that adults with transitioning genitalia also exhibited higher AR expression relative to females. Local androgen biosynthesis may also participate in genitalia transformation, as transitioning adults had greater 11β-HSD-like immunoreactivity in the epithelial layer of the dorsal lumen compared to both sexes. By administering an AR antagonist to adult males, we show AR is necessary to maintain male-typical morphology. In a species that is resistant to early sexual canalization, early androgenic differentiation mechanisms are consistent with other vertebrates and the tissue-specific regulation of AR expression appears to be repurposed in adulthood to allow for transitions between sexual phenotypes.

Transcriptomic and physiological changes in Eastern Mosquitofish (Gambusia holbrooki) after exposure to progestins and anti-progestagens

Endocrine active compounds (EACs) remain an important group of chemicals that require additional evaluation to determine their environmental impacts. While estrogens and androgens were previously demonstrated to impact organisms during environmental exposures, progestagens have recently been shown to have strong impacts on aquatic organisms. To gain an understanding of the impacts of these types of chemicals on aquatic species, experiments evaluating the mechanisms of action of progestagen exposure were conducted with the Eastern Mosquitofish (Gambusia holbrooki). The objective of this study was to conduct hepatic microarray analysis of male and female G. holbrooki exposed to progestins and anti-progestagens. In addition, we evaluated the ability of levonorgestrel, a synthetic progesterone (progestin), to induce anal fin elongation and to determine how anal fin growth is modulated during co-exposures with progesterone and androgen receptor antagonists. Gene expression analyses were conducted on male and female G. holbrooki exposed for 48h to the agonist levonorgestrel, the antagonist mifepristone, or a mixture of the two chemicals. Microarray analysis revealed that mifepristone does not act as an anti-progestagen in G. holbrooki in liver tissues, and that levonorgestrel elicits strong effects on the processes of embryo development and lipid transport. Levonorgestrel was also demonstrated to induce male secondary sexual characteristic formation in females, and co-exposure of either an androgen or levonorgestrel in the presence of the anti-androgen flutamide prevented anal fin elongation. These results provide indications as to the potential impacts of progestins, including non-target effects such as secondary sexual characteristic formation, and demonstrate the importance of this class of chemicals on aquatic organisms.

Aqueous exposure to the progestin, levonorgestrel, alters anal fin development and reproductive behavior in the eastern mosquitofish (Gambusia holbrooki)

Endogenous progestogens are important regulators of vertebrate reproduction. Synthetic progestins are components of human contraceptive and hormone replacement pharmaceuticals. Both progestogens and progestins enter the environment through a number of sources, and have been shown to cause profound effects on reproductive health in various aquatic vertebrates. Progestins are designed to bind human progesterone receptors, but they also have been shown to strongly activate androgen receptors in fish. Levonorgestrel (LNG) activates fish androgen receptors and induces development of male secondary sex characteristics in females of other species. Although behavior has been postulated to be a sensitive early indicator of exposure to certain environmental contaminants, no such research on the reproductive behavior of gestagen-exposed fish has been conducted to date. The goal of our study was to examine the exposure effects of a human contraceptive progestin, LNG, on the reproductive development and behavior of the viviparous eastern mosquitofish (Gambusia holbrooki). Internal fertilization is a requisite characteristic of viviparous species, and is enabled by an androgen driven elongation of the anal fin into the male gonopodium (i.e., phallus). In this study, we exposed adult mosquitofish to ethanol (EtOH control), 10ng/L, and 100ng/L LNG for 8d using a static replacement exposure design. After 8d, a subset of males and females from each treatment were examined for differences in the 4:6 anal fin ratio. In addition, paired social interaction trials were performed using individual control males and control females or females treated 10ng/L or 100ng/L LNG. Female mosquitofish exposed to LNG were masculinized as evidenced by the elongation of the anal fin rays, a feature normal to males and abnormal to females. LNG caused significant increases in the 4:6 anal fin ratios of female mosquitofish in both the 10ng/L and 100ng/L treatments, although these differences were not significant between the two treatments. LNG caused significant increases in the 4:6 anal fin ratio of males exposed to 100ng/L, with no effects observed in the 10ng/L treatment. In addition, the reproductive behavior of control males paired with female mosquitofish exposed to 100ng/L LNG was also altered, for these males spent more time exhibiting no reproductive behavior, had decreased attending behavior, and a lower number of gonopodial thrusts compared to control males paired to control female mosquitofish. Given the rapid effects on both anal fin morphology and behavior observed in this study, the mosquitofish is an excellent sentinel species for the detection of exposure to LNG and likely other 19-nortestosterone derived contraceptive progestins in the environment.

Sequence Evolution and Expression of the Androgen Receptor and Other Pathway-Related Genes in a Unisexual Fish, the Amazon Molly, Poecilia formosa, and Its Bisexual Ancestors

The all-female Amazon molly (Poecilia formosa) originated from a single hybridization of two bisexual ancestors, Atlantic molly (Poecilia mexicana) and sailfin molly (Poecilia latipinna). As a gynogenetic species, the Amazon molly needs to copulate with a heterospecific male, but the genetic information of the sperm-donor does not contribute to the next generation, as the sperm only acts as the trigger for the diploid eggs' embryogenesis. Here, we study the sequence evolution and gene expression of the duplicated genes coding for androgen receptors (ars) and other pathway-related genes, i.e., the estrogen receptors (ers) and cytochrome P450, family19, subfamily A, aromatase genes (cyp19as), in the Amazon molly, in comparison to its bisexual ancestors. Mollies possess-as most other teleost fish-two copies of the ar, er, and cyp19a genes, i.e., arα/arβ, erα/erβ1, and cyp19a1 (also referred as cyp19a1a)/cyp19a2 (also referred to as cyp19a1b), respectively. Non-synonymous single nucleotide polymorphisms (SNPs) among the ancestral bisexual species were generally predicted not to alter protein function. Some derived substitutions in the P. mexicana and one in P. formosa are predicted to impact protein function. We also describe the gene expression pattern of the ars and pathway-related genes in various tissues (i.e., brain, gill, and ovary) and provide SNP markers for allele specific expression research. As a general tendency, the levels of gene expression were lowest in gill and highest in ovarian tissues, while expression levels in the brain were intermediate in most cases. Expression levels in P. formosa were conserved where expression did not differ between the two bisexual ancestors. In those cases where gene expression levels significantly differed between the bisexual species, P. formosa expression was always comparable to the higher expression level among the two ancestors. Interestingly, erβ1 was expressed neither in brain nor in gill in the analyzed three molly species, which implies a more important role of erα in the estradiol synthesis pathway in these tissues. Furthermore, our data suggest that interactions of steroid-signaling pathway genes differ across tissues, in particular the interactions of ars and cyp19as.

Variation in the ontogeny of sex steroid levels between latitudinal populations of the medaka

INTRODUCTION

Sex steroids mediate the expression of sexual dimorphism during ontogeny, and populations that differ in the magnitudes of sexual dimorphism may accordingly differ in the ontogenetic patterns of their sex steroid levels. The medaka, Oryzias latipes species complex, shows geographic variation in the magnitude of sexual dimorphism with respect to the lengths of their anal and dorsal fins; dimorphism is greater in low-latitude populations than in high-latitude populations. However, sexual differences in the ontogenetic dynamics of sex steroids, and its interpopulation variation, have not been examined.

RESULTS

We measured testosterone (T), estradiol-17β (E2), and 11-ketotestosterone (11-KT) concentrations throughout ontogeny of laboratory-reared fish from two latitudinal populations: Aomori (northern) and Okinawa (southern). In both populations, the levels of all three steroids were high during early ontogenetic stages and decreased with growth. After reaching about 15 mm in standard length, when sexual dimorphisms in fin lengths became apparent, steroid levels increased and tended to plateau. Sexual differences in the steroid levels were observed only in the later ontogenetic stages; T and 11-KT levels were higher in males, while E2 levels were higher in females. Accordingly, interpopulation differences also became clearer; the southern fish tended to show higher T levels and lower E2 levels than the northern fish.

CONCLUSIONS

The ontogenetic patterns of sex steroid levels paralleled the ontogeny of anal and dorsal fins in the two latitudinal populations, suggesting that interpopulation variation in the degree of sexual dimorphisms in fin lengths is mediated by sex steroid-dependent regulation of fin elongation.

Neofunctionalization of Androgen Receptor by Gain-of-Function Mutations in Teleost Fish Lineage

Steroid hormone receptor family provides an example of evolution of diverse transcription factors through whole-genome duplication (WGD). However, little is known about how their functions have been evolved after the duplication. Teleosts present a good model to investigate an accurate evolutionary history of protein function after WGD, because a teleost-specific WGD (TSGD) resulted in a variety of duplicated genes in modern fishes. This study focused on the evolution of androgen receptor (AR) gene, as two distinct paralogs, ARα and ARβ, have evolved in teleost lineage after TSGD. ARα showed a unique intracellular localization with a higher transactivation response than that of ARβ. Using site-directed mutagenesis and computational prediction of protein-ligand interactions, we identified two key substitutions generating a new functionality of euteleost ARα. The substitution in the hinge region contributes to the unique intracellular localization of ARα. The substitution on helices 10/11 in the ligand-binding domain possibly modulates hydrogen bonds that stabilize the receptor-ligand complex leading to the higher transactivation response of ARα. These substitutions were conserved in Acanthomorpha (spiny-rayed fish) ARαs, but not in an earlier branching lineage among teleosts, Japanese eel. Insertion of these substitutions into ARs from Japanese eel recapitulates the evolutionary novelty of euteleost ARα. These findings together indicate that the substitutions generating a new functionality of teleost ARα were fixed in teleost genome after the divergence of the Elopomorpha lineage. Our findings provide a molecular explanation for an adaptation process leading to generation of the hyperactive AR subtype after TSGD.

Identification and Characterization of the Androgen Receptor From the American Alligator, Alligator mississippiensis

Androgens are essential for the development, reproduction, and health throughout the life span of vertebrates, particularly during the initiation and maintenance of male sexual characteristics. Androgen signaling is mediated by the androgen receptor (AR), a member of the steroid nuclear receptor superfamily. Mounting evidence suggests that environmental factors, such as exogenous hormones or contaminants that mimic hormones, can disrupt endocrine signaling and function. The American alligator (Alligator mississippiensis), a unique model for ecological research in that it exhibits environment-dependent sex determination, is oviparous and long lived. Alligators from a contaminated environment exhibit low reproductive success and morphological disorders of the testis and phallus in neonates and juveniles, both associated with androgen signaling; thus, the alterations are hypothesized to be related to disrupted androgen signaling. However, this line of research has been limited because of a lack of information on the alligator AR gene. Here, we isolated A mississippiensis AR homologs (AmAR) and evaluated receptor-hormone/chemical interactions using a transactivation assay. We showed that AmAR responded to all natural androgens and their effects were inhibited by cotreatment with antiandrogens, such as flutamide, p,p'-dichlorodiphenyldichloroethylene, and vinclozolin. Intriguingly, we found a spliced form of the AR from alligator cDNA, which lacks seven amino acids within the ligand-binding domain that shows no response to androgens. Finally, we have initial data on a possible dominant-negative function of the spliced form of the AR against androgen-induced AmAR.

Evaluation of the 5α-reductase inhibitor finasteride on reproduction and gonadal development in medaka, Oryzias latipes

5-α reductase (5αR) inhibitors have an anti-androgenic effect in mammals because they inhibit the conversion of testosterone to the potent androgen, dihydrotestosterone. Finasteride is a type-2 5αR inhibitor that is used as a human pharmaceutical for the treatment of prostate cancer, benign prostate hyperplasia and male pattern baldness. This study evaluated the impacts of finasteride (50, 500 and 5000μg/L) on the development and reproduction of medaka (Oryzias latipes) exposed continuously over multiple generations (F0, F1 and F2). The exposure was initiated with reproductively mature fish (F0 generation) and continued until the hatching of the F2 generation. There were no significant effects on survival, fecundity or fertility in the F0 (50, 500, 5000μg/L) and F1 (50, 500μg/L) generations. The F1 generation exposed to 5000μg/L exhibited significant mortality. Histopathology of the gonads demonstrated that medaka and pre-clinical species respond similarly to finasteride exposure. Intersex condition and maldeveloped gonads were observed in F0 generation males exposed to 5000μg/L and F1 generation males exposed to 500μg/L. F1 generation males exposed to 500μg/L displayed reduced gonadosomatic index with an increased incidence of testicular degeneration. Males in both generations exhibited an increased incidence of Leydig cell hyperplasia at concentrations ⩾500μg/L. F0 generation females exposed to 5000μg/L exhibited increased gonadosomatic index. An increased prevalence of accelerated post-ovulatory follicle involution was observed in females at concentrations ⩾500μg/L in both generations. The gonadal changes induced by finasteride support the idea that 5-α reductase inhibition impacts androgen signaling in fish. Results from this study are discussed in the context of differential expression of the androgen receptor between species of fish.

Beta-naphthoflavone-inducedCYP1A expression in the guppy Jenynsia multidentata: Time-dependent response, anesthetic MS-222 effect and fin analysis

Cytochrome P450 1A (CYP1A) expression in fish is used as a biomarker of exposure to organic contaminants, such PAHs, PCBs and dioxins, in the aquatic environment. South American guppy fish Jenynsia multidentata were exposed to the prototypical aryl hydrocarbon receptor (AHR) agonist beta-naphthoflavone (BNF; 1μM) and the fins were biopsied to characterize different aspects of CYP1A induction. RTq-PCR was used to quantify CYP1A mRNA levels in fish tissues. CYP1A induction in the gill, liver and anal fin (gonopodium) occurred within the first hour of waterborne exposure to BNF and persisted throughout 2, 4, 8, 24, 48 and 96h compared to controls (DMSO vehicle; p<0.05). The organ-specific temporal pattern of induction was marked by mRNA levels consistently augment as duration of exposure increases and tend to a sustained induction from 24h to 96h for gill and liver (∼15-fold and ∼50-fold over control, respectively). In gonopodium, there was a maximum CYP1A mRNA level at 4h (∼34-fold over control). Basal CYP1A mRNA levels and its induction following BNF exposure were not affected by administration of a chemical anesthetic (fish immersion in 100mgl(-1) MS-222 for 2-5min) in the gill, liver, gonopodium, dorsal or tail fin (p<0.05). In an ex vivo assay, in which small pieces of biopsied fins were exposed to BNF for 4h, high CYP1A induction was observed in the tail and gonopodium (∼49-fold and ∼69-fold, respectively) but not in the dorsal fin compared to controls. To our knowledge, this is the first study to show that a 1h waterborne exposure to an AHR agonist is sufficient to cause CYP1A induction in fish organs and fins. The present study added new information to the field regarding the use of MS-222 as an anesthetic on fish and the analysis of biopsied fins as an alternative non-lethalex vivo assay for evaluating the CYP1A biomarker in fish. This observation could be useful for planning fish toxicological bioassays and biomonitoring studies on the aquatic environments in South America.

Differential ligand selectivity of androgen receptors α and β from Murray-Darling rainbowfish (Melanotaenia fluviatilis)

Androgen receptors (ARs) mediate the physiological effects of androgens in vertebrates. In fishes, AR-mediated pathways can be modulated by aquatic contaminants, resulting in the masculinisation of female fish or diminished secondary sex characteristics in males. The Murray-Darling rainbowfish (Melanotaenia fluviatilis) is a small-bodied freshwater teleost used in Australia as a test species for environmental toxicology research. We determined concentration-response profiles for selected agonists and antagonists of rainbowfish ARα and ARβ using transient transactivation assays. For both ARα and ARβ, the order of potency of natural agonists was 11-ketotestosterone (11-KT)>5α-dihydrotestosterone>testosterone>androstenedione. Methyltestosterone was a highly potent agonist of both receptors relative to 11-KT. The relative potency of the veterinary growth-promoting androgen, 17β-trenbolone, varied by more than a factor of 5 between ARα and ARβ. The non-steroidal anti-androgen bicalutamide exhibited high inhibitory potency relative to the structurally related model anti-androgen, flutamide. The inhibitory potency of the agricultural fungicide, vinclozolin, was approximately 1.7-fold relative to flutamide for ARα, but over 20-fold in the case of ARβ. Fluorescent protein tagging of ARs showed that the rainbowfish ARα subtype is constitutively localised to the nucleus, while ARβ is cytoplasmic in the absence of ligand, an observation which agrees with the reported subcellular localisation of AR subtypes from other teleost species. Collectively, these data suggest that M. fluviatilis ARα and ARβ respond differently to environmental AR modulators and that in vivo sensitivity to contaminants may depend on the tissue distribution of the AR subtypes at the time of exposure.

Transcriptomics of two evolutionary novelties: how to make a sperm-transfer organ out of an anal fin and a sexually selected "sword" out of a caudal fin

Swords are exaggerated male ornaments of swordtail fishes that have been of great interest to evolutionary biologists ever since Darwin described them in the Descent of Man (1871). They are a novel sexually selected trait derived from modified ventral caudal fin rays and are only found in the genus Xiphophorus. Another phylogenetically more widespread and older male trait is the gonopodium, an intromittent organ found in all poeciliid fishes, that is derived from a modified anal fin. Despite many evolutionary and behavioral studies on both traits, little is known so far about the molecular mechanisms underlying their development. By investigating transcriptomic changes (utilizing a RNA-Seq approach) in response to testosterone treatment in the swordtail fish, Xiphophorus hellerii, we aimed to better understand the architecture of the gene regulatory networks underpinning the development of these two evolutionary novelties. Large numbers of genes with tissue-specific expression patterns were identified. Among the "sword genes" those involved in embryonic organ development, sexual character development and coloration were highly expressed, while in the gonopodium rather more morphogenesis-related genes were found. Interestingly, many genes and genetic pathways are shared between both developing novel traits derived from median fins: the sword and the gonopodium. Our analyses show that a larger set of gene networks was co-opted during the development and evolution of the "older" gonopodium than in the "younger," and morphologically less complex trait, the sword. We provide a catalog of candidate genes for future efforts to dissect the development of those sexually selected exaggerated male traits in swordtails.

Brain nonapeptide and gonadal steroid responses to deprivation of heterosexual contact in the black molly

Fish may respond to different social situations with changes in both physiology and behaviour. A unique feature of fish is that social interactions between males and females strongly affect the sexual characteristics of individuals. Here we provide the first insight into the endocrine background of two phenomena that occur in mono-sex groups of the black molly (Poecilia sphenops): masculinization in females and same-sex sexual behaviour, manifested by gonopodial displays towards same-sex tank mates and copulation attempts in males. In socially controlled situations, brain neurohormones impact phenotypic sex determination and sexual behaviour. Among these hormones are the nonapeptides arginine vasotocin (AVT) and isotocin (IT), counterparts of the well-known mammalian arginine vasopressin and oxytocin, respectively. To reveal potential hormone interactions, we measured the concentrations of bioactive AVT and IT in the brain, along with those of the sex steroids 17β-estradiol and 11-ketotestosterone in the gonads, of females, masculinized females, males displaying same-sex sexual behaviour and those who did not. These data were supplemented by morphological and histological analyses of the gonads. Correlations between brain nonapeptides and gonadal steroids strongly suggest a cross talk between hormonal systems. In the black molly, the masculinization process was associated with the production of brain AVT and gonadal steroids, whereas same-sex sexual behaviour involves both brain nonapeptides, but neither of the sex steroids. This study extends current knowledge of endocrine control of phenotypic sex and sexual behaviour in fish and for the first time links brain nonapeptides with the occurrence of male-male sexual behaviour in lower vertebrates.

Ontogenetic stage-specific quantitative trait loci contribute to divergence in developmental trajectories of sexually dimorphic fins between medaka populations

Sexual dimorphism can evolve when males and females differ in phenotypic optima. Genetic constraints can, however, limit the evolution of sexual dimorphism. One possible constraint is derived from alleles expressed in both sexes. Because males and females share most of their genome, shared alleles with different fitness effects between sexes are faced with intralocus sexual conflict. Another potential constraint is derived from genetic correlations between developmental stages. Sexually dimorphic traits are often favoured at adult stages, but selected against as juvenile, so developmental decoupling of traits between ontogenetic stages may be necessary for the evolution of sexual dimorphism in adults. Resolving intralocus conflicts between sexes and ages is therefore a key to the evolution of age-specific expression of sexual dimorphism. We investigated the genetic architecture of divergence in the ontogeny of sexual dimorphism between two populations of the Japanese medaka (Oryzias latipes) that differ in the magnitude of dimorphism in anal and dorsal fin length. Quantitative trait loci (QTL) mapping revealed that few QTL had consistent effects throughout ontogenetic stages and the majority of QTL change the sizes and directions of effects on fin growth rates during ontogeny. We also found that most QTL were sex-specific, suggesting that intralocus sexual conflict is almost resolved. Our results indicate that sex- and age-specific QTL enable the populations to achieve optimal developmental trajectories of sexually dimorphic traits in response to complex natural and sexual selection.

Exposure to paper mill effluent at a site in North Central Florida elicits molecular-level changes in gene expression indicative of progesterone and androgen exposure

Endocrine disrupting compounds (EDCs) are chemicals that negatively impact endocrine system function, with effluent from paper mills one example of this class of chemicals. In Florida, female Eastern mosquitofish (Gambusia holbrooki) have been observed with male secondary sexual characteristics at three paper mill-impacted sites, indicative of EDC exposure, and are still found at one site on the Fenholloway River. The potential impacts that paper mill effluent exposure has on the G. holbrooki endocrine system and the stream ecosystem are unknown. The objective of this study was to use gene expression analysis to determine if exposure to an androgen receptor agonist was occurring and to couple this analysis with in vitro assays to evaluate the presence of androgen and progesterone receptor active chemicals in the Fenholloway River. Focused gene expression analyses of masculinized G. holbrooki from downstream of the Fenholloway River paper mill were indicative of androgen exposure, while genes related to reproduction indicated potential progesterone exposure. Hepatic microarray analysis revealed an increase in the expression of metabolic genes in Fenholloway River fish, with similarities in genes and biological processes compared to G. holbrooki exposed to androgens. Water samples collected downstream of the paper mill and at a reference site indicated that progesterone and androgen receptor active chemicals were present at both sites, which corroborates previous chemical analyses. Results indicate that G. holbrooki downstream of the Fenholloway River paper mill are impacted by a mixture of both androgens and progesterones. This research provides data on the mechanisms of how paper mill effluents in Florida are acting as endocrine disruptors.

A new model army: Emerging fish models to study the genomics of vertebrate Evo-Devo

Many fields of biology--including vertebrate Evo-Devo research--are facing an explosion of genomic and transcriptomic sequence information and a multitude of fish species are now swimming in this "genomic tsunami." Here, we first give an overview of recent developments in sequencing fish genomes and transcriptomes that identify properties of fish genomes requiring particular attention and propose strategies to overcome common challenges in fish genomics. We suggest that the generation of chromosome-level genome assemblies--for which we introduce the term "chromonome"--should be a key component of genomic investigations in fish because they enable large-scale conserved synteny analyses that inform orthology detection, a process critical for connectivity of genomes. Orthology calls in vertebrates, especially in teleost fish, are complicated by divergent evolution of gene repertoires and functions following two rounds of genome duplication in the ancestor of vertebrates and a third round at the base of teleost fish. Second, using examples of spotted gar, basal teleosts, zebrafish-related cyprinids, cavefish, livebearers, icefish, and lobefin fish, we illustrate how next generation sequencing technologies liberate emerging fish systems from genomic ignorance and transform them into a new model army to answer longstanding questions on the genomic and developmental basis of their biodiversity. Finally, we discuss recent progress in the genetic toolbox for the major fish models for functional analysis, zebrafish, and medaka, that can be transferred to many other fish species to study in vivo the functional effect of evolutionary genomic change as Evo-Devo research enters the postgenomic era.

Bmp7 and Lef1 are the downstream effectors of androgen signaling in androgen-induced sex characteristics development in medaka

Androgens play key roles in the morphological specification of male type sex attractive and reproductive organs, whereas little is known about the developmental mechanisms of such secondary sex characters. Medaka offers a clue about sexual differentiation. They show a prominent masculine sexual character for appendage development, the formation of papillary processes in the anal fin, which has been induced in females by exogenous androgen exposure. This current study shows that the development of papillary processes is promoted by androgen-dependent augmentation of bone morphogenic protein 7 (Bmp7) and lymphoid enhancer-binding factor-1 (Lef1). Androgen receptor (AR) subtypes, ARα and ARβ, are expressed in the distal region of outgrowing bone nodules of developing papillary processes. Development of papillary processes concomitant with the induction of Bmp7 and Lef1 in the distal bone nodules by exposure to methyltestosterone was significantly suppressed by an antiandrogen, flutamide, in female medaka. When Bmp signaling was inhibited in methyltestosterone-exposed females by its inhibitor, dorsomorphin, Lef1 expression was suppressed accompanied by reduced proliferation in the distal bone nodules and retarded bone deposition. These observations indicate that androgen-dependent expressions of Bmp7 and Lef1 are required for the bone nodule outgrowth leading to the formation of these secondary sex characteristics in medaka. The formation of androgen-induced papillary processes may provide insights into the mechanisms regulating the specification of sexual features in vertebrates.

A dual role for SHH during phallus development in a marsupial

The mammalian phallus arises from identical primordia in both sexes and is patterned in part by the key morphogen Sonic hedgehog (SHH). We have investigated SHH and other morphogens during phallus development in the tammar wallaby. In this marsupial, testis differentiation and androgen production occurs just after birth, but it takes a further 50-60 days before the phallus becomes sexually dimorphic. One day before birth, SHH was expressed in both sexes in the urethral epithelium. In males, there was a marked upregulation of SHH, GLI2, and AR at day 50 postpartum, a time when testicular androgen production falls. SHH, GLI2, and AR were downregulated in female pouch young treated with androstanediol from days 24-50, but not when treatments were begun at day 29, suggesting an early window of androgen sensitivity. SHH, GLI2, and AR expression in the phallus of males castrated at day 23 did not differ from controls, but there was an increase in SHH and GLI2 and a decrease in FGF8 and BMP4 expression when the animals were castrated at day 29. These results suggest that the early patterning by SHH is androgen-independent followed by an androgen-dependent window of sensitivity and a sharp rise in SHH expression after androgen withdrawal at day 50.

Retinoic acid is involved in the metamorphosis of the anal fin into an intromittent organ, the gonopodium, in the green swordtail (Xiphophorus hellerii)

In poeciliid fish the male anal fin has been transformed into a gonopodium, an intromittent organ required for internal fertilization. Elevated testosterone levels induce metamorphosis of a subset of anal fin rays to grow and form the specialized terminal structures of the gonopodium. The molecular mechanisms underlying these processes are largely unknown. Here, we investigated whether retinoic acid (RA) signaling is involved in gonopodium development in the swordtail Xiphophorus hellerii. We showed that aldh1a2, a RA synthesizing enzyme, and the RA receptors, rar-ga and rar-gb, are expressed in anal fins during metamorphosis. aldh1a2 expression is regulated by testosterone in a concentration-dependent manner and is up-regulated in both hormone-induced and naturally developing gonopodia. Androgen receptor (ar), a putative regulator of gonopodial development, is co-expressed with aldh1a2 and the RA receptors in gonopodial rays. Importantly, experimental increase of RA signaling promoted growth of the gonopodium and increased the number of new segments. Based on gene expression analyses and pharmacological manipulation of gonopodium development, we show that the RA signaling pathway is activated in response to androgen signaling and promotes fin ray growth and development during the metamorphosis of the anal fin into the gonopodium.

Seasonal reproduction of male Gambusia holbrooki (eastern mosquitofish) from two Florida lakes

Sixteen monthly collections of adult male Gambusia holbrooki (eastern mosquitofish) were obtained from two lakes in central Florida, USA. Lake Woodruff and Lake Apopka are 36 miles apart, but differ in several environmental parameters. Compared with Lake Woodruff, Lake Apopka is warmer, more shallow in sampling areas (particularly during drought conditions; approximately 15-90 cm in Lake Apopka versus 60-120 cm in Lake Woodruff), more turbid, and more heavily contaminated with nutrients and industrial and agricultural chemicals. Here, we present detailed information on seasonal reproduction patterns in mosquitofish in their native range and compare patterns between fish from the two lakes. Male mosquitofish were reproductively active from spring through fall. Spermatogenesis, which is regulated in part by 11-ketotestosterone, ceased in October, and fish stored spermatozoa through the winter for immediate fertilization of offspring in the spring. Compared with Lake Woodruff, fish from Lake Apopka tended to be larger and have longer gonopodia and greater gonado- and hepato-somatic indices (GSI and HSI). High GSI in Apopka fish correlated with greater spermatid production, but fewer mature spermatozoa and either the same or lower sperm counts and sperm viability. Taken together, these observations suggest that differentiation of spermatids to spermatozoa is disrupted in Apopka fish, leading to reductions in fertility in some months. Delivery of sperm to females could also be affected in Apopka fish, which exhibit lower prevalence of efferent duct tissue in the testes during the summer.

Custom microarray construction and analysis for determining potential biomarkers of subchronic androgen exposure in the Eastern Mosquitofish (Gambusia holbrooki)

BACKGROUND

The eastern mosquitofish (Gambusia holbrooki) has the potential to become a bioindicator organism of endocrine disrupting chemicals (EDCs) due to its androgen-driven secondary sexual characteristics. However, the lack of molecular information on G. holbrooki hinders its use as a bioindicator coupled with biomarker data. While traditional gene-by-gene approaches provide insight for biomarker development, a holistic analysis would provide more rapid and expansive determination of potential biomarkers. The objective of this study was to develop and utilize a mosquitofish microarray to determine potential biomarkers of subchronic androgen exposure. To achieve this objective, two specific aims were developed: 1) Sequence a G. holbrooki cDNA library, and 2) Use microarray analysis to determine genes that are differentially regulated by subchronic androgen exposure in hepatic tissues of 17β-trenbolone (TB) exposed adult female G. holbrooki.

RESULTS

A normalized library of multiple organs of male and female G. holbrooki was prepared and sequenced by the Illumina GA IIx and Roche 454 XLR70. Over 30,000 genes with e-value ≤ 10⁻⁴ were annotated and 14,758 of these genes were selected for inclusion on the microarray. Hepatic microarray analysis of adult female G. holbrooki exposed to the vehicle control or 1 μg/L of TB (a potent anabolic androgen) revealed 229 genes upregulated and 279 downregulated by TB (one-way ANOVA, p < 0.05, FDR α = 0.05, fold change > 1.5 and < -1.5). Fifteen gene ontology biological processes were enriched by TB exposure (Fisher's Exact Test, p < 0.05). The expression levels of 17β-hydroxysteroid dehydrogenase 3 and zona pellucida glycoprotein 2 were validated by quantitative polymerase chain reaction (qPCR) (Student's t-test, p < 0.05).

CONCLUSIONS

Coupling microarray data with phenotypic changes driven by androgen exposure in mosquitofish is key for developing this organism into a bioindicator for EDCs. Future studies using this array will enhance knowledge of the biology and toxicological response of this species. This work provides a foundation of molecular knowledge and tools that can be used to delve further into understanding the biology of G. holbrooki and how this organism can be used as a bioindicator organism for endocrine disrupting pollutants in the environment.

Effects of 17β-trenbolone on Eastern and Western mosquitofish (Gambusia holbrooki and G. affinis) anal fin growth and gene expression patterns

The Eastern and Western mosquitofish (Gambusia holbrooki and G. affinis) are potential bioindicator organisms for endocrine disruptors. Male mosquitofish have an elongated anal fin (gonopodium) used for internal fertilization whose formation is driven by androgens. Normal female mosquitofish have a normal, rounded anal fin which undergoes elongation into a gonopodium structure when female mosquitofish are exposed to androgenic chemicals. Significant issues with using mosquitofish as a bioindicator include the lack of knowledge on how anal fin growth in females corresponds to endpoints relevant to biological integrity and the lack of information on the molecular pathways that regulate anal fin growth. The objectives of this study were to understand how androgen-induced anal fin elongation relates to changes in endpoints related to the female reproductive system and to understand how anal fin elongation occurs in androgen-exposed female mosquitofish. To achieve these objectives, adult female G. holbrooki were exposed to a vehicle control or one of three doses of the androgen 17β-trenbolone (TB) at nominal concentrations of 0.1, 1 or 10 μg TB/L. Anal fin measurements were taken and livers were used for quantitative polymerase chain reaction analysis of vitellogenin (vtg) mRNA expression at multiple time points. 10 μg TB/L induced anal fin elongation after 7 days of treatment (one-way ANOVA, p<0.05) as did 0.1 and 1 μg TB/L at later time points (one-way ANOVA, p<0.05). 10 μg TB/L significantly reduced hepatic vtg gene expression at all time points assessed (one-way ANOVA, p<0.05). There was no correlation between anal fin elongation levels and vtg gene expression (Spearman's ρ, p>0.05). In a separate experiment, female G. holbrooki and G. affinis were exposed to the vehicle control or 1 μg TB/L. Anal fins were used for qualitative gene expression analysis of the genes sonic hedgehog (shh), muscle segment homeobox C (msxC), and fibroblast growth factor receptor 1 (fgfr1) by in situ hybridization. Shh was expressed in the distal tip of the gonopodium while msxC and fgfr1 were more widely expressed along the same anal fin rays during androgen exposure. These data provide insight into the molecular pathways involved in anal fin elongation and pave the way for future work toward developing the mosquitofish into a bioindicator organism for endocrine disruptors.

Screening for potential effects of endocrine-disrupting chemicals in peri-urban creeks and rivers in Melbourne, Australia using mosquitofish and recombinant receptor-reporter gene assays

Sexually mature male mosquitofish (Gambusia holbrooki) were collected from various sites around Melbourne in 2009 to evaluate the performance of gonopodial indices as a biomarker for endocrine disruption in Melbourne's waterways. The mosquitofish indices assessed were body length (BL), gonopodial length (GL)/BL ratio, ray 4:6 ratio and the absence or presence of hooks and serrae, and these varied between sites. The study was complemented by measurements of estrogenic, retinoid, thyroid and aryl hydrocarbon (AhR) receptor activities of the water. Male mosquitofish were 16.3-21.5 mm in length, and although there was a statistically significant positive relationship showing that bigger fish had longer gonopodia than small fish (r2 = 0.52, p < 0.001), there were few significant differences in GL/BL ratio of fish between sites. Measured estrogenic activity was mostly in the range 0.1-1.7 ng/L EEQ, with one site having much higher levels (~12 ng/L EEQ). Aryl hydrocarbon (AhR) receptor activity was observed in all water samples (7-180 ng/L βNF EQ), although there was no consistent pattern in the level of AhR activity observed, i.e., 'clean' sites were as likely to return a high AhR activity response as urban or wastewater treatment plant (WWTP)-impacted sites. There was no correlation between measurements of receptor actvity and gonopodial length (GL):BL ratio and BL. We conclude that the mosquitofish gonopodia only fulfills part of the criteria for biomarker selection for screening. The mosquitofish indices assessed were cheap and easy-to-perform procedures; however, there is no baseline data from the selected sites to evaluate whether differences in the morpholical indices observed at a site were a result of natural selection in the population or due to estrogenic exposure.

Reproductive impairment of a viviparous fish species inhabiting a freshwater system with anthropogenic impact

The potential threat to animal reproduction by contaminated freshwater systems posed the necessity to identify and develop bioindicators and biomarkers to be used for screening and evaluation of the effects in organisms. The main goal of this work was to determine, through histological analyses and changes in gonopodium morphology, whether a freshwater system polluted by anthropogenic activities-sewage, agricultural, and industrial-could cause alterations at the organ level. We also propose the live-bearing fish, Jenynsia multidentata, as a species suitable to study the effects of contaminated aquatic environments. We compared male fish sampled at two different stations in Suquía River basin (Córdoba, Argentina), both differing in degree of pollution, through liver and testis histology and gonopodial morphometric parameters. The water quality, based on the physicochemical characteristics of the studied stations, varied markedly with a decrease in water quality at the downstream site (station 2). At the highest polluted area, detrimental effects on liver and testis were evidenced on histological analysis. Male individuals from station 2 also presented noticeable structural changes of the anal fin, such as a straight gonopodium and abnormal tip area. The present results demonstrate that a freshwater system polluted by the impacts of anthropogenic activities has detrimental effects to J. multidentata. The alterations registered in individuals from the polluted station indicate an impairment of male reproductive performance and imply a risk for other live-bearing species as well as the entire biodiversity. We consider J. multidentata a sentinel species that is useful to evaluate the potential risk present in the studied basin not only to itself but to other species as well.

Molecular mechanisms of external genitalia development

External genitalia development occurs through a combination of hormone independent, hormone dependent, and endocrine pathways. Perturbation of these pathways can lead to abnormal external genitalia development. We review human and animal mechanisms of normal and abnormal external genitalia development, and we evaluate abnormal mechanisms that lead to hypospadias. We also discuss recent laboratory findings that further our understanding of animal models of hypospadias.

Androgen receptor-beta mRNA levels in different tissues in breeding and post-breeding male and female sticklebacks, Gasterosteus aculeatus

BACKGROUND

Androgens induce male characters by activating androgen receptors (AR). Previous quantitative studies on AR in fishes have been limited to few tissues and/or a single season/reproductive state. The aim of this investigation was to study the possible role of AR-beta expression levels in the control of male traits in the three-spined stickleback. To that end, AR-beta expression levels in major tissues in breeding and post-breeding male and female sticklebacks were examined.

METHODS

AR-beta mRNA levels were quantified in ten tissues; eye, liver, axial muscle, heart, brain, intestine, ovary, testis, kidney and pectoral muscle in six breeding and post-breeding males and females using reverse transcription quantitative PCR.

RESULTS

Breeding in contrast to post-breeding males built nests and showed secondary sexual characters (e.g. kidney hypertrophy) and elevated androgen levels. Post-breeding females had lower ovarian weights and testosterone levels than breeding females. AR-beta was expressed in all studied tissues in both sexes and reproductive states with the highest expression in the gonads and in the kidneys. The kidney is an androgen target organ in sticklebacks, from which breeding males produce the protein spiggin, which is used in nest-building. There was also high AR-beta expression in the intestine, an organ that appears to take over hyperosmo-regulation in fresh water when the kidney hypertrophies in mature males and largely loses this function. The only tissue that showed effects of sex or reproductive state on AR-beta mRNA levels was the kidneys, where post-breeding males displayed higher AR-beta mRNA levels than breeding males.

CONCLUSION

The results indicate that changes in AR-beta mRNA levels play no or little role in changes in androgen dependent traits in the male stickleback.

Endocrine-disrupting effects of spironolactone in female western mosquitofish, Gambusia affinis

The discovery of pharmaceuticals in effluent from wastewater treatment plants and drug manufacturing facilities and in receiving waters has raised environmental concern. Because these compounds are ending up in the environment, it is important to investigate the effects of these compounds on wildlife as well as humans. The present study used a fish model to investigate the endocrine-disrupting effects of spironolactone (SPL), an aldosterone antagonist used as a diuretic, but which also exhibits antiandrogenic effects in humans. A dose-response study measured the effects of SPL on anal fin ray elongation, an androgen-dependent secondary sex trait, and expression of the vitellogenin gene, an estrogen-dependent trait, in female western mosquitofish, Gambusia affinis. Fish were exposed to SPL in the water for 35 d at four nominal concentrations: 10, 100, 250, and 500 nM (4.2, 41.7, 104.1, and 208.3 µg/L, respectively) via the static renewal method. Masculinization of females, as evidenced by development of an elongated and modified anal fin, was observed in the fish exposed to the three highest concentrations. Anal fin elongation was observed in the group exposed to the lowest SPL concentration, but without the development of a tip apparatus. These results confirm the results of a preliminary study that, in contrast to antiandrogenic effects seen in humans, SPL has androgenic and/or antiestrogenic activity in a fish.

Cloning, pharmacological characterization, and expression analysis of Atlantic salmon (Salmo salar L.) nuclear progesterone receptor

To better understand the role(s) of progestogens during early stages of spermatogenesis, we carried out studies on the nuclear progesterone receptor (Pgr) of the Atlantic salmon. Its open-reading frame shows the highest similarity with other piscine Pgr proteins. When expressed in mammalian cells, salmon Pgr exhibited progestogen-specific, dose-dependent induction of reporter gene expression, with 17α,20β-dihydroxy-4-pregnen-3-one (DHP) showing the highest potency. We then analyzed testicular pgr mRNA and DHP plasma levels in animals during the onset of spermatogenesis, which were exposed to natural light or to constant light, to induce significant differences in testis growth. Grouping of the animals according to their progress through spermatogenesis showed that testicular pgr mRNA levels as well as DHP plasma levels first increased when germ cells had reached the stage of late type B spermatogonia and further increased when entered meiosis, i.e. when spermatocytes were present. However, in situ hybridization studies revealed that pgr mRNA expression was restricted to Sertoli cells, with a strong signal in Sertoli cells contacting type A/early type B spermatogonia, while Sertoli cells contacting larger germ cell clones with further differentiated stages (e.g. late type B spermatogonia) were less intensely/not stained. We conclude that the increase in pgr mRNA levels per pair of testis reflects, at least in part, the increased number of Sertoli cells enveloping type A and early type B spermatogonia. We propose that Sertoli cell-expressed Pgr may mediate DHP-stimulated early steps in spermatogenesis in Atlantic salmon, such as an increase in the number of new spermatogonial cysts.