Endocrine control of sexual behavior in teleost fish

Multiple stressors disrupt sex hormones and fitness outcomes: effects of hypoxia and turbidity on an African cichlid fish

Freshwater organisms face a complex array of environmental stressors that can negatively affect endocrine function and subsequent fitness outcomes. Hypoxia and turbidity are two environmental stressors that are increasing due to human activities that could lead to endocrine disruption and reduced reproductive output. Our research addresses how hypoxia and elevated turbidity affect traits related to reproductive success, specifically sex hormone concentrations, investment in reproductive tissues and body size. We used wild fish from two populations (a river and a swamp) of an African cichlid, Pseudocrenilabrus multicolor, to produce offspring that were reared in a full factorial split brood rearing experiment (hypoxic/normoxic × clear/turbid). River and swamp populations represent divergent habitat types with respect to the stressors of interest, being well-oxygenated but turbid or hypoxic and clear, respectively. Overall, we found evidence for plastic responses to both stressors. Specifically, we found that there was an interactive effect of oxygen and turbidity on testosterone in males from both populations. Additionally, males of both populations reared under hypoxic conditions were significantly smaller in both mass and standard length than those raised under normoxic conditions and invested less in reproductive tissues (quantified as gonadosomatic index). Hypoxia and turbidity are experienced naturally by this species, and these environmental stressors did not affect the number of eggs laid by females when experienced in the absence of another stressor (i.e. normoxic/turbid or hypoxic/clear). However, there was an interactive effect of hypoxia and turbidity, as females reared and maintained under this treatment combination laid fewer eggs. This research underscores the importance of considering the possibility of stressor interactions when determining how anthropogenic stressors affect fitness outcomes.

Reproductive morphophysiology of an iteroparous batch spawner fish: from early follicle development to ovulation

Batch spawner fishes develop successive clutches of oocytes which allows them to participate in many reproductive cycles during their adult life (iteroparous) and spawn in multiple events within each breeding cycle. Here, ovarian follicular development was morpho-functionally analyzed in females of the iteroparous batch spawner fish Gymnocorymbus ternetzi. To obtain better insights into the reproductive morpho-physiology in batch spawners, the objective of this research was to analyze the dynamics of the follicular development, with its hormonal regulation between two active reproduction events. We found that over 16 days, follicles progressed asynchronously to chromatin nucleolus, Primary and Secondary growth stages of oogenesis with progressive secretion of 17β-estradiol (E2). During the end of secondary growth, the increase in 17α,20β-dihydroxy-4-pregnen-3-one (17,20β-p) was measured relative to the maturation process of the ovarian follicles (e.g., nuclear migration and its rupture during the resumption of meiosis). Interestingly, an additional increase in E2 was observed after fish reproduction, probably related to the recruitment of new batch follicles for secondary growth. We also measured the high values of multiple condition factor post-reproduction measurements, reflecting more energy invested during the pre-reproductive process. We also quantified high concentrations of 17,20β-p, probably related to the recruitment of a new batch of oogonia to meiosis, presumably secreted by post-ovulatory follicles, after fish reproduction. We finally found that fish without exposure to reproductive stimulus developed a regression phase at day 24, characterized by massive follicle atresia, that allow them to recycle energy and constitutive materials of the follicles invested during oogenesis for another reproductive cycle.

Environmentally relevant estrogens impaired spermatogenesis and sexual behaviors in male and female zebrafish

Environmental estrogens (EEs) are found extensively in natural waters and negatively affect fish reproduction. Research on the reproductive toxicity of EEs mixtures in fish at environmentally relevant concentrations is scarce. In this study, adult male zebrafish were exposed for 60 days to EES (a mixture of EEs), EE2-low (5.55 ng/L, with an estrogenic potency equal to EES), and EE2-high (11.1 ng/L). After exposure, the expression levels of vtg1, vtg3, and esr1 in the livers in EES-treated fish remained unaltered, whereas they were significantly increased in EE2-treated fish. Both EE2-high and EES exposures notably reduced the gonad somatic index and sperm count. A disrupted spermatogenesis was also observed in the testes of EE2-high- and EES-exposed fish, along with an alteration in the expression of genes associated with spermatogonial proliferation (pcna, nanog), cell cycle transition (cyclinb1, cyclind1), and meiosis (aldh1a2, cyp26a1, sycp3). Both EE2 and EES significantly lowered plasma 11-ketotestosterone levels in males, likely by inhibiting the expression level of genes for its synthesis (scc, cyp17a1 and cyp11b2), and increased 17β-estradiol (E2) levels, possibly through upregulating the expression of cyp19a1a. A significant increase in tnfrsf1a expression and the tnfrsf1a/tnfrsf1b ratio in EE2-high and EES-treated males also suggests increased apoptosis via the extrinsic pathway. Further investigation showed that both EE2-high and EES diminished the sexual behavior of male fish, accompanied with reduced E2 levels in the brain and the expression of genes in the kisspeptin/gonadotropin-releasing hormone system. Interestingly, the sexual behavior of unexposed females paired with treated males was also reduced, indicating a synergistic effect. This study suggests that EES have a more severe impact on reproduction than EE2-low, and EEs could interfere not only with spermatogenesis in fish, but also with the sexual behaviors of both exposed males and their female partners, thereby leading to a more significant disruption in fish reproduction.

Copper-oxide nanoparticles effects on goldfish (Carassius auratus): Lethal toxicity, haematological, and biochemical effects

The advancement of nanotechnology and the widespread use of nanoparticles (NPs) in various industries have highlighted the importance of studying the potential harmful effects of nanomaterials on organisms. This study aimed to evaluate the lethal toxicity thresholds of Copper Oxide Nanoparticles (CuO-NPs). The investigation focused on examining the sub-lethal toxicity effects of CuO-NPs on blood parameters, as well as their influence on the gill tissue and liver of goldfish (Carassius auratus). Goldfish were exposed to varying concentrations of CuO-NPs (10, 20, 30, 40, 60, 80, and 100 mg/L) for 96 h. The Probit software was employed to determine the LC50 (lethal concentration causing 50% fish mortality) by monitoring and documenting fish deaths at 24, 48, 72, and 96-hour intervals. Subsequently, sub-lethal concentrations of 5% LC50 (T1), 10% LC50 (T2), and 15% LC50 (T3) of CuO-NPs were administered based on the LC50 level to investigate their effects on haematological parameters, encompassing the number of red blood cells and white blood cells, hematocrit and haemoglobin levels, mean corpuscular volume, mean corpuscular haemoglobin and mean corpuscular haemoglobin concentration. Additionally, histopathological examinations were conducted on the gill and liver tissues of the studied fish. Results indicated concentration-response of fish mortalities. In general, changes in the blood biochemical parameters of fish exposed to sub-lethal concentrations of CuO-NPs included a significant decrease in leukocyte count and glucose level and an increase in protein and triglyceride levels. Furthermore, an escalation in tissue damage such as gill apical and basal hyperplasia, lamellae attachment, squamous cell swelling, blood cell infiltration, and cellular oedema in gills tissue. and bleeding, increased sinusoidal space, necrosis, lateralization of the nucleus, cell swelling, and water retention in the liver. The findings showed dose-dependent increasing toxicity in goldfish specimens exposed to CuO-NPs.

The decision of male medaka to mate or fight depends on two complementary androgen signaling pathways

Adult male animals typically court and attempt to mate with females, while attacking other males. Emerging evidence from mice indicates that neurons expressing the estrogen receptor ESR1 in behaviorally relevant brain regions play a central role in mediating these mutually exclusive behavioral responses to conspecifics. However, the findings in mice are unlikely to apply to vertebrates in general because, in many species other than rodents and some birds, androgens-rather than estrogens-have been implicated in male behaviors. Here, we report that male medaka (Oryzias latipes) lacking one of the two androgen receptor subtypes (Ara) are less aggressive toward other males and instead actively court them, while those lacking the other subtype (Arb) are less motivated to mate with females and conversely attack them. These findings indicate that, in male medaka, the Ara- and Arb-mediated androgen signaling pathways facilitate appropriate behavioral responses, while simultaneously suppressing inappropriate responses, to males and females, respectively. Notably, males lacking either receptor retain the ability to discriminate the sex of conspecifics, suggesting a defect in the subsequent decision-making process to mate or fight. We further show that Ara and Arb are expressed in intermingled but largely distinct populations of neurons, and stimulate the expression of different behaviorally relevant genes including galanin and vasotocin, respectively. Collectively, our results demonstrate that male teleosts make adaptive decisions to mate or fight as a result of the activation of one of two complementary androgen signaling pathways, depending on the sex of the conspecific that they encounter.

Estrogenic influences on agonistic behavior in teleost fishes

Teleost fishes show an extraordinary diversity of sexual patterns, social structures, and sociosexual behaviors. Sex steroid hormones are key modulators of social behaviors in teleosts as in other vertebrates and act on sex steroid receptor-containing brain nuclei that form the evolutionarily conserved vertebrate social behavior network (SBN). Fishes also display important differences relative to tetrapod vertebrates that make them particularly well-suited to study the physiological mechanisms modulating social behavior. Specifically, fishes exhibit high levels of brain aromatization and have what has been proposed to be a lifelong, steroid hormone dependent plasticity in the neural substrates mediating sociosexual behavior. In this review, we examine how estrogenic signaling modulates sociosexual behaviors in teleosts with a particular focus on agonistic behavior. Estrogens have been shown to mediate agonistic behaviors in a broad range of fishes, from sexually monomorphic gonochoristic species to highly dimorphic sex changers with alternate reproductive phenotypes. These similarities across such diverse taxa contribute to a growing body of evidence that estrogens play a crucial role in the modulation of aggression in vertebrates. As analytical techniques and genomic tools rapidly advance, methods such as LC-MS/MS, snRNAseq, and CRISPR-based mutagenesis show great promise to further elucidate the mechanistic basis of estrogenic effects on social behavior in the diverse teleost lineage.

Let's talk about sex: Mechanisms of neural sexual differentiation in Bilateria

In multicellular organisms, sexed gonads have evolved that facilitate release of sperm versus eggs, and bilaterian animals purposefully combine their gametes via mating behaviors. Distinct neural circuits have evolved that control these physically different mating events for animals producing eggs from ovaries versus sperm from testis. In this review, we will describe the developmental mechanisms that sexually differentiate neural circuits across three major clades of bilaterian animals-Ecdysozoa, Deuterosomia, and Lophotrochozoa. While many of the mechanisms inducing somatic and neuronal sex differentiation across these diverse organisms are clade-specific rather than evolutionarily conserved, we develop a common framework for considering the developmental logic of these events and the types of neuronal differences that produce sex-differentiated behaviors. This article is categorized under: Congenital Diseases > Stem Cells and Development Neurological Diseases > Stem Cells and Development.

Mating behaviors in ovoviviparous black rockfish (Sebastes schlegelii): molecular function of prostaglandin E2 as both a hormone and pheromone

Prostaglandins (PGs) are profound hormones in teleost sexual behavior, especially in mating. PGs act as pheromones that affect the olfactory sensory neurons of males, inducing the initiation of a series of mating behaviors. However, the molecular mechanism by which PGs trigger mating behavior in ovoviviparous teleosts is still unclear. In the present study, we employed the ovoviviparous black rockfish (Sebastes schlegelii), an economically important marine species whose reproductive production is limited by incomplete fertilization, as a model species. The results showed that when the dose of PGE2 was higher than 10 nmol/L, a significant (P < 0.05) increase in mating behaviors was observed. Dual-fluorescence in situ hybridization indicated that PGE2 could fire specific neurons in different brain regions and receptor cells in the olfactory sac. After combining with specific neurons in the central nervous system (CNS), a series of genes related to reproduction are activated. The intracerebroventricular administration of PGE2 significantly increased lhb levels (P < 0.05) in both sexes. Moreover, steroidogenesis in gonads was also affected, inducing an increase (P < 0.05) in E2 levels in males and T levels in females. PGE2 levels were also increased significantly (P < 0.05) in both sexes. The present study revealed that PGE2 can activate mating behavior in black rockfish in both hormone and pheromone pathways, leading to variations in sex steroid levels and activation of reproductive behaviors. Our results provide not only novel insight into the onset of mating behaviors in ovoviviparous teleosts but also solutions for the incomplete fertilization caused by natural mating in cage aquaculture.

Supplementary Information

The online version contains supplementary material available at 10.1007/s42995-023-00214-w.

Circadian Rhythm and Endocrinological Control on the Swimming and Sand Burrowing Behaviors of Japanese Sand Lances Ammodytes spp. (Uranoscopiformes, Ammodytidae)

In diurnal and nocturnal organisms, daily activity is regulated by the perception of environmental stimuli and circadian rhythms, which enable organisms to maintain their essential behaviors. The Japanese sand lances genus Ammodytes are coastal marine fish that exhibit unique nocturnal sand burrowing behavior. To elucidate the extrinsic and intrinsic regulation of this behavior and its endocrinological basis, we conducted a series of rearing experiments under various light conditions and hormone administrations. Under a light-dark photoperiod, the fish showed three types of behavior: sand buried, head-exposed from sand, and swimming/feeding. During the transition from dark to light periods, the fish first showed head exposure, followed by swimming and foraging, and buried themselves in the sand immediately after shifting to the dark period. Under constant light conditions, fish exhibited swimming behavior during the period corresponding to the acclimated light period. In addition, swimming did not occur under constant dark conditions but head exposure was observed at the time of the dark-light transition during acclimation. These observations indicate that the essential behavior of sand lances is regulated by both light and circadian rhythms. Subsequently, a melatonin-containing diet promoted the onset of burrowing in 10 to 120 min in a dose-dependent manner at 0.3-128 µg/g-diet, suggesting the direct behavioral regulation by this hormone. These findings suggest that the behavior of sand lances is strictly regulated by an intrinsic mechanism and that melatonin is a regulatory endocrine factor that induces burrowing behavior.

Artificial light at night disrupts male dominance relationships and reproductive success in a model fish species

Environmental light is perceived and anticipated by organisms to synchronize their biological cycles. Therefore, artificial light at night (ALAN) disrupts both diurnal and seasonal biological rhythms. Reproduction is a complex physiological process involving integration of environmental signals by the brain, and release of endocrine signals by the pituitary that regulate gametogenesis and spawning. In addition, males from many species form a dominance hierarchy that, through a combination of aggressive and protective behavior, influences their reproductive success. In this study, we investigated the effect of ALAN and continuous daylight on the behavior and fitness of male fish within a dominance hierarchy using a model fish, the Japanese medaka. In normal light/dark cycles, male medaka establish a hierarchy with the dominant males being more aggressive and remaining closer to the female thus limiting the access of subordinate males to females during spawning. However, determination of the paternity of the progeny revealed that even though subordinate males spend less time with the females, they are, in normal light conditions, equally successful at producing progeny due to an efficient sneaking behavior. Continuous daylight completely inhibited the establishment of male hierarchy, whereas ALAN did not affect it. Nonetheless, when exposed to ALAN, subordinate males fertilize far fewer eggs. Furthermore, we found that when exposed to ALAN, subordinate males produced lower quality sperm than dominant males. Surprisingly, we found no differences in circulating sex steroid levels, pituitary gonadotropin levels, or gonadosomatic index between dominant and subordinate males, neither in control nor ALAN condition. This study is the first to report an effect of ALAN on sperm quality leading to a modification of male fertilization success in any vertebrate. While this work was performed in a model fish species, our results suggest that in urban areas ALAN may impact the genetic diversity of species displaying dominance behavior.

Breaking Through the Bottleneck: Krogh's Principle in Behavioral Neuroendocrinology and the Potential of Gene Editing

In 1929, August Krogh wrote that for every question in biology, there is a species or collection of species in which pursuing such questions is the most appropriate for achieving the deepest insights. Referred to as "Krogh's Principle," these words are a guiding force for many biologists. In practice, Krogh's principle might guide a biologist interested in studying bi-parental care to choose not to use lab mice, in which the female does most of the parenting, but instead study species in which bi-parental care is present and clearly observable, such as in certain poison dart frogs. This approach to pursuing biological questions has been fruitful, with more in-depth insights achievable with new technologies. However, up until recently, an important limitation of Krogh's principle for biologists interested in the functions of certain genes, was certain techniques were only available for a few traditional model organisms such as lab mice, fruit flies (Drosophila melanogaster), zebrafish (Danio rerio) and C. elegans (Caenorhabditis elegans), in which testing the functions of molecular systems on biological processes can be achieved using genetic knockout (KO) and transgenic technology. These methods are typically more precise than other approaches (e.g., pharmacology) commonly used in nontraditional model organisms to address similar questions. Therefore, some of the most in-depth insights into our understanding of the molecular control of these mechanisms have come from a small number of genetically tractable species. Recent advances in gene editing technology such as CRISPR (Clustered Regularly Interspersed Short Palindromic Repeats)/Cas9 gene editing as a laboratory tool has changed the insights achievable for biologists applying Krogh's principle. In this review, we will provide a brief summary on how some researchers of nontraditional model organisms have been able to achieve different levels of experimental precision with limited genetic tractability in their non-traditional model organism in the field of behavioral neuroendocrinology, a field in which understanding tissue and brain-region specific actions of molecules of interest has been a major goal. Then, we will highlight the exciting potential of Krogh's principle using discoveries made in a popular model species of social behavior, the African cichlid fish Astatotilapia burtoni. Specifically, we will focus on insights gained from studies of the control of social status by sex steroid hormones (androgens and estrogens) in A. burtoni that originated during field observations during the 1970s, and have recently culminated in novel insights from CRISPR/Cas9 gene editing in laboratory studies. Our review highlighting discoveries in A. burtoni may function as a roadmap for others using Krogh's principle aiming to incorporate gene editing into their research program. Gene editing is thus a powerful complimentary laboratory tool researchers can use to yield novel insights into understanding the molecular mechanisms of physiology and behavior in non-traditional model organisms.

Zebrafish gonad mutant models reveal neuroendocrine mechanisms of brain sexual dimorphism and male mating behaviors of different brain regions

BACKGROUND

Sexually dimorphic mating behaviors differ between sexes and involve gonadal hormones and possibly sexually dimorphic gene expression in the brain. However, the associations among the brain, gonad, and sexual behavior in teleosts are still unclear. Here, we utilized germ cells-free tdrd12 knockout (KO) zebrafish, and steroid synthesis enzyme cyp17a1-deficient zebrafish to investigate the differences and interplays in the brain-gonad-behavior axis, and the molecular control of brain dimorphism and male mating behaviors.

METHODS

Tdrd12+/-; cyp17a1+/- double heterozygous parents were crossed to obtain tdrd12-/-; cyp17a1+/+ (tdrd12 KO), tdrd12+/+; cyp17a1-/- (cyp17a1 KO), and tdrd12-/-; cyp17a1-/- (double KO) homozygous progenies. Comparative analysis of mating behaviors were evaluated using Viewpoint zebrafish tracking software and sexual traits were thoroughly characterized based on anatomical and histological experiments in these KOs and wild types. The steroid hormone levels (testosterone, 11-ketotestosterone and 17β-estradiol) in the brains, gonads, and serum were measured using ELISA kits. To achieve a higher resolution view of the differences in region-specific expression patterns of the brain, the brains of these KOs, and control male and female fish were dissected into three regions: the forebrain, midbrain, and hindbrain for transcriptomic analysis.

RESULTS

Qualitative analysis of mating behaviors demonstrated that tdrd12-/- fish behaved in the same manner as wild-type males to trigger oviposition behavior, while cyp17a1-/- and double knockout (KO) fish did not exhibit these behaviors. Based on the observation of sex characteristics, mating behaviors and hormone levels in these mutants, we found that the maintenance of secondary sex characteristics and male mating behavior did not depend on the presence of germ cells; rather, they depended mainly on the 11-ketotestosterone and testosterone levels secreted into the brain-gonad regulatory axis. RNA-seq analysis of different brain regions revealed that the brain transcript profile of tdrd12-/- fish was similar to that of wild-type males, especially in the forebrain and midbrain. However, the brain transcript profiles of cyp17a1-/- and double KO fish were distinct from those of wild-type males and were partially biased towards the expression pattern of the female brain. Our results revealed important candidate genes and signaling pathways, such as synaptic signaling/neurotransmission, MAPK signaling, and steroid hormone pathways, that shape brain dimorphism and modulate male mating behavior in zebrafish.

CONCLUSIONS

Our results provide comprehensive analyses and new insights regarding the endogenous interactions in the brain-gonad-behavior axis. Moreover, this study revealed the crucial candidate genes and neural signaling pathways of different brain regions that are involved in modulating brain dimorphism and male mating behavior in zebrafish, which would significantly light up the understanding the neuroendocrine and molecular mechanisms modulating brain dimorphism and male mating behavior in zebrafish and other teleost fish.

Olfactory receptor OR52N2 for PGE2 in mediation of guppy courtship behaviors

Prostaglandins (PGs) are a type of physiologically active unsaturated fatty acids. As an important sex pheromone, PGs play a vital role in regulating the reproductive behaviors of species by mediating nerve and endocrine responses. In this study, guppy (Poecilia reticulate) was used as the model specie to detect the function of PGE₂ in inducing the onset of courtship behaviors. Our results showed that adding PGE₂ into the water environment could activate the courtship behavior of male guppy, indicating that the peripheral olfactory system mediated the PGE₂ function. Thereafter, the open reading frame (ORF) of olfactory receptor or52n2 was cloned, which was 936 bp in length, coding 311 amino acids. As a typical G protein-coupled receptor, OR52N2 had a conservative seven α-helix transmembrane domains. To confirm the regulatory relationship between OR52N2 and PGE₂, dual-luciferase reporter assay was employed to verify the activation of downstream CREB signaling pathways. Results showed that PGE₂ significantly enhanced CRE promoter activity in or52n2 ORF transient transfected HEK-293 T cells. Finally, localization of or52n2 mRNA were observed in ciliated receptor cells of the olfactory epithelium using in situ hybridization. Our results provide a novel insight into sex pheromone signaling transduction in reproductive behavior.

Neural Control of Sexual Behavior in Fish

Many vertebrate species show breeding periods and exhibit series of characteristic species-specific sexual behaviors only during the breeding period. Here, secretion of gonadal sex hormones from the mature gonads has been considered to facilitate sexual behaviors. Thus, the sexual behavior has long been considered to be regulated by neural and hormonal mechanisms. In this review, we discuss recent progress in the study of neural control mechanisms of sexual behavior with a focus on studies using fish, which have often been the favorite animals used by many researchers who study instinctive animal behaviors. We first discuss control mechanisms of sexual behaviors by sex steroids in relation to the anatomical studies of sex steroid-concentrating neurons in various vertebrate brains, which are abundantly distributed in evolutionarily conserved areas such as preoptic area (POA) and anterior hypothalamus. We then focus on another brain area called the ventral telencephalic area, which has also been suggested to contain sex steroid-concentrating neurons and has been implicated in the control of sexual behaviors, especially in teleosts. We also discuss control of sex-specific behaviors and sexual preference influenced by estrogenic signals or by olfactory/pheromonal signals. Finally, we briefly summarize research on the modulatory control of motivation for sexual behaviors by a group of peptidergic neurons called terminal nerve gonadotropin-releasing hormone (TN-GnRH) neurons, which are known to be especially developed in fishes among various vertebrate species.

Embryonic benzophenone-3 exposure inhibited fertility in later-life female zebrafish and altered developmental morphology in offspring embryos

Benzophenone-3 (BP3), an organic UV filter widely used in personal care products, is ubiquitous in aquatic environments. Previous studies have shown that BP3 can interfere with oocytes development in the ovary. The current study was conducted to evaluate the effects of embryonic BP3 exposure on reproductive outcomes in later life. Zebrafish embryos were exposed to different concentrations of BP3 (0, 1, 10, 100 μg/L) for 5 days in the developmental stage and subsequently fed for 4 months without any toxins. The body length, body weight, and ovary weight in F0 female adult zebrafish and morphology indices in F1 offspring embryos were measured. The reproductive behaviors of adult zebrafish were recorded by a digital camera. HE staining was used to estimate the development of oocytes and the proportion of different phases was calculated. qPCR was used to detect the expression levels of reproduction-related genes of the hypothalamic-pituitary-gonadal (HPG) axis. Our findings revealed that the body length and body weight were not changed with embryonic BP3 exposure, but BP3 exposure inhibited the development and maturation of ovaries in later-life female zebrafish, accompanied by an increased proportion of follicles in the primary growth and early vitellogenic stages, and a decline in the full-growth stage in ovaries. Meanwhile, reduced egg production, delayed hatching rate, altered somite count and increased mortality rate were observed at 100 μg/L in offspring embryos. Behavioral results showed that BP3 exposure reduced the frequency of chasing, touching, entering the spawning area, and the duration of fish entering the spawning area later in life, qPCR analysis showed that the expression levels of reproduction-related genes of the HPG axis were downregulated in females, following a decreasing trend in plasma E2 and 11-KT levels. These results suggested that embryonic BP3 exposure negatively affected the fertility of fish and the development of their offspring embryos, which may cause potential risks to aquatic ecosystems and human health.

Social behaviors as welfare indicators in teleost fish

Animal welfare is a key issue not only for aquaculture industry and food production, but also for daily husbandry practices in research topics related to physiology in wild and farmed animals. In this context, teleost fish constitute interesting models to assess alternative welfare indicators because of their wide diversity in reproductive and social structures. Any framework for assessing teleost fish welfare needs to account for the physiological mechanisms involved in each species as a first step. A comprehensive approach should also take into account how these physiological and behavioral parameters can be altered by environmental enrichment considering the specific requirements in each case and identifying intrinsic biological characteristics of individual species. This review will show how cortisol and sex steroids regulate social behavior in teleost fish, and how different aspects of social behavior can be employed as welfare indicators according to specific characteristics in each case. This article will consider evidence in teleost fish, including cichlids, characids and cyprinids with different reproductive strategies and social structures (e.g., territorial social hierarchies or shoaling behavior). Neotropical species will be particularly emphasized. The main laboratory-based animal welfare indicators are cortisol, a classical stress hormone, together with sex steroids. Considering that the endocrine landscape is intrinsically related to social behavior, reproductive and agonistic behavioral traits such as aggression, anxiety and courtship are key elements to assess welfare under housing and culture conditions. This review highlights the importance of assessing physiological mechanisms and identifying behavioral characteristics in teleost fish, especially in Neotropical species, as a baseline to understand which environmental enrichment can improve animal welfare in each individual species.

Prostaglandin E2 receptor Ptger4b regulates female-specific peptidergic neurons and female sexual receptivity in medaka

In vertebrates, female receptivity to male courtship is highly dependent on ovarian secretion of estrogens and prostaglandins. We recently identified female-specific neurons in the medaka (Oryzias latipes) preoptic area that express Npba, a neuropeptide mediating female sexual receptivity, in response to ovarian estrogens. Here we show by transcriptomic analysis that these neurons express a multitude of neuropeptides, in addition to Npba, in an ovarian-dependent manner, and we thus termed them female-specific, sex steroid-responsive peptidergic (FeSP) neurons. Our results further revealed that FeSP neurons express a prostaglandin E₂ receptor gene, ptger4b, in an ovarian estrogen-dependent manner. Behavioral and physiological examination of ptger4b-deficient female medaka found that they exhibit increased sexual receptivity while retaining normal ovarian function and that their FeSP neurons have reduced firing activity and impaired neuropeptide release. Collectively, this work provides evidence that prostaglandin E₂/Ptger4b signaling mediates the estrogenic regulation of FeSP neuron activity and female sexual receptivity.

Prostaglandin E₂ signaling mediates the estrogenic regulation of peptidergic neuronal activity and female receptivity via the ptger4b gene pathway in Japanese rice fish.

The masculinization steroid milieu caused by fluorene-9-bisphenol disrupts sex-typical courtship behavior in female zebrafish (Danio rerio)

In vertebrates, the behavior of congenital sex differences between males and females is highly dependent on steroid signals and hormonal milieu. The presence of endocrine disrupting chemicals (EDCs) in the environment generally plays a similar role to sex hormones, so its interference with aquatic organism population stability can not be ignored and is worth studying. Fluorene-9-bisphenol (BHPF) has been clarified as an endocrine disruptor on organisms by several studies but its mechanism in perturbation of courtship behavior of female zebrafish is not clear. Here, we proposed an automated multi-zebrafish tracking method quantifying the courtship process and reported that zebrafish females exposed to BHPF, are not receptive to males but rather court females, and lose normal ovarian function with an altered sex steroid milieu. Our results showed that BHPF damaged 17β-estradiol synthesis by down-regulation of sox3 and cyp19a1a, linking apoptosis with ovary development and female fecundity. The down-regulated expression of estrogen signaling through an estrogen receptor, esr2b, caused the induction of masculinization of female courtship behavior and sexual preference in zebrafish females after BHPF treatment. This process might be mediated by inhibiting the transcription of a neuropeptide B (npb) in the brain. Our study reveals that the estrogen signaling pathway may play an important role in classical courtship behavior and sexual preference of zebrafish. This study provided evidence that anti-estrogenic chemical exposure caused adverse effects on the regulation of the brain-gonad-estrogen axis of aquatic organisms, which should be of concern and highlighted the importance of controlling environmental contamination.

Cloning of gonadotropin Gph-alpha, FSH-beta and LH-beta subunits and seasonal profiles of steroid hormones in wild-caught Nile perch, Lates niloticus

The Nile perch (np; Lates niloticus) is a freshwater teleost species with a potential for aquaculture in freshwater surroundings. However, wild-caught breeders have persistently failed to spawn spontaneously in captivity. Cloning of the gonadotropin subunits and analysing seasonal variation in reproductive hormone levels for a 1-year period were done to gain knowledge on the physiological basis underlying the reproductive biology of np. The β-follicle-stimulating hormone (FSH-β) and β-luteinizing hormone (LH-β) subunits and their common α-glycoprotein (Gph-α) subunit were cloned using 3' and 5' RACE-PCR. The nucleotide sequences of the npgph-α, npfsh-β, and nplh-β subunits were 664, 580 and 675 nucleotides in length, encoding peptides of 124, 120 and 148 amino acids, respectively. The deduced amino acid sequence of each mature subunit showed high similarity with its counterparts in other teleost. Sequence analysis showed that npFSH-β is more similar to higher vertebrate FSH-βs than to higher vertebrate LH-βs. Heterologous immunoassay was calibrated to analyse pituitary LH levels. While the LH immunoassay showed parallelism of npLH with that of tilapia (ta), no parallelism for FSH was found. Levels of pituitary LH were higher in females at gonadal stages of vitellogenic oocytes, mature secondary oocytes and mature tertiary oocytes with migrating nucleus than in pre-vitellogenic oocytes and early and late perinucleolus oocytes. Using competitive steroid ELISA, variations in the levels of the steroid hormones 11-ketotestosterone (11-KT) in males and E2 in females were characterized in relation to month and reproductive index of Nile perch. Our findings show that in females, gonadosomatic index and plasma E2 were highly correlated (R² = 0.699, n = 172) and peaked from September to November while in males, the gonadosomatic index and plasma 11-KT peaked from October to November. In female fish, both steroid hormones were detected in the plasma but greatly varied in concentrations. E2 in particular, increased with the developmental stage of the gonads. The levels of steroid hormones, E2 and 11-KT in females and males respectively increased with fish size (total lengths) and suggest that females mature at a body length of 40-59 cm than their counter part males that mature at a total length of 60-70 cm. Taken together, we describe seasonal endocrine differences in wild-caught adult Nile perch which could potentially be exploited to manipulate the reproductive axis in cultured breeders.

Morphological description of gametes in cave and surface populations of Astyanax mexicanus (De Filippi, 1853)

The Mexican tetra Astyanax mexicanus presents two contrasting morphs, a widely distributed surface morph and a cave-adapted morph. These cave-adapted morphs have evolved independently from two different lineages (i.e. 'old' and 'new' lineages); therefore, this model system gives a unique opportunity to explore parallel adaptive evolution in biological traits. The present study corresponds to the first morphological description of the Astyanax mexicanus maturation process of the spermatozoa and oocytes, using thermal and hormonal stimuli to promote spermatogenesis and oogenesis, considering surface and cave morphs from both lineages. We corroborate the relevance of thermal and hormonal stimuli to promote gamete maturation. The hormone Ovaprim (GnRHa + Domperidone) is an effective promoter of ovarian development, maturation end in oocytes and spawning in Astyanax mexicanus. The sperm morphology of Astyanax mexicanus includes the sperm head, the midpiece, and tail or flagellum. We found differences in the spermatozoan total length between environments (F = 9.929, P = 0.05) and linages (F = 49.86, P = 0.005). The oocytes showed a spherical conformation with a mean diameter of 822.4 ± 194.1 μm for the surface populations, and 604.6 ± 38.3 µm for the cave populations. The oocyte chorion presents ridges and grooves that are arranged radially towards the micropyle. A plug in the micropyle zone was observed after fertilization, confirmed by the outer membrane of the chorion, which provides some weak adhesiveness to the substrate. We observed differences in chorion thickness between the contrasting environmental conditions. This is the first morphological characterization of the Sótanos Vázquez, Escondido and Tigre, which previous to this study were only known from speleological expeditions, with no previous biological information available.

Anguillid Eels as a Model Species for Understanding Endocrinological Influences on the Onset of Spawning Migration of Fishes

Simple Summary

Endocrine regulation has been thought to play a major role in the onset of migration. Anguillid eels provide a good model for studying the onset mechanisms of migrations to breeding areas, because the process of the onset of migration occurs in inland waters. In this review, we summarize information about the silvering process in anguillid eels and the dynamics of mRNA expression of neurohormones and pituitary hormones, thyroid hormones, and sex steroids associated with the onset of the spawning migration. We also provide new results. Because 11-KT drastically increases during silvering, the role of 11-KT in the onset of spawning migration was discussed in detail.

Abstract

Anguillid eels are the iconic example of catadromous fishes, because of their long-distance offshore spawning migrations. They are also a good model for research on the onset mechanisms of migrations to breeding areas, because the migrations begin in inland waters. When eels transform from yellow eels to silver eels, it is called silvering. Silver eels show various synchronous external and internal changes during silvering, that include coloration changes, eye-size increases, and gonadal development, which appear to be pre-adaptations to the oceanic environment and for reproductive maturation. A strong gonadotropic axis activation occurs during silvering, whereas somatotropic and thyrotropic axes are not activated. Among various hormones, 11-ketotestosterone (11-KT) drastically increases during spawning migration onset. Gradual water temperature decreases simulating the autumn migratory season, inducing 11-KT increases. Administration of 11-KT appeared to cause changes related to silvering, such as early-stage oocyte growth and eye enlargement. Moreover, 11-KT may be an endogenous factor that elevates the migratory drive needed for the spawning migration onset. These findings suggested that water temperature decreases cause 11-KT to increase in autumn and this induces silvering and increases migratory drive. In addition, we newly report that 11-KT is associated with a corticotropin-releasing hormone that influences migratory behavior of salmonids. This evidence that 11-KT might be among the most important factors in the spawning migration onset of anguillid eels can help provide useful knowledge for understanding endocrinological mechanisms of the initiation of spawning migrations.

Petroleum-derived naphthenic acids disrupt hormone-dependent sexual behaviours in male Western clawed frogs

Naphthenic acids (NAs), the carboxylic acids found in petroleum, are of emerging concern as they contaminate coastlines after oil spills, leech into freshwater ecosystems of oil sands areas and have wide industrial applications. They are acutely toxic in fish and tadpoles and may be endocrine disruptors at sublethal levels. We characterized androgen-dependent courtship behaviours and their disruption by NAs in male Western clawed frogs, Silurana tropicalis. Courtship primarily consists of males producing low trills and achieving amplexus, a mating position where a male clasps a female. Adult males were exposed for 5 days to 20 mg/l NA and injected with human chorionic gonadotropin to induce calling. The duration of calling activity was significantly reduced by NA exposure. Other acoustic parameters such as dominant frequency, click rate and trill length were not affected. Vocalization and amplexus were both inhibited after NA exposure and restored after 2 weeks of recovery in clean water. To determine possible disruption at the level of the testes, the effects of NA exposure on gene expression of key players in steroidogenesis was determined. Exposure to NAs decreased srd5a on average by ~ 25%. The enzyme 5α-reductase, encoded by srd5a, converts testosterone to its more bioactive form 5α-dihydrotestosterone (DHT), so NAs may be affecting this steroidogenic step. However, the observed upregulation of lhr, star and cyp17a1 suggests that NA-exposed males may be attempting to counteract the reduced potential to produce DHT. Yet, these NA-exposed frogs have dramatically reduced calling duration, so the observed upregulation of star and cyp17a1 is decoupled from the vocalizations. Calling duration and the ability of males to amplex females is reversibly disrupted by NA exposure, implying that environmental reduction and removal of NAs may help improve habitability of contaminated ecosystems.

Involvement of IGF-1R-PI3K-AKT-mTOR pathway in increased number of GnRH3 neurons during androgen-induced sex reversal of the brain in female tilapia

The neuroplastic mechanism of sex reversal in the fish brain remains unclear due to the difficulty in identifying the key neurons involved. Mozambique tilapia show different reproductive behaviours between sexes; males build circular breeding nests while females hold and brood fertilized eggs in their mouth. In tilapia, gonadotropin-releasing hormone 3 (GnRH3) neurons, located in the terminal nerve, regulate male reproductive behaviour. Mature males have more GnRH3 neurons than mature females, and these neurons have been indicated to play a key role in the androgen-induced female-to-male sex reversal of the brain. We aimed to elucidate the signalling pathway involved in the androgen-induced increase in GnRH3 neurons in mature female tilapia. Applying inhibitors to organotypic cultures of brain slices, we showed that the insulin-like growth factor (IGF)-1 receptor (IGF-1R)/PI3K/AKT/mTOR pathway contributed to the androgen-induced increase in GnRH3 neurons. The involvement of IGF-1 and IGF-1R in 11-ketotestosterone (11-KT)-induced development of GnRH3 neurons was supported by an increase in Igf-1 mRNA shortly after 11-KT treatment, the increase of GnRH3 neurons after IGF-1 treatment and the expression of IGF-1R in GnRH3 neurons. Our findings highlight the involvement of IGF-1 and its downstream signalling pathway in the sex reversal of the tilapia brain.

Role of Habenula in Social and Reproductive Behaviors in Fish: Comparison With Mammals

Social behaviors such as mating, parenting, fighting, and avoiding are essential functions as a communication tool in social animals, and are critical for the survival of individuals and species. Social behaviors are controlled by a complex circuitry that comprises several key social brain regions, which is called the social behavior network (SBN). The SBN further integrates social information with external and internal factors to select appropriate behavioral responses to social circumstances, called social decision-making. The social decision-making network (SDMN) and SBN are structurally, neurochemically and functionally conserved in vertebrates. The social decision-making process is also closely influenced by emotional assessment. The habenula has recently been recognized as a crucial center for emotion-associated adaptation behaviors. Here we review the potential role of the habenula in social function with a special emphasis on fish studies. Further, based on evolutional, molecular, morphological, and behavioral perspectives, we discuss the crucial role of the habenula in the vertebrate SDMN.

Familiarity reduces aggression but does not modify acoustic communication in pairs of Nile tilapia (Oreochromis niloticus) and black-chinned tilapia (Sarotherodon melanotheron)

Reproduction involves multiple complex behaviours, and the effects of familiarity on such social interactions are seldom described in fish. This is particularly true for sound production and communication within aggressive or non-aggressive context. This study explores the effects of a common garden rearing without parental care of two closely related cichlid species (Nile tilapia Oreochromis niloticus and black-chinned tilapia Sarotherodon melanotheron) on their sound production features and social interactions. After 9 months in common garden rearing, from embryonic stage to first maturity, sound production and associated behaviours were recorded on specimens of the two species in intraspecific and interspecific pairings. The authors found that fish were able to produce the same kind of sounds as those recorded in similar context for their parents. Drum sounds were associated to chasing, lateral attack and courtship in O. niloticus and only to fleeing or avoidance in S. melanotheron. Specific grunts were produced in chasing, after biting and in nest building by O. niloticus, and specific rolling sounds were associated to courtship in S. melanotheron. Sound production and behaviours were not correlated to sex steroid levels, but the number of sounds recorded in aggressive context was correlated to dominance in O. niloticus. The authors conclude that one generation of common garden rearing does not modify sound features, which remain specific and innate in the two cichlids. Despite the familiarity, O. niloticus remained dominant on S. melanotheron, but the aggressiveness between the two species decreased.

Identification of neurohypophysial hormones and the role of VT in the parturition of pregnant seahorses (Hippocampus erectus)

Neurohypophysial hormones regulate the reproductive behavior of teleosts; however, their role in the gestation and parturition of ovoviviparous fishes with male pregnancy (syngnathids) remains to be demonstrated. In the present study, the complementary DNA (cDNA) sequences of arginine vasotocin (VT) and isotocin (IT) from the lined seahorse (Hippocampus erectus) were cloned and identified. We observed that the mature core peptides of seahorse VT and IT were conserved among teleosts. In the phylogenic tree, seahorse VT and IT were clustered independently with teleost VT and IT. The tissue distribution patterns of VT and IT were similar, and both were highly expressed in the brain, gills, and gonads. Interestingly, they were also expressed to some extent in the brood pouch. In situ hybridization revealed that VT and IT messenger RNA (mRNA) signals in the brain were mainly located in the preoptic area region of the hypothalamus. Intraperitoneal administration of the VT core peptide to pregnant seahorses induced premature parturition, stimulated gonadotropin release, increased serum estrogen levels, and decreased prolactin secretion. Moreover, VT injection upregulated the mRNA expression of the membrane estrogen receptor in the brood pouch. In summary, neurohypophysial hormones promote premature parturition by regulating estrogen synthesis through the hypothalamus-pituitary-gonad axis.

Dynamics of Spermatogenesis and Change in Testicular Morphology under 'Mating' and 'Non-Mating' Conditions in Medaka (Oryzias latipes)

Here, we report that the gross morphology of the testes changes under 'non-mating' or 'mating' conditions in medaka (Oryzias latipes). During these conditions, an efferent duct expands and a histological unit of spermatogenesis, the lobule, increases its number under 'non-mating' conditions. Based on BrdU labeling experiments, lower mitotic activity occurs in gonial cells under 'non-mating' conditions, which is consistent with the reduced number of germ cell cysts. Interestingly, the total number of type A spermatogonia was maintained, regardless of the mating conditions. In addition, the transition from mitosis to meiosis may have been retarded under the 'non-mating' conditions. The minimum time required for germ cells to become sperm, from the onset of commitment to spermatogenesis, was approximately 14 days in vivo. The time was not found to significantly differ between 'non-mating' and 'mating' conditions. The collective data suggest the presence of a mechanism wherein the homeostasis of spermatogenesis is altered in response to the mating conditions.

The endocrine disruptor 17β-trenbolone alters the relationship between pre- and post-copulatory sexual traits in male mosquitofish (Gambusia holbrooki)

It is now well-established that reproduction in wildlife can be disrupted by anthropogenic environmental changes, such as chemical pollution. However, very little is known about how these pollutants might affect the interplay between pre- and post-copulatory mechanisms of sexual selection. Here, we investigated the impacts of 21-day exposure of male eastern mosquitofish (Gambusia holbrooki) to a field-realistic level (average measured concentration: 11 ng/L) of the endocrine-disrupting chemical 17β-trenbolone (17β-TB) on pre- and post-copulatory reproductive traits. We examined male reproductive behaviour by testing the time spent near a female behind a partition, as well as the number of copulation attempts made, and the time spent chasing a female in a free-swimming context. Sperm traits were also assayed for all males. We found that exposure of male fish to 17β-TB altered the relationship between key pre- and post-copulatory reproductive traits. Furthermore, 17β-TB-exposed males had, on average, a higher percentage of motile sperm, and performed fewer copulation attempts than unexposed males. However, there was no overall effect of 17β-TB exposure on either the time males spent associating with or chasing females. Taken together, our findings demonstrate the potential for chemical pollutants to affect both pre- and post-copulatory sexual traits, and the interplay between these mechanisms of sexual selection in contaminated wildlife.

ADAMTS1 is regulated by the EP4 receptor in the zebrafish ovary

Prostaglandins (PGs) are a class of fatty-acid derived hormones that are essential in ovulation of teleosts, but their exact role remains unknown. One putative target of PGs in ovulation is regulation of the expression of members of the A Disintegrin and Metalloproteinase with Thrombospondin motifs (ADAMTS) family, which are implicated in follicular rupture. This study investigated the regulation of ADAMTS, other proteases, and their inhibitors in response to treatment with PGE₂ or PGF_2α. Four members of the ADAMTS family, ADAMTS1, ADAMTS5, ADAMTS9, and ADAMTS16 were shown to be expressed in the ovary of zebrafish, but only adamts1 was upregulated in full-grown follicles following treatment with PGE₂. Inhibitors of the PG receptors EP1 and EP2 had no effect on PGE₂-stimulated adamts1 expression, while treatment of full-grown follicles with both PGE₂ and GW627368x, an inhibitor of EP4 function, prevented the PGE₂-induced increase in adamts1 expression. Treatment of full-grown follicles with the maturation-inducing hormone 17α,20β-dihydroxy-4-pregnen-3-one (17,20β-P) in vitro had no effect on the expression of adamts1 mRNA. These findings suggest that expression of ADAMTS1 in zebrafish ovarian follicles is regulated by the prostaglandin PGE₂ via the EP4 series prostaglandin receptor.

Is the use of recombinant cGnRH may be a future alternative to control the fish spawning? Let us go with the goldfish example

The use of recombinant gonadotropin-releasing hormone (rGnRH) has very rarely been tested in fish to promote spawning. This study evaluated the impact of recombinant chicken gonadotropin-releasing hormone (rcGnRH) with metoclopramide on the release of sex steroids and final maturation induction in goldfish (Carassius auratus) broodstock. For this purpose, goldfish broodstock was divided into four groups and treated with 0.9% NaCl with 20 mg/kg metoclopramide (Met) (C); 10 μg/kg body weight (BW) rcGnRH with 20 mg/kg metoclopramide (rcGn10); 15 μg/kg BW rcGnRH with 20 mg/kg metoclopramide (rcGn15); and 20 μg/kg BW rcGnRH with 20 mg/kg metoclopramide (rcGn20). The capability of the rcGnRH for eliciting biological response was tested in vivo by evaluating the changes of 17β estradiol (E2), testosterone (T), and 17α, 20β-dihydroxy-4-pregnen-3-one (DHP) and the induced spawning. Blood samples were obtained at 0 h, 12 h, and 24 h after injection. The rcGn10, rcGn15, and rcGn20 treatments induced lower E2 concentration, especially 24 h post-injection. T levels were significantly higher in rcGn10, rcGn15, and rcGn20 treatments 12 h post-injection than at 0 h and then decreased at 24 h post-injection. Furthermore, the rcGnRH tested significantly enhanced DHP secretion in rcGn10, rcGn15, and rcGn20 treatments 12 h post-injection before a decline at 24 h post-injection. No significant difference between the sampling times was found in the C treatment for the 3 sex steroids tested. The results also displayed that rcGnRH at 10-20 µg/kg of body weight can trigger spawning with the highest speed and efficiency of spawning at 20 µg/kg. The obtained results represent a possible strategy for enhancing the artificial reproduction and ovulation of broodstock fish by rGnRH and further support the use of recombinant hormones to promote reproduction in aquaculture.

A role for lakes in revealing the nature of animal movement using high dimensional telemetry systems

Movement ecology is increasingly relying on experimental approaches and hypothesis testing to reveal how, when, where, why, and which animals move. Movement of megafauna is inherently interesting but many of the fundamental questions of movement ecology can be efficiently tested in study systems with high degrees of control. Lakes can be seen as microcosms for studying ecological processes and the use of high-resolution positioning systems to triangulate exact coordinates of fish, along with sensors that relay information about depth, temperature, acceleration, predation, and more, can be used to answer some of movement ecology's most pressing questions. We describe how key questions in animal movement have been approached and how experiments can be designed to gather information about movement processes to answer questions about the physiological, genetic, and environmental drivers of movement using lakes. We submit that whole lake telemetry studies have a key role to play not only in movement ecology but more broadly in biology as key scientific arenas for knowledge advancement. New hardware for tracking aquatic animals and statistical tools for understanding the processes underlying detection data will continue to advance the potential for revealing the paradigms that govern movement and biological phenomena not just within lakes but in other realms spanning lands and oceans.

Cystic proliferation of germline stem cells is necessary to reproductive success and normal mating behavior in medaka

The production of an adequate number of gametes is necessary for normal reproduction, for which the regulation of proliferation from early gonadal development to adulthood is key in both sexes. Cystic proliferation of germline stem cells is an especially important step prior to the beginning of meiosis; however, the molecular regulators of this proliferation remain elusive in vertebrates. Here, we report that ndrg1b is an important regulator of cystic proliferation in medaka. We generated mutants of ndrg1b that led to a disruption of cystic proliferation of germ cells. This loss of cystic proliferation was observed from embryogenic to adult stages, impacting the success of gamete production and reproductive parameters such as spawning and fertilization. Interestingly, the depletion of cystic proliferation also impacted male sexual behavior, with a decrease of mating vigor. These data illustrate why it is also necessary to consider gamete production capacity in order to analyze reproductive behavior.

Estrogen receptor 2b is the major determinant of sex-typical mating behavior and sexual preference in medaka

Male and female animals typically display innate sex-specific mating behaviors, which, in vertebrates, are highly dependent on sex steroid signaling. While estradiol-17β (E2) signaling through estrogen receptor 2 (ESR2) serves to defeminize male mating behavior in rodents, the available evidence suggests that E2 signaling is not required in teleosts for either male or female mating behavior. Here, we report that female medaka deficient for Esr2b, a teleost ortholog of ESR2, are not receptive to males but rather court females, despite retaining normal ovarian function with an unaltered sex steroid milieu. Thus, contrary to both prevailing views in rodents and teleosts, E2/Esr2b signaling in the brain plays a decisive role in feminization and demasculinization of female mating behavior and sexual preference in medaka. Further behavioral testing showed that mutual antagonism between E2/Esr2b signaling and androgen receptor-mediated androgen signaling in adulthood induces and actively maintains sex-typical mating behaviors and preference. Our results also revealed that the female-biased sexual dimorphism in esr2b expression in the telencephalic and preoptic nuclei implicated in mating behavior can be reversed between males and females by altering the sex steroid milieu in adulthood, likely via mechanisms involving direct E2-induced transcriptional activation. In addition, Npba, a neuropeptide mediating female sexual receptivity, was found to act downstream of E2/Esr2b signaling in these brain nuclei. Collectively, these functional and regulatory mechanisms of E2/Esr2b signaling presumably underpin the neural mechanism for induction, maintenance, and reversal of sex-typical mating behaviors and sexual preference in teleosts, at least in medaka.

Suppression and recovery of reproductive behavior induced by early life exposure to mercury in zebrafish

While mercury (Hg)-induced reproductive impairments have been demonstrated in fishes, the effects of exposure to Hg²⁺ during early life stages on the reproductive behavior in adulthood and the persistency of these effects in the next generation remain largely unknown. In this study, zebrafish embryos were exposed to 0.6, 3, or 15 μg·L^-1 Hg²⁺ for 5 days and then reared for an additional 115 days in clean water, from which embryos were obtained and cultured in clean water for 120 days as the F₁ generation. Increased Hg levels in brains and decreased survival and growth were observed in individuals exposed to Hg²⁺ during early life stages. Early life exposure to Hg²⁺ reduced the frequency of touching in males as well as the frequency and duration of visits to the spawning area by females, males, or both sexes simultaneously, and resulted in lesser spawning and fertilization. Moreover, early life exposure to Hg²⁺ interfered with the transcription of genes encoding neuropeptides and hormones related to reproduction, which could be responsible for diminished sexual behavior and reduced reproductive outcomes. In the F₁ generation, such alterations were not observed in either females or males, indicating that the disruption of normal patterns of reproductive behavior caused by early life exposure to Hg²⁺ did not persist and was recovered. Overall, this study demonstrated that exposure to Hg²⁺ during early life stages suppressed the reproductive behavior of adult fish but this disruption could be recovered in the F₁ generation.

Fish reproductive biology - Reflecting on five decades of fundamental and translational research

Driven by the broad diversity of species and physiologies and by reproduction-related bottlenecks in aquaculture, the field of fish reproductive biology has rapidly grown over the last five decades. This review provides my perspective on the field during this period, integrating fundamental and applied developments and milestones. Our basic understanding of the brain-pituitary-gonadal axis led to overcoming the failure of farmed fish to ovulate and spawn in captivity, allowing us to close the fish life cycle and establish a predictable, year-round production of eggs. Dissecting the molecular and hormonal mechanisms associated with sex determination and differentiation drove technologies for producing better performing mono-sex and reproductively-sterile fish. The growing contingent of passionate fish biologists, together with the availability of innovative platforms such as transgenesis and gene editing, as well as new models such as the zebrafish and medaka, have generated many discoveries, also leading to new insights of reproductive biology in higher vertebrates including humans. Consequently, fish have now been widely accepted as vertebrate reproductive models. Perhaps the best testament of the progress in our discipline is demonstrated at the International Symposia on Reproductive Physiology of Fish (ISRPF), at which our scientific family has convened every four years since the grandfather of the field, the late Ronald Billard, organized the inaugural 1977 meeting in Paimpont, France. As the one person who has been fortunate enough to attend all of these meetings since their inception, I have witnessed first-hand the astounding evolution of our field as we capitalized on the molecular and biotechnological revolutions in the life sciences, which enabled us to provide a higher resolution of fish reproductive and endocrine processes, answer more questions, and dive into deeper comprehension. Undoubtedly, the next (five) decades will be similarly exciting as we continue to integrate physiology with genomics, basic and translational research, and the small fish models with the aquacultured species.

Dissection of the Ovulatory Process Using ex vivo Approaches

Ovulation is a unique physiological phenomenon that is essential for sexual reproduction. It refers to the entire process of ovarian follicle responses to hormonal stimulation resulting in the release of mature fertilization-competent oocytes from the follicles and ovaries. Remarkably, ovulation in different species can be reproduced out-of-body with high fidelity. Moreover, most of the molecular mechanisms and signaling pathways engaged in this process have been delineated using in vitro ovulation models. Here, we provide an overview of the major molecular and cytological events of ovulation observed in frogs, primarily in the African clawed frog Xenopus laevis, using mainly ex vivo approaches, with the focus on meiotic oocyte maturation and follicle rupture. For the purpose of comparison and generalization, we also refer extensively to ovulation in other biological species, most notoriously, in mammals.

Perfluorooctane sulfonate exposure alters sexual behaviors and transcriptions of genes in hypothalamic-pituitary-gonadal-liver axis of male zebrafish (Danio rerio)

BACKGROUND

Perfluorooctane sulfonate (PFOS) has been reported to be widely distributed in the environment and wildlife with persistence. PFOS has various biological toxicity, especially disturbing the endocrine system. However, few studies have systematically evaluated its effect on sexual behaviors alteration and reproduction-related genes. This study was performed to assess the effect of PFOS exposure on sexual behaviors and genes in hypothalamic-pituitary-gonadal-liver (HPGL) axis in adult zebrafish.

METHODS

Male adult zebrafish were exposed to PFOS (0, 2, 20, and 200 μg/L) and 5 μg/L estradiol (E₂) continuously for 21 days. Sexual behaviors were analyzed by zebrafish behavior tracking system and the mRNA levels of HPGL-related genes was detected by RT-qPCR.

RESULTS

Body weight of the fish was increased in 2, 200 μg/L PFOS and E₂ groups, and body length was increased with exposure to 2 μg/L PFOS and E₂. The hepatic-somatic index was decreased significantly after 2 and 20 μg/L PFOS treatments. Highest PFOS (200 μg/L) and E₂ exposure impaired standard zebrafish sexual behaviors significantly such as chasing, nose-tail and tail-touching. In brains, the genes gonadotropin-releasing hormone (GnRH), gonadotropin-releasing hormone receptor (GnRHr) were down-regulated with exposure to PFOS with linear trend and E₂ exposure, and follicle-stimulating hormone and luteinizing hormone were also down-regulated with exposure to 20 and 200 μg/L PFOS. In livers, the genes vitellogenin 1 and 3 were upregulated with some concentrations of PFOS and E₂, but estrogenic receptor α, β2 were upregulated in any concentration of PFOS and E₂. In testes, the expressions of follicle-stimulating hormone receptor, luteinizing hormone receptor, and androgen receptor genes were all significantly down-regulated with any exposure concentration of PFOS and E₂.

CONCLUSIONS

PFOS may alter the zebrafish reproductive system by disrupting endocrine activity and impairing sexual behaviors.

Acoustic signals produced by Nile tilapia Oreochromis niloticus and black-chinned tilapia Sarotherodon melanotheron during intra- and interspecific pairings

We characterised, for the first-time, the sound production of black-chinned tilapia Sarotherodon melanotheron and show differences with that of Nile tilapia Oreochromis niloticus in a hybridization pairing context. Although both species were able to produce drum sounds, they showed different acoustic features. Drum sounds were produced in aggressive (chasing or lateral attack) and non-aggressive (courtship) contexts by O. niloticus but only in aggressive situations (fleeing or avoidance) by S. melanotheron. The second type of sounds produced by O. niloticus were grunts, produced in both aggressive (chasing and after biting) and non-aggressive contexts (nest building). The second type of sound produced by S. melanotheron was a rolling sound, produced only during courtship. Each species was able to produce common sounds (drum) and species-specific sounds (grunts and rolling). This implies that species can communicate without being able to understand each other because the sounds emitted may probably have different significance. Drumming corresponded only to aggressivity in S. melanotheron, whereas this was not true for O. niloticus. 11-ketotestosterone (11-kt) levels were significantly higher in male O. niloticus than male S. melanotheron, but there was no significant correlation between 11-kt or estradiol concentrations and the number of sounds produced in aggressive or non-aggressive behavioural contexts in either species. During interspecies interactions, O. niloticus drum sounds are likely considered to be aggressive by S. melanotheron and could potentially constitute a reproductive barrier between the two species.

Long-Term Pharmaceutical Contamination and Temperature Stress Disrupt Fish Behavior

Natural environments are subject to a range of anthropogenic stressors, with pharmaceutical pollution being among the fastest-growing agents of global change. However, despite wild animals living in complex multi-stressor environments, interactions between pharmaceutical exposure and other stressors remain poorly understood. Accordingly, we investigated effects of long-term exposure to the pervasive pharmaceutical contaminant fluoxetine (Prozac) and acute temperature stress on reproductive behaviors and activity levels in the guppy (Poecilia reticulata). Fish were exposed to environmentally realistic fluoxetine concentrations (measured average: 38 or 312 ng/L) or a solvent control for 15 months using a mesocosm system. Additionally, fish were subjected to one of three acute (24 h) temperature treatments: cold stress (18 °C), heat stress (32 °C), or a control (24 °C). We found no evidence for interactive effects of fluoxetine exposure and temperature stress on guppy behavior. However, both stressors had independent impacts. Fluoxetine exposure resulted in increased male coercive copulatory behavior, while fish activity levels were unaffected. Under cold-temperature stress, both sexes were less active and males exhibited less frequent reproductive behaviors. Our results demonstrate that long-term exposure to a common pharmaceutical pollutant and acute temperature stress alter fundamental fitness-related behaviors in fish, potentially shifting population dynamics in contaminated ecosystems.

Personality traits change after an opportunity to mate

There is growing evidence that personality traits can change throughout the life course in humans and nonhuman animals. However, the proximate and ultimate causes of personality trait change are largely unknown, especially in adults. In a controlled, longitudinal experiment, we tested whether a key life event for adults--mating--can cause personality traits to change in female threespine sticklebacks. We confirmed that there are consistent individual differences in activity, sociability and risk-taking, and then compared these personality traits among three groups of females: (i) control females; (ii) females that had physically mated, and (iii) females that had socially experienced courtship but did not mate. Both the physical experience of mating and the social experience of courtship caused females to become less willing to take risks and less social. To understand the proximate mechanisms underlying these changes, we measured levels of excreted steroids. Both the physical experience of mating and the social experience of courtship caused levels of dihydroxyprogesterone (17α,20β-P) to increase, and females with higher 17α,20β-P were less willing to take risks and less social. These results provide experimental evidence that personality traits and their underlying neuroendocrine correlates are influenced by formative social and life-history experiences well into adulthood.

Disruption of male mating strategies in a chemically compromised environment

A leading source of endocrine-disrupting chemicals (EDCs) in the environment is run-off of veterinary pharmaceuticals used in agriculture, including hormonal growth promotants (HGPs). Despite being banned in various countries, HGP use is still common in beef production around the world. The androgenic steroid 17β-trenbolone (17β-TB) is a HGP that commonly enters surface waters via livestock effluent run-off. Here, we used a flow-through system to expose wild-caught adult male guppies (Poecilia reticulata) to an environmentally realistic level of 17β-TB (average measured concentration = 2 ng/L) for 21 days. We then compared the response of exposed and unexposed males to sequentially presented large and small stimulus (unexposed) females. Due to a positive size-fecundity relationship, larger females are generally expected to be preferred by males. While we found no evidence that the size of a previously encountered female affected the amount of courtship or coercive 'sneak' mating behaviour performed by males during the second presentation, males from both exposure treatments conducted more frequent courting events towards larger females during both presentations, suggesting an absolute preference for greater female size. Further, across both presentations, 17β-TB exposure caused a shift in male mating strategy towards increased coercive sneaking behaviour, although male sequential investment into mating effort was not impacted at the tested dosage. In combination, our findings demonstrate that exposure to a field-realistic level of a widespread agricultural pollutant alters male mating strategies in fish, and contribute to a growing understanding of sub-lethal impacts of chemical contaminants on complex behaviours in wildlife.

A conceptual framework for understanding sexual differentiation of the teleost brain

Vertebrate brains are sexually differentiated, giving rise to differences in various physiological and behavioral phenotypes between the sexes. In developing mammals and birds, the neural substrate underlying sex-dependent physiology and behavior undergoes an irreversible process of sexual differentiation due to the effects of perinatal gonadal steroids and sex chromosome complement. The differentiated neural substrate is then activated in the adult by the sex-specific steroid milieu to facilitate the expression of sex-typical phenotypes. However, this well-established concept does not hold for teleost fish, whose sexual phenotypes (behavioral or otherwise) are highly labile throughout life and can be reversed even in adulthood. Indeed, the available evidence suggests that, in teleosts, neither gonadal steroids early in development nor the sex chromosome complement contribute much to brain sexual differentiation; instead, steroids in adulthood serve to both differentiate the neural substrate and activate it to elicit sex-typical phenotypes in a transient and reversible manner. Evidence further suggests that marked sexual dimorphisms and adult steroid-dependent lability in the neural expression of sex steroid receptors constitute the primary molecular basis for sexual differentiation and lability of the teleost brain. The consequent sexually dimorphic but reversible steroid sensitivity in response to the adult steroid milieu may enable the teleost brain to maintain lifelong sexual lability and to undergo phenotypic sex reversal.

Paternal Exposure to Carbamazepine Impacts Zebrafish Offspring Reproduction Over Multiple Generations

Chronic low-concentration chemical exposures may have both direct health outcomes on adults and indirect effects on their offspring. Using zebrafish, we examined the impacts of chronic, low-concentration carbamazepine (CBZ) exposure on a suite of male reproductive endpoints in the parents and four generations of offspring reared in clean water. CBZ is one of the most frequently detected pharmaceutical residues in water, is a histone deacetylase inhibitor in mammals, and is reported to lower androgens in mammals and fish. Exposure of adult zebrafish to 10 μg/L CBZ for 6 weeks decreased reproductive output, courtship and aggressive behaviors, 11-ketotestosterone (11KT), and sperm morphology but did not impact milt volume or sperm swimming speed. Pairwise breeding generated lineages of offspring with both parents exposed and two lineages where only one parent was exposed; the control lineage had unexposed parents. Reproductive output and male reproductive indices were assessed in F₁-F₄ offspring to determine whether parental CBZ exposure had transgenerational impacts. The offspring of CBZ-exposed males had lower 11KT, reproductive output, altered courtship, aggression, and sperm morphology compared to the lineage from unexposed parents. Our results indicate that parental carbamazepine exposure history impacts the unexposed progeny up to the F₄ generations and that paternal, but not maternal, exposure is most important for the reproductive health of male offspring.

Investigating the role of prostaglandin receptor isoform EP4b in zebrafish ovulation

Prostaglandins (PGs) are a class of fatty acid-derived hormones that play an essential role in the regulation of ovulation of teleosts. This study investigated the various isoforms of ovarian PG receptors in the zebrafish ovary and their role in ovulation. Using real time qPCR, six PG receptor isoforms (ptger1a, ptger1b, ptger2a, ptger4a, ptger4b, and ptgfr) were shown to be expressed in the ovary. Only the PG receptor isoform ptger4b was upregulated at the time of ovulation in vivo, or following treatment in vivo with Ovaprim, which contains a gonadotropin releasing hormone analogue and a dopamine receptor antagonist and stimulates ovulation. Treatment of full-grown follicles with the maturation-inducing hormone 17α,20β-dihydroxy-4-pregnen-3-one (17,20βP) in vitro also induced expression of EP4b mRNA. Females ovulate in vivo after injection with Ovaprim, or injection with Ovaprim and inhibitors of EP1 (ONO-8130) or EP2 (TG4-155) function; they do not ovulate when injected with Ovaprim and an EP4 inhibitor (GW237368x). These findings suggest that the EP4 receptor, in particular the EP4b isoform, is essential for ovulation.

Identification of sex differences in zebrafish (Danio rerio) brains during early sexual differentiation and masculinization using 17α-methyltestoterone

Sexual behavior in teleost fish is highly plastic. It can be attributed to the relatively few sex differences found in adult brain transcriptomes. Environmental and hormonal factors can influence sex-specific behavior. Androgen treatment stimulates behavioral masculinization. Sex dimorphic gene expression in developing teleost brains and the molecular basis for androgen-induced behavioral masculinization are poorly understood. In this study, juvenile zebrafish (Danio rerio) were treated with 100 ng/L of 17 alpha-methyltestosterone (MT) during sexual development from 20 days post fertilization to 40 days and 60 days post fertilization. We compared brain gene expression patterns in MT-treated zebrafish with control males and females using RNA-Seq to shed light on the dynamic changes in brain gene expression during sexual development and how androgens affect brain gene expression leading to behavior masculinization. We found modest differences in gene expression between juvenile male and female zebrafish brains. Brain aromatase (cyp19a1b), prostaglandin 3a synthase (ptges3a), and prostaglandin reductase 1 (ptgr1) were among the genes with sexually dimorphic expression patterns. MT treatment significantly altered gene expression relative to both male and female brains. Fewer differences were found among MT-treated brains and male brains compared to female brains, particularly at 60 dpf. MT treatment upregulated the expression of hydroxysteroid 11-beta dehydrogenase 2 (hsd11b2), deiodinase, iodothyronine, type II (dio2), and gonadotrophin releasing hormones (GnRH) 2 and 3 (gnrh2 and gnrh3) suggesting local synthesis of 11-ketotestosterone, triiodothyronine, and GnRHs in zebrafish brains which are influenced by androgens. Androgen, estrogen, prostaglandin, thyroid hormone, and GnRH signaling pathways likely interact to modulate teleost sexual behavior.

Activation of the brain-pituitary-gonadotropic axis in the black porgy Acanthopagrus schlegelii during gonadal differentiation and testis development and effect of estradiol treatment

Previous studies revealed an estradiol (E2)-dependent peak in brain activity, including neurosteroidogenesis and neurogenesis in the black porgy during the gonadal differentiation period. The brain-pituitary-gonadotropic axis is a key regulator of reproduction and may also be involved in gonadal differentiation, but its activity and potential role in black porgy during the gonadal differentiation period is still unknown. The present study analyzed the expression of regulatory factors involved in the gonadotropic axis at the time of gonadal differentiation (90, 120, 150 days after hatching [dah]) and subsequent testicular development (180, 210, 300 dah). In agreement with previous studies, expression of brain aromatase cyp19a1b peaked at 120 dah, and this was followed by a gradual increase during testicular development. The expression of gonadotropin subunits increased slightly but not significantly during gonadal differentiation and then increased significantly at 300 dah. In contrast, the expression of brain gnrh1 and pituitary gnrh receptor 1 (gnrhr1) exhibited a pattern with two peaks, the first at 120 dah, during the period of gonadal differentiation, and the second peak during testicular development. Gonad fshr and lhcgr increased during gonadal differentiation period with highest transcript level in prespawning season during testicular development. This suggests that the early activation of brain gnrh1, pituitary gnrhr1 and gths, and gonad gthrs might be involved in the control of gonadal differentiation. E2 treatment increased brain cyp19a1b expression at each sampling time, in agreement with previous studies in black porgy and other teleosts. E2 also significantly stimulated the expression of pituitary gonadotropin subunits at all sampling times, indicating potential E2-mediated steroid feedback. In contrast, no significant effect of E2 was observed on gnrh1. Moreover, treatment of AI or E2 had no statistically significant effect on brain gnrh1 transcription levels during gonadal differentiation. This indicated that the early peak of gnrh1 expression during the gonadal differentiation period is E2-independent and therefore not directly related to the E2-dependent peak in brain neurosteroidogenesis and neurogenesis also occurring during this period in black porgy. Both E2-independent and E2-dependent mechanisms are thus involved in the peak expression of various genes in the brain of black porgy at the time of gonadal differentiation.