Aromatase regulates aggression in the African cichlid fish Astatotilapia burtoni

Triclosan, an antimicrobial drug, induced reproductive impairment in the freshwater fish, Anabas testudineus (Bloch, 1792)

Triclosan (TCS), an antimicrobial drug, is known to occupy different compartments in aquatic ecosystems. The present study focused to evaluate the reproductive toxicity of triclosan, at environmentally relevant (0.009 and 9 μg L-1) and sublethal (176.7 μg L-1) concentrations for 90 days in the pre-spawning phase of the fish, Anabas testudineus. The reproductive biomarkers, namely, gonadal steroidogenic enzymes, expression of aromatic genes, levels of serum gonadotropins, sex hormones, and histology of gonads were analyzed. The weight of the animal, brain weights along with gonadosomatic index decreased while mucus deposition increased significantly at all concentrations of triclosan as the primary defensive mechanism to prevent the entry of toxicants. Triclosan disrupted gonadal steroidogenesis as evidenced by a reduction in the activities of gonadal steroidogenic enzymes. The expressions of cyp19a1a and cyp19a1b genes were up-regulated in the brain of both sexes and testis, while down-regulated in the ovary indicating estrogenic effects of the compound. The endocrine-disrupting effects of triclosan were confirmed. The current results suggest that chronic exposure to triclosan altered reproductive endpoints thereby impairing normal reproductive functions in fish.

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.

Androgen receptor deficiency is associated with reduced aromatase expression in the ventromedial hypothalamus of male cichlids

Social behaviors are regulated by sex steroid hormones, such as androgens and estrogens. However, the specific molecular and neural processes modulated by steroid hormones to generate social behaviors remain to be elucidated. We investigated whether some actions of androgen signaling in the control of social behavior may occur through the regulation of estradiol synthesis in the highly social cichlid fish, Astatotilapia burtoni. Specifically, we examined the expression of cyp19a1, a brain-specific aromatase, in the brains of male A. burtoni lacking a functional ARα gene (ar1), which was recently found to be necessary for aggression in this species. We found that cyp19a1 expression is higher in wild-type males compared to ar1 mutant males in the anterior tuberal nucleus (ATn), the putative fish homolog of the mammalian ventromedial hypothalamus, a brain region that is critical for aggression across taxa. Using in situ hybridization chain reaction, we determined that cyp19a1+ cells coexpress ar1 throughout the brain, including in the ATn. We speculate that ARα may modulate cyp19a1 expression in the ATn to govern aggression in A. burtoni. These studies provide novel insights into the hormonal mechanisms of social behavior in teleosts and lay a foundation for future functional studies.

Embryonic exposures to chemicals acting on brain aromatase lead to different locomotor effects in zebrafish larvae

Pathways underlying neurodevelopmental effects of endocrine disruptors (EDs) remain poorly known. Expression of brain aromatase (aroB), responsible for estrogen production in the brain of teleosts, is regulated by estrogenic EDs and could play a role in their behavioral effects. We exposed zebrafish eleutheroembryos (0-120 h post-fertilization) to various concentrations of 16 estrogenic chemicals (incl. bisphenols and contraceptives), and of 2 aroB inhibitors. Behavior was monitored using a photomotor response test procedure. Both aroB inhibitors (clotrimazole and prochloraz) and a total of 6 estrogenic EDs induced significant behavioral alterations, including DM-BPA, BPC and BPS-MPE, three bisphenol substitutes which behavioral effects were, to our knowledge, previously unknown. However, no consensus was reported on the effects among tested substances. It appears that behavioral changes could not be linked to groups of substances defined by their specificity or potency to modulate aroB expression, or by their structure. Altogether, behavioral effects of estrogenic EDs in 120 h post-fertilization larvae appear unrelated to aroB but are nonetheless not to be neglected in the context of environmental safety.

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.

Mutation of brain aromatase disrupts spawning behavior and reproductive health in female zebrafish

Aromatase (Cyp19a1) is the steroidogenic enzyme that converts androgens into bioactive estrogens, and hence is in a pivotal position to mediate reproduction and sexual behavior. In teleosts, there are two aromatase paralogs: cyp19a1a that is highly expressed in granulosa and Leydig cells in the gonads with critical function in sexual differentiation of the ovary, and cyp19a1b that is highly expressed in radial glial cells in the brain with unknown roles in reproduction. Cyp19a1 ^-/- mutant zebrafish lines were used to investigate the importance of the cyp19a1 paralogs for spawning behavior and offspring survival and early development. Mutation of cyp19a1b was found to increase the latency to the first oviposition in females. Mutation of cyp19a1b in females also increased the number of eggs spawned; however, significantly more progeny died during early development resulting in no net increase in female fecundity. This finding suggests a higher metabolic cost of reproduction in cyp19a1b ^-/- mutant females. In males, the combined mutation of both cyp19a1 paralogs resulted in significantly lower progeny survival rates, indicating a critical function of cyp19a1 during early larval development. These data establish the specific importance of cyp19a1b for female spawning behavior and the importance of the cyp19a1 paralogs for early larval survival.

Fighting cichlids: An integrated multimodal analysis to understand female and male aggression in Cichlasoma dimerus

Aggression has been historically linked to males and androgen levels and, even if females from different species also display aggressive behavior, female aggression is still widely understudied. The aim of the present work is to disentangle how sex differences in social plasticity can be explained by sex steroid hormone levels, gonadal state and/or morphometric characteristics. In this context, we performed intrasexual dyadic encounters to identify social plasticity after acquiring a winner or loser status in males and females of Cichlasoma dimerus. This integral analysis suggests that the reproductive and hormonal variables analyzed explain the behavioral variation among winner and loser males and females, and that there are significant differences between sexes and contest outcome when individual morphometric variables are excluded from the analysis. Interestingly, there are no sex differences in aggressive and submissive behaviors, and clustering into winners and losers is mainly explained by specific behavioral displays, such as bites, chases, approaches, passive copings, and escapes. Correlation heatmaps show a positive correlation between estradiol with aggression and a negative correlation with submission, suggesting estrogens may have a dual role regulating agonistic behavior. Finally, these results suggest that size difference can help to understand aggression in females but not in males, and that assessment of the opponent's body size is important to understand aggression also before the initiation of the contest in both sexes. Overall, this study constitutes an integral approach adding insights into the importance of reproductive and hormonal variables to understand social plasticity in males and females.

Phylogenomic and expression analysis of Colossoma macropomum cyp19a1a and cyp19a1b and their non-classical role in tambaqui sex differentiation

The genes coding for Cytochrome P450 aromatase (cyp19a1a and cyp19a1b) and estrogen (E₂) receptors (esr1, esr2a and esr2b) play a conserved role in ovarian differentiation and development among teleosts. Classically, the "gonad form" of aromatase, coded by the cyp19a1a, is responsible for the ovarian differentiation in genetic females via ligation and activation of the Esr, which mediates the endocrine and exocrine signaling to allow or block the establishment of the feminine phenotype. However, in neotropical species, studies on the molecular and endocrine processes involved in gonad differentiation as well as on the effects of sex modulators are recent and scarce. In this study, we combined in silico analysis, real-time quantitative PCR (qPCR) assay and quantification of E₂ plasma levels of differentiating tambaqui (Colossoma macropomum) to unveil the roles of the paralogs cypa19a1a and cyp19a1b during sex differentiation. Although the synteny of each gene is very conserved among characids, the genomic environment displays striking differences in comparison to model teleost species, with many rearrangements in cyp19a1a and cyp19a1b adjacencies and transposable element traces in both regulatory regions. The high dissimilarity (DI) of SF-1 binding motifs in cyp19a1a (DI = 10.06 to 14.90 %) and cyp19a1b (DI = 8.41 to 13.50 %) regulatory region, respectively, may reflect in an alternative pathway in tambaqui. Indeed, while low transcription of cyp19a1a was detected prior to sex differentiation, the expression of cyp19a1b and esr2a presented a large variation at this phase, which could be associated with sex-specific differential expression. Histological analysis revealed that anti-estradiol treatments did not affect gonadal sex ratios, although Fadrozole (50 mg kg^-1 of food) reduced E₂ plasma levels (p < 0,005) as well cyp19a1a transcription; and tamoxifen (200 mg kg^-1 of food) down regulated both cyp19a1a and cyp19a1b but did not influence E₂ levels. Altogether, our results bring into light new insights about the evolutionary fate of cyp19a1 paralogs in neotropical fish, which may have generated uncommon roles for the gonadal and brain forms of cyp19a1 genes and the unexpected lack of effect of endocrine disruptors on tambaqui sexual differentiation.

Transcriptomes of testis and pituitary from male Nile tilapia (O. niloticus L.) in the context of social status

African cichlids are well established models for studying social hierarchies in teleosts and elucidating the effects social dominance has on gene expression. Ascension in the social hierarchy has been found to increase plasma levels of steroid hormones, follicle stimulating hormone (Fsh) and luteinizing hormone (Lh) as well as gonadosomatic index (GSI). Furthermore, the expression of genes related to gonadotropins and steroidogenesis and signaling along the brain-pituitary-gonad axis (BPG-axis) is affected by changes of an animal’s social status. In this study, we use RNA-sequencing to obtain an in-depth look at the transcriptomes of testes and pituitaries from dominant and subordinate male Nile tilapia living in long-term stable social hierarchies. This allows us to draw conclusions about factors along the brain-pituitary-gonad axis that are involved in maintaining dominance over weeks or even months. We identify a number of genes that are differentially regulated between dominant and subordinate males and show that in high-ranking fish this subset of genes is generally upregulated. Genes differentially expressed between the two social groups comprise growth factors, related binding proteins and receptors, components of Wnt-, Tgfβ- and retinoic acid-signaling pathway, gonadotropin signaling and steroidogenesis pathways. The latter is backed up by elevated levels of 11-ketotestosterone, testosterone and estradiol in dominant males. Luteinizing hormone (Lh) is found in higher concentration in the plasma of long-term dominant males than in subordinate animals. Our results both strengthen the existing models and propose new candidates for functional studies to expand our understanding of social phenomena in teleost fish.

A neural circuit perspective on brain aromatase

This review explores the role of aromatase in the brain as illuminated by a set of conserved network-level connections identified in several vertebrate taxa. Aromatase-expressing neurons are neurochemically heterogeneous but the brain regions in which they are found are highly-conserved across the vertebrate lineage. During development, aromatase neurons have a prominent role in sexual differentiation of the brain and resultant sex differences in behavior and human brain diseases. Drawing on literature primarily from birds and rodents, we delineate brain regions that express aromatase and that are strongly interconnected, and suggest that, in many species, aromatase expression essentially defines the Social Behavior Network. Moreover, in several cases the inputs to and outputs from this core Social Behavior Network also express aromatase. Recent advances in molecular and genetic tools for neuroscience now enable in-depth and taxonomically diverse studies of the function of aromatase at the neural circuit level.

Endocrine and neuroendocrine regulation of social status in cichlid fishes

Position in a dominance hierarchy profoundly impacts group members' survival, health, and reproductive success. Thus, understanding the mechanisms that regulate or are associated with an individuals' social position is important. Across taxa, various endocrine and neuroendocrine signaling systems are implicated in the control of social rank. Cichlid fishes, with their often-limited resources of food, shelter, and mates that leads to competition, have provided important insights on the proximate and ultimate mechanisms related to establishment and maintenance of dominance hierarchies. Here we review the existing information on the relationships between endocrine (e.g., circulating hormones, gonadal and other tissue measures) and neuroendocrine (e.g., central neuropeptides, biogenic amines, steroids) systems and dominant and subordinate social rank in male cichlids. Much of the current literature is focused on only a few representative cichlids, particularly the African Astatotilapia burtoni, and several other African and Neotropical species. Many hormonal regulators show distinct differences at multiple biological levels between dominant and subordinate males, but generalizations are complicated by variations in experimental paradigms, methodological approaches, and in the reproductive and parental care strategies of the study species. Future studies that capitalize on the diversity of hierarchical structures among cichlids should provide insights towards better understanding the endocrine and neuroendocrine mechanisms contributing to social rank. Further, examination of this topic in cichlids will help reveal the selective pressures driving the evolution of endocrine-related phenotypic traits that may facilitate an individual's ability to acquire and maintain a specific social rank to improve survival and reproductive success.

Association of androgens and estrogens with agonistic behavior in the annual fish Austrolebias reicherti

Agonistic behavior governs the settlement of conflicts among conspecifics for limiting resources. Sex steroids play a critical role in the regulation of agonistic behavior which in turn may produce modulations in hormone titres. In this study we analyzed the association of androgens and estrogens with agonistic behavior in the annual fish Austrolebias reicherti. This native species inhabits temporary ponds that dry out completely during summer, having one of the shortest lifespans among vertebrates. They are highly sexually dimorphic and have a single breeding season during which they reproduce continuously. Here we measured plasma levels of 11-ketotestosterone (11KT) and 17β-estradiol (E₂) in adult males after the resolution of a social conflict and assessed the role of the aromatase conversion of testosterone (T) to E₂ in male aggression. Winners had higher levels of 11KT than losers yet; winner 11KT levels did not differ from those of males not exposed to a social challenge. E₂ levels did not show differences among winners, losers or control males. However, fights under the aromatase inhibitor Fadrozole were overall less aggressive than control fights. Our results suggest an androgen response to losing a conflict and that the conversion of T to E₂ is involved in the regulation of aggressive behavior. Annual fish extreme life history may give new insights on hormone-behavior interactions.

Sex Determination and Differentiation in Teleost: Roles of Genetics, Environment, and Brain

The fish reproductive system is a complex biological system. Nonetheless, reproductive organ development is conserved, which starts with sex determination and then sex differentiation. The sex of a teleost is determined and differentiated from bipotential primordium by genetics, environmental factors, or both. These two processes are species-specific. There are several prominent genes and environmental factors involved during sex determination and differentiation. At the cellular level, most of the sex-determining genes suppress the female pathway. For environmental factors, there are temperature, density, hypoxia, pH, and social interaction. Once the sexual fate is determined, sex differentiation takes over the gonadal developmental process. Environmental factors involve activation and suppression of various male and female pathways depending on the sexual fate. Alongside these factors, the role of the brain during sex determination and differentiation remains elusive. Nonetheless, GnRH III knockout has promoted a male sex-biased population, which shows brain involvement during sex determination. During sex differentiation, LH and FSH might not affect the gonadal differentiation, but are required for regulating sex differentiation. This review discusses the role of prominent genes, environmental factors, and the brain in sex determination and differentiation across a few teleost species.

Nuclear androgen and progestin receptors inversely affect aggression and social dominance in male zebrafish (Danio rerio)

Aggression is a fundamental behavior displayed universally among animal species, but hyper- or hypo-aggressiveness can be maladaptive with negative consequences for individuals and group members. While the social and ecological significance of aggression is well understood, the specific neurobiological and hormonal mechanisms responsible for mediating aggression have not been fully elucidated. Previous studies have shown a relationship between aggressive acts and circulating gonadal steroids, but whether classical nuclear steroid receptors regulate aggression in animals is still uncertain. We examined whether the nuclear androgen receptor (Ar) and nuclear progestin receptor (Pgr) were necessary for aggressive behaviors and maintenance of a dominance relationship in male zebrafish (Danio rerio). Dyadic social interactions of Ar knockout (ArKO), Pgr knockout (PgrKO) and wildtype (WT) controls were observed for two weeks (2-weeks). ArKO zebrafish were significantly less aggressive and had a less defined dominance relationship, whereas PgrKO dominant zebrafish were significantly and persistently more aggressive with a robust dominance relationship. Our results demonstrate the importance of nuclear steroid hormone receptors in regulating aggression of adult male zebrafish and provide new models for understanding of the mechanisms of aggression.

Sexual Dimorphism in Aggression: Sex-Specific Fighting Strategies Across Species

Aggressive behavior is thought to have evolved as a strategy for gaining access to resources such as territory, food, and potential mates. Across species, secondary sexual characteristics such as competitive aggression and territoriality are considered male-specific behaviors. However, although female–female aggression is often a behavior that is displayed almost exclusively to protect the offspring, multiple examples of female–female competitive aggression have been reported in both invertebrate and vertebrate species. Moreover, cases of intersexual aggression have been observed in a variety of species. Genetically tractable model systems such as mice, zebrafish, and fruit flies have proven extremely valuable for studying the underlying neuronal circuitry and the genetic architecture of aggressive behavior under laboratory conditions. However, most studies lack ethological or ecological perspectives and the behavioral patterns available are limited. The goal of this review is to discuss each of these forms of aggression, male intrasexual aggression, intersexual aggression and female intrasexual aggression in the context of the most common genetic animal models and discuss examples of these behaviors in other species.

Fish as a model in social neuroscience: conservation and diversity in the social brain network

Mechanisms for fish social behaviours involve a social brain network (SBN) which is evolutionarily conserved among vertebrates. However, considerable diversity is observed in the actual behaviour patterns amongst nearly 30000 fish species. The huge variation found in socio-sexual behaviours and strategies is likely generated by a morphologically and genetically well-conserved small forebrain system. Hence, teleost fish provide a useful model to study the fundamental mechanisms underlying social brain functions. Herein we review the foundations underlying fish social behaviours including sensory, hormonal, molecular and neuroanatomical features. Gonadotropin-releasing hormone neurons clearly play important roles, but the participation of vasotocin and isotocin is also highlighted. Genetic investigations of developing fish brain have revealed the molecular complexity of neural development of the SBN. In addition to straightforward social behaviours such as sex and aggression, new experiments have revealed higher order and unique phenomena such as social eavesdropping and social buffering in fish. Finally, observations interpreted as 'collective cognition' in fish can likely be explained by careful observation of sensory determinants and analyses using the dynamics of quantitative scaling. Understanding of the functions of the SBN in fish provide clues for understanding the origin and evolution of higher social functions in vertebrates.

Zebrafish cyp17a1 knockout reveals that androgen-mediated signaling is important for male brain sex differentiation

Brain sex differentiation is a complex process, wherein genes and steroid hormones act to induce specific gender brain differentiation. Testosterone (T) derived from the gonads has been linked to neural circuit modeling in a sex-specific manner. Previously, we have shown that cyp17a1 knockout (KO) zebrafish have low plasma androgen levels, and display compromised male-typical mating behaviors. In this study, we demonstrated that treatment of cyp17a1 KO males with T or 11-ketotestosterone (11-KT) is sufficient to rescue mating impairment by restoring the male-typical secondary sex characters (SSCs) and mating behaviors, confirming an essential role of androgen in maintaining SSCs and mating behaviors. Brain steroid hormone analysis revealed that cyp17a1 KO fish have reduced levels of T and 11-KT. We performed RNA sequencing on brain samples of control and cyp17a1 KO male zebrafish to get insights regarding the impact of cyp17a1 KO on gene expression pattern, and to correlate it with the observed disruption of male-typical mating behaviors. Transcriptome analysis of cyp17a1 KO males showed a differential gene expression when compared to control males. In total, 358 genes were differentially regulated between control males and KO males. Important genes including brain aromatase (cyp19a1b), progesterone receptor (pgr), deiodinase (dio2), and insulin-like growth factor 1 (igf1) that are involved in brain functions, as well as androgen response genes including igf1, frem1a, elovl1a, pax3a, mmp13b, hsc70, ogg1 were regulated. RT-qPCR analysis following rescue of cyp17a1 KO with T and 11-KT further suggested that androgen-mediated signaling is disrupted in the cyp17a1 KO fish. Our results indicated that cyp17a1 KO fish have an incomplete masculinization and altered brain gene expression, which could be due to decreased androgen levels.

The estrogenic pathway modulates non-breeding female aggression in a teleost fish

Aggressive behaviors are widespread among animals and are critical in the competition for resources. The physiological mechanisms underlying aggression have mostly been examined in breeding males, in which gonadal androgens, acting in part through their aromatization to estrogens, have a key role. There are two alternative models that contribute to further understanding hormonal mechanisms underlying aggression: aggression displayed in the non-breeding season, when gonadal steroids are low, and female aggression. In this study we approach, for the first time, the modulatory role of estrogens and androgens upon non-breeding aggression in a wild female teleost fish. We characterized female aggression in the weakly electric fish Gymnotus omarorum and carried out acute treatments 1 h prior to agonistic encounters in dyads treated with either an aromatase inhibitor or an antagonist of androgen receptors. Anti-androgen treatment had no effect on behavior whereas acute aromatase inhibition caused a strong distortion of aggressive behavior. Territorial non-breeding aggression was robust and depended on rapid estrogen actions to maintain high levels of aggression, and ultimately reach conflict resolution from which dominant/subordinate status emerged. Our results, taken together with our own reports in males and the contributions from non-breeding aggression in bird and mammal models, suggest a common strategy involving fast-acting estrogens in the control of this behavior across species. In addition, further analysis of female non-breeding aggression may shed light on potential sexual differences in the fine tuning of social behaviors.

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.

Rapid effects of testosterone on social decision-making in a monogamous California mice (Peromyscus californicus)

Social animals must cope with challenges and opportunities by adjusting how they react to a salient stimulus. Here we use California mice (Peromyscus californicus) and investigate the mechanisms underlying social decision-making by studying (i) rapid effects of testosterone (T) pulses on a male's decisions to approach a novel male (challenge) versus a receptive female (opportunity), and (ii) whether social experience shapes how such effects are manifested. In Experiment 1, we found that sexually naïve males administered saline injections preferentially approached unfamiliar females over unfamiliar males, in contrast, 10 min after receiving a single T-injection, males expressed a preference for approaching unfamiliar males. Such an effect of T only occurred in sexually naïve males, but not pair-bonded males, suggesting that the rapid effects of T on approach behavior may rely on the pair-bonding experiences. Experiment 2 investigated social decision-making across three repeated exposures to the challenge/opportunity situations. Only the initial decision, approach to the challenge, predicted future aggressive behaviors, and such an effect relied on the rapid actions of T. We also found that experience with the controlled challenge situation (the male intruder was restrained behind a wire mesh) dampened the approach to the male side (potential threat) when later exposed to the same conditions. This suggests that a resident's motivation to defend against a threatening individual may decrease as the threat posed by the "neighbors" is reduced. Overall rapid effects of post-encounter T pulses may play important roles in influencing behavioral decisions during social interactions.

Time matters! Developmental shift in gene expression between the head and the trunk region of the cichlid fish Astatotilapia burtoni

BACKGROUND

Differential gene expression can be translated into differing phenotypic traits. Especially during embryogenesis, specific gene expression networks regulate the development of different body structures. Cichlid fishes, with their impressive phenotypic diversity and propensity to radiate, are an emerging model system in the genomics era. Here we set out to investigate gene expression throughout development in the well-studied cichlid fish Astatotilapia burtoni, native to Lake Tanganyika and its affluent rivers.

RESULTS

Combining RNA-sequencing from different developmental time points as well as integrating adult gene expression data, we constructed a new genome annotation for A. burtoni comprising 103,253 transcripts (stemming from 52,584 genomic loci) as well as a new reference transcriptome set. We compared our transcriptome to the available reference genome, redefining transcripts and adding new annotations. We show that about half of these transcripts have coding potential. We also characterize transcripts that are not present in the genome assembly. Next, using our newly constructed comprehensive reference transcriptome, we characterized differential gene expression through time and showed that gene expression is shifted between different body parts. We constructed a gene expression network that identified connected genes responsible for particular phenotypes and made use of it to focus on genes under potential positive selection in A. burtoni, which were implicated in fin development and vision.

CONCLUSIONS

We provide new genomic resources for the cichlid fish Astatotilapia burtoni, which will contribute to its further establishment as a model system. Tracing gene expression through time, we identified gene networks underlying particular functions, which will help to understand the genetic basis of phenotypic diversity in cichlids.

Hormonal regulation of social ascent and temporal patterns of behavior in an African cichlid

For many species, social rank determines which individuals perform certain social behaviors and when. Higher ranking or dominant (DOM) individuals maintain status through aggressive interactions and perform courtship behaviors while non-dominant (ND) individuals do not. In some species ND individuals ascend (ASC) in social rank when the opportunity arises. Many important questions related to the mechanistic basis of social ascent remain to be answered. We probed whether androgen signaling regulates social ascent in male Astatotilapia burtoni, an African cichlid whose social hierarchy can be readily controlled in the laboratory. As expected, androgen receptor (AR) antagonism abolished reproductive behavior during social ascent. However, we discovered multiple AR- and status-dependent temporal behavioral patterns that typify social ascent and dominance. AR antagonism in ASC males increased the time between successive behaviors compared to DOM males. Socially ascending males, independent of AR activation, were more likely than DOM males to follow aggressive displays with another aggressive display. Further analyses revealed differences in the sequencing of aggressive and courtship behaviors, wherein DOM males were more likely than ASC males to follow male-directed aggression with courtship displays. Strikingly, this difference was driven mostly by ASC males taking longer to transition from aggression to courtship, suggesting ASC males can perform certain DOM-typical temporal behavioral patterns. Our results indicate androgen signaling is necessary for social ascent and hormonal signaling and social experience may shape the full suite of DOM-typical behavioral patterns.

Effect of Fadrozole Exposure on Socioreproductive Behaviors and Neurochemical Parameters in Betta splendens

Induction of all-male population in Siamese fighting fish, Betta splendens, has potential application in ornamental fish trade. In addition, the sexually dimorphic nature of aggressive behavior exhibited by this species has made it into an emerging model for behavioral studies. Fadrozole, an aromatase inhibitor, which has been used widely in masculinization, has captivated us to use it in this study. Twenty one days postfertilization (dpf), B. splendens fry were subjected to discrete immersion treatment with various concentrations of fadrozole, and eventually, analyses of various socioreproductive behaviors and analyses of stress markers such as dopamine in brain samples, sex hormones, cortisol, and glucose in plasma samples were performed. We observed that 91% of 50 μg/L fadrozole treated fish developed as males. Interestingly, reproductive analyses of these males gave rise to two subgroups (A and B). Subsequent sociobehavioral analyses demonstrated a timid and subdued behavior in subgroup B males. Furthermore, estimation of stress markers such as dopamine levels in the brain tissue, cortisol, and glucose levels in blood plasma and sex hormone levels in blood plasma exhibited an endocrine disruption-mediated stress leading to altered behavior in these males. These findings will help in understanding the fadrozole-mediated masculinization and behavioral alterations following endocrine disruption.

Astatotilapia burtoni: A Model System for Analyzing the Neurobiology of Behavior

Most biomedical research is performed using a very limited number of "model" species. In part, this has resulted from a combination of full genomes, manipulation of genes, and short generation times in these species. However, the advent of low-cost sequencing and gene editing in any organism has increased the use of nontraditional organisms. Many scientists have paraphrased the adage by Krogh [ Krogh , A. ( 2018 ) Science 70 , 200 - 204 ] that for many biological problems some species will prove to be most convenient and useful to study. In particular, using organisms most suited to the specific research question can lead to novel insights about fundamental physiological, neurobiological, immunological, and neuroendocrine systems that can advance our understanding of the well-being and health of humans. In addition, such studies have led to new ideas about the evolution and mechanisms that control social behavior. Fishes constitute about 50% of all vertebrate species and are the most diverse vertebrate radiation. Here we review behavioral and neurobiological discoveries of plasticity in social behavior resulting from analysis of an African cichlid fish, showing how its unique behavioral system has facilitated a broad range of discoveries. For many future questions, Astatotilapia burtoni and other cichlid fishes may be ideally suited to study as we advance our understanding of the neural basis of social decisions.

Can estrogens be considered as key elements of the challenge hypothesis? The case of intrasexual aggression in a cichlid fish

Territorial aggression has been widely studied in males and it has been historically suggested that androgens are key mediators of this behavior. However, more recent evidence suggests that it is the aromatization to estrogens, rather than androgens themselves, that is key to regulating this behavior. Females also display aggressive behaviors, but the physiological regulation of female aggression is still understudied when compared to males. In this context, the challenge hypothesis postulates that male-male aggressive interactions stimulate the production of androgens in males in periods of social instability. Here we determine plasma sex steroid levels in Cichlasoma dimerus to assess whether estrogens are related to aggressive behavior and to test the challenge hypothesis in both males and females. We set-up challenge trials as intrasexual dyadic encounters and determined androgen and estrogen levels before and after the trial in both winners and losers. Even though there were no differences in initial estradiol-17β plasma levels between male winners and losers, initial levels were higher (p = .046) in female winners than in losers, while there were no differences in testosterone or 11-ketotestosterone levels. After trials, both males and females showed elevated levels of estradiol-17β and both androgens, but only males exhibited a significant 1.45, 5.42 and 3.2-fold increase in estradiol-17β, testosterone and 11-ketotestosterone, respectively (p = .023, p = .016, p = .018). Moreover, changes in circulating levels of estradiol-17β in females after the trials do not depend on their reproductive status or on the outcome of the contest. We suggest that female aggression is associated with initial levels of estradiol-17β, and that the challenge hypothesis, originally defined for androgens, could also be extended to estrogens.

The skeletal ontogeny of Astatotilapia burtoni - a direct-developing model system for the evolution and development of the teleost body plan

Background

The experimental approach to the evolution and development of the vertebrate skeleton has to a large extent relied on “direct-developing” amniote model organisms, such as the mouse and the chicken. These organisms can however only be partially informative where it concerns secondarily lost features or anatomical novelties not present in their lineages. The widely used anamniotes Xenopus and zebrafish are “indirect-developing” organisms that proceed through an extended time as free-living larvae, before adopting many aspects of their adult morphology, complicating experiments at these stages, and increasing the risk for lethal pleiotropic effects using genetic strategies.

Results

Here, we provide a detailed description of the development of the osteology of the African mouthbrooding cichlid Astatotilapia burtoni, primarily focusing on the trunk (spinal column, ribs and epicentrals) and the appendicular skeleton (pectoral, pelvic, dorsal, anal, caudal fins and scales), and to a lesser extent on the cranium. We show that this species has an extremely “direct” mode of development, attains an adult body plan within 2 weeks after fertilization while living off its yolk supply only, and does not pass through a prolonged larval period.

Conclusions

As husbandry of this species is easy, generation time is short, and the species is amenable to genetic targeting strategies through microinjection, we suggest that the use of this direct-developing cichlid will provide a valuable model system for the study of the vertebrate body plan, particularly where it concerns the evolution and development of fish or teleost specific traits. Based on our results we comment on the development of the homocercal caudal fin, on shared ontogenetic patterns between pectoral and pelvic girdles, and on the evolution of fin spines as novelty in acanthomorph fishes. We discuss the differences between “direct” and “indirect” developing actinopterygians using a comparison between zebrafish and A. burtoni development.

Dissecting the Transcriptional Patterns of Social Dominance across Teleosts

In many species, under varying ecological conditions, social interactions among individuals result in the formation of dominance hierarchies. Despite general similarities, there are robust differences among dominance hierarchies across species, populations, environments, life stages, sexes, and individuals. Understanding the proximate mechanisms underlying the variation is an important step toward understanding the evolution of social behavior. However, physiological changes associated with dominance, such as gonadal maturation and somatic growth, often complicate efforts to identify the specific underlying mechanisms. Traditional gene expression analyses are useful for generating candidate gene lists, but are biased by choice of significance cut-offs and difficult to use for between-study comparisons. In contrast, complementary analysis tools allow one to both test a priori hypotheses and generate new hypotheses. Here we employ a meta-analysis of high-throughput expression profiling experiments to investigate the gene expression patterns that underlie mechanisms and evolution of behavioral social phenotypes. Specifically, we use a collection of datasets on social dominance in fish across social contexts, sex, and species. Using experimental manipulation to produce female dominance hierarchies in the cichlid Astatotilapia burtoni, heralded as a genomic model of social dominance, we generate gene lists, and assess molecular gene modules. In the dominant female gene expression profile, we demonstrate a strong pattern of up-regulation of genes previously identified as having male-biased expression and furthermore, compare expression biases between male and female dominance phenotypes. Using a threshold-free approach to identify correlation throughout ranked gene lists, we query previously published datasets associated with maternal behavior, alternative reproductive tactics, cooperative breeding, and sex-role reversal to describe correlations among these various neural gene expression profiles associated with different instances of social dominance. These complementary approaches capitalize on the high-throughput gene expression profiling from similar behavioral phenotypes in order to address the mechanisms associated with social dominance behavioral phenotypes.

Do sex reversal procedures differentially affect agonistic behaviors and sex steroid levels depending on the sexual genotype in Nile tilapia?

In Nile tilapia Oreochromis niloticus, phenotypic males and females with different sexual genotypes (XX, XY, YY) have particular behavioral and physiological traits. Compared to natural XX females and XY males, XY and YY females and XX males expressed higher level of aggressiveness that could be related to higher levels of 17β-estradiol and 11-ketotestosterone, respectively. Our results suggest that the presence of a Y chromosome increases aggressiveness in females. However, since the same relationship between aggressiveness and the Y chromosome is not observed in males, we can hypothesize that the differences in aggressiveness are not directly dependent on the genotype but on the sex reversal procedures applied on young fry during their sexual differentiation to produce these breeders. These hormonal treatments could have permanently modified the development of the brain and consequently influenced the behavior of adults independently of their genotype. In both hypotheses (genotype or sex reversal influence), the causes of behavioral modifications have to be searched in an early modification of the brain sexual differentiation.

Acute embryonic anoxia exposure favours the development of a dominant and aggressive phenotype in adult zebrafish

Eutrophication and climate change are increasing the incidence of severe hypoxia in fish nursery habitats, yet the programming effects of hypoxia on stress responsiveness in later life are poorly understood. In this study, to investigate whether early hypoxia alters the developmental trajectory of the stress response, zebrafish embryos were exposed to 4 h of anoxia at 36 h post-fertilization and reared to adults when the responses to secondary stressors were assessed. While embryonic anoxia did not affect basal cortisol levels or the cortisol response to hypoxia in later life, it had a marked effect on the responses to a social stressor. In dyadic social interactions, adults derived from embryonic anoxia initiated more chases, bit more often, entered fewer freezes and had lower cortisol levels. Adults derived from embryonic anoxia also performed more bites towards their mirror image, had lower gonadal aromatase gene expression and had higher testosterone levels. We conclude that acute embryonic anoxia has long-lasting consequences for the hormonal and behavioural responses to social interactions in zebrafish. Specifically, we demonstrate that acute embryonic anoxia favours the development of a dominant and aggressive phenotype, and that a disruption in sex steroid production may contribute to the programming effects of environmental hypoxia.

To fight or mate? Hormonal control of sex recognition, male sexual behavior and aggression in the gecko lizard

Squamate reptiles are a highly diversified vertebrate group with extensive variability in social behavior and sexual dimorphism. However, hormonal control of these traits has not previously been investigated in sufficient depth in many squamate lineages. Here, we studied the hormonal control of male sexual behavior, aggressiveness, copulatory organ (hemipenis) size and sex recognition in the gecko Paroedura picta, comparing ovariectomized females, ovariectomized females treated with exogenous dihydrotestosterone (DHT), ovariectomized females treated with exogenous testosterone (T), control females and males. The administration of both T and DHT led to the expression of male-typical sexual behavior in females. However, in contrast to T, increased circulating levels of DHT alone were not enough to initiate the full expression of male-typical offensive aggressive behavior and development of hemipenes in females. Ovariectomized females were as sexually attractive as control females, which does not support the need for the demasculinization of the cues used for sex recognition by ovarian hormones as suggested in other sauropsids. On the other hand, our results point to the masculinization of the sex recognition cues by male gonadal androgens. Previously, we also demonstrated that sexually dimorphic growth is controlled by ovarian hormones in P. picta. Overall, it appears that individual behavioral and morphological sexually-dimorphic traits are controlled by multiple endogenous pathways in this species. Variability in the endogenous control of particular traits could have permitted their disentangling during evolution and the occurrence of (semi)independent changes across squamate phylogeny.

Pattern of aromatase mRNA expression in the brain of a weakly electric fish, Apteronotus leptorhynchus

Aromatase is a steroidogenic enzyme involved in the conversion of testosterone into estradiol. Teleosts are unique among vertebrates in possessing two distinct aromatase genes that show different expression patterns within the body. Since the brain is the essential organ underlying the control of behavior, an understanding of the expression pattern of aromatase in the brain can help to identify neural circuits and behaviors that are most likely to be affected by aromatase activity. In addition, identifying species differences in aromatase expression in the brain can further our understanding of divergence in behaviors regulated by local estradiol production and estrogen signaling. Apteronotus leptorhynchus is a species of weakly electric fish in which little is known about sex steroid expression within the brain and its role in electric signaling behavior. The goal of this study was to identify the mRNA expression pattern of aromatase in the brain of A. leptorhynchus. Aromatase mRNA was detected in several parts of the forebrain and in the pituitary gland; however, no aromatase expression was detected in the midbrain or hindbrain. These findings in A. leptorhynchus support a role for aromatase activity in reproduction, but no direct role in electric signaling behavior in non-breeding fish. The findings of this study help to broaden the basis for making phylogenetic comparisons of aromatase expression across teleost lineages as well as different signaling systems, and provide information on behaviors and neural circuits that are potentially affected by local estradiol production in A. leptorhynchus.

A game of two? Gene expression analysis of brain (cyp19a1b) and gonadal (cyp19a1a) aromatase in females of a Neotropical cichlid fish through the parental care period and removal of the offspring

For many species parental behavior is essential for the survival of the offspring. While the ultimate causes of teleost parental behavior have been widely studied, comparatively little is known about its proximate causes. The aim of this study was to analyze the yet unexplored, potential dual role of brain and gonadal aromatases, the enzymes responsible for the conversion of androgens to estrogens in the brains and gonads of teleosts, respectively, on the different stages of the maternal care period of the biparental cichlid Cichlasoma dimerus, locally known as chanchita. By immunohistochemistry we analyzed the neural distribution of brain aromatase and observed it exclusively within the forebrain, including areas involved in the regulation of parental behavior. We next analyzed the gene expression of brain aromatase in the brain, and gonadal aromatase in the ovary, of female chanchitas through the parental care period. To further characterize the physiological environment associated to maternal care, we also evaluated sex steroid levels (17β-estradiol, testosterone and 11-ketotestoterone) and ovarian follicle percentage. The onset of parental behavior specifically downregulated sex steroids synthesis and the rate of ovarian maturation, as denoted by a more than 10-fold decrease in steroid levels and delayed detection of mature follicles in females with offspring, compared to females which eggs were removed. Gene expression levels of both aromatases were independent of maternal care at the evaluated time points, even though they varied during the parental care period.

Dynamics of ovarian maturation throughout the reproductive cycle of the Neotropical cichlid fishCichlasomadimerus(Teleostei, Cichliformes)

In this study, we analyzed gene expression profiles, plasma steroids concentrations, and gonadal morphology throughout the reproductive cycle of female Cichlasoma dimerus (Heckel, 1840), a monogamous cichlid fish exhibiting social hierarchies. Fish were analyzed at six phases encompassing their annual cycle, namely resting (during the nonreproductive period), prespawning, 30 h post spawning, 4 days post spawning, 10 days post spawning, and subordinate (during the reproductive period). The histological and histomorphometric analysis showed that C. dimerus exhibits asynchronous ovarian development. Similar to resting females, subordinate females showed low gonadosomatic index, reduced expression levels of vitellogenin (vtgAb), zona pellucida (zpB), gonadal aromatase (cyp19a1A), and low concentrations of plasma sex steroids, thus indicating that social intimidation by dominant conspecifics elicited reproductive arrest. In reproductively active females, a direct positive correlation between plasma estradiol, vtgAb expression, percentage of late vitellogenic oocytes, and gonadosomatic index was observed. These parameters were maximal at the prespawning phase, decreased at 30 h post spawning and 4 days post spawning, and then reached a peak at 10 days post spawning. Our results indicate that female C. dimerus become spawning capable after 10 days post spawning, coincidently with the shortest time interval between successive spawns recorded in captivity.

Effect of dietary administration of letrozole and tamoxifen on gonadal development, sex differentiation and biochemical changes in common carp (Cyprinus carpio L.)

The effect of letrozole and tamoxifen on the specific growth rate (SGR; % day(-1)), gonado-somatic index (GSI), total haemoglobin (g%), gonadal and serum protein as well as lipid, sex differentiation and 17β-oestradiol levels were studied in sexually undifferentiated Cyprinus carpio fingerlings 30 days post fertilisation (30 dpf) for 60 days. Results showed decreased GSI with tamoxifen treatment whereas letrozole increased it. There were reduced protein, lipid, triglyceride and cholesterol levels after treatment with tamoxifen and letrozole during gonadal development. Tamoxifen (200mgkg(-1) feed) induced 82.5% masculinisation, whereas letrozole in the same dose produced 98.5% males. Gonadal 17β-oestradiol significantly declined from 86.0±1.41pg per 100mg (control) to 45.5±1.94pg per 100mg with tamoxifen and 36.0±0.72pg per 100mg with letrozole treatment. Similarly, serum 17β-oestradiol levels also decreased after tamoxifen and letrozole treatments. Testicular development in 37.8% of fish treated with tamoxifen and letrozole was found to be more advanced (spermatocytes) than in the control (spermatogonium); however, there was reduced ovarian growth and increased atresia. It was concluded that letrozole and tamoxifen both significantly affect sex differentiation and gonadal maturity in C. carpio leading to the production of sex-reversed males, yet the effect of letrozole was more potent.

From molecule to behavior: Brain aromatase (cyp19a1b) characterization, expression analysis and its relation with social status and male agonistic behavior in a Neotropical cichlid fish

The enzyme aromatase, responsible for the conversion of C19 androgens to C18 estrogens, exists as two paralogue copies in teleost fish: Cyp19a1a mostly expressed in the gonads, referred as gonadal aromatase, and Cyp19a1b, mostly expressed in the brain, accordingly known as brain aromatase. The neural localization of Cyp19a1b is greatly contained within the social behavior network and mesolimbic reward system in fish, suggesting a strong role of estrogen synthesis in the regulation of social behavior. In this work we aimed to analyze the variation in cyp19a1b expression in brain and pituitary of males of a highly social cichlid, Cichlasoma dimerus (locally known as chanchita), and its relation with inter-individual variability in agonistic behavior in a communal social environment. We first characterized chanchita's cyp19a1b mRNA and deduced amino acid sequence, which showed a high degree of conservation when compared to other teleost brain aromatase sequences, and its tissue expression patterns. Within the brain, Cyp19a1b was solely detected at putative radial glial cells of the forebrain, close to the brain ventricles. We then studied the relative expression levels of cyp19a1b by Real Time PCR in the brain and pituitary of males of different social status, territorial vs. non-territorial, and its relationship with an index of agonistic behavior. We found that even though, brain aromatase expression did not differ between types of males, pituitary cyp19a1b expression levels positively correlated with the index of agonistic behavior. This suggests a novel role of the pituitary in the regulation of social behavior by local estrogen synthesis.

A Journey through the Gonadotropin-Inhibitory Hormone System of Fish

Gonadotropin-inhibitory hormone (GnIH) is a hypothalamic neuropeptide that belongs to the RFamide peptide family and was first identified in the quail brain. From the discovery of avian GnIH, orthologous GnIH peptides have been reported in a variety of vertebrates, including mammals, amphibians, teleosts and agnathans, but also in protochordates. It has been clearly established that GnIH suppresses reproduction in avian and mammalian species through its inhibitory actions on brain GnRH and pituitary gonadotropins. In addition, GnIH also appears to be involved in the regulation of feeding, growth, stress response, heart function and social behavior. These actions are mediated via G protein-coupled GnIH receptors (GnIH-Rs), of which two different subtypes, GPR147 and GPR74, have been described to date. With around 30,000 species, fish represent more than one-half of the total number of recognized living vertebrate species. In addition to this impressive biological diversity, fish are relevant because they include model species with scientific and clinical interest as well as many exploited species with economic importance. In spite of this, the study of GnIH and its physiological effects on reproduction and other physiological processes has only been approached in a few fish species, and results obtained are in some cases conflicting. In this review, we summarize the information available in the literature on GnIH sequences identified in fish, the distribution of GnIH and GnIH-Rs in central and peripheral tissues, the physiological actions of GnIH on the reproductive brain-pituitary-gonadal axis, as well as other reported effects of this neuropeptide, and existing knowledge on the regulatory mechanisms of GnIH in fish.

Building the case for a novel teleost model of non-breeding aggression and its neuroendocrine control

In vertebrates, aggression has been traditionally associated with high levels of circulating androgens in breeding males. Nevertheless, the centrality of androgens as primary modulators of aggression is being reconsidered in at least in two particular cases: (1) territorial aggression outside the breeding season, and (2) aggression by females. We are developing the weakly electric fish, Gymnotus omarorum, as a novel, advantageous model system to address these two alternative forms of aggression. This species displays a short, escalated contest, after which a clear hierarchical status emerges. Subordination of individuals involves three sequential decisions: interruptions of their electric discharges, retreats, and chirps. These decisions are influenced by both size asymmetry between contenders and aggression levels of dominants. Both females and males are aggressive, and do not differ in fighting ability nor in the value placed on the resource. Aggression is completely independent of gonadal hormones: dominance status is unrelated to circulating androgen and estrogen levels, and gonadectomy in males does not affect aggression. Nevertheless, estrogenic pathways participate in the modulation of this non-breeding aggression. Our results parallel those put forth in other taxa, heightening the value of G. omarorum as a model to identify commonalities in neuroendrocrine strategies of vertebrate aggression control.

Variations on a theme: Genomics of sex determination in the cichlid fish Astatotilapia burtoni

Background

Sex chromosomes change more frequently in fish than in mammals or birds. However, certain chromosomes or genes are repeatedly used as sex determinants in different members of the teleostean lineage. East African cichlids are an enigmatic model system in evolutionary biology representing some of the most diverse extant vertebrate adaptive radiations. How sex is determined and if different sex-determining mechanisms contribute to speciation is unknown for almost all of the over 1,500 cichlid species of the Great Lakes. Here, we investigated the genetic basis of sex determination in a cichlid from Lake Tanganyika, Astatotilapia burtoni, a member of the most species-rich cichlid lineage, the haplochromines.

Results

We used RAD-sequencing of crosses for two populations of A. burtoni, a lab strain and fish caught at the south of Lake Tanganyika. Using association mapping and comparative genomics, we confirmed male heterogamety in A. burtoni and identified different sex chromosomes (LG5 and LG18) in the two populations of the same species. LG5, the sex chromosome of the lab strain, is a fusion chromosome in A. burtoni. Wnt4 is located on this chromosome, representing the best candidate identified so far for the master sex-determining gene in our lab strain of A. burtoni.

Conclusions

Cichlids exemplify the high turnover rate of sex chromosomes in fish with two different chromosomes, LG5 and LG18, containing major sex-determining loci in the two populations of A. burtoni examined here. However, they also illustrate that particular chromosomes are more likely to be used as sex chromosomes. Chromosome 5 is such a chromosome, which has evolved several times as a sex chromosome, both in haplochromine cichlids from all Great Lakes and also in other teleost fishes.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-016-3178-0) contains supplementary material, which is available to authorized users.

Two types of dominant male cichlid fish: behavioral and hormonal characteristics

Male African cichlid fish, Astatotilapia burtoni, have been classified as dominant or subordinate, each with unique behavioral and endocrine profiles. Here we characterize two distinct subclasses of dominant males based on types of aggressive behavior: (1) males that display escalating levels of aggression and court females while they establish a territory, and (2) males that display a stable level of aggression and delay courting females until they have established a territory. To profile differences in their approach to a challenge, we used an intruder assay. In every case, there was a male-male confrontation between the resident dominant male and the intruder, with the intruder quickly taking a subordinate role. However, we found that dominant males with escalating aggression spent measurably more time attacking subordinates than did dominant males with stable aggression that instead increased their attention toward the females in their tank. There was no difference in the behavior of intruders exposed to either type of dominant male, suggesting that escalating aggression is an intrinsic characteristic of some dominant males and is not elicited by the behavior of their challengers. Male behavior during the first 15 min of establishing a territory predicts their aggressive class. These two types of dominant males also showed distinctive physiological characteristics. After the intruder assay, males with escalating aggression had elevated levels of 11-ketotestosterone (11-KT), testosterone, estradiol, and cortisol, while those with stable aggression did not. These observations show that the same stimulus can elicit different behavioral and endocrine responses among A. burtoni dominant males that characterize them as either escalating or stable aggressive types. Our ability to identify which individuals within a population have escalating levels of aggressive responses versus those which have stable levels of aggressive responses when exposed to the same stimulus, offers a potentially powerful model for investigating the underlying molecular mechanisms that modulate aggressive behavior.

UV-filter benzophenone-3 inhibits agonistic behavior in male Siamese fighting fish (Betta splendens)

Benzophenone-3 (BP-3) is a widely used organic UV-filter compound. Despite the frequent occurrence of BP-3 in aquatic environments, little is known about its effect on fish behavior. The aim of this study was to investigate the endocrine disrupting effects of BP-3 in male fighting fish (Betta splendens) with a focus on agonistic behavior. Male fighting fish were exposed to 10, 100, and 1000 μg/L BP-3, as well as a solvent control (0.1% ethanol) and a positive control (100 ng/L 17α-ethynylestradiol, EE2), for 28 days. At the beginning and the end of exposure, standard length and body mass of the fish were measured for calculating the condition factor (CF). In addition, spontaneous swimming activity (total distance moved) and agonistic behavior (maximum velocity and duration of opercular display in front of a mirror) were also quantified. At the end of exposure, the fish gonads were sampled for gonadosomatic index (GSI) measurement and histology. After the exposure, CF was significantly decreased in the 1000 μg/L BP-3 groups. Spontaneous swimming activity was not affected. However, maximum velocity was significantly reduced in the EE2 and 1000 μg/L BP-3 treatments; duration of opercular display was significantly decreased in the EE2 and 10 and 1000 μg/L BP-3 treatments. GSI was not significantly different between groups. There was a slight but statistically significant decrease of relative proportion of mature spermatozoa in testicular tissue in the 100 μg/L BP-3 treatment. Collectively, our results demonstrate that BP-3 can disrupt agonistic behavior of male fighting fish, indicating the endocrine disrupting activity of this compound.

Removal of reproductive suppression reveals latent sex differences in brain steroid hormone receptors in naked mole-rats, Heterocephalus glaber

BACKGROUND

Naked mole-rats are eusocial mammals, living in large colonies with a single breeding female and 1-3 breeding males. Breeders are socially dominant, and only the breeders exhibit traditional sex differences in circulating gonadal steroid hormones and reproductive behaviors. Non-reproductive subordinates also fail to show sex differences in overall body size, external genital morphology, and non-reproductive behaviors. However, subordinates can transition to breeding status if removed from their colony and housed with an opposite-sex conspecific, suggesting the presence of latent sex differences. Here, we assessed the expression of steroid hormone receptor and aromatase messenger RNA (mRNA) in the brains of males and females as they transitioned in social and reproductive status.

METHODS

We compared in-colony subordinates to opposite-sex subordinate pairs that were removed from their colony for either 1 day, 1 week, 1 month, or until they became breeders (i.e., produced a litter). Diencephalic tissue was collected and mRNA of androgen receptor (Ar), estrogen receptor alpha (Esr1), progesterone receptor (Pgr), and aromatase (Cyp19a1) was measured using qPCR. Testosterone, 17β-estradiol, and progesterone from serum were also measured.

RESULTS

As early as 1 week post-removal, males exhibited increased diencephalic Ar mRNA and circulating testosterone, whereas females had increased Cyp19a1 mRNA in the diencephalon. At 1 month post-removal, females exhibited increased 17β-estradiol and progesterone. The largest changes in steroid hormone receptors were observed in breeders. Breeding females had a threefold increase in Cyp19a1 and fivefold increases in Esr1 and Pgr, whereas breeding males had reduced Pgr and increased Ar.

CONCLUSIONS

These data demonstrate that sex differences in circulating gonadal steroids and hypothalamic gene expression emerge weeks to months after subordinate animals are removed from reproductive suppression in their home colony.

Reproductive and parental care physiology of Cichlasoma dimerus males

The South American cichlid fish Cichlasoma dimerus presents a high breeding frequency and biparental care of the eggs and larvae. The male parental care period was divided in four different phases according to the developmental degree of the offspring: pre-spawning activity (MP, day 0), guarding eggs (ME, one day after fertilization (1 DAF)), guarding hatched larvae (MHL, 3 DAF), and guarding swimming larvae (MSL, 8 DAF). The aim of this study was to characterize male reproductive physiology by measuring steroid hormone plasma levels and analyzing testes cellular composition. Males exhibiting pre-spawning activity showed 8.4 times higher 11-ketotestosterone and 5.63 times higher testosterone levels than MHL. No differences were observed in estradiol and cortisol levels among the different phases. The cellular composition of the testes varied during the reproductive and parental care periods. Testes of MP were composed of 50% of spermatozoa, whereas spermatogonia type B and spermatocytes were predominant in the subsequent parental phases. A morphometric analysis of Leydig cells nuclear area revealed that MP and ME's Leydig cells averaged 1.27 times larger than that those of MHL and MSL and was positively correlated with circulating 11-KT and T levels. Hence, C. dimerus males showed important changes in its hormonal profiles and testicular cellular composition throughout the reproductive and parental care period.

Disruption of the rainbow trout reproductive endocrine axis by the polycyclic aromatic hydrocarbon benzo[a]pyrene

Successful reproduction in salmonids depends on a complex and highly regulated interplay between the pharmacokinetics and pharmacodynamics of naturally circulating sex steroids. The effects of a single intraperitoneal administration of the model PAH benzo[a]pyrene (B[a]P) on the kinetics of circulating levels of estradiol and testosterone through 7d post-injection in mature male and female rainbow trout (Oncorhynchus mykiss) in pre-spawning and spawning condition were investigated. Detailed measurements of the time course of injected E2 and excretion into the bile followed by pharmacokinetic modeling techniques were used to aid in identifying the potential mechanism of ED caused by B[a]P exposure. Plasma E2 and T concentrations were reduced significantly in both male and female trout. Administration of the GnRH analogue des-Gly(10)[D-Ala(6)]LH-RH-ethylamide, to induce spawning steroid profiles increased plasma E2 concentrations in control females, but not in B[a]P-treated fish. The mechanism underlying reductions in sex steroids in pre-spawning and spawning salmonids appears to be unrelated to the induction of P450 and related biotransformation enzymes by B[a]P. Induced biotransformation enzyme activities did not result in altered [(3)H]estradiol pharmacokinetics (e.g. terminal half-life) or elimination of steroid in bile, suggesting that B[a]P alters plasma E2 and T concentrations by other ED mechanisms in an anti-estrogenic manner.