A Neural Basis for Control of Cichlid Female Reproductive Behavior by Prostaglandin F2α

Olfactory GnRH3 crypt sensory neurons transduce sex pheromone signals to induce male courtship behavior in zebrafish

Olfactory activation of neuroendocrine pathways plays vital roles in many organisms for reproduction and survival. The importance of gonadotropin-releasing hormone (GnRH) neurons for reproduction is well-established but little is known about whether they can directly sense and transmit sex pheromone signals. We have uncovered the migration path and distribution pattern of a new GnRH neuronal population that fulfills this role. GnRH3 neurons arise from the region located beneath olfactory placode, undergo bidirectional migration along the olfactory nerve, and cell bodies lie within the olfactory epithelium, olfactory bulb and hypothalamus. These olfactory epithelial GnRH3 neurons express ora4, the olfactory receptor that detects pheromones. GnRH3-OB neurons with olfactory epithelial GnRH3 neurons ablation failed to respond to the waterborne post-ovulatory sex pheromone prostaglandin F2α (PGF2α). GnRH3 neurons in gnrh3-/- mutants have a reduced basal firing rate leading to abnormal responses to PGF2α. Male gnrh3-/- zebrafish exhibit deficiencies in courtship behavior and a decreased capacity to compete and spawn with females. These findings indicate that GnRH3-OE neurons function as crypt sensory neurons transducing sex pheromone-encoded information critical to reproductive success.

Prostaglandin E₂ activates the brain-pituitary axis via olfactory pathways in male Bostrychus sinensis

Prostaglandin E₂ (PGE₂) has been identified as a key sex pheromone in male Bostrychus sinensis, yet its molecular and neural mechanisms remain unclear. Here, we performed transcriptome sequencing on male B. sinensis brains following exposure to 50 nM PGE₂ to uncover genes and pathways involved in reproductive regulation. RNA-seq analysis revealed significant upregulation of key genes associated with the activation of the brain-pituitary axis. RT-PCR validation further confirmed the significant upregulation the expression of gnrh1 and kiss2 in the brain, as well as lhβ mRNA levels in the pituitary, supporting activation of the reproductive axis. To further elucidate the role of kiss2 in this regulatory pathway, we synthesized the core peptide of BsKiss2-12 and examined its functional effects. Administration of BsKiss2-12 (1μg/g) significantly increased the gnrh1 and kiss1ra mRNA levels in the brain, along with lhβ expression in the pituitary. Additionally, c-fos induction and DiI tracing experiments demonstrated that PGE₂ activated olfactory sensory neurons, relaying signals from the olfactory epithelium to the olfactory bulb and higher brain centers implicated in reproductive behavior. Collectively, our findings reveal that key molecular and neural mechanisms by which the sex pheromone PGE₂ modulates the reproductive axis in male B. sinensis.

Neuromodulation in the fish brain for reproductive success

In most teleosts, appropriate sexual behaviors and sexual maturation are essential for reproductive success. Most fish display their unique behavioral patterns for mating. These behaviors are thought to be regulated in the brain by sex steroid hormones since sexual behaviors are displayed only by sexually mature fish. In addition, recent studies have reported that neuropeptides, which are peptides released from neurons and modulate neural activities via their specific receptors in the brain, also play a key role in regulating sexual behavior. On the other hand, not only sexual behavior but also feeding behavior is important for reproductive function since sexual maturation requires sufficient nutrition. Especially feeding-related peptides, a type of neuropeptides, are thought to modulate feeding behavior. Thus, it is conceivable that neuropeptides are crucial modulators in the brain for reproductive success. This review summarizes recent advances in the knowledge of the neuromodulatory systems involved in sexual and feeding behaviors by neuropeptides and gonadal hormones.

A pheromone receptor in cichlid fish mediates attraction to females but inhibits male parental care

Reproductive behaviors differ across species, but the mechanisms that control variation in mating and parental care systems remain unclear. In many animal species, pheromones guide mating and parental care. However, it is not well understood how vertebrate pheromone signaling evolution can lead to new reproductive behavior strategies. In fishes, prostaglandin F2α (PGF2α) drives mating and reproductive pheromone signaling in fertile females, but this pheromonal activity appears restricted to specific lineages, and it remains unknown how a female fertility pheromone is sensed for most fish species. Here, we utilize single-cell transcriptomics and CRISPR gene editing in a cichlid fish model to identify and test the roles of key genes involved in olfactory sensing of reproductive cues. We find that a pheromone receptor, Or113a, detects fertile cichlid females and thereby promotes male attraction and mating behavior, sensing a ligand other than PGF2α. Furthermore, while cichlid fishes exhibit extensive parental care, for most species, care is provided solely by females. We find that males initiate mouthbrooding parental care if they have disrupted signaling in ciliated sensory neurons due to cnga2b mutation or if or113a is inactivated. Together, these results show that distinct mechanisms of pheromonal signaling drive reproductive behaviors across taxa. Additionally, these findings indicate that a single pheromone receptor has gained a novel role in behavior regulation, driving avoidance of paternal care among haplochromine cichlid fishes. Lastly, a sexually dimorphic, evolutionarily derived parental behavior is controlled by central circuits present in both sexes, while olfactory signals gate this behavior in a sex-specific manner.

Mate choice in the brain: species differ in how male traits 'turn on' gene expression in female brains

Mate choice plays a fundamental role in speciation, yet we know little about the molecular mechanisms that underpin this crucial decision-making process. Stickleback fish differentially adapted to limnetic and benthic habitats are reproductively isolated and females of each species use different male traits to evaluate prospective partners and reject heterospecific males. Here, we integrate behavioural data from a mate choice experiment with gene expression profiles from the brains of females actively deciding whether to mate. We find substantial gene expression variation between limnetic and benthic females, regardless of behavioural context, suggesting general divergence in constitutive gene expression patterns, corresponding to their genetic differentiation. Intriguingly, female gene co-expression modules covary with male display traits but in opposing directions for sympatric populations of the two species, suggesting male displays elicit a dynamic neurogenomic response that reflects known differences in female preferences. Furthermore, we confirm the role of numerous candidate genes previously implicated in female mate choice in other species, suggesting evolutionary tinkering with these conserved molecular processes to generate divergent mate preferences. Taken together, our study adds important new insights to our understanding of the molecular processes underlying female decision-making critical for generating sexual isolation and speciation.

Androgen receptor alpha deficiency impacts aromatase expression in the female cichlid brain

Steroid hormones bind to specific receptors that act as transcription factors to modify gene expression in the brain to regulate physiological and behavioural processes. The specific genes controlled by steroid hormones in the brain are not fully known. Identifying these genes is integral to establishing a comprehensive understanding of how hormones impact physiology and behaviour. A popular organism for answering this question is the cichlid fish Astatotilapia burtoni. Recently, CRISPR/Cas9 was used to engineer A. burtoni that lack functional androgen receptor (AR) genes encoding ARα. ARα mutant male A. burtoni produced fewer aggressive displays and possessed reduced expression of the gene encoding brain-specific aromatase, cyp19a1, in the ventromedial hypothalamus (VMH), an aggression locus. As a follow-up, we investigated whether ARα deficiency affected cyp19a1 expression in female A. burtoni using the same genetic line. We find that female A. burtoni possessing one or two non-functional ARα alleles had much higher expression of cyp19a1 in the preoptic area (POA), while females with one non-functional ARα allele possessed lower expression of cyp19a1 in the putative fish homologue of the bed nucleus of the stria terminalis (BNST). Thus, ARα may have a sex-specific role in modifying cyp19a1 expression in the teleost POA and BNST, regions that underlie sex differences across vertebrates.

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.

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.

Sex diversity in the 21st century: Concepts, frameworks, and approaches for the future of neuroendocrinology

Sex is ubiquitous and variable throughout the animal kingdom. Historically, scientists have used reductionist methodologies that rely on a priori sex categorizations, in which two discrete sexes are inextricably linked with gamete type. However, this binarized operationalization does not adequately reflect the diversity of sex observed in nature. This is due, in part, to the fact that sex exists across many levels of biological analysis, including genetic, molecular, cellular, morphological, behavioral, and population levels. Furthermore, the biological mechanisms governing sex are embedded in complex networks that dynamically interact with other systems. To produce the most accurate and scientifically rigorous work examining sex in neuroendocrinology and to capture the full range of sex variability and diversity present in animal systems, we must critically assess the frameworks, experimental designs, and analytical methods used in our research. In this perspective piece, we first propose a new conceptual framework to guide the integrative study of sex. Then, we provide practical guidance on research approaches for studying sex-associated variables, including factors to consider in study design, selection of model organisms, experimental methodologies, and statistical analyses. We invite fellow scientists to conscientiously apply these modernized approaches to advance our biological understanding of sex and to encourage academically and socially responsible outcomes of our work. By expanding our conceptual frameworks and methodological approaches to the study of sex, we will gain insight into the unique ways that sex exists across levels of biological organization to produce the vast array of variability and diversity observed in nature.

Genome editing in East African cichlids and tilapias: state-of-the-art and future directions

African cichlid fishes of the Cichlidae family are a group of teleosts important for aquaculture and research. A thriving research community is particularly interested in the cichlid radiations of the East African Great Lakes. One key goal is to pinpoint genetic variation underlying phenotypic diversification, but the lack of genetic tools has precluded thorough dissection of the genetic basis of relevant traits in cichlids. Genome editing technologies are well established in teleost models like zebrafish and medaka. However, this is not the case for emerging model organisms, such as East African cichlids, where these technologies remain inaccessible to most laboratories, due in part to limited exchange of knowledge and expertise. The Cichlid Science 2022 meeting (Cambridge, UK) hosted for the first time a Genome Editing Workshop, where the community discussed recent advances in genome editing, with an emphasis on CRISPR/Cas9 technologies. Based on the workshop findings and discussions, in this review we define the state-of-the-art of cichlid genome editing, share resources and protocols, and propose new possible avenues to further expand the cichlid genome editing toolkit.

Pheromone Perception in Fish: Mechanisms and Modulation by Internal Status

Pheromones are chemical signals that facilitate communication between animals, and most animals use pheromones for reproduction and other forms of social behavior. The identification of key ligands and olfactory receptors used for pheromonal communication provides insight into the sensory processing of these important cues. An individual's responses to pheromones can be plastic, as physiological status modulates behavioral outputs. In this review, we outline the mechanisms for pheromone sensation and highlight physiological mechanisms that modify pheromone-guided behavior. We focus on hormones, which regulate pheromonal communication across vertebrates including fish, amphibians, and rodents. This regulation may occur in peripheral olfactory organs and the brain, but the mechanisms remain unclear. While this review centers on research in fish, we will discuss other systems to provide insight into how hormonal mechanisms function across taxa.

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.

Knock-out of vasotocin reduces reproductive success in female zebrafish, Danio rerio

The vertebrate nonapeptide vasotocin/vasopressin is evolutionarily highly conserved and acts as neuromodulator and endocrine/paracrine signaling molecule. Circumstantial and mechanistic evidence from pharmacological manipulations of the vasotocin system in several teleost fishes suggest sex- and species-specific reproductive roles of vasotocin. While effects of vasotocin on teleost reproductive physiology involve both courtship behaviors and the regulation of the hypothalamic-pituitary-gonadal (HPG) axes, comprehensive studies investigating behavioral and physiological reproductive consequences of genetic ablation of vasotocin in a genetically tractable fish model, such as the zebrafish, are currently lacking. Here, we report the generation of homozygous CRISPR/Cas9-based vasotocin gene knock-out zebrafish. Breeding pairs of vasotocin knock-out fish produce significantly fewer fertilized eggs per clutch compared to wildtype fish, an effect coincident with reduced female quivering courtship behavior. Crossbreeding experiments reveal that this reproductive phenotype is entirely female-dependent, as vasotocin-deficient males reproduce normally when paired with female wild-type fish. Histological analyses of vasotocin knock-out ovaries revealed an overall reduction in oocytes and differential distribution of oocyte maturation stages, demonstrating that the reproductive phenotype is linked to oocyte maturation and release. Ovarian hormone quantification and gene expression analysis in mutant fish indicated reduced synthesis of Prostaglandin F2α, a hormone involved in ovarian maturation, egg release and regulation of female courtship behavior in some cyprinids. However, acute injection of vasotocin did not rescue the female mutant reproductive phenotype, suggesting a contribution of organizational effects of vasotocin. Together, this study provides further support for emerging roles of vasotocin in female teleost reproduction in an important teleost model species.

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.

Function of secretoneurin in regulating the expression of reproduction-related genes in ovoviviparous black rockfish (Sebastes schlegelii)

Secretoneurin (SN), a conserved peptide derived from secretogranin-2 (scg2), also known as secretogranin II or chromogranin C, plays an important role in regulating gonadotropin in the pituitary, which affects the reproductive system. This study aimed to clarify the mode of action of scg2 in regulating gonad development and maturation and the expression of mating behavior-related genes. Two scg2 cDNAs were cloned from the ovoviviparity teleost black rockfish (Sebastes schlegelii). In situ hybridization detected positive scg2 mRNA signals in the telencephalon and hypothalamus, where sgnrh and kisspeptin neurons were reported to be located and potentially regulated by scg2. In vivo, intracerebral ventricular injections of synthetic black rockfish SNa affected brain cgnrh, sgnrh, kisspeptin1, pituitary lh and fsh and gonad steroidogenesis-related gene expression levels with sex dimorphism. In vitro, a similar effect was found in primary cultured brain and pituitary cells. Thus, SN could contribute to the regulation of gonadal development, as well as reproductive behaviors, including mating and parturition.

A progestin regulates the prostaglandin pathway in the neuroendocrine system in female mudskipper Boleophthalmus pectinirostris

Sex hormones regulate the reproductive cycle through brain-pituitary axis, but the molecular mechanism is still enigmatic. In the reproductive season, the mudskipper Boleophthalmus pectinirostris possesses a semilunar periodicity spawning rhythm, which coincides with the semilunar periodicity variations in 17α-hydroxyprogesterone, the precursor of 17α,20β-dihydroxy-4-pregnen-3-one (DHP), a sexual progestin in teleosts. In the present study, we investigated in vitro the brain transcriptional differences between DHP-treated tissues and control groups using RNA-seq. Differential expression analysis revealed that 2700 genes significantly differentially expressed, including 1532 up-regulated and 1168 down-regulated genes. The majority of prostaglandin pathway-related genes were dramatically up-regulated, especially the prostaglandin receptor 6 (ptger6). Tissue distribution analysis revealed that ptger6 gene was ubiquitously expressed. In situ hybridization results showed that ptger6, nuclear progestin receptor (pgr), and DHP-induced c-fos mRNA were co-expressed in the ventral telencephalic area, the ventral nucleus of ventral telencephalic area, the anterior part of parvocellular preoptic nucleus, the magnocellular part of magnocellular preoptic nucleus, the ventral zone of periventricular hypothalamus, the anterior tubercular nucleus, the periventricular nucleus of posterior tuberculum, and the torus longitudinalis. DHP significantly enhanced promoter activities of ptger6 via Pgr. Together, this study suggested that DHP regulates the prostaglandin pathway in the neuroendocrine system of teleost fish.

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.

No evidence for sex chromosomes in natural populations of the cichlid fish Astatotilapia burtoni

Sex determination (SD) is not conserved among teleost fishes and can even differ between populations of the same species. Across the outstandingly species-rich fish family Cichlidae, more and more SD systems are being discovered. Still, the picture of SD evolution in this group is far from being complete. Lake Tanganyika and its affluent rivers are home to Astatotilapia burtoni, which belongs to the extremely successful East African cichlid lineage Haplochromini. Previously, in different families of an A. burtoni laboratory strain, an XYW system and an XY system have been described. The latter was also found in a second laboratory strain. In a laboratory-reared family descending from a population of the species' southern distribution, a second XY system was discovered. Yet, an analysis of sex chromosomes for the whole species distribution is missing. Here, we examined the genomes of 11 natural populations of A. burtoni, encompassing a wide range of its distribution, for sex-linked regions. We did not detect signs of differentiated sex chromosomes and also not the previously described sex chromosomal systems present in laboratory lines, suggesting different SD systems in the same species under natural and (long-term) artificial conditions. We suggest that SD in A. burtoni is more labile than previously assumed and consists of a combination of non-genetic, polygenic, or poorly differentiated sex chromosomes.

East African cichlid fishes

Cichlid fishes are a very diverse and species-rich family of teleost fishes that inhabit lakes and rivers of India, Africa, and South and Central America. Research has largely focused on East African cichlids of the Rift Lakes Tanganyika, Malawi, and Victoria that constitute the biodiversity hotspots of cichlid fishes. Here, we give an overview of the study system, research questions, and methodologies. Research on cichlid fishes spans many disciplines including ecology, evolution, physiology, genetics, development, and behavioral biology. In this review, we focus on a range of organismal traits, including coloration phenotypes, trophic adaptations, appendages like fins and scales, sensory systems, sex, brains, and behaviors. Moreover, we discuss studies on cichlid phylogenies, plasticity, and general evolutionary patterns, ranging from convergence to speciation rates and the proximate and ultimate mechanisms underlying these processes. From a methodological viewpoint, the last decade has brought great advances in cichlid fish research, particularly through the advent of affordable deep sequencing and advances in genetic manipulations. The ability to integrate across traits and research disciplines, ranging from developmental biology to ecology and evolution, makes cichlid fishes a fascinating research system.

Prostaglandin F2α drives female pheromone signaling in cichlids, revealing a basis for evolutionary divergence in olfactory signaling

Pheromones play essential roles in reproduction in many species. Prostaglandin F_2α (PGF_2α) acts as a female reproductive hormone and as a sex pheromone in some species. An olfactory receptor (OR) for PGF_2α was recently discovered in zebrafish, but this signaling pathway is evolutionarily labile. To understand the evolution of signals that attract males to fertile females, we used the African cichlid Astatotilapia burtoni and found that adult males strongly prefer fertile female odors. Injection of a prostaglandin synthesis inhibitor abolishes this attractivity of fertile females, indicating these hormones are necessary for pheromonal signaling. Unlike zebrafish, A. burtoni males are insensitive to PGF_2α, but they do exhibit strong preference for females injected with PGF_2α. This attractiveness is independent of the PGF_2α hormonal receptor Ptgfr, indicating that this pheromone signaling derives from PGF_2α metabolization into a yet-undiscovered pheromone. We further discovered that fish that are insensitive to PGF_2α lack an ortholog for the OR Or114 that zebrafish use to detect PGF_2α. These results indicate that PGF_2α itself does not directly induce male preference in cichlids. Rather, it plays a vital role that primes females to become attractive via an alternative male OR.

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

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

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.

Behavior and neural activation patterns of nonredundant visual and acoustic signaling during courtship in an African cichlid fish

Animals evolve mechanisms to send and receive communication signals through multiple sensory channels during crucial behavioral contexts like aggression and reproduction. This assures the transmission of important context-dependent signals that supply either the same (redundant) or different (nonredundant) information to the receiver. Despite the importance of multimodal communication, there are relatively few species in which information on sender signals and receiver responses are known. Further, little is known about where context-dependent unimodal and multimodal information is processed in the brain to produce adaptive behaviors. We used the African cichlid, Astatotilapia burtoni, to investigate how unimodal and multimodal signals are processed within the female brain in a reproductive context. During courtship, dominant males produce low frequency sounds in conjunction with visual displays (quivers) directed towards receptive gravid females. We compared affiliation behaviors and neural activation patterns in gravid females exposed to visual, acoustic, and visual-acoustic signals from courting dominant males. Females displayed reduced affiliation in auditory only conditions, but similar affiliation during visual and visual-acoustic conditions, demonstrating that visual-acoustic signaling from males is nonredundant but vision dominates. Using the neural activation marker cfos, we identified differential activation in specific socially-relevant brain nuclei between unimodal and multimodal conditions and distinct neural co-activation networks associated with each sensory context. Combined with our previous work on chemosensory signaling, we propose that A. burtoni represents a valuable vertebrate model for studying context-dependent behavioral and neural decision making associated with nonredundant multimodal communication.

Individual experience as a key to success for the cuckoo catfish brood parasitism

Brood parasites are involved in coevolutionary arms races with their hosts, whereby adaptations of one partner elicit the rapid evolution of counter-adaptations in the other partner. Hosts can also mitigate fitness costs of brood parasitism by learning from individual or social experience. In brood parasites, however, the role of learning can be obscured by their stealthy behaviour. Cuckoo catfish (Synodontis multipunctatus) parasitise clutches of mouthbrooding cichlids in Lake Tanganyika and are the only non-avian obligate brood parasites among vertebrates. We experimentally demonstrate that cuckoo catfish greatly enhance their efficiency in parasitising their hosts as they learn to overcome host defences. With increasing experience, cuckoo catfish increased their parasitism success by greater efficiency through improved timing and coordination of intrusions of host spawnings. Hence, within the coevolutionary arms races, brood parasites learn to overcome host defences during their lifetime.

The importance of learning for brood parasites is explored using cuckoo catfish. The catfish increase their parasitism success as they gain experience, mainly by improving their social coordination and timing of intrusions to cichlid host spawnings.

Expressions of melanopsins in telencephalon imply their function in synchronizing semilunar spawning rhythm in the mudskipper Boleophthalmus pectinirostris

The mudskipper Boleophthalmus pectinirostris inhabits intertidal mudflats, exhibiting semilunar reproductive rhythms. To investigate whether melanopsin is possibly involved in the synchronization of the semilunar spawning rhythm in the female mudskipper, we first cloned all four melanopsin subtypes (opn4m1, opn4m3, opn4x1, opn4x2) in B. pectinirostris. Results from RTq-PCR showed that significantly higher transcription levels of all four melanopsin subtypes were observed in the eyes rather than other tissues. In brain, all four melanopsin subtypes were also detectable in different regions, including the telencephalon, in which the expression of melanopsin has not been reported in other teleosts. The transcription levels of opn4m3 and opn4x1 in the telencephalon exhibited a daily fluctuation pattern. When females entered the spawning season, opn4m1 and opn4x1 transcript levels increased significantly in the telencephalon. During the spawning season, the transcript levels of opn4m3 and opn4x1 in the telencephalon appeared to have a cyclic pattern associated with semilunar periodicity, exhibiting two cycles with a peak around the first or the last lunar quarters. Results from ISH showed that, opn4x1 mRNA was localized in the medial of dorsal telencephalic area, dorsal nucleus of ventral telencephalic area (Vd), ventral nucleus of ventral telencephalic area (Vv), anterior part of parvocellular preoptic nucleus, magnocellular part of the magnocellular preoptic nucleus (PMmc), habenular and ventral zone of hypothalamus. Intriguingly, gnrh3 mRNA was also located in Vd, Vv and PMmc. Taken together, our results suggested that melanopsins, e.g. opn4x1, expressed in the telencephalon might mediate semilunar spawning activity in the female mudskipper.

Cyclooxygenases of ovoviviparous black rockfish (Sebastes schlegelii): Cloning, tissue distribution and potential role in mating and parturition

Prostaglandins are a series of unsaturated fatty acids that play critical roles in regulating reproductive events. The prostaglandins endoperoxide H synthases-1/2 (PGHS-1/2; also named cyclooxygenases-1/2, COX-1/2) catalyse the commitment step in prostaglandin synthesis. However, the of the cox genes in teleosts, especially ovoviviparous teleosts, is still unclear. The aim of the present study was to determine the potential role of cox genes in mating and parturition behaviour using black rockfish (Sebastes schlegelii) as a model species. Two transcripts, cox1 and cox2, were cloned. The phylogenetic analysis results revealed that both cox genes were closely related to mammalian coxs. qPCR analyses of their tissue distribution showed that cox1 was mainly expressed in the heart in both sexes, while cox2 was mainly expressed in the testis and ovary. Detection of cox expression in samples from reproductive-related stages further showed that both cox genes may play important roles in mating and parturition processes. In situ hybridization further detected positive cox mRNA signals in the testis and ovary, where they are known to be involved in mating and parturition behaviour. These data suggest that cox1 and cox2 are crucial in inducing mating, gonad regeneration and parturition behaviour.

High-speed camera recordings uncover previously unidentified elements of zebrafish mating behaviors integral to successful fertilization

The mating behavior of teleost fish consists of a sequence of stereotyped actions. By observing mating of zebrafish under high-speed video, we analyzed and characterized a behavioral cascade leading to successful fertilization. When paired, a male zebrafish engages the female by oscillating his body in high frequency (quivering). In response, the female pauses swimming and bends her body (freezing). Subsequently, the male contorts his trunk to enfold the female’s trunk. This behavior is known as wrap around. Here, we found that wrap around behavior consists of two previously unidentified components. After both sexes contort their trunks, the male adjusts until his trunk compresses the female’s dorsal fin (hooking). After hooking, the male trunk slides away from the female’s dorsal fin, simultaneously sliding his pectoral fin across the female’s gravid belly, stimulating egg release (squeezing/spawning). Orchestrated coordination of spawning presumably increases fertilization success. Surgical removal of the female dorsal fin inhibited hooking and the transition to squeezing. In a neuromuscular mutant where males lack quivering, female freezing and subsequent courtship behaviors were absent. We thus identified traits of zebrafish mating behavior and clarified their roles in successful mating.

In vitro follicle culture shows that progestagens are the maturation-inducing hormones (MIH) and possible regulators of the ovulation-mediating hormone PGE2 in female Eurasian perch Perca fluviatilis

In European aquaculture, Eurasian perch, Perca fluviatilis L., is perceived as one of the most highly valuable freshwater fish species and a strong candidate for the development of freshwater aquaculture. In the pursuit of improving the quality of reproduction in this domesticated species, investigating the hormones mediating the final oocyte maturation (FOM) is therefore indispensable. But, the exact nature of the maturation-inducing hormone (MIH) in Eurasian perch is unknown. To further validate the existence of a maturation-inducing activity behind potential hormonal candidates in this species, we in vitro tested a group of nine hormones: cortisol (Co), 11-deoxycortisol (11-D), corticosterone (coS), 11-deoxycorticosterone (DOC), 17α,20βdihydroxy-4-pregnen-3-one (DHP) and 17α,20β,21 trihydroxy-4-pregnen-3-one (THP), prostaglandin E₂ (PGE2), estradiol-17β (E2) and testosterone (T), in their ability to trigger FOM advancement and the production of sex steroids potentially involved in FOM. Using mature female perch, two in vitro experiments were conducted with oocytes at the start of the FOM. The follicles were incubated for 62 h in Cortland media with and without human chorionic gonadotropin (hCG). By the end of the incubation, only DHP and THP triggered the full advancement in FOM even at low doses with the effect of DHP being in vivo validated. However, the de novo productions of E2 and DHP were not shown to be regulated by either of the MIH candidates. Progestagens are hence more credible candidates as MIH than corticosteroids in Eurasian perch. Our in vitro study also revealed that both PGE2 and DHP are strongly associated with ovulation and that PGE2 might have slightly contributed to such DHP activity.

Manipulation of the Tyrosinase gene permits improved CRISPR/Cas editing and neural imaging in cichlid fish

Direct tests of gene function have historically been performed in a limited number of model organisms. The CRISPR/Cas system is species-agnostic, offering the ability to manipulate genes in a range of models, enabling insights into evolution, development, and physiology. Astatotilapia burtoni, a cichlid fish from the rivers and shoreline around Lake Tanganyika, has been extensively studied in the laboratory to understand evolution and the neural control of behavior. Here we develop protocols for the creation of CRISPR-edited cichlids and create a broadly useful mutant line. By manipulating the Tyrosinase gene, which is necessary for eumelanin pigment production, we describe a fast and reliable approach to quantify and optimize gene editing efficiency. Tyrosinase mutants also remove a major obstruction to imaging, enabling visualization of subdermal structures and fluorophores in situ. These protocols will facilitate broad application of CRISPR/Cas9 to studies of cichlids as well as other non-traditional model aquatic species.

Reproductive- and Social-State Plasticity of Multiple Sensory Systems in a Cichlid Fish

Intra- and inter-sexual communications are vital to the survival and reproductive success of animals. In species that cycle in and out of breeding or other physiological condition, sensory function can be modulated to optimize communication at crucial times. Little is known, however, about how widespread this sensory plasticity is across taxa, whether it occurs in multiple senses or both sexes within a species, and what potential modulatory substances and substrates are involved. Thus, studying modulation of sensory communication in a single species can provide valuable insights for understanding how sensory abilities can be altered to optimize detection of salient signals in different sensory channels and social contexts. The African cichlid fish Astatotilapia burtoni uses multimodal communication in social contexts such as courtship, territoriality, and parental care and shows plasticity in sensory abilities. In this review, we synthesize what is known about how visual, acoustic, and chemosensory communication is used in A. burtoni in inter- and intra-specific social contexts, how sensory funtion is modulated by an individual's reproductive, metabolic, and social state, and discuss evidence for plasticity in potential modulators that may contribute to changes in sensory abilities and behaviors. Sensory plasticity in females is primarily associated with the natural reproductive cycle and functions to improve detection of courtship signals (visual, auditory, chemosensory, and likely mechanosensory) from high-quality males for reproduction. Plasticity in male sensory abilities seems to function in altering their ability to detect the status of other males in the service of territory ownership and future reproductive opportunities. Changes in different classes of potential modulators or their receptors (steroids, neuropeptides, and biogenic amines) occur at both peripheral sensory organs (eye, inner ear, and olfactory epithelium) and central visual, olfactory, and auditory processing regions, suggesting complex mechanisms contributing to plasticity of sensory function. This type of sensory plasticity revealed in males and females of A. burtoni is likely more widespread among diverse animals than currently realized, and future studies should take an integrative and comparative approach to better understand the proximate and ultimate mechanisms modulating communication abilities across taxa.

Hormonal Prostaglandin F2α Mediates Behavioral Responsiveness to a Species-Specific Multi-component Male Hormonal Sex Pheromone in a Female Fish

Although hormonally-derived female sex pheromones have been well described in approximately a dozen species of teleost fish, only a few male sex pheromones have been characterized and the neuroendocrine underpinnings of behavioral responsiveness to them is not understood. Herein, we describe a study that addresses this question using the goldfish, Carassius auratus, an important model species of how hormones drive behavior in egg-laying teleost fishes. Our study had four components. First, we examined behavioral responsiveness of female goldfish and found that when injected with prostaglandin F2α (PGF2α), a treatment that drives female sexual receptivity, and found that they became strongly and uniquely attracted to the odor of conspecific mature males, while non-PGF2α-treated goldfish did not discern males from females. Next, we characterized the complexity and specificity of the male pheromone by examining the responsiveness of PGF2α-treated females to the odor of either mature male conspecifics or male common carp odor, as well as their nonpolar and polar fractions. We found that the odor of male goldfish was more attractive than that of male common carp, and that its activity was attributable to both its nonpolar and polar fractions with the later conveying information on species-identity. Third, we hypothesized that androstenedione (AD), a 19-carbon sex steroid produced by all male fish might be the nonpolar fraction and tested whether PGF2α-treated goldfish were attracted to either AD alone or as part of a mixture in conspecific water. We found that while AD was inactive on its own, it became highly attractive when added to previously unattractive female conspecific water. Lastly, in a test of whether nonhormonal conspecific odor might determine species-specificity, we added AD to water of three species of fish and found that while AD made goldfish water strongly attractive, its effects on other species holding water were small. We conclude that circulating PGF2α produced at the time of ovulation induces behavioral sensitivity to a male sex pheromone in female goldfish and that this male pheromone is comprised of AD and a mixture of body metabolites. Because PGF2α commonly mediates ovulation and female sexual behavior in egg-laying fishes, and AD is universally produced by male fishes as a precursor to testosterone, we suggest that these two hormones may have similar roles mediating male-female behavior and communication in many species of fish.

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.

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.

Modular genetic control of social status in a cichlid fish

Social hierarchies are ubiquitous in social species and profoundly influence physiology and behavior. Androgens like testosterone have been strongly linked to social status, yet the molecular mechanisms regulating social status are not known. The African cichlid fish Astatotilapia burtoni is a powerful model species for elucidating the role of androgens in social status given their rich social hierarchy and genetic tractability. Dominant A. burtoni males possess large testes and bright coloration and perform aggressive and reproductive behaviors while nondominant males do not. Social status in A. burtoni is in flux, however, as males alter their status depending on the social environment. Due to a teleost-specific whole-genome duplication, A. burtoni possess two androgen receptor (AR) paralogs, ARα and ARβ, providing a unique opportunity to disentangle the role of gene duplication in the evolution of social systems. Here, we used CRISPR/Cas9 gene editing to generate AR mutant A. burtoni and performed a suite of experiments to interrogate the mechanistic basis of social dominance. We find that ARβ, but not ARα, is required for testes growth and bright coloration, while ARα, but not ARβ, is required for the performance of reproductive behavior and aggressive displays. Both receptors are required to reduce flees from females and either AR is sufficient for attacking males. Thus, social status in A. burtoni is inordinately dissociable and under the modular control of two AR paralogs. This type of nonredundancy may be important in facilitating social plasticity in A. burtoni and other species whose social status relies on social experience.

Neural and molecular mechanisms underlying female mate choice decisions in vertebrates

Female mate choice is a dynamic process that allows individuals to selectively mate with those of the opposite sex that display a preferred set of traits. Because in many species males compete with each other for fertilization opportunities, female mate choice can be a powerful agent of sexual selection, often resulting in highly conspicuous traits in males. Although the evolutionary causes and consequences of the ornamentation and behaviors displayed by males to attract mates have been well studied, embarrassingly little is known about the proximate neural mechanisms through which female choice occurs. In vertebrates, female mate choice is inherently a social behavior, and although much remains to be discovered about this process, recent evidence suggests the neural substrates and circuits underlying other fundamental social behaviors (such as pair bonding, aggression and parental care) are likely similarly recruited during mate choice. Notably, female mate choice is not static, as social and ecological environments can shape the brain and, consequently, behavior in specific ways. In this Review, we discuss how social and/or ecological influences mediate female choice and how this occurs within the brain. We then discuss our current understanding of the neural substrates underlying female mate choice, with a specific focus on those that also play a role in regulating other social behaviors. Finally, we propose several promising avenues for future research by highlighting novel model systems and new methodological approaches, which together will transform our understanding of the causes and consequences of female mate choice.

An updated field guide for snark hunting: Comparative contributions to behavioral neuroendocrinology in the era of model organisms

Studying neuroendocrine behavioral regulatory mechanisms in a variety of species across vertebrate groups is critical for determining how they work in natural contexts, how they evolved, and ultimately what can be generalized from them, potentially even to humans. All of the above are difficult, at best, if work within our field is exclusively done in traditional laboratory organisms. The importance of comparative approaches for understanding the relationships between hormones and behavior has been recognized and advocated for since our field's inception through a series of papers centered upon a poetic metaphor of Snarks and Boojums, all of which have articulated the benefits that come from studying a diverse range of species and the risks associated with a narrow focus on "model organisms." This mini-review follows in the footsteps of those powerful arguments, highlighting some of the comparative work since the latest interactions of the metaphor that has shaped how we think about three major conceptual frameworks within our field, two of them formalized - the Organization/Activation Model of sexual differentiation and the Social Brain Network - and one, context-dependency, that is generally associated with virtually all modern understandings of how hormones affect behavior. Comparative approaches are broadly defined as those in which the study of mechanism is placed within natural and/or evolutionary contexts, whether they directly compare different species or not. Studies are discussed in relation to how they have either extended or challenged generalities associated with the frameworks, how they have shaped subsequent work in model organisms to further elucidate neuroendocrine behavioral regulatory mechanisms, and how they have stimulated work to determine if and when similar mechanisms influence behavior in our own species.

Ecotype differences in aggression, neural activity and behaviorally relevant gene expression in cichlid fish

In Lake Malawi, two ecologically distinct lineages of cichlid fishes (rock- vs sand-dwelling ecotypes, each comprised of over 200 species) evolved within the last million years. The rock-dwelling species (Mbuna) are aggressively territorial year-round and males court and spawn with females over rocky substrate. In contrast, males of sand-dwelling species are not territorial and instead aggregate on seasonal breeding leks in which males construct courtship "bowers" in the sand. However, little is known about how phenotypic variation in aggression is produced by the genome. In this study, we first quantify and compare behavior in seven cichlid species, demonstrating substantial ecotype and species differences in unconditioned mirror-elicited aggression. Second, we compare neural activity in mirror-elicited aggression in two representative species, Mchenga conophoros (sand-dwelling) and Petrotilapia chitimba (rock-dwelling). Finally, we compare gene expression patterns between these two species, specifically within neurons activated during mirror aggression. We identified a large number of genes showing differential expression in mirror-elicited aggression, as well as many genes that differ between ecotypes. These genes, which may underly species differences in behavior, include several neuropeptides, genes involved in the synthesis of steroid hormones and neurotransmitter activity. This work lays the foundation for future experiments using this emerging genetic model system to investigate the genomic basis of evolved species differences in both brain and behavior.

Examining the disconnect between prolactin and parental care in avian brood parasites

Prolactin is often referred to as the "parental hormone" but there are examples in which prolactin and parental behavior are disconnected. One intriguing example is in avian obligate brood parasites; species exhibiting high circulating prolactin but no parental care. To understand this disconnect, we examined transcriptional and behavioral responses to prolactin in brown-headed (Molothrus ater) and bronzed (M aeneus) brood parasitic cowbirds. We first examine prolactin-dependent regulation of transcriptome wide gene expression in the preoptic area (POA), a brain region associated with parental care across vertebrates. We next examined prolactin-dependent abundance of seven parental care-related candidate genes in hypothalamic regions that are prolactin-responsive in other avian species. We found no evidence of prolactin sensitivity in cowbirds in either case. To understand this prolactin insensitivity, we compared prolactin receptor transcript abundance between parasitic and nonparasitic species and between prolactin treated and untreated cowbirds. We observed significantly lower prolactin receptor transcript abundance in brown-headed but not bronzed cowbird POA compared with a nonparasite and no prolactin-dependent changes in either parasitic species. Finally, estrogen-primed female brown-headed cowbirds with or without prolactin treatment exhibited significantly greater avoidance of nestling begging stimuli compared with untreated birds. Taken together, our results suggest that modified prolactin receptor distributions in the POA and surrounding hypothalamic regions disconnect prolactin from parental care in brood parasitic cowbirds.

Utilizing the blind cavefish Astyanax mexicanus to understand the genetic basis of behavioral evolution

Colonization of novel habitats often results in the evolution of diverse behaviors. Comparisons between individuals from closely related populations that have evolved divergent behaviors in different environments can be used to investigate behavioral evolution. However, until recently, functionally connecting genotypes to behavioral phenotypes in these evolutionarily relevant organisms has been difficult. The development of gene editing tools will facilitate functional genetic analysis of genotype–phenotype connections in virtually any organism, and has the potential to significantly transform the field of behavioral genetics when applied to ecologically and evolutionarily relevant organisms. The blind cavefish Astyanax mexicanus provides a remarkable example of evolution associated with colonization of a novel habitat. These fish consist of a single species that includes sighted surface fish that inhabit the rivers of Mexico and southern Texas and at least 29 populations of blind cavefish from the Sierra Del Abra and Sierra de Guatemala regions of Northeast Mexico. Although eye loss and albinism have been studied extensively in A. mexicanus, derived behavioral traits including sleep loss, alterations in foraging and reduction in social behaviors are now also being investigated in this species to understand the genetic and neural basis of behavioral evolution. Astyanax mexicanus has emerged as a powerful model system for genotype–phenotype mapping because surface and cavefish are interfertile. Further, the molecular basis of repeated trait evolution can be examined in this species, as multiple cave populations have independently evolved the same traits. A sequenced genome and the implementation of gene editing in A. mexicanus provides a platform for gene discovery and identification of the contributions of naturally occurring variation to behaviors. This review describes the current knowledge of behavioral evolution in A. mexicanus with an emphasis on the molecular and genetic underpinnings of evolved behaviors. Multiple avenues of new research that can be pursued using gene editing tools are identified, and how these will enhance our understanding of behavioral evolution is discussed.

Evolutionary Dynamics of Structural Variation at a Key Locus for Color Pattern Diversification in Cichlid Fishes

Color patterns in African cichlid fishes vary spectacularly. Although phylogenetic analysis showed already 30 years ago that many color patterns evolved repeatedly in these adaptive radiations, only recently have we begun to understand the genomic basis of color variation. Horizontal stripe patterns evolved and were lost several times independently across the adaptive radiations of Lake Victoria, Malawi, and Tanganyika and regulatory evolution of agouti-related peptide 2 (agrp2/asip2b) has been linked to this phenotypically labile trait. Here, we asked whether the agrp2 locus exhibits particular characteristics that facilitate divergence in color patterns. Based on comparative genomic analyses, we discovered several recent duplications, insertions, and deletions. Interestingly, one of these events resulted in a tandem duplication of the last exon of agrp2. The duplication likely precedes the East African radiations that started 8-12 Ma, is not fixed within any of the radiations, and is found to vary even within some species. Moreover, we also observed variation in copy number (two to five copies) and secondary loss of the duplication, illustrating a surprising dynamic at this locus that possibly promoted functional divergence of agrp2. Our work suggests that such instances of exon duplications are a neglected mechanism potentially involved in the repeated evolution and diversification that deserves more attention.

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.

The Future of Gene-Guided Neuroscience Research in Non-Traditional Model Organisms

Natural variations across animals in form, function, and behavior have long been sources of inspiration to scientists. Despite this, experimentalists focusing on the neural bases of behavior have increasingly focused on a select few model species. This consolidation is motivated primarily by the availability of resources and technologies for manipulation in these species. Recent years have witnessed a proliferation of experimental approaches that were developed primarily in traditional model species, but that may in principle be readily applied to any species. High-throughput sequencing, CRISPR gene editing, transgenesis, and other technologies have enabled new insights through their deployment in non-traditional model species. The availability of such approaches changes the calculation of which species to study, particularly when a trait of interest is most readily observed in a non-traditional model organism. If these technologies are widely adopted in many new species, it promises to revolutionize the field of neuroethology.

Reproductive state-dependent plasticity in the visual system of an African cichlid fish

Visual communication is used widely across the animal kingdom to convey crucial information about an animals' identity, reproductive status, and sex. Although it is well-demonstrated that auditory and olfactory sensitivity can change with reproductive state, fewer studies have tested for plasticity in the visual system, a surprising detail since courtship and mate choice behaviors in many species are largely dependent on visual signals. Here, we tested for reproductive state-dependent plasticity in the eye of the cichlid fish Astatotilapia burtoni using behavioral, gene expression, neural activation, and electrophysiology techniques. Males court ovulated females more intensely than gravid females, and ovulated females were more responsive to male courtship behaviors than gravid females. Using electroretinography to measure visual sensitivity in dark-adapted fish, we revealed that gravid, reproductively-ready females have increased visual sensitivity at wavelengths associated with male courtship coloration compared to non-gravid females. After ovulation was hormonally induced, female's spectral sensitivity further increased compared to pre-injection measurements. This increased sensitivity after hormone injection was absent in non-gravid females and in males, suggesting an ovulation-triggered increase in visual sensitivity. Ovulated females had higher mRNA expression levels of reproductive neuromodulatory receptors (sex-steroids; gonadotropins) in the eye than nonovulated females, whereas males had similar expression levels independent of reproductive/social state. In addition, female mate choice-like behaviors positively correlated with expression of gonadotropin system receptors in the eye. Collectively, these data provide crucial evidence linking endocrine modulation of visual plasticity to mate choice behaviors in females.

CRISPR/CAS9 mutagenesis of a single r-opsin gene blocks phototaxis in a marine larva

Many marine animals depend upon a larval phase of their life cycle to locate suitable habitat, and larvae use light detection to influence swimming behaviour and dispersal. Light detection is mediated by the opsin genes, which encode light-sensitive transmembrane proteins. Previous studies suggest that r-opsins in the eyes mediate locomotory behaviour in marine protostomes, but few have provided direct evidence through gene mutagenesis. Larvae of the marine annelid Capitella teleta have simple eyespots and are positively phototactic, although the molecular components that mediate this behaviour are unknown. Here, we characterize the spatio-temporal expression of the rhabdomeric opsin genes in C. teleta and show that a single rhabdomeric opsin gene, Ct-r-opsin1, is expressed in the larval photoreceptor cells. To investigate its function, Ct-r-opsin1 was disrupted using CRISPR/CAS9 mutagenesis. Polymerase chain reaction amplification and DNA sequencing demonstrated efficient editing of the Ct-r-opsin1 locus. In addition, the pattern of Ct-r-opsin1 expression in photoreceptor cells was altered. Notably, there was a significant decrease in larval phototaxis, although the eyespot photoreceptor cell and associated pigment cell formed normally and persisted in Ct-r-opsin1-mutant animals. The loss of phototaxis owing to mutations in Ct-r-opsin1 is similar to that observed when the entire photoreceptor and pigment cell are deleted, demonstrating that a single r-opsin gene is sufficient to mediate phototaxis in C. teleta. These results establish the feasibility of gene editing in animals like C. teleta, and extend previous work on the development, evolution and function of the C. teleta visual system . Our study represents one example of disruption of animal behaviour by gene editing through CRISPR/CAS9 mutagenesis, and has broad implications for performing genome editing studies in a wide variety of other understudied animals.

One gene, multiple alleles: insights into the microevolution of pigmentation polymorphisms

Understanding how phenotypic variation is generated and maintained, and the evolutionary forces that shape these processes is the main goal of evolutionary biology. Great progress has been made in uncovering the genetic basis of morphological diversity, yet little is known about both the genetics and developmental basis of discrete polymorphisms segregating in wild populations. Exploring variation in developmental mechanisms at the population level can address the long-standing question of whether the mechanisms of change are the same at the micro- and macroevolutionary scale. This integration has been difficult mainly because the study of the evolution of developmental mechanisms and population genetics remain separate (Genetics, 195, 625 and 2013). In this issue of Molecular Ecology, Roberts et al. (Molecular Ecology and 2017) make a significant contribution towards bridging this gap by studying the genetic and developmental basis of an extremely variable pigmentation pattern. A polymorphic blotched coloration is common among females of four genera of Lake Malawi cichlids. The presence of this phenotype associates with a noncoding SNP upstream of the transcription factor pax7a (Science, 326, 998 and 2009). The authors describe in detail the morphs' pigmentation development, showing that phenotypic differences result from alterations in pigment cell development and survival. Next, using controlled crosses and population genetics studies, they identified three putative pax7a dominant blotch alleles that are associated with specific morphs. These different alleles lead to higher levels of pax7a transcript that correlate with different pigment cell composition. Finally, sequence comparison of the locus within populations and between species revealed a common origin of the allele controlling the blotched morph followed by a pattern of sequential appearance of derived alleles that gave rise to morph diversity. The coupling of the evolutionary history of this allelic series with the developmental analysis of the phenotype paves the way for a mechanistic understanding of morphological innovation and diversification.

Loss-of-function approaches in comparative physiology: is there a future for knockdown experiments in the era of genome editing?

Loss-of-function technologies, such as morpholino- and RNAi-mediated gene knockdown, and TALEN- and CRISPR/Cas9-mediated gene knockout, are widely used to investigate gene function and its physiological significance. Here, we provide a general overview of the various knockdown and knockout technologies commonly used in comparative physiology and discuss the merits and drawbacks of these technologies with a particular focus on research conducted in zebrafish. Despite their widespread use, there is an ongoing debate surrounding the use of knockdown versus knockout approaches and their potential off-target effects. This debate is primarily fueled by the observations that, in some studies, knockout mutants exhibit phenotypes different from those observed in response to knockdown using morpholinos or RNAi. We discuss the current debate and focus on the discrepancies between knockdown and knockout phenotypes, providing literature and primary data to show that the different phenotypes are not necessarily a direct result of the off-target effects of the knockdown agents used. Nevertheless, given the recent evidence of some knockdown phenotypes being recapitulated in knockout mutants lacking the morpholino or RNAi target, we stress that results of knockdown experiments need to be interpreted with caution. We ultimately argue that knockdown experiments should not be discontinued if proper control experiments are performed, and that with careful interpretation, knockdown approaches remain useful to complement the limitations of knockout studies (e.g. lethality of knockout and compensatory responses).

Evolutionary diversity as a catalyst for biological discovery

The tremendous diversity of animal behaviors has inspired generations of scientists from an array of biological disciplines. To complement investigations of ecological and evolutionary factors contributing to behavioral evolution, modern sequencing, gene editing, computational and neuroscience tools now provide a means to discover the proximate mechanisms upon which natural selection acts to generate behavioral diversity. Social behaviors are motivated behaviors that can differ tremendously between closely related species, suggesting phylogenetic plasticity in their underlying biological mechanisms. In addition, convergent evolution has repeatedly given rise to similar forms of social behavior and mating systems in distantly related species. Social behavioral divergence and convergence provides an entry point for understanding the neurogenetic mechanisms contributing to behavioral diversity. We argue that the greatest strides in discovering mechanisms contributing to social behavioral diversity will be achieved through integration of interdisciplinary comparative approaches with modern tools in diverse species systems. We review recent advances and future potential for discovering mechanisms underlying social behavioral variation; highlighting patterns of social behavioral evolution, oxytocin and vasopressin neuropeptide systems, genetic/transcriptional "toolkits," modern experimental tools, and alternative species systems, with particular emphasis on Microtine rodents and Lake Malawi cichlid fishes.

High levels of circulating prostaglandin F2α associated with ovulation stimulate female sexual receptivity and spawning behavior in the goldfish (Carassius auratus)

This study tested the hypothesis that blood-borne prostaglandin F_2α (PGF_2α) produced at the time of ovulation by female goldfish, a typical scramble-spawning, egg-laying cyprinid fish, functions as a hormone which stimulates female sexual receptivity, behavior, and pheromone release, thereby synchronizing female mating behavior with egg availability. We conducted 5 experiments. First, we tested whether PGF_2α is found in the blood of female fish and if it increases at the time of ovulation. Using gas chromatography-mass spectrometry, we found that circulating PGF_2α was approximately 1 ng/ml prior to ovulation, increased over 50-fold within 3 h of ovulation and returned to preovulatory values after spawning and egg release. Ovulated fish also released over 2 ng/h of PGF_2α and 800 ng/h of 15-keto-PGF_2α, a metabolite of PGF_2α - both compounds with known pheromonal function. Second, we tested how closely levels of circulating PGF_2α tracked the timing of ovulation by sampling fish at the time of ovulation and discovered that PGF_2α increased within 15 min of ovulation, peaked after 9 h, and fell to basal levels as fish spawned and released their eggs. Third, we tested whether an interaction between eggs and the reproductive tract serves as a source of circulating PGF_2α and its relationship with female sexual receptivity by injecting ovulated eggs (or an egg-substitute) into the reproductive tract of females stripped of ovulated eggs. We found both of these treatments elicited measurable increases in plasma PGF_2α as well as female sexual behavior. A fourth experiment showed that indothemacin, a PG synthase inhibitor, blocked both PGF_2α increase and female sexual behavior in egg-substitute-injected fish. Finally, we tested the relationship between the expression of female behavior and PGF_2α in PGF_2α-injected fish and found that circulating PGF_2α levels closely paralleled behavior, rising within 15 min and peaking at 45 min. Together, these experiments establish that PGF_2α functions as a behavioral blood-borne hormone in the goldfish, suggesting it likely has similar activity in other related, externally-fertilizing fishes.