Cichlid Fish Astatotilapia Burtoni Research Articles

Early-life social environment alters juvenile behavior and neuroendocrine function in a highly social cichlid fish

Early-life experiences can shape adult behavior, with consequences for fitness and health, yet fundamental questions remain unanswered about how early-life social experiences are translated into variation in brain and behavior. The African cichlid fish Astatotilapia burtoni, a model system in social neuroscience, is well known for its highly plastic social phenotypes in adulthood. Here, we rear juveniles in either social groups or pairs to investigate the effects of early-life social environments on behavior and neuroendocrine gene expression. We find that both juvenile behavior and neuroendocrine function are sensitive to early-life effects. Behavior robustly co-varies across multiple contexts (open field, social cue investigation, and dominance behavior assays) to form a behavioral syndrome, with pair-reared juveniles towards the end of syndrome that is less active and socially interactive. Pair-reared juveniles also submit more readily as subordinates. In a separate cohort, we measured whole brain expression of stress and sex hormone genes. Expression of glucocorticoid receptor 1a was elevated in group-reared juveniles, supporting a highly-conserved role for the stress axis mediating early-life effects. The effect of rearing environment on androgen receptor α and estrogen receptor α expression was mediated by treatment duration (1 vs. 5 weeks). Finally, expression of corticotropin-releasing factor and glucocorticoid receptor 2 decreased significantly over time. Rearing environment also caused striking differences in gene co-expression, such that expression was tightly integrated in pair-reared juveniles but not group-reared or isolates. Together, this research demonstrates the important developmental origins of behavioral phenotypes and identifies potential behavioral and neuroendocrine mechanisms.

Social context influences sensorimotor gating in female African cichlid fish Astatotilapia burtoni.

Disruption in prepulse inhibition (PPI), a sensorimotor gating phenomenon found in many species, has been associated with various psychiatric disorders in humans. Social defeat has been identified as a mediator of naturally evoked reductions of PPI in African cichlid fish Astatotilapia burtoni where males reversibly alter social status and their sensorimotor gating abilities. Here we investigated A. burtoni females, which establish a male-like social hierarchy with dominant (DOM) and subordinate (SUB) individuals when housed in communities without males. We asked if DOM and SUB females demonstrate socially induced PPI differences comparable to their male DOM and SUB counterparts. Results suggest that social defeat reduced PPI in SUB females as compared to DOM females (p = 0.033) and mixed-sex community female controls (p = 0.017). However, socially defeated females in same-sex communities remained proactive when engaging in antagonistic behaviors, which appears beneficial in avoiding substantial reductions in PPI as seen in reactive, socially defeated males. In open field swimming tests, SUB females exhibited increased anxiety-related behavior (thigmotaxis) as compared to females from mixed-sex communities (COM). Taken together, our results emphasize social defeat is a reliable modulator of PPI independent of sex, and anxiety related to social defeat might be a factor in mediating PPI plasticity.

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

BackgroundDifferential 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.ResultsCombining 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.ConclusionsWe 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.

Social rank, color morph, and social network metrics predict oxidative stress in a cichlid fish

Dominance hierarchies are a fundamental part of social systems in many species and social rank can influence access to resources and impact health and physiology. While social subordination is a profound stressor, few studies consider the social stress experienced by dominant males due to constantly needing to defend their dominance status through costly aggressive displays. Recent studies suggest that in species that use body coloration to signal status, these costs may also be color morph-specific. Our study examines the link between the social rank, intensity of territorial defense, body coloration, and oxidative stress in males of the color polymorphic cichlid fish Astatotilapia burtoni where males are either blue or yellow. We studied behavior in naturalistic communities and examined circulating reactive oxygen metabolites (ROMs) and antioxidant defenses. We found that dominant males experience higher concentrations of circulating ROMs without notably increasing their antioxidant defenses, but this effect was not related to color morph. Aggression and social network ties predicted oxidative stress in a morph-specific manner, with yellow but not blue males showing signs of increased oxidative damage with increasing agonistic effort. In contrast to expectation, oxidative stress was not influenced by cortisol or testosterone levels. We conclude that oxidative stress is instrumental to understanding the costs and benefits of high social rank. Living in a social hierarchy can be stressful even for those on top. While past work has established the stress involved in being a low-ranking member of a hierarchy, we show that while high-ranking males attain definite advantages in terms of decreased stress hormone levels and increased reproductive opportunities, they also pay for their increased activity level with increased risk of cellular damage.

Glial Cells in the Fish Retinal Nerve Fiber Layer Form Tight Junctions, Separating and Surrounding Axons

In the retina of teleost fish, cell addition continues throughout life involving proliferation and axonal growth. To study how this is achieved in a fully functioning retina, we investigated the nerve fiber layer (NFL) of the cichlid fish Astatotilapia burtoni for components that might regulate the extracellular environment. We hypothesized that growing axons are surrounded by different cell structures than signal conducting axons. Using immunohistochemistry and freeze fracture electron microscopy we found that the endfeet of Müller cells (MCs) expressed aquaporin-4 but not in high densities as in mammals. The presence of this water channel indicates the involvement of MCs in water homeostasis. Remarkably, we discovered conspicuous tight junctions in the retinal NFL. These tight junctions formed branching strands between myelin-like wrappings of ganglion cell axons that differed morphologically from any known myelin, and also an elaborate meshwork on large membrane faces between axons. We speculated that these tight junctions have additional functions than solely facilitating nerve conductance. Immunostainings against the adaptor protein ZO-1 labeled the NFL as did antibodies against the mammalian claudin-1, 3, and 19. Performing PCR analysis, we showed expression of claudin-1, 3, 5a, 5b, 9, 11, and 19 in the fish retina, claudins that typically occur at brain barriers or myelin. We could show by immunostains for doublecortin, a marker for differentiating neurons, that new axons are not surrounded by the myelin-like wrappings but only by the endfeet of MCs. We hypothesize that the tight junctions in the NFL of fish might contribute to the separation of an extracellular space around axons facilitating conductance, from a growth-promoting environment. For a functional test we applied Evans Blue dye to eye cup preparations which showed a retention of the dye in the NFL. This indicates that these remarkable tight junctions can indeed act as a diffusion barrier.

Color change and pigmentation in a color polymorphic cichlid fish

In many haplochromine cichlids, body coloration is an important communication cue during social interactions. In some cichlids, individuals can change color, but we have little information about the underlying physiological mechanisms. We examined the regulation of coloration in the color polymorphic cichlid fish Astatotilapia burtoni where males are either blue or yellow. Previous studies implicated the melanocortin system, a neuroendocrine center that regulates pigmentation and the stress response, in regulating the color polymorphism in this species. We found that both blue and yellow males express a high density of yellow xanthophores. Dispersal of xanthophore pigments in both yellow and blue morphs occurred within minutes in a dose-dependent manner. Similarly, exogenous α-melanocyte stimulating hormone (α-MSH, a melanocortin hormone) increased yellowness of the body in a dose-independent fashion. We observed many color changes in males housed in social communities with the proportion of yellow males increasing over the 2-week experimental period. However, color phenotype or color change was not influenced by experimental alteration of the stability of the social hierarchy. The effects of α-MSH suggest that the melanocortin system contributes to the polymorphism in coloration in A. burtoni but the role of social interactions and social stress in regulating color remains unclear.

Neural and behavioural correlates of repeated social defeat

Dominance hierarchies are common across the animal kingdom and have important consequences for reproduction and survival. Animals of lower social status cope with repeated social defeat using proactive and reactive behaviours. However, there remains a paucity of information on how an individual’s coping behaviours changes over time or what neural mechanisms are involved. We used a resident-intruder paradigm in the African cichlid fish Astatotilapia burtoni to investigate the neural correlates of these two opposing behaviour groups. Fish initially used both proactive and reactive behaviours, but had a dramatic increase in use of proactive behaviours during the third interaction, and this was followed by cessation of proactive behaviours and exclusive use of reactive coping. By quantifying neural activation in socially-relevant brain regions, we identify a subset of brain nuclei, including those homologous to the mammalian amygdala, showing higher activation in fish displaying proactive but not reactive behaviours. Fish displaying reactive behaviours had greater neural activation in the superior raphe, suggesting a possible conserved function during social defeat across vertebrates. These data provide the first evidence on the involvement of specific brain regions underlying proactive and reactive coping in fishes, indicating that these nuclei have conserved functions during social defeat across taxa.

Distribution and female reproductive state differences in orexigenic and anorexigenic neurons in the brain of the mouth brooding African cichlid fish, Astatotilapia burtoni.

Integration of reproduction and metabolism is necessary for species survival. While the neural circuits controlling energy homeostasis are well-characterized, the signals controlling the relay of nutritional information to the reproductive axis are less understood. The cichlid fish Astatotilapia burtoni is ideal for studying the neural regulation of feeding and reproduction because females cycle between a feeding gravid state and a period of forced starvation while they brood developing young inside their mouths. To test the hypothesis that candidate neuropeptide-containing neurons known to be involved in feeding and energy homeostasis in mammals show conserved distribution patterns, we performed immunohistochemistry and in situ hybridization to localize appetite-stimulating (neuropeptide Y, NPY; agouti-related protein, AGRP) and appetite-inhibiting (cocaine and amphetamine-regulated transcript, CART; pro-opiomelanocortin, pomc1a) neurons in the brain. NPY, AGRP, CART, and pomc1a somata showed distribution patterns similar to other teleosts, which included localization to the lateral tuberal nucleus (NLT), the putative homolog of the mammalian arcuate nucleus. Gravid females also had larger NPY and AGRP neurons in the NLT compared to brooding females, but brooding females had larger pomc1a neurons compared to gravid females. Hypothalamic agrp mRNA levels were also higher in gravid compared to brooding females. Thus, larger appetite-stimulating neurons (NPY, AGRP) likely promote feeding while females are gravid, while larger pomc1a neurons may act as a signal to inhibit food intake during mouth brooding. Collectively, our data suggest a potential role for NPY, AGRP, POMC, and CART in regulating energetic status in A. burtoni females during varying metabolic and reproductive demands.

Socially induced plasticity in sensorimotor gating in the African cichlid fish Astatotilapia burtoni.

Deficits in prepulse inhibition (PPI), social defeat and social withdrawal are hallmark features of several neurological and neuropsychiatric disorders. However, the link between social environment and PPI i.e., the possible role of social defeat in driving PPI plasticity, is far from clear. Here we explored these questions in the African cichlid fish Astatotilapia burtoni, where males exist as two distinct yet reversible phenotypes. In fish communities, DOMs exhibit frequent aggressive and territorial behaviors, threatening and attacking SUBs, which respond either by engaging in fights and fleeing, or by avoiding interaction with DOMs altogether. Social phenotypes were selected using focal observations of dominant and submissive behaviors. Tests of auditory PPI showed markedly decreased PPI in SUBs as compared to DOMs at prepulse/pulse interstimulus interval of ISI 50ms. Interestingly, further analysis showed the PPI reduction in SUBs was driven by males with low social interactivity. Testing males before and after social transitions revealed increasing and decreasing PPI in ascending and descending males, respectively. In an open field paradigm, SUBs also showed higher levels of wall hugging (thigmotaxis) and freezing when compared to DOMs i.e., an increase in anxiety-related behavior. Together the results suggest distinct yet reversible behavioral PPI phenotypes in A. burtoni males, and that social defeat drives PPI plasticity. The fact that PPI deficits are readily reversible by status change implies PPI plasticity may reflect an adaptive response to challenges in the social environment.

Neuromolecular Regulation of Aggression Differs by Social Role during Joint Territory Defense.

In response to a territory intrusion, neighboring males of the African cichlid fish Astatotilapia burtoni engage in aggressive joint territory defense in a manner that depends on their social role. Here, we examine the possible function of several neuroendocrine and neuromodulator pathways previously implicated in the regulation of complex social behavior. We find that the neuromolecular regulation of aggression during joint territory defense is very much dependent on an individual's role in this context. In neighbors but not in residents, aggression is correlated to gene expression in the medial part of the dorsal telencephalon (area Dm), the putative homolog to the mammalian basolateral amygdala. This correlation is strikingly high for expression of the serotonin receptor 5-HT2c, suggesting the serotonin system is important in regulating context-dependent behavior. Furthermore, by examining candidate gene expression co-variance patterns in area Dm and in the lateral part of the dorsal telencephalon (area Dl), the putative homolog to the mammalian hippocampus, we identify two main patterns: gene expression is co-regulated within, but not across, brain regions, and co-regulation is synergistic rather than antagonistic. Our results highlight the critical effect of social context on both behavior and its neuromolecular basis.

The melanocortin system regulates body pigmentation and social behaviour in a colour polymorphic cichlid fish.

The melanocortin system is a neuroendocrine system that regulates a range of physiological and behavioural processes. We examined the extent to which the melanocortin system simultaneously regulates colour and behaviour in the cichlid fish Astatotilapia burtoni We found that yellow males are more aggressive than blue males, in line with previous studies. We then found that exogenous α-melanocyte-stimulating hormone (α-MSH) increases yellowness of the body and dispersal of xanthophore pigments in both morphs. However, α-MSH had a morph-specific effect on aggression, with only blue males showing an increase in the rate of aggression. Exogenous agouti signalling peptide (ASIP), a melanocortin antagonist, did not affect coloration but reduced the rate of aggression in both colour morphs. Blue males had higher cortisol levels than yellow males. Neural gene expression of melanocortin receptors (mcr) and ligands was not differentially regulated between colour morphs. In the skin, however, mc1r and pro-opiomelanocortin (pomc) β were upregulated in blue males, while asip 1 was upregulated in yellow males. The effects of α-MSH on behaviour and body coloration, combined with morph-specific regulation of the stress response and the melanocortin system, suggest that the melanocortin system contributes to the polymorphism in behaviour and coloration in A. burtoni.

Variation of anal fin egg-spots along an environmental gradient in a haplochromine cichlid fish.

Male secondary sexual traits are targets of inter- and/or intrasexual selection, but can vary due to a correlation with life-history traits or as by-product of adaptation to distinct environments. Trade-offs contributing to this variation may comprise conspicuousness toward conspecifics versus inconspicuousness toward predators, or between allocating resources into coloration versus the immune system. Here, we examine variation in expression of a carotenoid-based visual signal, anal-fin egg-spots, along a replicate environmental gradient in the haplochromine cichlid fish Astatotilapia burtoni. We quantified egg-spot number, area, and coloration; applied visual models to estimate the trait's conspicuousness when perceived against the surrounding tissue under natural conditions; and used the lymphocyte ratio as a measure for immune activity. We find that (1) males possess larger and more conspicuous egg-spots than females, which is likely explained by their function in sexual selection; (2) riverine fish generally feature fewer but larger and/or more intensely colored egg-spots, which is probably to maintain signal efficiency in intraspecific interactions in long wavelength shifted riverine light conditions; and (3) egg-spot number and relative area correlate with immune defense, suggesting a trade-off in the allocation of carotenoids. Taken together, haplochromine egg-spots feature the potential to adapt to the respective underwater light environment, and are traded off with investment into the immune system.

Reproductive and metabolic state differences in olfactory responses to amino acids in a mouth brooding African cichlid fish.

Olfaction mediates many crucial life-history behaviors such as prey detection, predator avoidance, migration and reproduction. Olfactory function can also be modulated by an animal's internal physiological and metabolic states. While this is relatively well studied in mammals, little is known about how internal state impacts olfaction in fishes, the largest and most diverse group of vertebrates. Here we apply electro-olfactograms (EOGs) in the African cichlid fish Astatotilapia burtoni to test the hypothesis that olfactory responses to food-related cues (i.e. l-amino acids; alanine and arginine) vary with metabolic, social and reproductive state. Dominant males (reproductively active, reduced feeding) had greater EOG magnitudes in response to amino acids at the same tested concentration than subordinate males (reproductively suppressed, greater feeding and growth rates). Mouth brooding females, which are in a period of starvation while they brood fry in their mouths, had greater EOG magnitudes in response to amino acids at the same tested concentration than both recovering and gravid females that are feeding. Discriminant function analysis on EOG magnitudes also grouped the male (subordinate) and female (recovering, gravid) phenotypes with higher food intake together and distinguished them from brooding females and dominant males. The slope of the initial negative phase of the EOG also showed intra-sexual differences in both sexes. Our results demonstrate that the relationship between olfaction and metabolic state observed in other taxa is conserved to fishes. For the first time, we provide evidence for intra-sexual plasticity in the olfactory response to amino acids that is influenced by fish reproductive, social and metabolic state.

Cognitive skills and the evolution of social systems.

How do animal social skills influence evolution? Complex animal social behaviors require many cognitive skills including individual recognition and observational learning. For social systems to evolve, these abilities need to be transmitted genetically or culturally and supported by the evolution of underlying neural systems. Because animal skill sets are so varied, it seems best to describe animal cognitive behaviors as being a social calculus that can change with experience, which has evolved to match and facilitate the complexity of the social system where it arose. That is, acquiring and using social information in response to a rapidly changing complex world leads to social competence enabling success in essential behavioral interactions. Here, we describe the remarkable suite of social skills discovered in the African cichlid fish Astatotilapia burtoni, including an attention hierarchy, male deception, transitive inference, the mechanistic bases of social dominance, female mate choice and the neural control of female reproductive behavior. The social calculus of this species is presented as an example of a potential causal factor in the evolution of sophisticated social behavior necessary for the evolutionary success of their social system.

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

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

Social plasticity in non-territorial male African cichlid fish Astatotilapia burtoni

The African cichlid fish Astatotilapia burtoni represents a valuable model system for studies of social decision-making due to its socially mediated phenotypic plasticity. The males reversibly transition in social status from reproductively dominant and territorial (DOM) to submissive and non-territorial (SUB). Males are traditionally categorized into these two behavioral phenotypes by observational scoring. There is evidence, however, that this dichotomous categorization might not capture the behavioral plasticity displayed by individuals transitioning between SUB and DOM status. To test this concept, we used focal observations of intrasexual conflict behavior in fish communities combined with a modified analysis of the ethogram typically used in A. burtoni. Results revealed a cluster of males close to the crossover point between SUB and DOM status as defined by the traditional dominance index. These intermediate males showed the highest frequency of intrasexual conflict behaviors, distinct behavioral responses to threats, and body pigment signaling displays that distinguish them from prototypical SUBs and DOMs. As such, our results provide a noninvasive behavioral metric to categorize A. burtoni males into three groups, thus further capturing the complex social dynamic of this model organism.

Localization of glutamatergic, GABAergic, and cholinergic neurons in the brain of the African cichlid fish, Astatotilapia burtoni.

Neural communication depends on release and reception of different neurotransmitters within complex circuits that ultimately mediate basic biological functions. We mapped the distribution of glutamatergic, GABAergic, and cholinergic neurons in the brain of the African cichlid fish Astatotilapia burtoni using in situ hybridization to label vesicular glutamate transporters (vglut1, vglut2.1, vglut3), glutamate decarboxylases (gad1, gad2), and choline acetyltransferase (chat). Cells expressing the glutamatergic markers vgluts 1-3 show primarily nonoverlapping distribution patterns, with the most widespread expression observed for vglut2.1, and more restricted expression of vglut1 and vglut3. vglut1 is prominent in granular layers of the cerebellum, habenula, preglomerular nuclei, and several other diencephalic, mesencephalic, and rhombencephalic regions. vglut2.1 is widely expressed in many nuclei from the olfactory bulbs to the hindbrain, while vglut3 is restricted to the hypothalamus and hindbrain. GABAergic cells show largely overlapping gad1 and gad2 expression in most brain regions. GABAergic expression dominates nuclei of the subpallial ventral telencephalon, while glutamatergic expression dominates nuclei of the pallial dorsal telencephalon. chat-expressing cells are prominent in motor cranial nerve nuclei, and some scattered cells lie in the preoptic area and ventral part of the ventral telencephalon. A localization summary of these markers within regions of the conserved social decision-making network reveals a predominance of either GABAergic or glutamatergic cells within individual nuclei. The neurotransmitter distributions described here in the brain of a single fish species provide an important resource for identification of brain nuclei in other fishes, as well as future comparative studies on circuit organization and function. J. Comp. Neurol. 525:610-638, 2017. © 2016 Wiley Periodicals, Inc.

Changes of Sexual Behaviors in Rapamycin-injected Cichlid Fish Astatotilapia burtoni Males.

Cichlid fish species exhibit characteristic sexual behaviors according to not only reproductive stages but also social status. In a reproductive season, Astatotilapia burtoni males compete for females and a small number of dominant winners finally obtain the chance of spermiation. In addition to the characteristic behaviors, the dominant males have relatively bigger gonadotropin-releasing hormone 1 (GnRH1) neurons in the preoptic area (POA) of brain compared to those of subordinate males. Although the stimulatory effect of GnRH1 in vertebrate reproduction is well established, little is known about the triggering signal pathway to control GnRH1 neurons and GnRH1-mediated sexual behavior. In the present study, we evaluated the potential effect of TOR inhibitor rapamycin in relation to the cichlid male behaviors and GnRH1 neuron. After 14 h and 26 h of intraventricular injection of rapamycin, behavior patterns of chasing and courtship display did not show significant changes between rapamycin- and DMSO-injected males. Behaviors of spawning site entry increased in rapamycininjected fish at 26 h post-injection than at 14 h post-injection significantly (P<0.05). Meanwhile, there was a tendency that GnRH1 neurons’ soma size in the POA shrank by rapamycin injection, whereas the testes did not show notable changes. Taken together, these results suggest the possible role of TOR signal on GnRH1-mediated sexual behavior in cichlid dominant males, although further biological characterization of the TOR signaling pathway will be required to clarify this matter.

Cobalt Chloride Treatment Used to Ablate the Lateral Line System Also Impairs the Olfactory System in Three Freshwater Fishes.

Fishes use multimodal signals during both inter- and intra-sexual displays to convey information about their sex, reproductive state, and social status. These complex behavioral displays can include visual, auditory, olfactory, tactile, and hydrodynamic signals, and the relative role of each sensory channel in these complex multi-sensory interactions is a common focus of neuroethology. The mechanosensory lateral line system of fishes detects near-body water movements and is implicated in a variety of behaviors including schooling, rheotaxis, social communication, and prey detection. Cobalt chloride is commonly used to chemically ablate lateral line neuromasts, thereby eliminating water-movement cues to test for mechanosensory-mediated behavioral functions. However, cobalt acts as a nonspecific calcium channel antagonist and could potentially disrupt function of all superficially located sensory receptor cells, including those for chemosensing. Here, we examined whether CoCl2 treatment used to ablate the lateral line system also impairs olfaction in three freshwater fishes, the African cichlid fish Astatotilapia burtoni, goldfish Carassius auratus, and the Mexican blind cavefish Astyanax mexicanus. To examine the impact of CoCl2 on the activity of peripheral receptors, we quantified DASPEI fluorescence intensity of the olfactory epithelium from fish exposed to control and CoCl2 solutions. In addition, we examined brain activation in olfactory processing regions of A. burtoni immersed in either control or cobalt solutions. All three species exposed to CoCl2 had decreased DASPEI staining of the olfactory epithelium, and in A. burtoni, cobalt treatment caused reduced neural activation in olfactory processing regions of the brain. To our knowledge this is the first empirical evidence demonstrating that the same CoCl2 treatment used to ablate the lateral line system also impairs olfactory function. These data have important implications for the use of CoCl2 in future research and suggest that previous studies using CoCl2 should be reinterpreted in the context of both impaired mechanoreception and olfaction.

Inhibition of Aromatase Induces Partial Sex Change in a Cichlid Fish: Distinct Functions for Sex Steroids in Brains and Gonads

Sex steroids are major drivers of sexual development and also responsible for the maintenance of the established gender. Especially fishes exhibit great plasticity and less conservation in sex determination and sexual development compared to other vertebrate groups. In addition, fishes have a constant sex steroid production throughout their entire lifespan, which makes them particularly susceptible to interferences with the endogenous sex steroid system. This susceptibility has recently been used to show that inhibition of the key enzyme of estrogen synthesis, aromatase Cyp19a1, can induce functional sex reversal even in adult fish. Here, we investigated the impact of the aromatase inhibitor (AI) fadrozole in adult females of the East African cichlid fish Astatotilapia burtoni. Using gene expression, phenotypic measurements, behavioral experiments, and hormone measurements, we assessed if females treated with fadrozole develop a male-like phenotype. We found that AI treatment has a different effect on gene expression in the gonad compared to the brain, the 2 tissues mostly implicated in sexual development. In contrast to observations in other gonochoristic species, A. burtoni ovaries cannot be transformed into functional testis by AI. However, rapid changes towards a male-like phenotype can be induced with AI in coloration, hormone levels, and behavior.