Rayna M. Harris

The genomic substrate for adaptive radiation in African cichlid fish

Cichlid fishes are famous for large, diverse and replicated adaptive radiations in the Great Lakes of East Africa. To understand the molecular mechanisms underlying cichlid phenotypic diversity, we sequenced the genomes and transcriptomes of five lineages of African cichlids: the Nile tilapia (Oreochromis niloticus), an ancestral lineage with low diversity; and four members of the East African lineage: Neolamprologus brichardi/pulcher (older radiation, Lake Tanganyika), Metriaclima zebra (recent radiation, Lake Malawi), Pundamilia nyererei (very recent radiation, Lake Victoria), and Astatotilapia burtoni (riverine species around Lake Tanganyika). We found an excess of gene duplications in the East African lineage compared to tilapia and other teleosts, an abundance of non-coding element divergence, accelerated coding sequence evolution, expression divergence associated with transposable element insertions, and regulation by novel microRNAs. In addition, we analysed sequence data from sixty individuals representing six closely related species from Lake Victoria, and show genome-wide diversifying selection on coding and regulatory variants, some of which were recruited from ancient polymorphisms. We conclude that a number of molecular mechanisms shaped East African cichlid genomes, and that amassing of standing variation during periods of relaxed purifying selection may have been important in facilitating subsequent evolutionary diversification.

Predator exposure alters stress physiology in guppies across timescales.

In vertebrates, glucocorticoids mediate a wide-range of responses to stressors. For this reason, they are implicated in adaptation to changes in predation pressure. Trinidadian guppies (Poecilia reticulata) from high-predation environments have repeatedly and independently colonized and adapted to low-predation environments, resulting in parallel changes in life history, morphology, and behavior. We validated methods for non-invasive waterborne hormone sample collection in this species, and used this technique to examine genetic and environmental effects of predation on basal glucocorticoid (cortisol) levels. To examine genetic differences, we compared waterborne cortisol levels in high- and low-predation fish from two distinct population pairs. We found that fish from high-predation localities had lower cortisol levels than their low-predation counterparts. To isolate environmental influences, we compared waterborne cortisol levels in genetically similar fish reared with and without exposure to predator chemical cues. We found that fish reared with predator chemical cues had lower waterborne cortisol levels than those reared without. Comparisons of waterborne and whole-body cortisol levels demonstrated that populations differed in overall cortisol levels in the body, whereas rearing conditions altered the release of cortisol from the body into the water. Thus, evolutionary history with predators and lifetime exposure to predator cues were both associated with lower cortisol release, but depended on distinct physiological mechanisms.

Complex structural and regulatory evolution of the pro-opiomelanocortin gene family

The melanocortin system is a neuroendocrine machinery that has been associated with phenotypic diversification in a number of vertebrate lineages. Central to the highly pleiotropic melanocortin system is the pro-opiomelanocortin (pomc) gene family, a family of pre-prohormones that each give rise to melanocyte stimulating hormone (MSH), adrenocorticotropic releasing hormone (ACTH), β-lipotropin hormone, and β-endorphin. Here we examine the structure, tissue expression profile, and pattern of cis transcriptional regulation of the three pomc paralogs (α1, α2, and β) in the model cichlid fish Astatotilapia burtoni and other cichlids, teleosts, and mammals. We found that the hormone-encoding regions of pomc α1, pomc α2 and pomc β are highly conserved, with a few notable exceptions. Surprisingly, the pomc β gene of cichlids and pomacentrids (damselfish) encodes a novel melanocortin peptide, ε-MSH, as a result of a tandem duplication of the segment encoding ACTH. All three genes are expressed in the brain and peripheral tissues, but pomc α1 and α2 show a more spatially restricted expression profile than pomc β. In addition, the promoters of each pomc gene have diverged in nucleotide sequence, which may have facilitated the diverse tissue-specific expression profiles of these paralogs across species. Increased understanding of the mechanisms regulating pomc gene expression will be invaluable to the study of pomc in the context of phenotypic evolution.

Arginine vasotocin and androgen pathways are associated with mating system variation in North American cichlid fishes

Neuroendocrine pathways that regulate social behavior are remarkably conserved across divergent taxa. The neuropeptides arginine vasotocin/vasopressin (AVT/AVP) and their receptor V1a mediate aggression, space use, and mating behavior in male vertebrates. The hormone prolactin (PRL) also regulates social behavior across species, most notably paternal behavior. Both hormone systems may be involved in the evolution of monogamous mating systems. We compared AVT, AVT receptor V1a2, PRL, and PRL receptor PRLR1 gene expression in the brains as well as circulating androgen concentrations of free-living reproductively active males of two closely related North American cichlid species, the monogamous Herichthys cyanoguttatus and the polygynous Herichthys minckleyi. We found that H. cyanoguttatus males bond with a single female and together they cooperatively defend a small territory in which they reproduce. In H. minckleyi, a small number of large males defend large territories in which they mate with several females. Levels of V1a2 mRNA were higher in the hypothalamus of H. minckleyi, and PRLR1 expression was higher in the hypothalamus and telencephalon of H. minckleyi. 11-ketotestosterone levels were higher in H. minckleyi, while testosterone levels were higher in H. cyanoguttatus. Our results indicate that a highly active AVT/V1a2 circuit(s) in the brain is associated with space use and social dominance and that pair bonding is mediated either by a different, less active AVT/V1a2 circuit or by another neuroendocrine system.

Integrating resource defence theory with a neural nonapeptide pathway to explain territory-based mating systems

The ultimate-level factors that drive the evolution of mating systems have been well studied, but an evolutionarily conserved neural mechanism involved in shaping behaviour and social organization across species has remained elusive. Here, we review studies that have investigated the role of neural arginine vasopressin (AVP), vasotocin (AVT), and their receptor V1a in mediating variation in territorial behaviour. First, we discuss how aggression and territoriality are a function of population density in an inverted-U relationship according to resource defence theory, and how territoriality influences some mating systems. Next, we find that neural AVP, AVT, and V1a expression, especially in one particular neural circuit involving the lateral septum of the forebrain, are associated with territorial behaviour in males of diverse species, most likely due to their role in enhancing social cognition. Then we review studies that examined multiple species and find that neural AVP, AVT, and V1a expression is associated with territory size in mammals and fishes. Because territoriality plays an important role in shaping mating systems in many species, we present the idea that neural AVP, AVT, and V1a expression that is selected to mediate territory size may also influence the evolution of different mating systems. Future research that interprets proximate-level neuro-molecular mechanisms in the context of ultimate-level ecological theory may provide deep insight into the brain-behaviour relationships that underlie the diversity of social organization and mating systems seen across the animal kingdom.

Sex steroid hormones modulate responses to social challenge and opportunity in males of the monogamous convict cichlid, Amatitliana nigrofasciata

Steroid hormones play an important role in modulating behavioral responses to various social stimuli. It has been suggested that variation in the hormonal regulation of behavior across species is associated with social organization and/or mating system. In order to further elucidate the interplay of hormones and behavior in social situations, we exposed males of the monogamous convict cichlid Amatitliana nigrofasciata to three social stimuli: gravid female, intruder male, and a nonsocial stimulus. We used a repeated measure design to create behavioral profiles and explore how sex steroid hormones respond to and regulate social behavior. Results show distinct behavioral responses to different social situations, with circulating 11-ketotestosterone increasing in response to social stimuli. Pharmacological manipulations using specific androgen and estrogen receptor agonists and antagonists exposed complex control over digging behavior in the social opportunity context. In the social challenge context, aggressive behaviors decreased in response to blocking the androgen receptor pathway. Our results extend our understanding of sex steroid regulation of behavioral responses to social stimulation.

Social odors conveying dominance and reproductive information induce rapid physiological and neuromolecular changes in a cichlid fish

BackgroundSocial plasticity is a pervasive feature of animal behavior. Animals adjust the expression of their social behavior to the daily changes in social life and to transitions between life-history stages, and this ability has an impact in their Darwinian fitness. This behavioral plasticity may be achieved either by rewiring or by biochemically switching nodes of the neural network underlying social behavior in response to perceived social information. Independent of the proximate mechanisms, at the neuromolecular level social plasticity relies on the regulation of gene expression, such that different neurogenomic states emerge in response to different social stimuli and the switches between states are orchestrated by signaling pathways that interface the social environment and the genotype. Here, we test this hypothesis by characterizing the changes in the brain profile of gene expression in response to social odors in the Mozambique Tilapia, Oreochromis mossambicus. This species has a rich repertoire of social behaviors during which both visual and chemical information are conveyed to conspecifics. Specifically, dominant males increase their urination frequency during agonist encounters and during courtship to convey chemical information reflecting their dominance status.ResultsWe recorded electro-olfactograms to test the extent to which the olfactory epithelium can discriminate between olfactory information from dominant and subordinate males as well as from pre- and post-spawning females. We then performed a genome-scale gene expression analysis of the olfactory bulb and the olfactory cortex homolog in order to identify the neuromolecular systems involved in processing these social stimuli.ConclusionsOur results show that different olfactory stimuli from conspecifics’ have a major impact in the brain transcriptome, with different chemical social cues eliciting specific patterns of gene expression in the brain. These results confirm the role of rapid changes in gene expression in the brain as a genomic mechanism underlying behavioral plasticity and reinforce the idea of an extensive transcriptional plasticity of cichlid genomes, especially in response to rapid changes in their social environment.

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.

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.

Copy number variation in the melanocortin 4 receptor gene and alternative reproductive tactics the swordtail Xiphophorus multilineatus

Alternative reproductive tactics are powerful examples of how variation in genetics and physiology among individuals can lead to striking diversity in phenotype. In the swordtails (genus Xiphophorus), copy number variation (CNV) at the melanocortin 4 receptor (mc4r) locus is correlated with male body size, which in turn is correlated with male mating behavior. We measured the relationship between mc4r CNV, behavior, and 11-ketotesterone (11-KT) in X. multilineatus to determine whether mc4r CNV was associated with other components of male tactics in addition to body size. We confirmed the results of previous studies, showing that male size increases with mc4r CNV and that mating behavior toward females was size-dependent. We also examined agonistic behavior by exposing males to their mirror image and found that male-male displays behavior were size-dependent. Small males were less likely to exhibit an agonistic response, suggesting that alternative reproductive tactics span intrasexual and intersexual contexts. There was no significant association between mc4r CNV and behavior or 11-KT hormone titer. Mc4r CNV is thus associated with the variation in male body size, but not with other traits independent of body size.