Extended phenotypes offer a unique opportunity to experimentally manipulate and identify sources of selection acting on traits under natural conditions. The social cichlid fish Neolamprologus multifasciatus builds nests by digging up aquatic snail shells, creating an extended sexual phenotype that is highly amenable to experimental manipulation through addition of extra shells. Here, we find sources of both positive sexual selection and opposing natural selection acting on this trait; augmenting shell nests increases access to mates, but also increases social aggression and predation risk. Increasing the attractiveness of one male also changed social interactions throughout the social network and altered the entire community structure. Manipulated males produced and received more displays from neighbouring females, who also joined augmented male territories at higher rates than unmanipulated groups. However, males in more attractive territories received more aggression from neighbouring males, potentially as a form of social policing. We also detected a significant ecological cost of the ‘over-extended' phenotype; heterospecific predators usurped augmented nests at higher rates, using them as breeding sites and displacing residents. Using these natural experiments, we find that both social and ecological interactions generate clear sources of selection mediating the expression of an extended phenotype in the wild.
We examine the complex evolution of animal nervous systems and discuss the ramifications of this complexity for inferring the nature of early animals. Although reconstructing the origins of nervous systems remains a central challenge in biology, and the phenotypic complexity of early animals remains controversial, a compelling picture is emerging. We now know that the nervous system and other key animal innovations contain a large degree of homoplasy, at least on the molecular level. Conflicting hypotheses about early nervous system evolution are due primarily to differences in the interpretation of this homoplasy. We highlight the need for explicit discussion of assumptions and discuss the limitations of current approaches for inferring ancient phenotypic states.
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.
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.
Behavioral studies have often examined parental care by measuring phenotypic plasticity of behavior within a species. Phylogenetic studies have compared parental care among species, but only at broad categories (e.g., care vs. no care). Here we provide a detailed account that integrates phylogenetic analysis with quantitative behavioral data to better understand parental care behavior in the Cuatro Ciénegas cichlid, Herichthys minckleyi. We found that H. minckleyi occurs in a clade of sexually monochromatic or weakly dichromatic monogamous species, but that male and female H. minckleyi have dramatically different reproductive coloration patterns, likely as a result of sexual selection. Furthermore, we found that males are polygynous; large males guard large territories, and smaller males may attempt alternative mating tactics (sneaking). Finally, compared to the closely related monogamous Rio Grande cichlid, H. cyanoguttatus, males of H. minckleyi were present at their nests less often and performed lower rates of aggressive offspring defense, and females compensated for the absence of their mates by performing higher levels of offspring defense. Body color, mating system, and parental care in H. minckleyi appear to have evolved after it colonized Cuatro Ciénegas, and are likely a result of evolution in an isolated, stable environment.
Neuropeptides modulate many aspects of behavior and physiology in a broad range of animals. Arginine vasotocin (AVT) is implicated in mediating social behavior in teleost fish, although its specific role varies between species, sexes, life stages, and social context. To investigate whether the effects of AVT on behavior depend on social context, we used the African cichlid fish Astatotilapia burtoni, which is well-known for its remarkable behavioral plasticity. We pharmacologically manipulated the AVT system in established socially dominant and subordinate A. burtoni males, as well as in males ascending to dominance status in a socially unstable environment. Our results show that exogenous AVT causes a stress response, as evidenced by reduced behavioral activity and increased circulating levels of cortisol in established dominant and subordinate males. Administration of the AVT antagonist Manning compound, on the other hand, did not affect established subordinate or dominant males. However, AVT antagonist-treated males ascending from subordinate to dominant status exhibited reduced aggressive and increased courtship behavior compared to vehicle-treated animals. Finally, we measured circulating cortisol levels and brain gene expression levels of AVT and its behaviorally relevant V1a2 receptor in all three social phenotypes and found that plasma cortisol and mRNA levels of both genes were increased in ascending males compared to dominant and subordinate males. Our results provide a more detailed understanding of the role of the AVT system in the regulation of complex behavior in a dynamically changing social environment.
Discrete variation in reproductive behavior and physiology is observed in diverse taxa. Although it is known that most within-sex alternative reproductive tactics arise as a consequence of phenotypic plasticity, relatively little is known about differential neural gene expression among plastic alternative reproductive phenotypes. In the ocellated wrasse Symphodus ocellatus, males exhibit one of three alternative tactics (nesting, satellite, and sneaker) within a reproductive season, but switch tactics between years. Satellites and sneakers spawn parasitically in dominant (nesting) males' nests, but only nesting males provide parental care. Nesting and satellite males show transient cooperative defense of nests against sneakers. Here, we analyze circulating sex steroid hormone levels and neural gene expression profiles in these three male phenotypes and in females. 11-ketotestosterone (but not testosterone) was highest in nesting males, while estradiol was highest in females. Brain transcriptomes of satellites and females were most similar to each other and intermediate to nesting and sneaker males. Sneakers showed more total expression differences, whereas nesting males showed higher magnitude expression differences. Our findings reveal the surprising extent to which neural gene expression patterns vary across reproductive tactics that vary in a number of social traits, including aggression, territoriality, and cooperation, providing important insights into the molecular mechanisms that may underlie variation in cooperative and reproductive behavior.
Educated by his deep appreciation of nature, Darwin observed that "from so simple a beginning endless forms most beautiful" have arisen throughout the evolutionary history of life on earth (1). The spectacular diversity of orchids (2) and beetles (3) has long fascinated naturalists and casual observers alike. More recently, the adaptive radiations of Hawaiian drosophilids (4), Caribbean Anolis lizards (5), and African cichlid fishes (6) have become prime examples for understanding the mechanisms that enable diversification. Gene duplication and deletion are generally considered important evolutionary mechanisms that give rise to phenotypic diversity (7). Following gene duplication and loss, adaptation and speciation appear to proceed through a combination of both structural and cis-regulatory changes in one or more paralogous genes (8). Recent advances in sequencing technology have enabled researchers to make significant progress in understanding the molecular evolution that has facilitated diversification. In PNAS, Cortesi et al. (9) examine the evolution of vertebrate opsin genes as a spectacular example of how gene duplication and deletion events that affect spectral sensitivity have driven adaptation to diverse light environments and visual displays.
Social 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.
The neural and molecular mechanisms underlying social behavior - including their functional significance and evolution - can only be fully understood using data obtained under multiple social, environmental, and physiological conditions. Understanding the complexity of social behavior requires integration across levels of analysis in both laboratory and field settings. However, there is currently a disconnect between the systems studied in the laboratory versus the field. We argue that recent conceptual and technical advances provide exciting new opportunities to close this gap by making non-model organisms accessible to modern approaches in both laboratory and nature.
At menopause, the dramatic loss of ovarian estradiol (E2) necessitates the adaptation of estrogen-sensitive neurons in the hypothalamus to an estrogen-depleted environment. We developed a rat model to test the "critical window" hypothesis of the effects of timing and duration of E2 treatment after deprivation on the hypothalamic neuronal gene network in the arcuate nucleus and the medial preoptic area. Rats at 2 ages (reproductively mature or aging) were ovariectomized and given E2 or vehicle replacement regimes of differing timing and duration. Using a 48-gene quantitative low-density PCR array and weighted gene coexpression network analysis, we identified gene modules differentially regulated by age, timing, and duration of E2 treatment. Of particular interest, E2 status differentially affected suites of genes in the hypothalamus involved in energy balance, circadian rhythms, and reproduction. In fact, E2 status was the dominant factor in determining gene modules and hormone levels; age, timing, and duration had more subtle effects. Our results highlight the plasticity of hypothalamic neuroendocrine systems during reproductive aging and its surprising ability to adapt to diverse E2 replacement regimes.
Across animals, there is remarkable diversity in behavior. Modern genomic approaches have made it possible to identify the molecular underpinnings of varied behavioral phenotypes. By examining species with plastic phenotypes we have begun to understand the dynamic and flexible nature of neural transcriptomes and identified gene modules associated with variation in social and reproductive behaviors in diverse species. Importantly, it is becoming increasingly clear that some candidate genes and gene networks are involved in complex social behaviors across even divergent species, yet few comparative transcriptomics studies have been conducted that examine a specific behavior across species. We discuss the implications of a range of important and insightful studies that have increased our understanding of the neurogenomics of behavioral plasticity. Despite its successes, behavioral genomics has been criticized for its lack of hypotheses and causative insights. We propose here a novel avenue to overcome some of these short-comings by complementing "forward genomics" studies (i.e., from phenotype to behaviorally relevant gene modules) with a "reverse genomics" approach (i.e., manipulating novel gene modules to examine effects on behavior, hormones, and the genome itself) to examine the functional causes and consequences of differential gene expression patterns. We discuss how several established approaches (such as pharmacological manipulations of a novel candidate pathway, fine scale mapping of novel candidate gene expression in the brain, or identifying direct targets of a novel transcription factor of interest) can be used in combination with the analysis of the accompanying neurogenomic responses to reveal unexpected biological processes. The integration of forward and reverse genomics will move the field beyond statistical associations and yield great insights into the neural and molecular control of social behavior and its evolution.
Cichlid fishes are famous for large, diverse and replicated adaptive radiations in the Great Lakes of East Africa. To understand themolecularmechanisms underlying cichlid phenotypic diversity,wesequenced the genomesand transcriptomes of five lineages of Africancichlids: theNile tilapia (Oreochromis niloticus), an ancestral lineagewith low diversity; and four members of the East African lineage: Neolamprologus brichardi/pulcher (older radiation, Lake Tanganyika), Metriaclima zebra (recent radiation, LakeMalawi),Pundamilia nyererei (very recent radiation, LakeVictoria), andAstatotilapia burtoni (riverine species around Lake Tanganyika).Wefound 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 fromancient polymorphisms. Weconclude that a numberof molecular mechanisms shaped East African cichlid genomes, and that amassing of standing variation during periods of relaxed purifying selectionmayhave been important in facilitating subsequent evolutionary diversification.
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), ??-lipotropin hormone, and ??-endorphin. Here we examine the structure, tissue expression profile, and pattern of cis transcriptional regulation of the three pomc paralogs (??1, ??2, and ??) in the model cichlid fish Astatotilapia burtoni and other cichlids, teleosts, and mammals. We found that the hormone-encoding regions of pomc ??1, pomc ??2 and pomc ?? are highly conserved, with a few notable exceptions. Surprisingly, the pomc ?? gene of cichlids and pomacentrids (damselfish) encodes a novel melanocortin peptide, ??-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 ??1 and ??2 show a more spatially restricted expression profile than pomc ??. 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. ?? 2013 Elsevier Inc.
Social interactions are central to most animals and have a fundamental impact upon the phenotype of an individual. Social behavior (social interactions among conspecifics) represents a central challenge to the integration of the functional and mechanistic bases of complex behavior. Traditionally, studies of proximate and ultimate elements of social behavior have been conducted by distinct groups of researchers, with little communication across perceived disciplinary boundaries. However, recent technological advances, coupled with increased recognition of the substantial variation in mechanisms underlying social interactions, should compel investigators from divergent disciplines to pursue more integrative analyses of social behavior. We propose an integrative conceptual framework intended to guide researchers towards a comprehensive understanding of the evolution and maintenance of mechanisms governing variation in sociality.
Remarkable examples of social cognition have been described across a diverse range of species, yet surprisingly little is known about the neurobiological underpinnings of these behaviors. Recent studies suggest that the molecular pathways and neural networks that mediate social behavior have been relatively conserved across vertebrate evolution, suggesting that shared mechanisms may drive adaptive behavioral responses to social stimuli. Here, we review recent advances in the neurobiology of flexible and context-dependent social behaviors across vertebrate taxa, focusing on female mate choice, pair-bonding, and aggressive behavior. Furthermore, we highlight the outstanding opportunities for uncovering the mechanisms mediating cooperative behavior, an exemplar of social cognition. We suggest a framework for investigating context-dependent neural organization and the evoked neural response to social stimuli. ?? 2014 Elsevier Ltd.
Cichlid fishes are famous for large, diverse and replicated adaptive radiations in the Great Lakes of East Africa. To under- standthemolecularmechanismsunderlying cichlidphenotypic diversity,wesequencedthegenomesandtranscriptomes of fivelineages of Africancichlids: theNile tilapia (Oreochromis niloticus),anancestral lineagewithlowdiversity;andfour members of the East African lineage: Neolamprologus brichardi/pulcher (older radiation, Lake Tanganyika),Metriaclima zebra(recent radiation,LakeMalawi),Pundamilianyererei (veryrecentradiation,LakeVictoria),andAstatotilapiaburtoni (riverine species aroundLakeTanganyika).Wefound an excess of gene duplications in the East African lineagecompared to tilapia and other teleosts, an abundance of non-coding element divergence, accelerated coding sequence evolution, expression divergence associated with transposable element insertions,and regulationby novel microRNAs. In addition, we analysed sequence data from sixty individuals representing six closely related species from Lake Victoria, and show genome-widediversifying selectiononcodingandregulatoryvariants,someofwhichwererecruited fromancientpoly- morphisms.Weconclude that anumberof molecular mechanismsshaped East African cichlid genomes, and that amass- ing of standing variation during periods of relaxed purifying selectionmayhavebeenimportantin facilitating subsequent evolutionary diversification.
Efforts to understand nervous system structure and function have received new impetus from the federal Brain Research through Advancing Innovative Neurotechnologies (BRAIN) Initiative. Comparative analyses can contribute to this effort by leading to the discovery of general principles of neural circuit design, information processing, and gene-structure-function relationships that are not apparent from studies on single species. We here propose to extend the comparative approach to nervous system 'maps' comprising molecular, anatomical, and physiological data. This research will identify which neural features are likely to generalize across species, and which are unlikely to be broadly conserved. It will also suggest causal relationships between genes, development, adult anatomy, physiology, and, ultimately, behavior. These causal hypotheses can then be tested experimentally. Finally, insights from comparative research can inspire and guide technological development. To promote this research agenda, we recommend that teams of investigators coalesce around specific research questions and select a set of 'reference species' to anchor their comparative analyses. These reference species should be chosen not just for practical advantages, but also with regard for their phylogenetic position, behavioral repertoire, well-annotated genome, or other strategic reasons. We envision that the nervous systems of these reference species will be mapped in more detail than those of other species. The collected data may range from the molecular to the behavioral, depending on the research question. To integrate across levels of analysis and across species, standards for data collection, annotation, archiving, and distribution must be developed and respected. To that end, it will help to form networks or consortia of researchers and centers for science, technology, and education that focus on organized data collection, distribution, and training. These activities could be supported, at least in part, through existing mechanisms at NSF, NIH, and other agencies. It will also be important to develop new integrated software and database systems for cross-species data analyses. Multidisciplinary efforts to develop such analytical tools should be supported financially. Finally, training opportunities should be created to stimulate multidisciplinary, integrative research into brain structure, function, and evolution.