Behavioral plasticity in C. elegans: paradigms,circuits, genes

Life in the soil is an intellectual and practical challenge that the nematode Caenorhabditis elegans masters by utilizing 302 neurons. The nervous system assembled by these 302 neurons is capable of executing a variety of behaviors, some of respectable complexity. The simplicity of the nervous system, its thoroughly characterized structure, several sets of well-defined behaviors, and its genetic amenability combined with its isogenic background make C. elegans an attractive model organism to study the genetics of behavior. This review describes several behavioral plasticity paradigms in C. elegans and their underlying neuronal circuits and then goes on to review the forward genetic analysis that has been undertaken to identify genes involved in the execution of these behaviors. Lastly, the review outlines how reverse genetics and genomic approaches can guide the analysis of the role of genes in behavior and why and how they will complement the forward genetic analysis of behavior.     

Modular genetic control of sexually dimorphic behaviors

Sex hormones such as estrogen and testosterone are essential for sexually dimorphic behaviors in vertebrates. However, the hormone-activated molecular mechanisms that control the development and function of the underlying neural circuits remain poorly defined. We have identified numerous sexually dimorphic gene expression patterns in the adult mouse hypothalamus and amygdala. We find that adult sex hormones regulate these expression patterns in a sex-specific, regionally restricted manner, suggesting that these genes regulate sex typical behaviors. Indeed, we find that mice with targeted disruptions of each of four of these genes (Brs3, Cckar, Irs4, Sytl4) exhibit extremely specific deficits in sex specific behaviors, with single genes controlling the pattern or extent of male sexual behavior, male aggression, maternal behavior, or female sexual behavior. Taken together, our findings demonstrate that various components of sexually dimorphic behaviors are governed by separable genetic programs.     

Genetic basis of male sexual behavior

Male sexual behavior is increasingly the focus of genetic study in a variety of animals. Genetic analysis in the soil roundworm Caenorhabditis elegans and the fruit fly Drosophila melanogaster has lead to identification of genes and circuits that govern behaviors ranging from motivation and mate-searching to courtship and copulation. Some worm and fly genes have counterparts with related functions in higher animals and many more such correspondences can be expected. Analysis of mutations in mammals can potentially lead to insights into such issues as monogamous versus promiscuous sexual behavior and sexual orientation. Genetic analysis of sexual behavior has implications for understanding how the nervous system generates and controls a complex behavior. It can also help us to gain an appreciation of how behavior is encoded by genes and their regulatory sequences.     

Genes,environment and responsibility for violent behavior: “Whatever genes one has it is preferable that you are prevented from going around stabbing people”

For the legal system to function effectively people are generally viewed as autonomous actors able to exercise choice and responsible for their actions. It is conceivable that genetic traits associated with violent and antisocial behavior could call into question an affected individual's responsibility for acts of criminal violence. Evidence concerning genes associated with violent and antisocial behavior has been introduced in criminal courts in the USA and Italy, either alone or with associated environmental factors. One example of a “genetic defense” is based on low levels of monoamine oxidase A (MAOA) activity, with a prevalence of around 30% in Caucasian males. In countries with trial by jury it is particularly relevant to consider the views of publics on criminal liability and the significance they assign to evidence citing genetic influences on behavior. This article draws on largely qualitative research looking at participants' explanations of, and assigning of responsibility for, violent and antisocial behavior where environmental or genetic influences are claimed. Genetic factors were not viewed deterministically by participants but were considered by most to be irrelevant to personal responsibility. Notions of human agency, free will and choice were crucial to explanations of problem behaviors and ensured that offenders could be held responsible despite evidence on environmental and genetic factors.     

Conceptual and methodological issues in the genetics of mouse agonistic behavior

Currently, 36 genes have been reported to affect offensive behavior in male mice. Potentially, these genes could be used to analyze the mechanism of this behavior. But there are methodological flies in this conceptual ointment. The studies with these genes varied in the genetic background, the maternal environments, the postweaning housing, the strain or type of opponent, and the type of test. The effects of each of these on the genetics of offense are reviewed with examples. It is concluded that between-study variation in these environmental or experiential circumstances may make it difficult to impossible to relate the effect of one genetic variant to another and to use these to identify and relate the pathways for gene effects on offensive behaviors. For this reason, standardization of these conditions is recommended.     

Defining functional gene‐circuit interfaces in the mouse nervous system

Complexity in the nervous system is established by developmental genetic programs, maintained by differential genetic profiles and sculpted by experiential and environmental influence over gene expression. Determining how specific genes define neuronal phenotypes, shape circuit connectivity and regulate circuit function is essential for understanding how the brain processes information, directs behavior and adapts to changing environments. Mouse genetics has contributed greatly to current percepts of gene‐circuit interfaces in behavior, but considerable work remains. Large‐scale initiatives to map gene expression and connectivity in the brain, together with advanced techniques in molecular genetics, now allow detailed exploration of the genetic basis of nervous system function at the level of specific circuit connections. In this review, we highlight several key advances for defining the function of specific genes within a neural network .     

Changes in the Expression of Monoaminergic Genes under the Influence of Repeated Experience of Agonistic Interactions: From Behavior to Gene

The role of genetic and environmental factors as well as brain neurochemistry in regulating aggressive and submissive behaviors in animals are considered. We present a review of data on changes in brain monoaminergic activity (synthesis, catabolism, receptors) and on the expression of monoaminergic genes under repeated daily agonistic confrontations in male mice. A repeated experience of aggression was shown to result in the total activation of the dopaminergic systems and the inhibition of the serotonergic one. This was accompanied by a decrease in the mRNA level of the cathecol-O-methyltransferase gene in the midbrain and an increase of the mRNA level of the dopamine transporter and tyrosine hydroxylase genes in the ventral tegmental area of aggressive male mice. Repeated experience of social defeats produced dynamic changes in the serotonergic system of some brain areas and an increase of the mRNA level of the serotonin transporter and monoamine oxidase A genes in the midbrain raphe nuclei. Theoretical and methodological possibilities of the proposed ethological approach for studying molecular mechanisms of agonistic behavior are discussed in the context of the fundamental problem of investigating the ways of regulation from behavior to gene.     

A comprehensive genetic characterization of bacterial motility

We have developed a powerful experimental framework that combines competitive selection and microarray-based genetic footprinting to comprehensively reveal the genetic basis of bacterial behaviors. Application of this method to Escherichia coli motility identifies 95% of the known flagellar and chemotaxis genes, and reveals three dozen novel loci that, to varying degrees and through diverse mechanisms, affect motility. To probe the network context in which these genes function, we developed a method that uncovers genome-wide epistatic interactions through comprehensive analyses of double-mutant phenotypes. This allows us to place the novel genes within the context of signaling and regulatory networks, including the Rcs phosphorelay pathway and the cyclic di-GMP second-messenger system. This unifying framework enables sensitive and comprehensive genetic characterization of complex behaviors across the microbial biosphere.     

Genetic modules and networks for behavior: lessons from Drosophila

Behaviors are quantitative traits determined through actions of multiple genes and subject to genome-environment interactions. Early studies concentrated on analyzing the effects of single genes on behaviors, often generating views of simplified linear genetic pathways. The genome era has generated a profound paradigm shift enabling us to identify all the genes that contribute to expression of a behavioral phenotype, to investigate how they are organized as functional ensembles and to begin to identify polymorphisms that contribute to phenotypic variation and are targets for natural selection. Recent studies show that the genetic architecture of behavior is determined by dynamic and plastic modular networks of pleiotropic genes and that the behavioral phenotype manifests itself as an emergent property of such networks. Such networks are exquisitely sensitive to genetic background and sex effects. This review describes how Drosophila can serve as a model for uncovering fundamental principles of the genetic architecture of behavior.     

Changes in the expression of monoaminergic genes under the influence of repeated experience of agonistic interactions: from behavior to gene

The role of genetic and environmental factors as well as brain neurochemistry in regulating aggressive and submissive behaviors in animals are considered. We present a review of data on changes in brain monoaminergic activity (synthesis, catabolism, receptors) and on the expression of monoaminergetic genes under repeated daily agonistic confrontations in male mice. A repeated experience of aggression was shown to result in the total activation of the dopaminergic system and the inhibition of the serotonergic one. This was accompanied by a decrease in the mRNA level of the catechol-O-methyltransferase gene in the midbrain and an increase of the mRNA level of the dopamine transporter and tyrosine hydroxylase genes in the ventral tegmental area of aggressive male mice. Repeated experience of social defeats produced dynamic changes in the serotonergic system of some brain areas and an increase of the mRNA level of the serotonin transporter and monoamine oxidase A genes in the midbrain raphe nuclei. Theoretical and methodological possibilities of the proposed ethological approach for studying molecular mechanisms of agonistic behavior are discussed in the context of the fundamental problem of investigating the ways of regulation from behavior to gene.     

Genetics of alcohol consumption in Drosophila melanogaster

Individual variation in alcohol consumption in human populations is determined by genetic, environmental, social and cultural factors. In contrast to humans, genetic contributions to complex behavioral phenotypes can be readily dissected in Drosophila, where both the genetic background and environment can be controlled and behaviors quantified through simple high‐throughput assays. Here, we measured voluntary consumption of ethanol in ～3000 individuals of each sex from an advanced intercross population derived from 37 lines of the Drosophila melanogaster Genetic Reference Panel. Extreme quantitative trait loci mapping identified 385 differentially segregating allelic variants located in or near 291 genes at P < 10^?8. The effects of single nucleotide polymorphisms associated with voluntary ethanol consumption are sex‐specific, as found for other alcohol‐related phenotypes. To assess causality, we used RNA interference knockdown or P{MiET1} mutants and their corresponding controls and functionally validated 86% of candidate genes in at least one sex. We constructed a genetic network comprised of 23 genes along with a separate trio and a pair of connected genes. Gene ontology analyses showed enrichment of developmental genes, including development of the nervous system. Furthermore, a network of human orthologs showed enrichment for signal transduction processes, protein metabolism and developmental processes, including nervous system development. Our results show that the genetic architecture that underlies variation in voluntary ethanol consumption is sexually dimorphic and partially overlaps with genetic factors that control variation in feeding behavior and alcohol sensitivity. This integrative genetic architecture is rooted in evolutionarily conserved features that can be extrapolated to human genetic interaction networks.     

Behavior and mutagenesis screens: the importance of baseline analysis of inbred strains

Random mutagenesis as a means of identifying the function of genes has been used extensively in a variety of model organisms. Until recently it has been used primarily in the identification of single-gene traits that cause visible and developmental mutations. However, this genetic approach also has the power to identify genes that control complex biological systems such as behavior. Mutagenesis screens for behavioral mutations require careful consideration of many factors, including choice of both assays and background strains for use in mutagenesis and subsequent mapping of the affected gene or genes. This paper describes behavioral assays for monitoring motor coordination on the accelerating rotarod, anxiety-related behaviors in the elevated zero maze and sensorimotor reactivity, gating, and habituation of acoustic startle. These five physiological or neurological behaviors can represent potential endophenotypes for a variety of neurological and psychiatric disorders. The significant degree of strain- and sex-specific differences in the performance of four inbred strains of mice (C57BL/6J, C3HeB/FeJ, DBA/2J, and 129/SvlmJ) in these behavioral assays illustrates the importance of performing baseline analysis prior to behavioral mutagenesis screens and genetic mapping of selected mutations. Received: 16 December 1999 / Accepted: 17 December 1999     

Behavioral genetic approaches to visual system development and function in zebrafish

The zebrafish is a recent vertebrate model system that shows great potential for a genetic analysis of behavior. Early development is extraordinarily rapid, so that larvae already display a range of behaviors 5 days after fertilization. In particular the visual system develops precociously, supporting a number of visually mediated behaviors in the larva. This provides the opportunity to use these visually mediated behaviors to screen chemically mutagenized strains for defects in vision. Larval optokinetic and optomotor responses have already been successfully employed to screen for mutant strains with defects in the visual system. In the adult zebrafish a visually mediated escape response has proved useful for screening for dominant mutations of the visual system. Here, I summarize visually mediated behaviors of both larval and adult zebrafish and their applicability for genetic screens, and present, the approaches and results of visual behavior carried out to date.     

Control of male sexual behavior in Drosophila by the sex determination pathway

Understanding how genes influence behavior, including sexuality, is one of biology's greatest challenges. Much of the recent progress in understanding how single genes can influence behavior has come from the study of innate behaviors in the fruit fly Drosophila melanogaster. In particular, the elaborate courtship ritual performed by the male fly has provided remarkable insights into how the neural circuitry underlying sexual behavior--which is largely innate in flies--is built into the nervous system during development, and how this circuitry functions in the adult. In this review we will discuss how genes of the sex determination pathway in Drosophila orchestrate the developmental events necessary for sex-specific behaviors and physiology, and the broader lessons this can teach us about the mechanisms underlying the development of sex-specific neural circuitry.     

Heritability, correlations and in silico mapping of locomotor behavior and neurochemistry in inbred strains of mice

The midbrain dopamine system mediates normal and pathologic behaviors related to motor activity, attention, motivation/reward and cognition. These are complex, quantitative traits whose variation among individuals is modulated by genetic, epigenetic and environmental factors. Conventional genetic methods have identified several genes important to this system, but the majority of factors contributing to the variation remain unknown. To understand these genetic and environmental factors, we initiated a study measuring 21 behavioral and neurochemical traits in 15 common inbred mouse strains. We report trait data, heritabilities and genetic and non-genetic correlations between pheno-types. In general, the behavioral traits were more heritable than neurochemical traits, and both genetic and non-genetic correlations within these trait sets were high. Surprisingly, there were few significant correlations between the behavioral and the individual neurochemical traits. However, striatal serotonin and one measure of dopamine turnover (DOPAC/DA) were highly correlated with most behavioral measures. The variable accounting for the most variation in behavior was mouse strain and not a specific neurochemical measure, suggesting that additional genetic factors remain to be determined to account for these behavioral differences. We also report the prospective use of the in silico method of quantitative trait loci (QTL) analysis and demonstrate difficulties in the use of this method, which failed to detect significant QTLs for the majority of these traits. These data serve as a framework for further studies of correlations between different midbrain dopamine traits and as a guide for experimental cross designs to identify QTLs and genes that contribute to these traits.     

Behavioral and neurobiological implications of sex-determining factors in Drosophila

The function of the central nervous system as it controls sex-specific behaviors in Drosophila has been studied with renewed intensity, in the context of genetic factors that influence the development of sexually differentiated aspects of this insect. Three categories of genetic variations that cause anomalies in courtship and mating behaviors are discussed: (1) mutants isolated with regard to courtship defects, of which putatively courtship-specific variants such as the fruitless mutant are a subset; (2) general behavioral and neurological variants (including sensory and learning mutants), whose defects include subnormal reproductive performance; and (3) mutations of genes within the sex-determination regulatory hierarchy of Drosophila, the analysis of which has included studies of reproductive behavior. Recent studies of mutations in two of these categories have provided new insights into the control of neuronally based aspects of sex-specific behavior. The doublesex gene, the final factor acting in the sex-determination hierarchy, had been previously thought to regulate all aspects of sexual differentiation. Yet, it has been recently shown that doublesex does not control at least one neuronally-determined feature of sex-specific anatomy—a muscle in the male's abdomen, whose normal development is, however, dependent on the action of fruitless. These considerations prompted us to examine further (and in some cases re-examine) the influences exerted by sex-determination hierarchy genes on behavior. Our results—notably those obtained from assessments of doublesex mutations' effects on general reproductive actions and on a particular component of the courtship sequence (male “singing” behavior)—lead to the suggestion that there is a previously unrecognized branch within the sexdetermination hierarchy, which controls the differentiation of the male- and female- specific phenotypes of Drosophila. This new branch separates from the doublesex-related one immediately before the action of that gene (just after fransformer and transformer-2) and appears to control as least some aspects of neuronally determined sexual differentiation of males. © 1994 Wiley-Liss, Inc.     

The RNA world meets behavior: A-->I pre-mRNA editing in animals

Speculations on the genetic component of animal behavior have been fueled primarily by single-gene mutations that affect specific behaviors in model organisms. Pre-mRNA editing by adenosine deaminases acting on RNA (ADARs) provides an additional mechanism for introducing protein diversity and has primarily been observed in signaling components of the nervous system. Two recent reports of mutant mice and Drosophila deficient in ADAR activities provide further evidence that pre-mRNA editing has an ancient and primary role in the evolution of nervous system function and behavior.     

Modular genetic control of innate behaviors

Many complex behaviors are genetically hardwired. Based on previous findings on genetic control of mating and other behaviors in invertebrate and mammalian systems, I suggest that genetic control of complex behaviors is modular: first, dedicated genes specify different behavioral patterns; secondly, separable genetic networks govern distinct behavioral components. I speculate that modular genetic encoding of complex behaviors may in part reflect modularity in brain development and function. Editor's suggested further reading in BioEssays From songs to synapses: Molecular mechanisms of birdsong memory Abstract     

Looking for Food: Molecular Neuroethology of Invertebrate Feeding Behavior

The assignment of complex behavior of animals to the function of specific genes has seen significant advances in the past decade. The advent of modern tools of genetics and genomics permitted analyses that revealed a good number of neural system enriched genes whose products modulate, and whose polymorphism qualitatively or quantitatively influenced invertebrate feeding behavior. The most prominent of these genes are orthologues of foraging (for) and the neuropeptide Y (NPY)/NPY receptor. The former encodes a cyclic‐GMP‐dependent protein kinase, which functional genetics have been characterized in Drosophila melanogaster, Apis mellifera and Caenorhabditis elegans. Allelic variations and changes in the expression of the above genes could influence the initiation of particular feeding behaviors or related social phenotype. These genes have provided the first molecular insights towards feeding behavior in invertebrates. Besides detailed investigations into the neural pathways involved and mechanisms of function of the gene products, parallel studies in other animal models is imperative to understand ecological drivers of animal feeding behavior.