The mechanisms by which circadian pacemaker systems transmit timing information to control behavior are largely unknown. Here, we define two critical features of that mechanism in Drosophila. We first describe animals mutant for the pdf neuropeptide gene, which is expressed by most of the candidate pacemakers (LNv neurons). Next, we describe animals in which pdf neurons were selectively ablated. Both sets of animals produced similar behavioral phenotypes. Both sets entrained to light, but both were largely arrhythmic under constant conditions. A minority of each pdf variant exhibited weak to moderate free-running rhythmicity. These results confirm the assignment of LNv neurons as the principal circadian pacemakers controlling daily locomotion in Drosophila. They also implicate PDF as the principal circadian transmitter.

Complex brains and behaviors have occurred repeatedly within vertebrate classes throughout evolution. What adaptive pressures drive such changes? Both environmental and social features have been implicated in the expansion of select brain structures, particularly the telencephalon. East African cichlid fishes provide a superb opportunity to analyze the social and ecological correlates of neural phenotypes and their evolution. As a result of rapid, recent, and repeated radiations, there are hundreds of closely-related species available for study, with an astonishing diversity in habitat preferences and social behaviors. In this study, we present quantitative ecological, social, and neuroanatomical data for closely-related species from the (monophyletic) Ectodini clade of Lake Tanganyikan cichlid fish. The species differed either in habitat preference or social organization. After accounting for phylogeny with independent contrasts, we find that environmental and social factors differentially affect the brain, with environmental factors showing a broader effect on a range of brain structures compared to social factors. Five out of seven of the brain measures show a relationship with habitat measures. Brain size and cerebellar size are positively correlated with species number (which is correlated with habitat complexity); the medulla and olfactory bulb are negatively correlated with habitat measures. The telencephalon shows a trend toward a positive correlation with rock size. In contrast, only two brain structures, the telencephalon and hypothalamus, are correlated with social factors. Telencephalic size is larger in monogamous species compared to polygamous species, as well as with increased numbers of individuals; monogamy is also associated with smaller hypothalamic size. Our results suggest that selection or drift can act independently on different brain regions as the species diverge into different habitats and social systems.

Phenotypic plasticity is the development of different phenotypes from a single genotype, depending on the environment. Such plasticity is a pervasive feature of life, is observed for various traits and is often argued to be the result of natural selection. A thorough study of phenotypic plasticity should thus include an ecological and an evolutionary perspective. Recent advances in large-scale gene expression technology make it possible to also study plasticity from a molecular perspective, and the addition of these data will help answer long-standing questions about this widespread phenomenon. In this review, we present examples of integrative studies that illustrate the molecular and cellular mechanisms underlying plastic traits, and show how new techniques will grow in importance in the study of these plastic molecular processes. These techniques include: (i) heterologous hybridization to DNA microarrays; (ii) next generation sequencing technologies applied to transcriptomics; (iii) techniques for studying the function of noncoding small RNAs; and (iv) proteomic tools. We also present recent studies on genetic model systems that uncover how environmental cues triggering different plastic responses are sensed and integrated by the organism. Finally, we describe recent work on changes in gene expression in response to an environmental cue that persist after the cue is removed. Such long-term responses are made possible by epigenetic molecular mechanisms, including DNA methylation. The results of these current studies help us outline future avenues for the study of plasticity.

In the brain of the fly Drosophila melanogaster, ∼150 clock-neurons are organized to synchronize and maintain behavioral rhythms, but the physiological and neurochemical bases of their interactions are largely unknown. Here we reevaluate the cellular properties of these pacemakers by application of a novel genetic reporter and several phenotypic markers. First, we describe an enhancer trap marker called R32 that specifically reveals several previously undescribed aspects of the fly's central neuronal pacemakers. We find evidence for a previously unappreciated class of neuronal pacemakers, the lateral posterior neurons (LPNs), and establish anatomical, molecular, and developmental criteria to establish a subclass within the dorsal neuron 1 (DN1) group of pacemakers. Furthermore, we show that the neuropeptide IPNamide is specifically expressed by this DN1 subclass. These observations implicate IPNamide as a second candidate circadian transmitter in the Drosophila brain. Finally, we present molecular and anatomical evidence for unrecognized phenotypic diversity within each of four established classes of clock neurons. Indexing termsDrosophila; circadian clock; neuropeptides; PDF receptor; IPNamide; nplp1; glass Overt biological rhythms such as the predictable daily leaf movements of plants and the sleep/ wake cycle of animals are ultimately driven by molecular oscillations. Throughout the living world such oscillations are sustained by intracellular transcriptional/translational feedback loops (reviewed by Dunlap, 1999). In Drosophila many components of the molecular clock are known in detail. Products of the clock genes period (per), timeless (tim), Clock, cycle, vrille, and PAR domain protein 1 (Pdp1) form two interconnected and self-sustained transcriptional feedback loops, the kinetics of which are regulated by clock protein interactions and attendant kinases (recently reviewed by Hardin, 2004;Schöning and Staiger, 2005;Taghert and Lin, 2005). Although the core clock genes are expressed rhythmically throughout the fly's body, only a few small groups of clock-expressing neurons in the central brain (termed pacemakers) are necessary and sufficient for the organization and maintenance of rhythmic locomotor activity (reviewed by Hall, 2005;Helfrich-Förster, 2005). When these pacemaker *Correspondence to: Paul Taghert, Dept. Anatomy and Neurobiology, Washington University School of Medicine, 660 S. Euclid Ave., St. Louis, MO 63110. E-mail: taghertp@pcg.wustl.edu. Current address for S.C.P. Renn: Bauer Center for Genomics Research, Harvard University, 7 Divinity Ave., Rm. 249, Cambridge, MA 02138. NIH Public Access NIH-PA Author ManuscriptNIH-PA Author Manuscript NIH-PA Author Manuscript neurons of Drosophila are electrically silenced their molecular oscillations are lost under constant conditions, suggesting that electrical activity at the cell membrane is required for the endogenous molecular clockwork (Nitabach et al., 2002).Approximately 150 neurons express the dynamic molecular clockwork in the adult ...

Behaviour and physiology are regulated by both environment and social context. A central goal in the study of the social control of behaviour is to determine the underlying physiological, cellular and molecular mechanisms in the brain. The African cichlid fish Astatotilapia burtoni has long been used as a model system to study how social interactions regulate neural and behavioural plasticity. In this species, males are either socially dominant and reproductively active or subordinate and reproductively suppressed. This phenotypic difference is reversible. Using an integrative approach that combines quantitative behavioural measurements, functional genomics and bioinformatic analyses, we examine neural gene expression in dominant and subordinate males as well as brooding females. We confirm the role of numerous candidate genes that are part of neuroendocrine pathways and show that specific co-regulated gene sets (modules), as well as specific functional Gene Ontology categories, are significantly associated with dominance or reproductive state. Finally, even though the dominant and subordinate phenotypes are robustly defined, we find a surprisingly high degree of individual variation in the transcript levels of the very genes that are differentially regulated between these phenotypes. Our results demonstrate the molecular complexity in the brain underlying social behaviour, identify novel targets for future studies, validate many candidate genes, and exploit individual variation in order to gain biological insights.