This observation implies a highly polygenic and pleiotropic architecture for behaviors and is consistent with their modularity, that is complex ABT-737 ic50 behaviors can be dissected into constituent components, with overlapping genetic networks underlying each component (e.g. locomotion would be a constituent component of aggression as well as mating

behavior; Figure 1). Large phenotypic effects that arise from disruption of a single gene can lead to the identification of cellular pathways or mechanisms that are instrumental in enabling the behavior. A classic example is the elucidation of the regulatory feedback loops that control the circadian clock which largely came from studies on single gene mutations [23 and 24]. However, effects on downstream gene products and genes that regulate these feedback loops

and their interactions that lead to the ultimate expression of circadian behavior remain to be fully understood. Similarly, disruption of single genes, including tailless [ 25] and Tachykinin (Tk) and its receptor, Takr86C [ 26•], and inactivation of the neurons in which they are expressed have identified a neural circuit that contributes to aggression and originates in the pars intercerebralis, a brain structure also implicated in the control of sleep see more [ 27]. However, other brain regions, including the mushroom bodies [

28], dopaminergic C1GALT1 projections to the central complex [ 29], octopaminergic neurons in the suboesophageal ganglion [ 30] and input via the olfactory projection [ 31 and 32•] are also implicated in aggression. Furthermore, transposon insertions in as many as 57 genes from among 170 genes surveyed resulted either in reduced aggression or hyper-aggression [ 33]. Thus, the mechanisms that regulate aggression depend on integrated neural circuits and a complex and extensive underlying genetic architecture, in which disruption of any major single gene component can result in an abnormal behavioral phenotype. These examples illustrate how mutagenesis studies can provide significant insights into some of the underlying cellular mechanisms that drive behaviors. For any given behavioral phenotype, however, such studies have also led to a pantheon of genes with diverse annotated functions, each of which affects the phenotype, but without indication of how these genes interact together as a functional ensemble that gives rise to the phenotype. Putting these independent snippets of information in a common framework is the goal of systems genetics approaches (Figure 2).