| Aggressive behavior is observed throughout the animal kingdom, and can be used to obtain or defend mates, food, or territory, or to establish and maintain social structure. However, aggression can be selectively disadvantageous, in that it can result in bodily harm, can detract from time spent mating or foraging, and is metabolically expensive. Furthermore, aggression can reach pathological levels in humans, and is often a component of psychiatric disorders such as schizophrenia, conduct disorder, and Alzheimer's Disease. Aggressive behavior is a complex, quantitative trait, influenced by multiple interacting genes whose effects vary in magnitude and direction, and are sensitive to environmental perturbations. An understanding of the genetics underlying aggression is important from an evolutionary perspective and in the context of human health. We used a suite of approaches in Drosophila melanogaster to identify genes affecting aggressive behavior: artificial selection to derive lines of flies divergent in their aggression levels, followed by whole genome expression analysis; a screen of P-element insertional mutants and characterization of a subset of candidate genes; quantitative trait locus (QTL) mapping to identify segregating variation affecting aggression; and behavioral and transcriptional analysis of a panel of wild-derived inbred lines to assess natural genetic variation and correlated modules of genes.;Artificial selection for 25 generations followed by expression profiling revealed that a substantial portion of the genome -- more than 1,500 genes -- exhibited differential expression that was correlated with the selection response. These candidate genes have been implicated in a wide variety of biological processes and molecular functions, including metabolism, learning and memory, and nervous system development. Many are computationally predicted genes with no previous annotation. Functional tests of 19 P-element mutants confirmed 15 of these candidates, none of which have been previously implicated in aggressive behavior.;A screen of 170 P-element insertional mutants in a co-isogenic background identified 59 lines with aberrant aggressive behavior relative to the control. These genes fell into diverse gene ontology categories, including metabolism and localization. qPCR of nine candidate genes revealed disruption of gene expression throughout development. Morphometric analysis of the alpha and beta lobes of the mushroom bodies showed subtle yet significant differences in length and/or width relative to controls. P-element remobilization rescued the mutant behavioral phenotype in nearly every case, confirming these nine candidate genes as affecting aggression.;QTL mapping of introgression lines uncovered five regions contributing to variation in aggression levels between the two parental stocks. Deficiency complementation mapping was used to refine a region spanning two of these QTL, which fractionated into seven smaller regions. Mutant complementation tests to 58 positional candidate genes resulted in the identification of four genes affecting variation in aggressive behavior.;Finally, behavioral assessment of a reference panel of 40 wild-derived inbred lines revealed abundant natural variation in aggression. Transcriptome profiling was used to identify quantitative trait transcripts and single feature polymorphisms. Five hundred sixty unique genes met significance criteria in at least one of these categories, and these candidate genes have roles in biological processes such as olfaction, metabolism, and stress response. Tests of mutants in candidate genes suggest that this is a highly effective manner in which to identify genes influencing aggressive behavior. Genes implicated in affecting aggression through transcript abundance fell into nine correlated modules: genes in each module are potentially co-regulated, and may be functionally related. |
