Violent fruit flies

This paper uses relatively straightforward methods and produced interesting, and unexpected results. Too bad it would be unethical and would take forever to do this type of study in humans.

Molecular analysis of flies selected for aggressive behavior

Herman A Dierick, Ralph J. Greenspan

Nature Genetics- 38, 1023-1031 (2006)

Abstract: Aggressive behavior is pervasive throughout the animal kingdom, and yet very little is known about its molecular underpinnings. To address this problem, we have developed a population-based selection procedure to increase aggression in Drosophila melanogaster. We measured changes in aggressive behavior in the selected subpopulations with a new two-male arena assay. In only ten generations of selection, the aggressive lines became markedly more aggressive than the neutral lines. After 21 generations, the fighting index increased more than 30-fold. Using microarray analysis, we identified genes with differing expression levels in the aggressive and neutral lines as candidates for this strong behavioral selection response. We tested a small set of these genes through mutant analysis and found that one significantly increased fighting frequency. These results suggest that selection for increases in aggression can be used to molecularly dissect this behavior.

a passage from the conclusion:

The one gene that produced a direct effect on aggression encodes a cytochrome P450 (ref. 32). These enzymes are involved in a variety of fundamental physiological functions as varied as growth, development, reproduction, detoxification and pheromone recognition³⁷. Some members of the CYP6 family have been shown to be enriched in olfactory tissues in D. melanogaster and Phyllopertha diversa^{38, 39}, in which they might be involved in pheromone signaling. If the Cyp6a20 mutant is defective in pheromone degradation, its increased fighting frequency might be explained by abnormal sensitivity to male pheromones eliciting an aggressive response. Cyp6a20 has been shown to undergo circadian fluctuation⁴⁰. Our results, however, suggest that the Cyp6a20 effect is not due to a phase shift in circadian behavior but rather to an effect of this mutant on aggression.

Another notable change in the list of significant expression differences is the downregulation of Obp56a in the aggressive lines (Table 2), which represents one of the strongest responders in this selection experiment. Odor-binding proteins have also been implicated in pheromone signaling between flies and have been shown to affect complex behavior⁴¹. In this regard, in our population cage, we noticed a previously undescribed behavior in which males drag their genital area on the food surface while walking as if making a territorial mark (Supplementary Video 5). This abdomen dipping has been previously reported in other dipterans, in which it has been shown to be associated with territorial attraction of females^{25, 26}. Although premature, it is tempting to speculate that this marking might also act as a repellant to other males and that a decrease in gustatory detection of this repellant might make aggressive males less sensitive to it, eliminating one inhibitory component towards an eventual territorial collision.