Abstract

Directed behaviour emerges from neural integration of sensory stimuli, memory of prior experience and internal states. We seek an understanding of these conserved neural mechanisms using genetically-encoded tools and the relatively small brain of Drosophila. By temporally controlling neural function memories can be implanted and internal states altered so that most flies behave according to our direction. Such recent studies have revealed a role for distinct subsets of dopaminergic neurons that innervate the mushroom bodies in reward learning and the control of motivated fly behaviour. Therefore, the positive reinforcement system of flies is more similar to that of mammals than previously envisaged. One might interpret the relative ease of altering behaviour to indicate that everything is simple in the fly brain. However, complexity arises in unexpected ways. Cell-type specific gene expression profiling revealed transposable element expression in long-term memory relevant neuronsofthemushroom body. Importantly, brain-specific transposable element excision and insertion is heterogeneous within and between fly brains. Since neural expression and retrotransposition of LINE -1 transposable elements has been observed in mammals, it appears that genomic heterogeneity is a conserved feature of the brain. We propose that it may prove beneficial to specific cell-types and neural processes and could plausibly contribute to behavioural individuality.