In Drosophila melanogaster, the causal links among a complex behaviour, single neurons and single genes can be demonstrated through experimental manipulations. A key player in establishing the male courtship circuitry is the fruitless (fru) gene, the expression of which yields the FruM proteins in a subset of male but not female neurons. FruM probably regulates chromatin states, leading to single-neuron sex differences and, consequently, a sexually dimorphic circuitry. The mutual connections among fru-expressing neurons-including primary sensory afferents, central interneurons such as the P1 neuron cluster that triggers courtship, and courtship motor pattern generators-probably form the core portion of the male courtship circuitry.
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