Intense light unleashes male courtship behavior in wild-type Drosophila

Studies of the Drosophila courtship repertoire have elucidated several principles of the neurogenetic organization of complex behavior. Based on a multi-modal integration of olfactory, gustatory, visual, and acoustic cues, males perform stereotypic patterns of chasing, courtship song production, and copulation attempts (1, 2). Here we report a serendipitous finding that intense light (∼18 klx, comparable to direct sunlight) induces courtshipassociated motor activity in wild-type (WT) male flies, including chasing with wing extension and song production. The intensified male courtship behavior was directed to both male or female targets, as shown in both male-only and mixed sex arenas. However, in male-only chambers, we found extreme behavioral manifestations such as “chaining” and “wheeling”, which resembled previously reported homosexual behavior in certain mini-white+ constructs (3, 4) and fruitless mutants (5). Investigations into the required sensory systems, yielded results of strong implications. In visual system mutants with defective photo-sensitive or screening pigments (norpA), the phenomenon was absent. Even in sevenless mutants, where the high-acuity blue/UV channels are disrupted but motion sensitivity is preserved (6), light-triggered courtship was greatly attenuated. However, light-induced courtship was unhampered in flies lacking acoustic signals (with wings cut to disrupt song production). Strikingly, we observed unrestrained male-male courtship behavior in olfactory mutants (orco and sbl). Particularly, orco males displayed maximum courtship scores regardless of light intensity. Together our observations support the notion that male courtship is restrained by olfactory cues and can be unleashed by strong visual stimulation in Drosophila.