Innate spectral preferences and aversive visual learning reveal wavelength-dependent preferences and discrimination in Drosophila melanogaster

Chromatic information is detected by distinctly tuned photoreceptors and used in both innate and learned behaviors. Although the molecular identities and spectral sensitivities of these photoreceptors have been extensively characterized, how they combine to function remains elusive. We conducted innate spectral preference and visual learning assays in Drosophila melanogaster, a well-established model for exploring the neural mechanisms underlying behaviors. First, we assessed spectral preferences based on slow phototactic responses using combinations of monochromatic LED lights ranging from 402 nm to 630 nm. Flies showed a preference for moderately longer wavelengths (green to orange) over shorter wavelengths (violet to blue). Next, we performed an aversive heat conditioning assay to test wavelength discrimination using the same set of light combinations. Flies successfully learned to discriminate between light stimuli within the short to middle wavelengths (violet to green), but failed to discriminate stimuli within the middle to long wavelengths (green to red). Furthermore, these discrimination behaviors were compared to distances in photoreceptor space, suggesting that the photoreceptor space defined by the two opponent channels-short (Rh3/Rh4) vs. long (Rh6), and middle (Rh5) vs. short (Rh3/Rh4) and long (Rh6)-can effectively predict the behavioral results.