The Role of Pigments in Light Color Variation of the Firefly Photinus pyralis

Firefly light color does not appear to directly influence mate choice, but it seems to be under selection to enhance signal detectability by increasing contrast with the visual background. While luciferase has been considered the sole determinant of light color, populations of the Big Dipper firefly (Photinus pyralis) with identical luciferases display variation in emitted light color. Here, we examined whether P. pyralis fireflies use pigments to filter the light produced by luciferase and contribute to variation in light color across populations. If pigments influence light color, we predicted that genes involved with pigment biosynthesis would be expressed in light organs, and that pigment substrates could also be detected in firefly light organs. Since screening pigments are important for insect vision, we examined whether any pigment genes and/or transporters expressed in light organs were also expressed in eyes, thus providing a direct pigment-based mechanism for the reported tuning between emitted light color (light organ) and visual sensitivity (eyes). We identified 46 ommochrome and pterin pigment genes expressed in P. pyralis light organs and eyes, including those whose products could filter the light generated by luciferase and influence emitted light color. The shared pigments between light organs and eyes offer candidates for signal tuning. While we found no statistically significant differences between the pigment gene expression of P. pyralis fireflies in populations with yellower and greener signals, our data suggest several mechanisms for how pigments in the light organ could modify P. pyralis signal color.