Butterflies use color vision extensively to navigate the natural world. Their retinas are more complex than those found in Drosophila, where development and patterning has been heavily studied. Instead of the eight photoreceptors found in flies, butterflies have an additional ninth photoreceptor per ommatidium (“unit eye”). They also have three main types of ommatidia instead of the two distributed stochastically in the fly retina. We set out to determine how butterflies generate increased sensory receptor diversity to provide improved color vision, and more specifically, to determine how much of the retinal patterning network from Drosophila is reused or modified.
Using genome sequencing, gene expression, and CRISPR gene knock out we show that the regulatory network that defines photoreceptor subtypes in Drosophila is redeployed in butterflies (Papilio xuthus and Vanessa cardui) to generate additional subtypes. In Drosophila, a complex regulatory network combined with cell-cell signaling specifies photoreceptor subtypes within each ommatidium. Then, a stochastic decision of whether to express the transcription factor Spineless in R7 photoreceptors determines which of two types of ommatidia is specified. We find that the R7 marker Prospero is expressed in two photoreceptors per ommatidium in butterflies. CRISPR knock-out of Spineless shows that this gene also controls stochastic choice in each of the two R7s in butterflies, suggesting there is deep evolutionary conservation of stochastic patterning mechanisms. Having two stochastically distributed types of R7s allows for the specification of three ommatidial types instead of two, which in turn allowed for the evolution and deployment of additional opsins, tetrachromacy, and improved color vision. These efforts provide evidence that our extensive knowledge of patterning in the Drosophila visual system applies to other groups, and that adaptation for specific visual requirements can occur through modification of this network.