Connectome of the lamina reveals the circuit for early color processing in the visual pathway of a butterfly.

Connectomics has become a standard neuroscience methodology in a few model animals,1 with the visual system being a popular target of study.2-5 Combining connectomics with circuit and behavioral physiology, recent studies on the color vision of the fruit fly Drosophila melanogaster have focused on the mechanisms underlying early wavelength processing in the optic ganglia.6-8 However, the color vision capabilities of D. melanogaster are limited,9 compared with many flower-visiting insects.10,11 For example, a butterfly Papilio xuthus has six spectral classes of photoreceptors. Each ommatidium contains nine photoreceptors in one of three fixed combinations, making the eye an array of three spectrally distinct ommatidia types.12 Behaviorally, P. xuthus can detect 1 nm differences in light wavelength across the spectrum from ultraviolet to red, outperforming humans.13 What is the neuronal basis of such precise color vision? How does such a system evolve? Addressing these questions requires comparative studies at the circuit level. Here, we performed a connectome analysis in the first optic ganglion, the lamina, of P. xuthus. The lamina comprises cartridges, each typically containing nine photoreceptor axons from a single ommatidium and four second-order neurons. We found abundant inter-photoreceptor connections, which are absent in the lamina of D. melanogaster. We also identified connections between neighboring cartridges, particularly those receiving inputs from spectrally distinct ommatidia. The linear summation of synaptic connections well explains the spectral sensitivity of photoreceptors and second-order neurons in the lamina.