Evolution of Phototransduction Genes in Lepidoptera

Abstract Vision is underpinned by phototransduction, a signaling cascade that converts light energy into an electrical signal. Among insects, phototransduction is best understood in Drosophila melanogaster. A survey of phototransduction genes in four insect genomes found gains and losses between D. melanogaster and other insects; this study did not include lepidopterans. Diurnal butterflies and nocturnal moths occupy different light environments and have distinct eye morphologies, which might impact the expression of their phototransduction genes. Here, we used transcriptomics and phylogenetics to identify phototransduction genes that vary between D. melanogaster and Lepidoptera, and between moths and butterflies. Most phototransduction genes were conserved between D. melanogaster and Lepidoptera, with some exceptions. We found two lepidopteran opsins lacking a D. melanogaster ortholog, and using antibodies found that one, a candidate retinochrome which we name unclassified opsin (UnRh), is expressed in the crystaline cone cells and the pigment cells of the butterfly Heliconius melpomene. We also found differences between Lepidoptera and D. melanogaster phototransduction in diacylglycerol regulation where a lepidopteran paralog, DAGβ, may be taking on a role in vision. Lastly, butterflies express similar amounts of trp and trpl channel mRNAs, while moths express approximately 50x less trp. Since TRP/TRPL channels allow Ca2+ and Na+ influx this might explain why moths appear to express less Calx and Nckx30C Na+/Ca2+ channel mRNAs. Our findings suggest that while many single-copy D. melanogaster phototransduction genes are conserved in lepidopterans, phototransduction gene expression differences exist between moths and butterflies that may be linked to their visual light environment.