Evolutionary conservation of opsin gene expression patterns in the compound eyes of darkling beetles

Recent large-scale studies of opsin gene contents in representatives of the largest order of insects, the Coleoptera (beetles), revealed that the blue wavelength-sensitive (B) opsin subfamily is absent in this clade, while the ultraviolet- (UV) and long wavelength-sensitive (LW) opsin subfamilies are broadly conserved with gene duplications possibly reintroducing blue sensitivity in select subclades. Little is known yet, however, how opsin genes are expressed in the compound eyes of beetles. In a previous study, we analyzed opsin gene expression in the red flour beetle Tribolium castaneum, a member of the family of darkling beetles (Tenebrionidae), and found that a singleton LW opsin homolog is homogeneously expressed in all photoreceptors of the compound eye retina with a singleton UV opsin homolog being co-expressed in the R7 subtype photoreceptors. To probe for the evolutionary conservation of these expression patterns, we isolated complete opsin transcript sequences from three additional species in the subfamily Tenebrionidae (Tribolium confusum, Tenebrio molitor, Zophobas morio) and studied their expression via whole mount in situ hybridization in the pupal retina. These experiments revealed very similar, if not identical, photoreceptor subtype-specific expression patterns in all three species compared with T. castaneum. Documenting a deep conservation of photoreceptor subtype-specific opsin gene expression in this range of darkling beetles, our study provides a first point of reference for broader comparative studies of retinal organization in the Coleoptera.