Antennal RNAseq reveals odorant receptors with sex-biased expression in the common eastern firefly, Photinus pyralis

With their charismatic nighttime flashes, fireflies are a classic organismal system for studying the evolution of visual mating signals. However, across their diversity, fireflies employ a variety of mating strategies that include both chemical and visual signals. While phylogenetic evidence points to a common ancestor that relied on long-range pheromones, behavioral evidence suggests that light-dependent flashing fireflies do not use smell for mating. We tested this hypothesis by investigating the olfactory genetics of the dusk-active, light-using common eastern firefly, Photinus pyralis. In insects, odors are primarily detected by odorant receptor (OR) proteins embedded in the dendritic membranes of olfactory receptor neurons. If pheromones are part of mate signaling in light-using fireflies, then one or more OR genes should be upregulated in the antennae of the searching sex (males). We therefore annotated the complete suite of ORs in the genome of P. pyralis and measured expression of OR genes between the sexes using RNAseq.

We identified 102 ORs in the P. pyralis genome, including the conserved single-copy Orco. Our phylogenetic analysis showed lineage-specific OR diversification in P. pyralis relative to other beetle species. Differential expression analysis of male and female antennae and hind legs revealed that a subset of ORs were upregulated in antennae as compared to legs, suggesting a role in adult olfaction. Notably, PpyrOR6 was one of two genes, and the only OR, that was significantly upregulated between male and female antennae, suggesting a sex-specific role such as mate location.

These findings increase known diversity of insect ORs in an understudied beetle family and suggest that fireflies with flashing adults use multimodal signals during mating.