The royal perfume: Conserved compounds or diverse bouquets? A phylogenetic comparative study of queen pheromone evolution in carpenter ants

Pheromone evolution occurs at different rates depending on function. Rapid or saltational evolution is typical of sex pheromones, while gradual evolution and conservation of compound structures can occur for functions unrelated to mating. Queen pheromones are crucial elements for maintaining the characteristic reproductive hierarchy of social insect colonies and are usually found as a subset of cuticular lipids, mainly hydrocarbons. My study surveys queen-associated cuticular compounds and compares their evolution to that of the other cuticular lipids using the hyperdiverse, cosmopolitan, and ecologically diverse ant genus Camponotus (carpenter ants) as a model system. I hypothesized that the fundamental role of queen signals for colony structure in social insects would lead to their evolutionary conservation relative to other cuticular lipids. Queen-associated compounds were analyzed in terms of their relative volatility, their similarity in compound class composition relative to worker compounds, and the phylogenetic signal of many trait attributes, including compound branching patterns, saturation of the carbon chain, and structural complexity. Phylogenetic comparisons were made using an ultra-conserved elements phylogeny based on >1,200 loci, containing every colony and all 21 species in my study. To verify the signaling function of queen-associated lipids, I conducted bioassays to assess the ability of queen compounds to induce retinue formation, a queen-specific behavior. My results demonstrate rapid evolution of queen signal chemistry and relatively little conservation within the cuticular hydrocarbon trait. This may suggest a role of learning, rather than strictly innate responses to pheromones, a paradigm shift in how we think of signaler-receiver interactions.