Differential gene expression underlying the biosynthesis of Dufour's gland signals in Bombus impatiens

Pheromones regulating social behavior are one of the most explored phenomena in social insects. However, compound identity, biosynthesis and their genetic basis are known in only a handful of species. Here we examined the gene expression associated with pheromone biosynthesis of two main chemical classes: esters and terpenes, using the social bee Bombus impatiens. We conducted chemical and RNA-seq analyses of the Dufour's gland, an exocrine gland producing a plethora of pheromones regulating social behavior in hymenopteran species. The Dufour's gland contains mostly long-chained hydrocarbons, terpenes and esters that signal reproductive and social status in several bee species. In bumble bees, the Dufour's gland contains queen- and worker-specific esters, in addition to terpenes and terpene-esters only found in gynes and queens. These compounds are assumed to be synthesized de novo in the gland, however, their genetic basis is unknown. A whole transcriptome gene expression analysis of the gland in queens, gynes, queenless and queenright workers showed distinct transcriptomic profiles, with thousands of differentially expressed genes between the groups. Workers and queens express genes associated with key enzymes in the biosynthesis of wax esters, while queens and gynes preferentially express key genes in terpene biosynthesis. Overall, our data demonstrate gland-specific regulation of chemical signals associated with social behavior and identifies candidate genes and pathways regulating caste-specific chemical signals in social insects.