Three-dimensional morphology of the hypertrophied sex pheromone gland in a lek-mating carpenter bee (Xylocopa sonorina) revealed by micro computed tomography and scanning electron microscopy

Abstract

The evolution of long-range pheromonal communication is often facilitated by morphological adaptations in glandular structures. Male carpenter bees (Xylocopa) in four subgenera possess hypertrophied sex pheromone glands responsible for their long-distance mate attraction. The valley carpenter bee (Xylocopa sonorina) is known for its “dispersed lek” mating system, in which males lure females to mating territories by means of a sex pheromone produced in the dorsal mesosomal gland. The gland functions first as secretory tissue and then as a large, setal-lined storage vessel for the sex pheromone blend. Here, we characterized the morphology of this gland by collecting sexually active males and imaging the dorsal mesosomal gland using micro computed tomography (micro-CT) scanning. From these images, we reconstructed a detailed model, representing the first 3D visualization of the gland complex in its entirety, and the first quantification of its morphology. We additionally characterized the ultrastructure of the gland using scanning electron microscopy (SEM), and discuss the possible functional significance of setae lining the gland tubules. We found that the gland can occupy nearly 20% of the thoracic volume, a space typically filled entirely in other bees by large indirect flight muscles. This massive spatial and physiological investment in sex gland tissue reflects the critical importance of long-range pheromonal mate attraction in this lek-mating species.