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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.

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Data availability statement

All data generated or analyzed during this study are included in this published article and its supplementary file.

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Acknowledgements

We thank Brendon Belongia and Greg Groth of Exact Metrology, Madison, WI, for operation of the CT scanner. Our thanks to Zachary Michels for SEM micrographs of the male gland tubules and setae, and for operation of the Hitachi S-3400N scanning electron microscope. We thank Brenda Baggett of the University of Arizona’s Cancer Imaging Shared Resource for earlier exploratory micro-CT scans of male carpenter bee glands using an Inveon micro-CT scanner. We thank 3D artist Daniel Hornung of Tucson, AZ, for assistance with cleaning up the 3D model before the final creation of figures. We also thank Dr. Brian Smith and Dr. Hong Lei of Arizona State University for access to their Amira imaging software. A very special thanks to Kathryn Busby and her dragon log sculpture “Felix” that was home to the population of carpenter bees we studied. Finally, we thank two anonymous reviewers whose comments helped improve the manuscript.

Funding

This work was supported by an NSF GRFP award to MMO and by an NSF-EAR award to SLB (2050246) for support of the LaserChron Center at the University of Arizona.

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SB collected specimens and performed micro-CT and SEM imaging. MO and JAT processed image data. MO wrote the manuscript. MO, SB, JAT, and RM participated in manuscript editing and study design.

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Correspondence to Madeleine M. Ostwald.

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Ostwald, M.M., Alba-Tercedor, J., Minckley, R.L. et al. 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. Apidologie 53, 60 (2022). https://doi.org/10.1007/s13592-022-00967-w

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Keywords

  • Sex pheromone gland
  • Mating strategies
  • Xylocopa