Pangolin expression influences the development of a morphological novelty: beetle horns

Morphological diversity arises during development through the actions and interactions of diverse developmental pathways. Among those, the Wnt pathway is known to contribute to diverse developmental processes such as segmentation and the morphogenesis of appendages. Here, we characterize a transcription factor in the Wnt pathway, pangolin (pan), to investigate the role of Wnt signaling in the development of evolutionarily novel body structures: the horns of beetles. Beetle horns are highly diverse in size, shape, and number and develop principally from two major body regions: the head and prothorax. We investigate horns in two species of the genus Onthophagus using comparative in situ hybridization, larval RNA interference, and allometric measurements to analyze whether horn formation is regulated by pan and by extension the Wnt pathway. Our results illustrate that pan expression affects beetle horn growth in a species-, sex-, and location-specific manner in two morphologically distinct, yet closely-related, Onthophagus species.