The weapon trait is typically characterized by an exaggerated body part employed in intrasexual competition, such as male-male combat over mating partners. In the last two decades, EvoDevo studies have engaged in understanding the genetic basis and the evolutionary origin of this sexually selected weapon. However, it remains unclear to what extent factors in the molecular mechanism controlling organ growth regulate the sex- and organ-specific exaggeration of the weapon traits. To this end, we investigate the functions of two well-known organ-growth controlling pathways, Fat-signaling and Crumbs-Expanded modules, for weaponized mandible growth in the Japanese common stag beetle, Dorcus rectus. Here, we find that nine genes of these pathways are involved in mandible extension and morphogenesis along the proximodistal (PD) axis. Some of these genes control the morphogenesis of the distal portion of the leg and antenna and leg size as well. These results suggest that the common role of these genes in appendage growth may be recruited to the exaggerated growth of the Dorcus mandible as a functional module. In addition, our morphometrics shows that dachsous and expanded contribute to sex-linked growth of male mandibles, suggesting that these genes adjust mandible size and shape according to sex through integrating sexual information with organ growth mechanisms. Overall, the weaponized nature of the Dorcus mandible might have originated via the co-option of the Fat-signaling and Crumbs-Expanded module-related genes into mandible exaggeration along the PD axis.
Keywords: Dorcus rectus; exaggerated trait; morphogenesis; proximodistal axis; weapon trait.