doublesex regulates sexually dimorphic beetle horn formation by integrating spatial and temporal developmental contexts in the Japanese rhinoceros beetle Trypoxylus dichotomus

Abstract

AbstractMany scarab beetles have sexually dimorphic exaggerated horns that are an evolutionary novelty. Since the shape, number, size, and location of horns are highly diverged within Scarabaeidae, beetle horns are an attractive model for studying the evolution of sexually dimorphic and novel traits. In beetles including the Japanese rhinoceros beetle Trypoxylus dichotomus, the sex determination gene doublesex (dsx) plays a crucial role in sexually dimorphic horn formation during larval-pupal development. However, knowledge of when and how dsx drives the gene regulatory network (GRN) for horn formation to form sexually dimorphic horns during development remains elusive. To address this issue, we identified a Trypoxylus-ortholog of the sex determination gene, transformer (tra), that regulates sex-specific splicing of the dsx pre-mRNA, and whose loss of function results in sex transformation. By knocking down tra function at multiple developmental timepoints during larval-pupal development, we estimated the onset when the sex-specific GRN for horn formation is driven. In addition, we also revealed that dsx regulates different aspects of morphogenetic activities during the prepupal and pupal developmental stages to form appropriate morphologies of pupal head and thoracic horn primordia as well as those of adult horns. Based on these findings, we discuss the evolutionary developmental background of sexually dimorphic trait growth in horned beetles.\n\nAuthor Summary:\n\nBeetle horns are highly enriched in a particular family Scarabaeidae, although the shape, size and number of horns are diversified within the group. In addition, many scarab beetle horns are sexually dimorphic. It has been questioned how a particular group of beetles has originated and diversified evolutionary novel horns. Here we found the exact time when morphological sexual dimorphism of horn primordia appeared, estimated the onset of the developmental program for sexually dimorphic horn formation driven by Doublesex, and revealed that Doublesex regulates different aspects of cell activities of horn primordia depending on the spatiotemporal contexts. Our study provides our understanding regarding regulatory shifts in these mechanisms during the evolution of sexually dimorphic traits in horned beetles.