Genetical control of 2D pattern and depth of the primordial furrow that codes 3D shape of the rhinoceros beetle horn

Abstract

The head horn of the Asian rhinoceros beetle develops as extensively folded primordia before unfurling into its final 3D shape at the pupal molt. The information of the final 3D structure of the beetle horn is encoded in the folding pattern of the developing primordia. However, the developmental mechanism underlying epithelial folding of the primordia is unknown. In this study, we addressed this gap in our understanding of the developmental patterning of the 3D horn shape of beetles by focusing on the formation of surficial furrows that become the bifurcated 3D shape of the horn. By gene knockdown screening via RNAi, we found that knockdown of the gene Notch disturbed overall horn primordia furrow depth without affecting 2D furrow pattern. In contrast, knockdown of CyclinE altered 2D horn primordia furrow pattern without affecting furrow depth. From these results, depth and 2D pattern of primordial surficial furrow are likely to be regulated independently during the development and both of change can alter the final 3D shape. Author SummaryIn insects, some large structure is made under the old exoskeleton before the molting. Long horn of rhino-beetle is one of extreme cases. The beetle horn is compactly packed as furrowed primordia under the larval exoskeleton. At molting, the primordia is extended to form its final 3D horn shape as blowing up furrows like a balloon. This transformation from primordia to final horn does not required any living cell activities. Thus, characteristics of furrows of primordia actually determine the final 3D shape. However, molecular mechanisms and genetic basis of furrow formation is not well understood not only in beetle horn but also in any other insects. In this study, by using beetle horn as a model, we addressed what kind of genetic factors are contributed to primordial furrow formation. By gene knockdown screening, we found that knockdown of the gene Notch disturbed primordial furrow depth without affecting 2D furrow pattern. In contrast, knockdown of CyclinE altered 2D furrow pattern without affecting furrow depth. In both case, final horn shapes were disturbed. From these results, we concluded that both of the depth and 2D pattern of primordial furrow can contribute final shape, but their development is controlled independently.