Conservation, innovation, and the evolution of horned beetle diversity

Beetle horns represent an evolutionary novelty exhibiting remarkable diversity above and below the species level. Here, we show that four typical appendage patterning genes, extradenticle (exd), homothorax (hth), dachshund (dac), and Distal-less (Dll) are expressed in the context of the development of sexually dimorphic thoracic horns in three Onthophagus species. At least two of these genes, Dll and hth, exhibited expression patterns consistent with a conservation of patterning function during horn development relative to their known roles in the development of insect legs. exd, hth, and dac expression patterns during horn development were largely invariable across species or sexes within species. In contrast, Dll expression was far more discrete and exhibited consistent differences between sexes and species. Most importantly, differences in location and domain size of Dll expression tightly correlated with the degree to which prepupal horn primordia were retained or resorbed before the final adult molt. Our results lend further support to the hypothesis that the origin of beetle horns relied, at least in part, on the redeployment of already existing developmental mechanisms, such as appendage patterning processes and that changes in the exact location and domain size of Dll expression may represent important modifier mechanisms that modulate horn expression in different species or sexes. If correct, this would imply that certain components of genetic basis of horn development may be able to diversify rapidly within lineages and largely independent of phylogenetic distance. We present a first model that integrates presently available data on the genetic regulation of horn development and diversity.