Male-specific exaggerated horns are an evolutionary novelty and have diverged rapidly via intrasexual selection. Here, we investigated the function of the conserved sex-determination gene doublesex (dsx) in the Japanese rhinoceros beetle (Trypoxylus dichotomus) using RNA interference (RNAi). Our results show that the sex-specific T. dichotomus dsx isoforms have an antagonistic function for head horn formation and only the male isoform has a role for thoracic horn formation. These results indicate that the novel sex-specific regulation of dsx during horn morphogenesis might have been the key evolutionary developmental event at the transition from sexually monomorphic to sexually dimorphic horns.

As a first step in reconstructing the phylogeny of world stag beetles (Coleoptera: Lucanidae), phylogenetic relationships among the major members of Japanese stag beetles were explored by analyzing a sequence of 1030 nucleotides from the mitochondrial 16S ribosomal RNA (16S rRNA) gene. A total of 20 species and three additional subspecies representing 13 genera were examined to provide basic information on the phylogeny of world Lucanidae. The resultant phylogenetic tree indicates that the family Lucanidae is monophyletic, and contains two major lineages: one consists of the genera Platycerus, Aesalus, Ceruchus, and Nicagus, and the other includes Dorcus, Rhaetulus, Prosopocoilus, Aegus, Neolucanus, Prismognathus, Lucanus, Figulus, and Nigidius. Generic members of the latter lineage are further divided into the following four sublineages: i) Figulus and Nigidius; ii) Prismognathus and Lucanus; iii) Aegus and Neolucanus; and iv) Dorcus, Rhaetulus, and Prosopocoilus. These molecular phylogenetic relationships are used as a basis for a preliminary exploration of the evolution of sexual dimorphism in the shape of the mandible. The results of this investigation suggest that strong sexual dimorphism with well-developed mandibles in males evolved independently at least twice, once in the genus Aegus and once in the ancestor of the Lucanus-Prismognathus and Dorcus-Rhaetulus-Prosopocoilus clades. Alternatively, it is possible that sexual dimorphism of mandibles has undergone secondary loss in the genera Figulus and Nigidius.

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Molecular phylogenetic relationships among 25 species of the wood-feeding cockroach belonging to the genus Salganea Stål (Panesthiinae; Blaberidae) in Southeast Asia were analyzed based on the DNA sequence of the complete mitochondrial cytochrome oxidase II (COII) gene. Most basal relationships among species of Salganea are poorly resolved by both neighbor-joining and nonweighted parsimony analyses, suggesting the possibility of a hard polytomy due to a rapid and potentially simultaneous radiation early in the history of the genus. For more apical relationships, however, some interesting phylogenetic relationships were recognized. The monophyly of the two species groups, morio and foveolata, the former of which is distributed mainly in the Sunda lands (containing the Malay Peninsula, Sumatra, Java, and Borneo), whereas the latter is Sulawesi endemic, was strongly supported. Based on the inferred phylogenetic patterns and recent palaeogeographic scenario for Southeast Asia, it is suggested that a radiation of Salganea species occurred in Southeast Asia presumably in the early Tertiary, and several barriers against dispersal and gene flow, such as the formation of straits or high mountains, have arisen from the middle Tertiary.