Diversification of doublesex function underlies morph-, sex-, and species-specific development of beetle horns

Abstract

Sex-specific trait expression is frequently associated with highly variable, condition-dependent expression within sexes and rapid divergence among closely related species. Horned beetles are an excellent example for studying the molecular basis of these phenomena because horn morphology varies markedly among species, between sexes, and among alternative, nutritionally-cued morphs within sexes. In addition, horns lack obvious homology to other insect traits and provide a good opportunity to explore the molecular basis of the rapid diversification of a novel trait within and between species. Here we show that the sex-determination gene doublesex (dsx) underlies important aspects of horn development, including differences between sexes, morphs, and species. In male Onthophagus taurus, dsx transcripts were preferentially expressed in the horns of the large, horned morph, and RNAimediated knockdown of dsx dramatically altered male horn allometry by massively reducing horn development in large males, but not in smaller males. Conversely, dsx RNAi induced ectopic, nutrition-sensitive horn development in otherwise hornless females. Finally, in a closely related species (Onthophagus sagittarius) that has recently evolved a rare reversed sexual dimorphism, dsx RNAi revealed reversed as well as novel dsx functions despite an overall conservation of dsx expression. This suggests that rapid evolution of dsx functions has facilitated the transition from a regular sexual dimorphism to a reversed sexual dimorphism in this species. Our findings add beetle horns to existing examples of a close relationship between dsx and sexual trait development, and suggest that dsx function has been coopted to facilitate both the evolution of environmentally-cued intrasexual dimorphisms and rapid species divergences in a novel trait.cooption | polyphenism | evolutionary novelty | interchangeability | weak linkage

Dmrt1 induces the male pathway in a turtle with temperature-dependent sex determination

“…Recently, the Dmrt1 ortholog Doublesex (Dsx) has been identified as a key regulator of the male phenotype in the branchiopod crustacean Daphina magna, which exhibits environmental sex determination (ESD) (Kato et al, 2011). In beetles, Dsx is involved in nutritiondependent male sexual trait development (Gotoh et al, 2014;Kijimoto et al, 2012). All these observations suggest that, in spite of the diversity and plasticity of sex-determination mechanisms, DM domain genes are highly conserved in male sexual differentiation pathways between GSD, TSD and the broader category of ESD systems.…”

Hedgehog signaling enables nutrition-responsive inhibition of an alternative morph in a polyphenic beetle

“…To knockdown hh, ptc, and smo, dsRNA was generated as described in ref. 23. The region of interest was amplified and cloned using the StrataClone PCR Cloning Kit followed by BigDye Sequencing (Table S2 shows primer sequences).…”

“…Previous studies have shown that horn development is instructed by many of the same genes that regulate the formation of regular appendages, such as leg gap genes (14), that nutrition responsiveness of horn development may be coupled to insulin signaling (24,25), and that sex-and morph-specific horn exaggeration may be facilitated by doublesex (dsx), a master regulator of somatic sex determination (23,26,27). However, how beetle horns achieve their strikingly dimorphic expression in many species and the developmental mechanisms that enable the definition of a sharp body size threshold separating alternate morphs from what is presumed to be a continuous range of nutritional conditions are entirely unknown.…”

“…Our results suggest that recruitment of the Hh signaling pathway may have been a key step in the evolution of trait thresholds and the corresponding origin of alternative phenotypes and complex allometries. especially critical role in horned beetle diversification by (i) enabling the evolution of some of the most extreme examples of condition-dependent exaggerated male secondary sexual traits (21); (ii) facilitating the origin of horn polyphenisms and discrete hornless sneaker and horned fighter morphs adapted for alternative reproductive niches (18); (iii) establishing sharp threshold body sizes that decouple alternative developmental trajectories and enable the integration of two separate, complex, disparate male phenotypes from the same genome (16,22); and (iv) contributing to the enormous radiation of horned beetles through the evolution of morph-specific morphologies, behaviors, and physiologies and the diversification of size thresholds, perhaps best illustrated in the >2,000 species-rich genus Onthophagus (16,17,19,23).…”

Molecular cloning, characterization and expression analysis of a Doublesex gene from Daphnia carinata (Crustacea: Cladocera) during different reproductive stages

“…Dsx function has been co-opted to facilitate the evolution of environmentally cued intrasexual dimorphisms (Kijimoto et al, 2012), and it is known that Daphnia reproduction switches from parthenogenic to sexual females in response to environmental cues, so DapcaDsx2 expression levels differ between asexual and sexual reproduction (Liu et al, 2014). Taken together, these results clearly show that DapcaDsx2 mRNA has a specific expression pattern during the different reproductive stages, and that DapcaDsx2 mRNA levels are significantly higher during sexual than asexual reproduction, indicating that it exhibits gender-specific expression, which would play an important role in regulating reproductive transformation in D. carinata.…”