Developmental Link between Sex and Nutrition; doublesex Regulates Sex-Specific Mandible Growth via Juvenile Hormone Signaling in Stag Beetles

Abstract

Sexual dimorphisms in trait expression are widespread among animals and are especially pronounced in ornaments and weapons of sexual selection, which can attain exaggerated sizes. Expression of exaggerated traits is usually male-specific and nutrition sensitive. Consequently, the developmental mechanisms generating sexually dimorphic growth and nutrition-dependent phenotypic plasticity are each likely to regulate the expression of extreme structures. Yet we know little about how either of these mechanisms work, much less how they might interact with each other. We investigated the developmental mechanisms of sex-specific mandible growth in the stag beetle Cyclommatus metallifer, focusing on doublesex gene function and its interaction with juvenile hormone (JH) signaling. doublesex genes encode transcription factors that orchestrate male and female specific trait development, and JH acts as a mediator between nutrition and mandible growth. We found that the Cmdsx gene regulates sex differentiation in the stag beetle. Knockdown of Cmdsx by RNA-interference in both males and females produced intersex phenotypes, indicating a role for Cmdsx in sex-specific trait growth. By combining knockdown of Cmdsx with JH treatment, we showed that female-specific splice variants of Cmdsx contribute to the insensitivity of female mandibles to JH: knockdown of Cmdsx reversed this pattern, so that mandibles in knockdown females were stimulated to grow by JH treatment. In contrast, mandibles in knockdown males retained some sensitivity to JH, though mandibles in these individuals did not attain the full sizes of wild type males. We suggest that moderate JH sensitivity of mandibular cells may be the default developmental state for both sexes, with sex-specific Dsx protein decreasing sensitivity in females, and increasing it in males. This study is the first to demonstrate a causal link between the sex determination and JH signaling pathways, which clearly interact to determine the developmental fates and final sizes of nutrition-dependent secondary-sexual characters.

Latitudinal cline in weapon allometry and phenology of the European stag beetle

“…It is worth noting that both larval growth conditions and feeding resources (Moczek 2002) have an effect on the developmental mechanisms that regulate the elaboration of exaggerated structures in holometabolous insects (Shingleton et al 2008). Indeed, it has been demonstrated that in stag beetles, the development of SSC is closely associated with larval feeding conditions, the mediatory function of the juvenile hormone (Gotoh et al 2011) and the expression of transcription factors encoded by double-sex genes (Gotoh et al 2014). Larvae of the studied populations, described above, have grown in different woodland types (e.g., oak grove, chestnut) and have experienced different nutritional conditions.…”

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

“…Recent work has implicated the sex determination gene dsx in promoting horn formation in males subject to optimal nutritional condition in both the rhinoceros beetle Trypoxylus dichotomus (which exhibits a linear, highly positive allometry) and O. taurus (which exhibits a more derived sigmoidal allometry, including a sharp body size threshold separating alternative male morphs). In both taxa, dsx seems to play a critical role in the nutrition-dependent exaggeration of horn growth, and several studies suggest further that insulin signaling, juvenile hormone signaling, or a combination of the two may interact with dsx and sensitize dsx action to nutritional conditions (24,25,27). This research highlights a third highly conserved but previously overlooked pathway, the Hh signaling pathway, whichlike dsx signaling-has been co-opted into the regulation of horn development and acquired nutrition sensitivity in the process but exerts its regulatory function in, to the best of our knowledge, a thus far unprecedented manner, namely by actively inhibiting horn growth in low-nutrition individuals only (Fig.…”

“…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.…”

doublesex alters aggressiveness as a function of social context and sex in the polyphenic beetle Onthophagus taurus

“…In striking contrast, downregulation of dsxf in female O. taurus induces nutrition-responsive development of male-like head horns (Kijimoto et al, 2012) in typically hornless females. Thus, dsx plays a critical role in the regulation of both sexual dimorphism and male polyphenism in O. taurus , and independent studies in rhinoceros beetles (Ito et al, 2013) and stag beetles (Gotoh et al, 2014) have since confirmed that sex-specific dsx isoforms have repeatedly evolved to not just inhibit the formation of weapons in females, but also to disproportionately enhance their growth in high-nutrition males.…”