The transcriptomic basis of tissue- and nutrition-dependent sexual dimorphism in the beetle Onthophagus taurus.

Sexual dimorphism accounts for a large fraction of intraspecific diversity. However, not all traits are equally sexually dimorphic; instead, individuals are mosaics of tissues that vary in their ability to exhibit dimorphism. Furthermore, the degree of a trait's sexual dimorphism is frequently environment-dependent, with elaborate sexual dimorphism commonly being restricted to high nutritional conditions. Understanding the developmental basis and evolution of condition-dependent sexual dimorphism can be critically informed by determining - across tissues and nutritional conditions - what sex-biased genes are deployed and how they interact and translate into functional processes. Indeed, key theories concerning the evolution of condition-dependent sexually dimorphic traits rest on assumptions regarding their developmental genetic underpinnings, yet, have largely gone unexamined by empirical studies. Here, we provide such evidence by investigating the transcriptomic basis of tissue- and nutrition-dependent sexual dimorphism in the bull-headed dung beetle Onthophagus taurus. Our findings suggest (1) that generating morphological sexual dimorphism requires sex-biased gene expression in and developmental remodeling of both sexes, regardless of which sex exhibits externally visible trait exaggeration, (2) that although sexually dimorphic phenotypes are comprised of traits underlain by independent repertoires of sex-biased gene expression, they act similarly at a functional level, and (3) that sexual dimorphism and condition-dependence share common genetic underpinnings specifically in sexually-selected traits.