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

Abstract

Despite sharing nearly the same genome, individuals within the same species can vary drastically in both morphology and behaviour as a function of developmental stage, sex or developmental plasticity. Thus, regulatory processes must exist that enable the stage-, sex- or environment-specific expression of traits and their integration during ontogeny, yet exactly how trait complexes are co-regulated and integrated is poorly understood. In this study, we explore the developmental genetic basis of the regulation and integration of environment-dependent sexual dimorphism in behaviour and morphology in the horn-polyphenic dung beetle Onthophagus taurus through the experimental manipulation of the transcription factor doublesex (dsx). The gene dsx plays a profound role in the developmental regulation of morphological differences between sexes as well as alternative male morphs by inhibiting horn formation in females but enabling nutrition-responsive horn growth in males. Specifically, we investigated whether experimental downregulation of dsx expression affects male and female aggressive and courtship behaviours in two social contexts: interactions between individuals of the same sex and interactions between males and females. We find that dsx downregulation significantly alters aggressiveness in both males and females, yet does so differently for both sexes as a function of social context: dsxRNAi males exhibited elevated aggression towards males but showed reduced aggression towards females, whereas dsxRNAi females became more aggressive towards males, while their aggressiveness towards other females was unaffected. Moreover, we document unexpectedly high levels of female aggression independent of dsx treatment in both wild-type and control-injected individuals. Lastly, we found no effects of dsxRNAi on courtship and mating behaviours. We discuss the role of dsx in the regulation of sex-specific and plastic behaviours, the unexpectedly high levels of aggression of hornless dsxRNAi males in relation to the well-established description of the hornless sneaker phenotype and the potential ecological function of high female aggression.

Evolution and plasticity of morph‐specific integration in the bull‐headed dung beetle Onthophagus taurus

“…Polyphenic variation in mating strategies is frequently portrayed as highly polarized, with a ‘fighting’ morph showing male–male aggression and fighting behavior, whereas males that fall in the ‘sneaker’ category exclusively employ more clandestine tactics (Simpson, Sword, & Lo, 2011). Yet, in O. taurus , fighting behavior is not restricted to major males (Beckers et al., 2017; Moczek & Emlen, 2000). Specifically, if minor males encounter opponents of similar size, they will also engage in male–male combat identical in posture and motion to their horned male counterparts (Moczek & Emlen, 2000).…”

“…Nevertheless, the morph‐dependent integration in tibia size could relate to other aspects of polyphenic development as well. For example, during courtship males tap the female's elytra repeatedly with their forelegs (‘drumming’; see: Beckers et al., 2017; Kotiaho, 2002), and females exert preferences based on courtship vigor (Kotiaho, Simmons, & Tomkins, 2001; Simmons & Holley, 2011), therefore, tibia shape and integration may also function in the context of ornamentation for courtship, rather than weaponry for combat. Alternatively, or in addition, major males of Eastern US origin engage in significantly more paternal assistance during reproduction, including the digging of tunnels, than their minor male counterparts (Moczek, 1999), which could further shape the morph‐dependent integration in tibia size.…”

“…Phenotypic integration can be adaptive if natural selection favors the co‐occurrence of several traits via correlational selection or selection on certain functional trait combinations on the evolutionary, static, and ontogenetic levels (Cheverud, 1982, 1984; Klingenberg, 2014). Examples include evolutionary integration of ecological variation, life history, physiology, and behavior within and among species (Blanckenhorn et al., 2020; Réale et al., 2010; Ricklefs & Wikelski, 2002), developmental integration between morphology and behavior among individuals (Beckers, Kijimoto, & Moczek, 2017), or the ontogenetic integration between functionally related skeletal structures (Zelditch, 1988). In addition to such adaptive scenarios, trait covariation can also arise as a by‐product of shared developmental processes involved in the development of different traits, or arise neutrally by mere genetic correlation via pleiotropy or linkage (Lande, 1982; Lynch & Walsh, 1998).…”

Experimental evidence for the role of sexual selection in the evolution of cuticular hydrocarbons in the dung beetle, Onthophagus taurus

“…Although the mating system of O. taurus has been characterized by sexual selection on males (Kotiaho et al, 2001;Moczek & Emlen, 2000), females accrue benefits by mating with multiple mates (Garcia-Gonzalez & Simmons, 2007;McCullough et al, 2017;Simmons & Garcia-Gonzalez, 2008), particularly mates of high quality (Simmons & Holley, 2011). Recent behavioural observations have also revealed aggressive interactions among females of this species (Beckers, Kijimoto, & Moczek, 2017). This raises the possibility that females might compete for access to high-quality males and that males may bias their mating activity towards females with particular CHC profiles.…”

Serotonin signaling suppresses the nutrition‐responsive induction of an alternate male morph in horn polyphenic beetles

“…These morphologies and behaviors are the product of very different developmental and physiological processes: male horns are the products of epidermal outbuddings that are initiated and grown during late larval development, and their length is fully determined in early pupae, whereas adult behavior is most likely influenced by morphspecific differentiation of function in the adult brain, which does not come into existence and does not form connections with the peripheral nervous system until mid-to late-pupal development at the earliest. Kijimoto et al (2012) and Beckers, Kijimoto, and Moczek (2017) raised the possibility that one mechanism capable of coordinating the expression of both male morphology and male behavior in horned beetles is dsx: not only does the expression of the male dsx isoform promote the formation of horn growth, it also F I G U R E 3 Scaling relationship between body size (x-axis) and horn size (y-axis) for male Onthophagusgazella, O. nigriventris, andO. sagittarius treated with AMTP.…”