No abstract

The genetic mechanisms behind aggressive behaviors are important for understanding the formation of dominance hierarchies, and thus social systems in general. Studies into the effects of social experience and agonistic contest outcomes have shown significant changes in brain gene expression resulting from repeated winning and losing, as well as changing dominance rank, primarily in obligately social species. However, our knowledge of the genetic underpinnings of behavior in subsocial organisms is relatively poor, yet understanding the behavioral genetics of this simplest form of sociality provides the basis for understanding all other forms of social living. Here, we measured the effects of aggression on brain gene expression in the subsocial bee, Ceratina calcarata, in individuals that had experienced repeated winning, repeated losing, or a change in rank during repeated encounters. Consistent winning accounted for the majority of variation in brain gene expression, followed by changing rank over maintaining rank. Candidate genes for aggression are identified through comparative transcriptomics across 21 invertebrate and 6 vertebrate taxa. Lastly, we identified significantly over-represented cis-regulatory elements, namely transcription factor binding motifs deeply conserved across a wide range of taxa and broadly implicated in differential regulation of genes related to aggressive/dominant behavior. We present evidence for both genetic and cis-regulatory mechanisms for aggression that may have broad importance to social evolution.

Dominance hierarchies represent some of nature’s most rudimentary social structures, and aggression is key to their establishment in many animal species. Previous studies have focused on the relative influences of prior experience and physiological traits of individuals in determining social rank through aggression. Here we examine the behavioural potential for dominance hierarchy formation in the subsocial small carpenter bee, Ceratina calcarata. Both physiological traits and social experience were found to play partial roles in predicting future interactive behaviour in this species. Our results suggest that individual size is associated with dominance in initial encounters, while prior experience plays a larger role in predicting dominance in subsequent encounters. Social systems in the early stages of social evolution may well have followed these same predictive factors and these factors are key targets for future studies of social evolution and the behavioural origins of dominance hierarchies.