Keywords: Hox genes, evolution, development, preadaptations, rove beetles, symbiosis peer-reviewed) is the author/funder. All rights reserved. No reuse allowed without permission.The copyright holder for this preprint (which was not . http://dx.doi.org/10.1101/198945 doi: bioRxiv preprint first posted online Oct. 5, 2017; Introduction Symbiotic relationships pervade the natural world, but their evolution from a free-living existence is poorly understood. Explaining how a given symbiosis evolved and took on its precise form rests on explaining how the often intricate traits that mediate the relationship emerged developmentally and evolutionarily. In the Metazoa, numerous parasitic and mutualistic taxa bear specialized anatomical, physiological and neurobiological adaptations for engaging in interspecies interactions. Such multifarious traits are typically idiosyncratic, lineage-specific features: ant-tended nectary organs of lycaenid butterfly caterpillars (1), the sucking disks of remoras (2), or the neural differentiation of host and conspecific chatter by parasitic cowbirds (3). Inferring the origins of such features can be challenging, with many appearing as novelties, or deriving from complex or extreme phenotypic modifications that cloud their evolutionary histories. Preadaptations-genetic or phenotypic attributes that evolved prior to the symbiosis itself (also termed "exaptations": 4)-have proven useful for understanding the evolutionary starting material for functional traits in a variety of symbiotic relationships (5-7). Preadaptive traits may form the basis for rudimentary or facultative symbioses by predisposing interactions to occur between free-living species (so called "primary preadaptations;" 7). Preadaptations may also offer paths of least resistance to subsequent adaptation, biasing phenotypic change to certain preexisting traits as the rudimentary symbiosis evolves in intimacy ("secondary preadaptations"; (7).One clade that serves as a paradigm for understanding the evolution of animal symbioses are the rove beetles (Staphylinidae), currently recognized as the most species rich family in the Metazoa (>63,000 described species)(8). Most staphylinids are freeliving, predatory inhabitants of litter and soil (9, 10), but numerous independent lineages have evolved to live symbiotically inside social insect colonies, in particular those of ants (myrmecophiles) and termites (termitophiles) (11)(12)(13)(14)(15). Such taxa appear to behave primarily as social parasites: burdensome colony guests, which probably impose a cost on their hosts through resource exploitation and brood predation (7, 13). The ecologies of these species vary markedly, from opportunistic nest intruders that are attacked when detected by hosts, to socially integrated species that are accepted as apparent nestmates (16-21).peer-reviewed) is the author/funder. All rights reserved. No reuse allowed without permission.The copyright holder for this preprint (which was not . http://dx.doi.org/10.1101/198945 doi: bioRxiv preprint firs...