The molecular basis of venom resistance in a rattlesnake-squirrel predator-prey system

18 pages, 5 fig, 3 tables.Data that support the findings of this study are available in the Dryad Data Repository (https://doi.org/10.5061/dryad.wm37pvmjz)Understanding how interspecific interactions mould the molecular basis of adaptations in coevolving species is a long-sought goal of evolutionary biology. Venom in predators and venom resistance proteins in prey are coevolving molecular phenotypes, and while venoms are highly complex mixtures it is unclear if prey respond with equally complex resistance traits. Here, we use a novel molecular methodology based on protein affinity columns to capture and identify candidate blood serum resistance proteins ("venom interactive proteins" [VIPs]) in California Ground Squirrels (Otospermophilus beecheyi) that interact with venom proteins from their main predator, Northern Pacific Rattlesnakes (Crotalus o. oreganus). This assay showed that serum-based resistance is both population- and species-specific, with serum proteins from ground squirrels showing higher binding affinities for venom proteins of local snakes compared to allopatric individuals. Venom protein specificity assays identified numerous and diverse candidate prey resistance VIPs but also potential targets of venom in prey tissues. Many specific VIPs bind to multiple snake venom proteins and, conversely, single venom proteins bind multiple VIPs, demonstrating that a portion of the squirrel blood serum "resistome" involves broad-based inhibition of nonself proteins and suggests that resistance involves a toxin scavenging mechanism. Analyses of rates of evolution of VIP protein homologues in related mammals show that most of these proteins evolve under purifying selection possibly due to molecular constraints that limit the evolutionary responses of prey to rapidly evolving snake venom proteins. Our method represents a general approach to identify specific proteins involved in co-evolutionary interactions between species at the molecular level.HLG was supported by NSF Grant 1638872 during preparation of the manuscript. Research in the laboratory of J.J.C. was partly financed by grant BFU2017-89103-P from the Ministerio de Ciencia, Innovación y Universidades, Madrid (Spain). Fieldwork for serum and venom col-lection was funded by the American Museum of Natural History's Theodore Roosevelt Grant and American Society of Naturalist Graduate Student Research Grant, both to M.L.H. M.L.H. was sup-ported by an NSF Postdoctoral Research Fellowship 1711141 during preparation of the manuscriptPeer reviewe