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

Abstract

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 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 (venom interactive proteins [VIPs]) in California Ground Squirrels (Otospermophilus beecheyi) that interact with venom 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 from ground squirrels showing higher binding affinities for venom 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 and, conversely, single venom bind multiple VIPs, demonstrating that a portion of the squirrel blood serum resistome involves broad-based inhibition of nonself 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 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 involved in co-evolutionary interactions between species at the molecular level.