Give and take: testing for exchange of crotoxin in rock rattlesnakes (Crotalus lepidus) within and between species

Mellor, N. Jade

Njmello@g.clemson.edu

Parkinson, Chris

Department of Biological Sciences

Clemson University

Clemson, South Carolina USA

Snake venoms display a range of hemorrhagic (Type I) to neurotoxic (Type II) venoms. Type I venoms are characterized by the expression of snake venom metalloproteinases and low toxicity while Type II venoms are characterized by low snake venom metalloproteinases and high toxicity. Although many rattlesnake species have been classified as either Type I or Type II, population-level variation in venom phenotype has been documented in several species. Much of the variation in venom phenotype is driven by the presence or absence of Crotoxin (or its homologs, hereafter all referred to as CTx). CTx is a highly toxic heterodimeric beta-neurotoxin, and the main component of Type II venoms in many species. The phenomenon of population-level variation has been most widely studied in large-bodied lowland rattlesnakes (Crotalus scutulatus, C. helleri, and C. horridus). While it has been suspected in C. lepidus, CTx presence has not been genetically confirmed in the small-bodied montane clade. We used genomics and transcriptomics to test for the presence, distribution, and evolution of CTx in the Rock Rattlesnake (C. lepidus). Out of 104 individuals from across the species’ range, 17 were genetically confirmed to have CTx. Interestingly, CTx presence was not significantly associated with longitude, latitude, subspecies, or elevation. However, we did identify several climatic variables associated with CTx presence, including one which has been identified in previous studies on CTx expression variation. These findings expand our understanding of CTx evolution in rattlesnakes by providing insights on the phylogenetic distribution of CTx across rattlesnakes, the variation in crotoxin expression, and highlighting environments to which CTx may be locally adapted. Our results likely support previous hypotheses of an ancestral origin for crotoxin followed by independent sorting in lineages; therefore, future studies should focus on testing for the presence of CTx in other species of montane rattlesnakes.