Sequence Divergence in Venom Genes Within and Between Montane Pitviper (Viperidae: Crotalinae: Cerrophidion ) Species is Driven by Mutation–Drift Equilibrium

Snake venom can vary both among and within species. While some groups of New World pitvipers—such as rattlesnakes—have been well studied, very little is known about the venom of montane pitvipers ( Cerrophidion ) found across the Mesoamerican highlands. Compared to most well-studied rattlesnakes, which are widely distributed, the isolated montane populations of Cerrophidion may facilitate unique evolutionary trajectories and venom differentiation. Here, we describe the venom gland transcriptomes for populations of C. petlalcalensis , C. tzotzilorum , and C. godmani from Mexico, and a single individual of C. sasai from Costa Rica. We explore gene expression variation in Cerrophidion and sequence evolution of toxins within C. godmani specifically. Cerrophidion venom gland transcriptomes are composed primarily of snake venom metalloproteinases, phospholipase A _2 s (PLA _2 s), and snake venom serine proteases. Cerrophidion petlalcalensis shows little intraspecific variation; however, C. godmani and C. tzotzilorum differ significantly between geographically isolated populations. Interestingly, intraspecific variation was mostly attributed to expression variation as we did not detect signals of selection within C. godmani toxins. Additionally, we found PLA _2 -like myotoxins in all species except C. petlalcalensis , and crotoxin-like PLA _2 s in the southern population of C. godmani . Our results demonstrate significant intraspecific venom variation within C. godmani and C. tzotzilorum . The toxins of C. godmani show little evidence of directional selection where variation in toxin sequence is consistent with evolution under a model of mutation–drift equilibrium. Cerrophidion godmani individuals from the southern population may exhibit neurotoxic venom activity given the presence of crotoxin-like PLA _2 s; however, further research is required to confirm this hypothesis.