Evolution of the Western Rattlesnakes and diversification of their venoms

Mackessy, Stephen P.                      

Department of Biological Sciences      

University of Northern Colorado  

Greeley, Colorado USA

The Western Rattlesnake clade consists of several species, including C. cerberus, C. viridis, C. oreganus and C. scutulatus (hereafter CVOS), with combined ranges that span the western half of the United States, extending into south-central Mexico and southwestern Canada. The taxonomy of the group has been contentious and debated for some time, and understanding regional differentiation is complicated by numerous cases of range expansion/contraction, secondary contact and introgression in many areas, and current isolation due to geographic features. However, these same factors have produced a natural experiment scenario for evaluating their effects on venom phenotypes on a broad geographic scale. Over many years, we have collected venom samples broadly, focusing on specific taxa and then assembling this data into a larger cohesive group in order to identify trends that may occur among members of the CVOS clade. Ontogenetic variation in venom composition is common but is neither universal nor identical in various clades, and venom paedomorphosis appears to be limited to C. o. concolor and perhaps C. scutulatus, both of which are characterized by type II venoms (high toxicity, low SVMP activity) as adults. Most taxa produce venoms as adults with of a type I pattern (lower toxicity, higher proteolytic activities), though geographic trends are variable; Crotalus v. viridis, for example, shows disjunct compositional changes along a N-S axis, whereas C. cerberus venoms vary along an E-W axis, with greatest differences seen at extremes of distribution. Crotalus scutulatus shows an interesting pattern of type II (more common) or type I venoms, with several areas where “blended” venom phenotypes occur; venom type may be related to climatic variables. For C. v. viridis, these differences are likely associated with differential diets (more diverse in south) and perhaps several environmental parameters; for C. cerberus, insufficient diet data exists, but differences may be associated with edge-of-range effects.