Montane Rattlesnakes in México: Venoms of Crotalus tancitarensis and Related Species within the Crotalus intermedius Group.

The Crotalus intermedius group is a clade of rattlesnakes consisting of several species adapted to a high elevation habitat, primarily in México. Crotalus tancitarensis was previously classified as C. intermedius, until individuals occurring on Cerro Tancítaro in Michoacán, México, were reevaluated and classified as a new species (C. tancitarensis) based on scale pattern and geographic location. This study aimed to characterize the venom of C. tancitarensis and compare the venom profile to those of other species within the Crotalus intermedius group using gel electrophoresis, biochemical assays, reverse-phase high performance liquid chromatography, mass spectrometry, and lethal toxicity (LD50) assays. Results show that the venom profiles of species within the Crotalus intermedius group are similar, but with distinct differences in phospholipase A2 (PLA2), metalloproteinase PI (SVMP PI), and kallikrein-like serine proteinase (SVSP) activity and relative abundance. Proteomic analysis indicated that the highland forms produce venoms with 50-60 protein isoforms and a composition typical of type I rattlesnake venoms (abundant SVMPs, lack of presynaptic PLA2-based neurotoxins), as well as a diversity of typical Crotalus venom components such as serine proteinases, PLA2s, C-type lectins, and less abundant toxins (LAAOs, CRiSPs, etc.). The overall venom profile of C. tancitarensis appears most similar to C. transversus, which is consistent with a previous mitochondrial DNA analysis of the Crotalus intermedius group. These rattlesnakes of the Mexican highlands represent a radiation of high elevation specialists, and in spite of divergence of species in these Sky Island habitats, venom composition of species analyzed here has remained relatively conserved. The majority of protein family isoforms are conserved in all members of the clade, and as seen in other more broadly distributed rattlesnake species, differences in their venoms are largely due to relative concentrations of specific components.

Predator-prey interactions and venom composition in a high elevation lizard specialist, Crotalus pricei (Twin-spotted Rattlesnake).

The Twin-spotted Rattlesnake (Crotalus pricei) is a small lizard specialist restricted to higher montane habitat in the Sky Islands of Arizona and México. Though this species is restricted to high elevations and dispersal between mountaintops is impossible, few studies have investigated venom composition or the predator-prey relationship between C. p. pricei and its primary prey source, Yarrow's Spiny Lizard (Sceloporus jarrovii). Because of current isolation of populations in disjunct mountain ranges, it is possible that populations show local adaptation to prey and/or environmental conditions, as reflected by distinct venom phenotypes. Here we characterize venom composition of C. pricei from several different Sky Island mountain ranges and its relationship with S. jarrovii by using various analytical techniques and comparative toxicity tests. Results of venom analyses indicate that there is limited geographic variation in venom composition, occurring primarily in venoms of C. pricei from Durango, México, which had unusual PLA2 isoforms that are lacking from venoms of U.S. populations. Toxicity assays reveal that S. jarrovii has not developed resistance specific to C. p. pricei venom but does display a general tolerance to venom of several snakes in the genus Crotalus. These results provide insight into the evolutionary relationship between a lizard specialist and its natural prey, in addition to novel information on the venom composition of a little-studied species with a narrow range in the United States.