Ecology and Behavior of the Midget Faded Rattlesnake (Crotalus Oreganus Concolor) in Wyoming 1

The best of both worlds? Rattlesnake hybrid zones generate complex combinations of divergent venom phenotypes that retain high toxicity

Studying the consequences of hybridization between closely related species with divergent traits can reveal patterns of evolution that shape and maintain extreme trophic adaptations. Snake venoms are an excellent model system for examining the evolutionary and ecological patterns that underlie highly selected polymorphic traits. Here we investigate hybrid venom phenotypes that result from natural introgression between two rattlesnake species that express highly divergent venom phenotypes: Crotalus o. concolor and C. v. viridis. Though not yet documented, interbreeding between these species may lead to novel venom phenotypes with unique activities that break the typical trends of venom composition in rattlesnakes. The characteristics of these unusual phenotypes could unveil the roles of introgression in maintaining patterns of venom composition and variation, including the near ubiquitous dichotomy between neurotoxic or degradative venoms observed across rattlesnakes. We use RADseq data to infer patterns of gene flow and hybrid ancestry between these diverged lineages and link these genetic data with analyses of venom composition, biological activity, and whole animal model toxicity tests to understand the impacts of introgression on venom composition. We find that introgressed populations express admixed venom phenotypes that do not sacrifice biological activity (lethal toxicity) or overall abundance of dominant toxins compared to parental venoms. These hybridized venoms therefore do not represent a trade-off in functionality between the typical phenotypic extremes but instead represent a unique combination of characters whose expression appears limited to the hybrid zone.

Evidence that genomic incompatibilities and other multilocus processes impact hybrid fitness in a rattlesnake hybrid zone

Hybrid zones provide valuable opportunities to understand the genomic mechanisms that promote speciation by providing insight into factors involved in intermediate stages of speciation. Here, we investigate introgression in a hybrid zone between two rattlesnake species (Crotalus viridis and Crotalus oreganus concolor) that have undergone historical allopatric divergence and recent range expansion and secondary contact. We use Bayesian genomic cline models to characterize genomic patterns of introgression between these lineages and identify loci potentially subject to selection in hybrids. We find evidence for a large number of genomic regions with biased ancestry that deviate from the genomic background in hybrids (i.e., excess ancestry loci), which tend to be associated with genomic regions with higher recombination rates. We also identify suites of excess ancestry loci that show highly correlated allele frequencies (including conspecific and heterospecific combinations) across physically unlinked genomic regions in hybrids. Our findings provide evidence for multiple multilocus evolutionary processes impacting hybrid fitness in this system.

Rattlesnake migrations and the implications of thermal landscapes

BACKGROUND

The importance of thermal resources to terrestrial ectotherms has been well documented but less often considered in larger-scale analyses of habitat use and selection, such as those routinely conducted using standard habitat features such as vegetation and physical structure. Selection of habitat based on thermal attributes may be of particular importance for ectothermic species, especially in colder climates. In Canada, Western Rattlesnakes (Crotalus oreganus) reach their northern limits, with limited time to conduct annual migratory movements between hibernacula and summer habitat. We radio-tracked 35 male snakes departing from 10 different hibernacula. We examined coarse-scale differences in migratory movements across the region, and then compared the route of each snake with thermal landscapes and ruggedness GIS maps generated for different periods of the animals' active season.

RESULTS

We observed dichotomous habitat use (grasslands versus upland forests) throughout most of the species' northern range, reflected in different migratory movements of male snakes emanating from different hibernacula. Snakes utilizing higher-elevation forests moved further during the course of their annual migrations, and these snakes were more likely to use warmer areas of the landscape.

CONCLUSION

In addition to thermal benefits, advantages gained from selective migratory patterns may include prey availability and outbreeding. Testing these alternative hypotheses was beyond the scope of this study, and to collect the data to do so will require overcoming certain challenges. Still, insight into migratory differences between rattlesnake populations and the causal mechanism(s) of migrations will improve our ability to assess the implications of landscape change, management, and efficacy of conservation planning. Our findings suggest that such assessments may need to be tailored to individual dens and the migration strategies of their inhabitants. Additionally, local and landscape-scale migration patterns, as detected in this study, will have repercussions for snakes under climate-induced shifts in ecosystem boundaries and thermal regimes.

Snake-venom resistance as a mammalian trophic adaptation: lessons from didelphid marsupials

Mammals that prey on venomous snakes include several opossums (Didelphidae), at least two hedgehogs (Erinaceidae), several mongooses (Herpestidae), several mustelids, and some skunks (Mephitidae). As a group, these taxa do not share any distinctive morphological traits. Instead, mammalian adaptations for ophiophagy seem to consist only in the ability to resist the toxic effects of snake venom. Molecular mechanisms of venom resistance (as indicated by biochemical research on opossums, mongooses, and hedgehogs) include toxin-neutralizing serum factors and adaptive changes in venom-targeted molecules. Of these, toxin-neutralizing serum factors have received the most research attention to date. All of the toxin-neutralizing serum proteins discovered so far in both opossums and mongooses are human α1B-glycoprotein homologs that inhibit either snake-venom metalloproteinases or phospholipase A(2) myotoxins. By contrast, adaptive changes in venom-targeted molecules have received far less attention. The best-documented examples include amino-acid substitutions in mongoose nicotinic acetylcholine receptor that inhibit binding by α-neurotoxins, and amino-acid substitutions in opossum von Willebrand factor (vWF) that are hypothesized to weaken the bond between vWF and coagulopathic C-type lectins. Although multiple mechanisms of venom resistance are known from some species, the proteomic complexity of most snake venoms suggests that the evolved biochemical defences of ophiophagous mammals are likely to be far more numerous than currently recognized. Whereas most previous research in this field has been motivated by the potential for medical applications, venom resistance in ophiophagous mammals is a complex adaptation that merits attention from comparative biologists. Unfortunately, evolutionary inference is currently limited by ignorance about many relevant facts that can only be provided by future research.