title: Toxicity and population structure of the Rough-Skinned Newt (Taricha granulosa) outside the range of an arms race with resistant predators.
abstract: Species interactions, and their fitness consequences, vary across the geographic range of a coevolutionary relationship. This spatial heterogeneity in reciprocal selection is predicted to generate a geographic mosaic of local adaptation, wherein coevolutionary traits are phenotypically variable from one location to the next. Under this framework, allopatric populations should lack variation in coevolutionary traits due to the absence of reciprocal selection. We examine phenotypic variation in tetrodotoxin (TTX) toxicity of the Rough-Skinned Newt (Taricha granulosa) in regions of allopatry with its TTX-resistant predator, the Common Garter Snake (Thamnophis sirtalis). In sympatry, geographic patterns of phenotypic exaggeration in toxicity and toxin-resistance are closely correlated in prey and predator, implying that reciprocal selection drives phenotypic variation in coevolutionary traits. Therefore, in allopatry with TTX-resistant predators, we expect to find uniformly low levels of newt toxicity. We characterized TTX toxicity in northwestern North America, including the Alaskan panhandle where Ta. granulosa occur in allopatry with Th. sirtalis. First, we used microsatellite markers to estimate population genetic structure and determine if any phenotypic variation in toxicity might be explained by historical divergence. We found northern populations of Ta. granulosa generally lacked population structure in a pattern consistent with northern range expansion after the Pleistocene. Next, we chose a cluster of sites in Alaska, which uniformly lacked genetic divergence, to test for phenotypic divergence in toxicity. As predicted, overall levels of newt toxicity were low; however, we also detected unexpected among- and within-population variation in toxicity. Most notably, a small number of individuals contained large doses of TTX that rival means of toxic populations in sympatry with Th. sirtalis. Phenotypic variation in toxicity, despite limited neutral genetic divergence, suggests that factors other than reciprocal selection with Th. sirtalis likely contribute to geographic patterns of toxicity in Ta. granulosa.
authors.affiliation: Department of Biology University of Virginia Charlottesville Virginia.
authors.forename: Michael T J
authors.initials: MT
authors.lastname: Hague
authors.affiliation: Museum of Vertebrate Zoology University of California Berkeley California.
authors.forename: Leleña A
authors.initials: LA
authors.lastname: Avila
authors.affiliation: Department of Biology Utah State University, Uintah Basin Campus Vernal Utah.
authors.forename: Charles T
authors.initials: CT
authors.lastname: Hanifin
authors.affiliation: Victoria BC Canada.
authors.forename: W Andrew
authors.initials: WA
authors.lastname: Snedden
authors.affiliation: Department of Biology California State University Bakersfield California.
authors.forename: Amber N
authors.initials: AN
authors.lastname: Stokes
authors.affiliation: Department of Biology Utah State University Logan Utah.
authors.forename: Edmund D
authors.initials: ED
authors.lastname: Brodie
authors.affiliation: Department of Biology University of Virginia Charlottesville Virginia.
authors.forename: Edmund D
authors.initials: ED
authors.lastname: Brodie
keywords: Arms race; Taricha granulosa; coevolution; tetrodotoxin