Patterns of genetic differentiation in Thamnophis and Taricha from the Pacific Northwest

Abstract

Aim The co‐evolutionary interaction between the common garter snake, Thamnophis sirtalis, and the rough‐skinned newt, Taricha granulosa, takes place throughout much of the Pacific Northwest (North America). The biogeography of the Pacific Northwest has been heavily influenced by the last Pleistocene glaciation, which reached a maximum as late as 14,000 yr bp. We researched: (1) what type of population structure is present for garter snakes and newts, (2) whether the population structure of these species is consistent with a Pleistocene glaciation hypothesis, and (3) how population structure and migration possibly affect co‐evolution between these species. Location The Pacific Northwest of North America, specifically northern California, Oregon and Washington in the USA. Methods We sampled approximately 20 populations for each species from three different transects. Using microsatellite markers and tissue samples from both species, we quantified the population structure for both species. Individual‐based assignment tests were used to estimate contemporary migration rates. Results Both Th. sirtalis and Ta. granulosa exhibited little genetic differentiation among our study sites, even among those separated by large distances. Significant population structure was detected on multiple geographic scales. Differences in population structure were observed among transects and between garter snake and newt transects. Contemporary migration rate estimates indicate high levels of genetic exchange between populations. Main conclusions Prior to this study, little was known about the fine‐scale population structure of either species in this region. Patterns of population structure for garter snakes and newts reflect a shared biogeographical history affected by the Pleistocene glaciation in the Pacific Northwest. Both species apparently migrate frequently between populations, thus potentially retarding the process of adaptive co‐evolution. We find that populations from a northern coastal transect (Washington) are most likely to be locally adapted.

Discordant patterns of population structure for two co‐distributed snake species across a fragmented Ontario landscape

“…Further, an sPCA found no significant geographic structure suggesting that, in reality, garter snakes across our study area represent a single genetic population. This result is supported by previous studies that found low levels of genetic differentiation among subpopulations of T. sirtalis separated by large distances, in surveys using both microsatellites (Ridenhour et al, 2007) and allozymes (King & Lawson, 2001).…”

“…However, no individual for any garter snake locus failed to amplify implying that null alleles are not a pervasive problem. Previous tests by the researchers that developed or used the microsatellites used here reported no significant departures from HWE with the exception of TS1 and TS2 (McCracken et al, 1999;Prosser et al, 1999;Garner et al, 2004;Ridenhour et al, 2007). Bittner & King (2003) investigated the population structure of garter snakes from Lake Erie and southwestern Ontario using both allozymes and DNA microsatellites, including loci TS1 and TS2.…”

Toxicity and population structure of the Rough‐Skinned Newt (Taricha granulosa) outside the range of an arms race with resistant predators

“…() and Ridenhour et al. () also reported low estimates of population subdivision among sites in Oregon and Washington, suggesting that Ta. granulosa may exhibit low site fidelity.…”

Influence of montane isolation and refugia on population structure ofSorex palustrisin western North America

“…However, we found support for range expansion for only 3 populations. Low genetic variation in the Pacific Northwest has been attributed to range expansion following glacial recession (Edmands 2001;Ridenhour et al 2007).…”