Do genetic structure and landscape heterogeneity impact color morph frequency in a polymorphic salamander?

Abstract

Landscape heterogeneity plays an important role in population structure and divergence, particularly for species with limited vagility. Here, we used a landscape genetic approach to identify how landscape and environmental variables affect genetic structure and color morph frequency in a polymorphic salamander. The eastern red‐backed salamander, Plethodon cinereus, is widely distributed in northeastern North America and contains two common color morphs, striped and unstriped, that are divergent in ecology, behavior, and physiology. To quantify population structure, rates of gene flow, and genetic drift, we amplified 10 microsatellite loci from 648 individuals across 28 sampling localities. This study was conducted in northern Ohio, where populations of P. cinereus exhibit an unusually wide range of morph frequency variation. To test whether genetic distance was more correlated with morph frequency, elevation, canopy cover, waterways, ecological niche or geographic distance, we used resistance distance and least cost path analyses. We then examined whether landscape and environmental variables, genetic distance or geographic distance were correlated with variation in morph frequency. Tests for population structure revealed three genetic clusters across our sampling range, with one cluster monomorphic for the striped morph. Rates of gene flow and genetic drift were low to moderate across sites. Genetic distance was most correlated with ecological niche, elevation and a combination of landscape and environmental variables. In contrast, morph frequency variation was correlated with waterways and geographic distance. Thus, our results suggest that selection is also an important evolutionary force across our sites, and a balance between gene flow, genetic drift and selection interact to maintain the two color morphs.

Eastern red-backed salamanders: A comprehensive review of an undervalued model in evolution, ecology, & behavior

“…Using microsatellite markers, they find a complex interaction between gene flow, genetic drift, and natural selection working together to explain morph frequencies, similar to Kraemer et al (2016). Using ecological niche modeling, Hantak et al (2019) found waterways were the most important factor in explaining morph frequency variation, likely, as the authors suggest, because they are strong dispersal barriers.…”

“…At the time Petranka (1998) wrote his chapter on P. cinereus, there was evidence that striped morphs were more common in cooler climates (i.e., higher latitudes; Test, 1952;Lotter & Scott, 1977) and evidence that unstriped morphs had a physiology consistent with being more tolerant of warmer conditions than striped morphs (Moreno, 1989). However, more recent studies have identified exceptions to these general patterns and contradictory results that suggest high levels of local variation in both color phenotype frequency (e.g., Hantak et al, 2019) and color phenotype physiologies (e.g., Petruzzi, Niewiarowski, & Moore, 2006). For example, Petruzzi et al (2006) found that the patterns of physiological differences between color phenotypes first detected by (Moreno, 1989) in New Jersey were inconsistent and variable across several populations in Ohio.…”

“…Several studies since Petranka (1998), have explored the variation of color phenotypes in P. cinereus for reasons other than climate, three focusing on selection imposed by predators using clay models (Fitzpatrick et al, 2009;Kraemer, Serb, & Adams, 2016;Grant et al, 2018), and two focusing on genetic structure and landscape heterogeneity (Fisher-Reid et al, 2013;Hantak et al, 2019). Fitzpatrick et al (2009) found that avian predators will attack the more common morph when presented with striped and unstriped clay models at varying frequencies, suggesting negative frequency-dependent selection is acting to maintain color polymorphism.…”

“…It would be interesting to completely resurvey Highton's (1977) localities and follow Cosentino et al's (2017) modeling methods to determine if the same covariates explain the change in color phenotype frequency over time. Fisher-Reid et al (2013) and Hantak et al (2019) are two of several recent studies mentioned briefly above which have tried to leverage the technological advances in genetics since Petranka (1998) to understand variation color phenotype frequencies in P. cinereus. Williams, Highton, & Cooper (1968) were the first to note that the typical color morph ratio (70:30 striped:unstriped) is not observed on Long Island, New York.…”

“…Fisher-Reid et al (2013) documented a soil moisture gradient on Long Island that correlated with the change in morph frequency, suggesting selection may have played a role in driving the less fit morph extinct at the gradient extremes. In northern Ohio, Hantak et al (2019) focus on the population genetics from 28 localities which span a range of color phenotype frequencies from 100% striped in eastern Ohio to nearly 100% unstriped in western Ohio. Using microsatellite markers, they find a complex interaction between gene flow, genetic drift, and natural selection working together to explain morph frequencies, similar to Kraemer et al (2016).…”