Relative importance of isolation‐by‐environment and other determinants of gene flow in an alpine amphibian

Testing concordance and conflict in spatial replication of landscape genetics inferences

The degree to which landscape genetics findings can be extrapolated to different areas of a species range is poorly understood. Here, we used a broadly distributed ectothermic lizard (Sceloporus occidentalis, Western Fence lizard) as a model species to evaluate the full role of topography, climate, vegetation, and roads on dispersal and genetic differentiation. We conducted landscape genetics analyses with a total of 119 individuals in five areas within the Sierra Nevada mountain range. Genetic distances calculated from thousands of ddRAD markers were used to optimize landscape resistance surfaces and infer the effects of landscape and topographic features on genetic connectivity. Across study areas, we found a great deal of consistency in the primary environmental gradients impacting genetic connectivity, along with some site-specific differences, and a range in the proportion of genetic variance explained by environmental factors across study sites. High-elevation colder areas were consistently found to be barriers to gene flow, as were areas of high ruggedness and slope. High temperature seasonality and high precipitation during the winter wet season also presented a substantial barrier to gene flow in a majority of study areas. The effect of other landscape variables on genetic differentiation was more idiosyncratic and depended on specific attributes at each site. Across study areas, canyon valleys were always implicated as facilitators to dispersal and key features linking populations and maintaining genetic connectivity, though the relative importance varied in different areas. We emphasize that spatial data layers are complex and multidimensional, and careful consideration of spatial data correlation structure and robust analytic frameworks will be critical to our continued understanding of spatial genetics processes.

Alpine Extremophytes in Evolutionary Turmoil: Complex Diversification Patterns and Demographic Responses of a Halophilic Grass in a Central Asian Biodiversity Hotspot

Diversification and demographic responses are key processes shaping species evolutionary history. Yet we still lack a full understanding of ecological mechanisms that shape genetic diversity at different spatial scales upon rapid environmental changes. In this study, we examined genetic differentiation in an extremophilic grass Puccinellia pamirica and factors affecting its population dynamics among the occupied hypersaline alpine wetlands on the arid Pamir Plateau in Central Asia. Using genomic data, we found evidence of fine-scale population structure and gene flow among the localities established across the high-elevation plateau as well as fingerprints of historical demographic expansion. We showed that an increase in the effective population size could coincide with the Last Glacial Period, which was followed by the species demographic decline during the Holocene. Geographic distance plays a vital role in shaping the spatial genetic structure of P. pamirica alongside with isolation-by-environment and habitat fragmentation. Our results highlight a complex history of divergence and gene flow in this species-poor alpine region during the Late Quaternary. We demonstrate that regional climate specificity and a shortage of nonclimate data largely impede predictions of future range changes of the alpine extremophile using ecological niche modeling. This study emphasizes the importance of fine-scale environmental heterogeneity for population dynamics and species distribution shifts.

Host association, environment, and geography underlie genomic differentiation in a major forest pest

Diverse geographic, environmental, and ecological factors affect gene flow and adaptive genomic variation within species. With recent advances in landscape ecological modelling and high-throughput DNA sequencing, it is now possible to effectively quantify and partition their relative contributions. Here, we use landscape genomics to identify determinants of genomic differentiation in the forest tent caterpillar, Malacosoma disstria, a widespread and irruptive pest of numerous deciduous tree species in North America. We collected larvae from multiple populations across Eastern Canada, where the species experiences a diversity of environmental gradients and feeds on a number of different host tree species, including trembling aspen (Populus tremuloides), sugar maple (Acer saccharum), red oak (Quercus rubra), and white birch (Betula papyrifera). Using a combination of reciprocal causal modelling (RCM) and distance-based redundancy analyses (dbRDA), we show that differentiation of thousands of genome-wide single nucleotide polymorphisms (SNPs) among individuals is best explained by a combination of isolation by distance, isolation by environment (spatial variation in summer temperatures and length of the growing season), and differences in host association. Configuration of suitable habitat inferred from ecological niche models was not significantly related to genomic differentiation, suggesting that M. disstria dispersal is agnostic with respect to habitat quality. Although population structure was not discretely related to host association, our modelling framework provides the first molecular evidence of host-associated differentiation in M. disstria, congruent with previous documentation of reduced growth and survival of larvae moved between natal host species. We conclude that ecologically mediated selection is contributing to variation within M. disstria, and that divergent adaptation related to both environmental conditions and host association should be considered in ongoing research and management of this important forest pest.

Fine-scale genome-wide signature of Pleistocene glaciation in Thitarodes moths (Lepidoptera: Hepialidae), host of Ophiocordyceps fungus in the Hengduan Mountains

The Hengduan Mountains region is a biodiversity hotspot known for its topologically complex, deep valleys and high mountains. While landscape and glacial refugia have been evoked to explain patterns of inter-species divergence, the accumulation of intra-species (i.e. population level) genetic divergence across the mountain-valley landscape in this region has received less attention. We used genome-wide restriction site-associated DNA sequencing (RADseq) to reveal signatures of Pleistocene glaciation in populations of Thitarodes shambalaensis (Lepidoptera: Hepialidae), the host moth of parasitic Ophiocordyceps sinensis (Hypocreales: Ophiocordycipitaceae) or "caterpillar fungus" endemic to the glacier of eastern Mt. Gongga. We used moraine history along the glacier valleys to model the distribution and environmental barriers to gene flow across populations of T. shambalaensis. We found that moth populations separated by less than 10 km exhibited valley-based population genetic clustering and isolation-by-distance (IBD), while gene flow among populations was best explained by models using information about their distributions at the local last glacial maximum (LGM_L , 58 kya), not their contemporary distribution. Maximum likelihood lineage history among populations, and among subpopulations as little as 500 meters apart, recapitulated glaciation history across the landscape. We also found signals of isolated population expansion following the retreat of LGM_L glaciers. These results reveal the fine-scale, long-term historical influence of landscape and glaciation on the genetic structuring of populations of an endangered and economically important insect species. Similar mechanisms, given enough time and continued isolation, could explain the contribution of glacier refugia to the generation of species diversity among the Hengduan Mountains.

Population genetic differentiation and genomic signatures of adaptation to climate in an abundant lizard

Species distributed across climatic gradients will typically experience spatial variation in selection, but gene flow can prevent such selection from causing population genetic differentiation and local adaptation. Here, we studied genomic variation of 415 individuals across 34 populations of the common wall lizard (Podarcis muralis) in central Italy. This species is highly abundant throughout this region and populations belong to a single genetic lineage, yet there is extensive phenotypic variation across climatic regimes. We used redundancy analysis to, first, quantify the effect of climate and geography on population genomic variation in this region and, second, to test if climate consistently sorts specific alleles across the landscape. Climate explained 5% of the population genomic variation across the landscape, about half of which was collinear with geography. Linear models and redundancy analyses identified loci that were significantly differentiated across climatic regimes. These loci were distributed across the genome and physically associated with genes putatively involved in thermal tolerance, regulation of temperature-dependent metabolism and reproductive activity, and body colouration. Together, these findings suggest that climate can exercise sufficient selection in lizards to promote genetic differentiation across the landscape in spite of high gene flow.

Neo-sex chromosome evolution and phenotypic differentiation across an elevational gradient in horned larks (Eremophila Alpestris)

Genetic structure and phenotypic variation among populations is affected by both geographic distance and environmental variation across species' distributions. Understanding the relative contributions of isolation by distance (IBD) and isolation by environment (IBE) is important for elucidating population dynamics across habitats and ecological gradients. In this study, we compared phenotypic and genetic variation among Horned Lark (Eremophila alpestris) populations from 10 sites encompassing an elevational gradient from low-elevation desert scrub in Death Valley (285 a.s.l.) to high-elevation meadows in the White Mountains of the Sierra Nevada of California (greater than 3000 m a.s.l.). Using a ddRAD dataset of 28,474 SNPs aligned to a high-quality reference genome, we compared genetic structure with elevational, environmental, and spatial distance to quantify how different aspects of the landscape drive genomic and phenotypic differentiation in Horned Larks. We found larger-bodied birds were associated with sites that had less seasonality and higher annual precipitation, and longer spurs occurred in soils with more clay and silt content, less sand, and finer fragments. Larks have large neo-sex chromosomes, and we found that associations with elevation and environmental variation were much stronger among neo-sex chromosomes compared to autosomes. Furthermore, we found that putative chromosomal translocations, fusions, and inversions were associated with elevation and may underlie local adaptation across an elevational gradient in Horned Larks. Our results suggest that genetic variation in Horned Larks is affected more by IBD than IBE, but specific phenotypes and genomic regions-particually on neo-sex chromosomes-bear stronger associations with the environment.

Multiscale patterns of isolation by ecology and fine-scale population structure in Texas bobcats

Patterns of spatial genetic variation can be generated by a variety of ecological processes, including individual preferences based on habitat. These ecological processes act at multiple spatial and temporal scales, generating scale-dependent effects on gene flow. In this study, we focused on bobcats (Lynx rufus), a highly mobile, generalist felid that exhibits ecological and behavioral plasticity, high abundance, and broad connectivity across much of their range. However, bobcats also show genetic differentiation along habitat breaks, a pattern typically observed in cases of isolation-by-ecology (IBE). The IBE observed in bobcats is hypothesized to occur due to habitat-biased dispersal, but it is unknown if this occurs at other habitat breaks across their range or at what spatial scale IBE becomes most apparent. Thus, we used a multiscale approach to examine isolation by ecology (IBE) patterns in bobcats (Lynx rufus) at both fine and broad spatial scales in western Texas. We genotyped 102 individuals at nine microsatellite loci and used partial redundancy analysis (pRDA) to test if a suite of landscape variables influenced genetic variation in bobcats. Bobcats exhibited a latitudinal cline in population structure with a spatial signature of male-biased dispersal, and no clear barriers to gene flow. Our pRDA tests revealed high genetic similarity in similar habitats, and results differed by spatial scale. At the fine spatial scale, herbaceous rangeland was an important influence on gene flow whereas mixed rangeland and agriculture were significant at the broad spatial scale. Taken together, our results suggests that complex interactions between spatial-use behavior and landscape heterogeneity can create non-random gene flow in highly mobile species like bobcats. Furthermore, our results add to the growing body of data highlighting the importance of multiscale study designs when assessing spatial genetic structure.

Adaptation to elevation but limited local adaptation in an amphibian

We performed a reciprocal transplant experiment to estimate "parallel" adaptation to elevation and "unique" adaptation to local sites at the same elevation, using the frog Rana temporaria in the Swiss Alps. It is important to distinguish these two processes because they have different implications for population structure and ecological specialization. Larvae were reared from hatching to metamorphosis within enclosures installed in their pond of origin, in three foreign ponds at the same elevation, and in four ponds at different elevation (1500-2000 m higher or lower). There were two source populations from each elevation, and adults were held in a common environment for 1 year before they were crossed to produce offspring for the experiment. Fitness was a measure that integrated larval survival, development rate, and body size. Parallel adaptation to elevation was indicated by an advantage at the home elevation (11.5% fitness difference at low elevation and 47% at high elevation). This effect was stronger than that observed in most other studies, according to a survey of previous transplant experiments across elevation (N = 8 animal species and 71 plants). Unique local adaptation within elevational zones was only 0.3-0.7 times as strong as parallel adaptation, probably because gene flow is comparatively high among nearby wetlands at the same elevation. The home-elevation advantage may reduce gene flow across the elevational gradient and enable the evolution of habitat races specialized on elevation.

The Influence of Environmental Variation on the Genetic Structure of a Poison Frog Distributed Across Continuous Amazonian Rainforest

Biogeographic barriers such as rivers have been shown to shape spatial patterns of biodiversity in the Amazon basin, yet relatively little is known about the distribution of genetic variation across continuous rainforest. Here, we characterize the genetic structure of the brilliant-thighed poison frog (Allobates femoralis) across an 880-km-long transect along the Purus-Madeira interfluve south of the Amazon river, based on 64 individuals genotyped at 7609 single-nucleotide polymorphism (SNP) loci. A population tree and clustering analyses revealed 4 distinct genetic groups, one of which was strongly divergent. These genetic groups were concomitant with femoral spot coloration differences, which was intermediate within a zone of admixture between two of the groups. The location of these genetic groups did not consistently correspond to current ecological transitions between major forest types. A multimodel approach to quantify the relative influence of isolation-by-geographic distance (IBD) and isolation-by-environmental resistance (IBR) nevertheless revealed that, in addition to a strong signal of IBD, spatial genetic differentiation was explained by IBR primarily linked to dry season intensity (r2 = 8.4%) and canopy cover (r2 = 6.4%). We show significant phylogenetic divergence in the absence of obvious biogeographical barriers and that finer-scaled measures of genetic structure are associated with environmental variables also known to predict the density of A. femoralis.

Gene flow and climate-associated genetic variation in a vagile habitat specialist

Previous work in landscape genetics suggests that geographic isolation is of greater importance to genetic divergence than variation in environmental conditions. This is intuitive when configurations of suitable habitat are a dominant factor limiting dispersal and gene flow, but has not been thoroughly examined for habitat specialists with strong dispersal capability. Here, we evaluate the effects of geographic and environmental isolation on genetic divergence for a vagile invertebrate with high habitat specificity and a discrete dispersal life stage: Dod's Old World swallowtail butterfly, Papilio machaon dodi. In Canada, P. m. dodi are generally restricted to eroding habitat along major river valleys where their larval host plant occurs. A series of causal and linear mixed effects models indicate that divergence of genome-wide single nucleotide polymorphisms is best explained by a combination of environmental isolation (variation in summer temperatures) and geographic isolation (Euclidean distance). Interestingly, least-cost path and circuit distances through a resistance surface parameterized as the inverse of habitat suitability were not supported. This suggests that, although habitat associations of many butterflies are specific due to reproductive requirements, habitat suitability and landscape permeability are not equivalent concepts due to considerable adult vagility. We infer that divergent selection related to variation in summer temperatures has produced two genetic clusters within P. m. dodi, differing in voltinism and diapause propensity. Within the next century, temperatures are predicted to rise by amounts greater than the present-day difference between regions of the genetic clusters, potentially affecting the persistence of the northern cluster under continued climate change.