Spatial Patterns Of Genetic Diversity Research Articles

Geography alone cannot explain Tetranychus truncatus (Acari: Tetranychidae) population abundance and genetic diversity in the context of the center-periphery hypothesis.

The center-periphery hypothesis (CPH) states that the genetic diversity, genetic flow, and population abundance of a species are highest at the center of the species' geographic distribution. However, most CPH studies have focused on the geographic distance and have ignored ecological and historical effects. Studies using niche models to define the center and periphery of a distribution and the interactions among geographical, ecological, and historical gradients have rarely been done in the framework of the CPH, especially in biogeographical studies of animal species. Here, we examined the CPH for a widely distributed arthropod, Tetranychus truncatus (Acari: Tetranychidae), in eastern China using three measurements: geographic distance to the center of the distribution (geography), ecological suitability based on current climate data (ecology), and historical climate data from the last glacial maximum (history). We found that the relative abundances of different populations were more strongly related to ecology than to geography and history. Genetic diversity within populations and genetic differentiation among populations based on mitochondrial marker were only significantly related to history. However, the genetic diversity and population differentiation based on microsatellites were significantly related to all three CPH measurements. Overall, population abundance and genetic pattern cannot be explained very well by geography alone. Our results show that ecological gradients explain the variation in population abundance better than geographic gradients and historical factors, and that current and historical factors strongly influence the spatial patterns of genetic variation. This study highlights the importance of examining more than just geography when assessing the CPH.

Estimating seed dispersal distance: A comparison of methods using animal movement and plant genetic data on two primate-dispersed Neotropical plant species.

Seed dispersal distance (SDD) critically influences the survival of seedlings, spatial patterns of genetic diversity within plant populations, and gene flow among plant populations. In animal‐dispersed species, foraging behavior and movement patterns determine SDD. Direct observations of seed dispersal events by animals in natural plant populations are mostly constrained by the high mobility and low visibility of seed dispersers. Therefore, diverse alternative methods are used to estimate seed dispersal distance, but direct comparisons of these approaches within the same seed dispersal system are mostly missing.We investigated two plant species with different life history traits, Leonia cymosa and Parkia panurensis, exclusively dispersed by two tamarin species, Saguinus mystax and Leontocebus nigrifrons. We compared SDD estimates obtained from direct observations, genetic identification of mother plants from seed coats, parentage analysis of seedlings/saplings, and phenomenological and mechanistic modeling approaches.SDD derived from the different methods ranged between 158 and 201 m for P. panurensis and between 178 and 318 m for L. cymosa. In P. panurensis, the modeling approaches resulted in moderately higher estimates than observations and genotyping of seed coats. In L. cymosa, parentage analysis resulted in a lower estimate than all other methods. Overall, SDD estimates for P. panurensis (179 ± 16 m; mean ± SD) were significantly lower than for L. cymosa (266 ± 59 m; mean ± SD).Differences among methods were related to processes of the seed dispersal loop integrated by the respective methods (e.g., seed deposition or seedling distribution). We discuss the merits and limitations of each method and highlight the aspects to be considered when comparing SDD derived from different methodologies. Differences among plant species were related to differences in reproductive traits influencing gut passage time and feeding behavior, highlighting the importance of plant traits on animal‐mediated seed dispersal distance.

Diatraea saccharalis history of colonization in the Americas. The case for human-mediated dispersal.

The sugarcane borer moth, Diatraea saccharalis, is one of the most important pests of sugarcane and maize crops in the Western Hemisphere. The pest is widespread throughout South and Central America, the Caribbean region and the southern United States. One of the most intriguing features of D. saccharalis population dynamics is the high rate of range expansion reported in recent years. To shed light on the history of colonization of D. saccharalis, we investigated the genetic structure and diversity in American populations using single nucleotide polymorphism (SNPs) markers throughout the genome and sequences of the mitochondrial gene cytochrome oxidase (COI). Our primary goal was to propose possible dispersal routes from the putative center of origin that can explain the spatial pattern of genetic diversity. Our findings showed a clear correspondence between genetic structure and the geographical distributions of this pest insect on the American continents. The clustering analyses indicated three distinct groups: one composed of Brazilian populations, a second group composed of populations from El Salvador, Mexico, Texas and Louisiana and a third group composed of the Florida population. The predicted time of divergence predates the agriculture expansion period, but the pattern of distribution of haplotype diversity suggests that human-mediated movement was most likely the factor responsible for the widespread distribution in the Americas. The study of the early history of D. saccharalis promotes a better understanding of range expansion, the history of invasion, and demographic patterns of pest populations in the Americas.

Convergent evolution in floral morphology in a plant ring species, the Caribbean Euphorbia tithymaloides.

Ring species have long fascinated evolutionary biologists for their potential insights into lineage divergence and speciation across space. Few studies have investigated the potential for convergent or parallel evolution along the diverging fronts of ring species. We investigated a potential case of parallel floral variation in the Caribbean spurge Euphorbia tithymaloides, the only plant system with molecular support as a ring species. The terminal populations of each front, despite being the most divergent, exhibit such similar floral traits that they were originally considered each other's closest relative. We evaluated convergence in floral and leaf traits in relation to geography across 95 populations spanning the distribution of E. tithymaloides. We also reanalyzed available genetic data (from previous phylogenetic analyses) in an explicitly spatial framework. Floral morphology appears to have shifted in a convergent fashion along both geographic fronts of E. tithymaloides, resulting in shorter and more compact inflorescences in Antillean populations compared to the typical elongate "slipper-like" cyathia characteristic of the area of origin. Patterns of spatial genetic variation were more consistent with a two-fronted invasion of the Caribbean than with a simpler model of isolation-by-distance. Floral divergence in E. tithymaloides is consistent with convergent evolution along the two fronts of a ring species. We outline several (not mutually exclusive) mechanisms that could be driving patterns in morphology, including shifts toward generalized pollination with reduced reliance on hummingbirds, shifts in floral structure closely matching available hummingbird bill traits, and shifts toward increased selfing.

Phylogeography of the Neotropical epiphytic orchid, Brassavola nodosa: evidence for a secondary contact zone in northwestern Costa Rica.

Spatial patterns of genetic variation can reveal otherwise cryptic evolutionary and landscape processes. In northwestern Costa Rica, an approximately concordant genetic discontinuity occurs among populations of several plant species. We conducted phylogeographic analyses of an epiphytic orchid, Brassavola nodosa, to test for genetic discontinuity and to explore its underlying causes. We genotyped 18 populations with 19 nuclear loci and two non-coding chloroplast sequence regions. We estimated genetic diversity and structure, relative importance of pollen and seed dispersal, and divergence time to understand how genetic diversity was spatially partitioned. Nuclear genetic diversity was high with little differentiation among populations (GSTn = 0.065). In contrast, chloroplast haplotypes were highly structured (GSTc = 0.570) and reveal a discontinuity between northwestern and southeastern populations within Costa Rica. Haplotype differences suggest two formerly isolated lineages that diverged ~10,000-100,000 YBP. Haplotype mixing and greater genetic diversity occur in an intermediate transition zone. Patterns of nuclear and chloroplast data were consistent. Different levels of genetic differentiation for the two genomes reflect the relative effectiveness of biotic versus abiotic dispersers of pollen and seeds, respectively. Isolation of the two lineages likely resulted from the complex environmental and geophysical history of the region. Our results suggest a recent cryptic seed dispersal barrier and/or zone of secondary contact. We hypothesize that powerful northeasterly trade winds hinder movement of wind-borne seeds between the two regions, while the multi-directional dispersal of pollen by strong-flying sphinx moths resulted in lower differentiation of nuclear loci.

Genetic diversity and population structure of Kichulchoia multifasciata in South Korea

Kichulchoia multifasciata, a small-sized loach (Family Cobitidae) inhabiting only the Nakdong River on the Korean Peninsula, is generally found around well-oxygenated minor tributaries. Despite the scarcity, this species is not yet a legally protected species in South Korea, and the spatial pattern of genetic diversity, which is essential to create management strategies, has never been studied. In this study, three mitochondrial loci and ten microsatellites were used to analyze the genetic diversity and the structure among eight K. multifasciata populations collected from different tributaries. K. multifasciata populations showed a high level of mitochondrial variability, with above 0.9 of average haplotype diversity in all three loci analyzed. A substantial microsatellite polymorphism was also found. Weak genetic structuring among populations and isolation by distance were clearly revealed in both mitochondrial and microsatellite data, indicating the high levels of gene flow among the tributaries. However, a few of the populations exhibited a genetic signature that they have experienced the historical size decline. Our data suggest that unequal gene flow might be one of the causes; nevertheless, the possibility of effective population size reduction by other factors could not be completely excluded. Our genetic evidence can be the critical information needed to protect the populations from the genetic erosion in the Nakdong River that has been destroyed and fragmented with increasing anthropogenic interferences.

Predicting contemporary range‐wide genomic variation using climatic, phylogeographic and morphological knowledge in an ancient, unglaciated landscape

AbstractAimUnderstanding the spatial distribution of genetic variation across a species’ range is a central tenet in evolutionary biology and the ability to predict such patterns is valuable to conservation management. Genetic diversity and differentiation may be explained by present‐day niche centrality, historical processes, or morphological variation across a species’ range, yet few studies evaluate these factors concurrently. Moreover, there is a particular lack of knowledge of the influence of these factors in ancient, unglaciated landscapes of the Southern Hemisphere. We investigate these concepts for their influence upon spatial patterns of genetic variation in a widespread plant species in a southern biodiversity hotspot.LocationSouth‐western Australia.TaxonCalothamnus quadrifidus.MethodsWe use genotyping‐by‐sequencing to assess genome‐wide diversity and differentiation across the range of Calothamnus quadrifidus, a morphologically variable species that occurs over a wide geographic range in an ancient, unglaciated landscape. We correlated the patterns of genetic diversity and differentiation with those predicted under four hypotheses regarding effects of geographic centrality, climatic centrality, historical phylogeography and morphological subspecies.ResultsWe found spatial variation in genetic diversity and strong population structuring, and while each hypothesis was consistent with some of these patterns, no hypothesis predicted the full extent of genetic diversity or structure in C. quadrifidus.Main conclusionsOur results are indicative of a complex evolutionary history, with multiple factors shaping contemporary genetic variation in C. quadrifidus over time, rather than any one major driving factor. This is enabled by the greater capacity to build complex evolutionary signals in ancient, more stable landscapes like the southwest of Western Australia that are not as predictable as those within the younger, post‐glacial landscapes of the Northern Hemisphere. Finally, our study demonstrates the importance of using an interdisciplinary approach to understanding the spatial patterns of genetic variation across species’ ranges and the influence of ecological, geographical and historical factors.

Complementing the Pleistocene biogeography of European amphibians: Testimony from a southern Atlantic species

AbstractAimTo reconstruct the historical biogeography of Hyla molleri, a tree frog endemic to the Eurosiberian and Mediterranean bioclimatic zones in the Iberian Peninsula.LocationIberian Peninsula.MethodsWe used molecular data (mtDNA and species‐specific, polymorphic microsatellite loci) and species distribution modelling (SDM) from the Last Interglacial (LIG) to the present to characterize spatial patterns of genetic diversity in this species and assess their relationship with climatically favourable areas through time.ResultsGenetic diversity is heterogeneously distributed across the range of H. molleri, with two main genetic reservoirs located in (a) central and southern Portugal and (b) a fragmented area encompassing mountainous areas in northern Spain. According to SDM, the Iberian Peninsula has experienced a progressive and continuous decrease in climatically favourable areas for H. molleri since the LIG, especially in southern and eastern Iberia, where the species is currently absent. However, we found no correlation between areas that have remained climatically favourable since the LIG and current genetic diversity.Main conclusionsOur results suggest that the demographic history of H. molleri since the Pleistocene has been characterized by relative stability, contrasting with the large‐scale cycles of extinction‐recolonization inferred for other more thermophilous, co‐distributed amphibian species in Iberia. Accounting for discordant demographic responses to climatic changes across syntopic species provides new insights about the evolutionary history of amphibian communities in southern Europe.

Partitioning drivers of spatial genetic variation for a continuously distributed population of boreal caribou: Implications for management unit delineation.

Isolation by distance (IBD) is a natural pattern not readily incorporated into theoretical models nor traditional metrics for differentiating populations, although clinal genetic differentiation can be characteristic of many wildlife species. Landscape features can also drive population structure additive to baseline IBD resulting in differentiation through isolation‐by‐resistance (IBR). We assessed the population genetic structure of boreal caribou across western Canada using nonspatial (STRUCTURE) and spatial (MEMGENE) clustering methods and investigated the relative contribution of IBD and IBR on genetic variation of 1,221 boreal caribou multilocus genotypes across western Canada. We further introduced a novel approach to compare the partitioning of individuals into management units (MU) and assessed levels of genetic connectivity under different MU scenarios. STRUCTURE delineated five genetic clusters while MEMGENE identified finer‐scale differentiation across the study area. IBD was significant and did not differ for males and females both across and among detected genetic clusters. MEMGENE landscape analysis further quantified the proportion of genetic variation contributed by IBD and IBR patterns, allowing for the relative importance of spatial drivers, including roads, water bodies, and wildfires, to be assessed and incorporated into the characterization of population structure for the delineation of MUs. Local population units, as currently delineated in the boreal caribou recovery strategy, do not capture the genetic variation and connectivity of the ecotype across the study area. Here, we provide the tools to assess fine‐scale spatial patterns of genetic variation, partition drivers of genetic variation, and evaluate the best management options for maintaining genetic connectivity. Our approach is highly relevant to vagile wildlife species that are of management and conservation concern and demonstrate varying degrees of IBD and IBR with clinal spatial genetic structure that challenges the delineation of discrete population boundaries.

Postglacial range expansion shaped the spatial genetic structure in a marine habitat‐forming species: Implications for conservation plans in the Eastern Adriatic Sea

AbstractAimUnderstanding how historical and contemporary processes shaped and maintain spatial patterns of genetic diversity is a major goal for conservation biologists. Here, we characterized the pattern of neutral genetic diversity and we inferred underlying processes in the habitat‐forming octocoral Paramuricea clavata in the Adriatic Sea, a peculiar phylogeographic region of the Mediterranean Sea.LocationEastern coast of the Adriatic Sea.MethodsWe genotyped seven microsatellites in 454 individuals of P. clavata from 13 populations recolonized after the Last Glacial Maximum (LGM). We estimated the levels of contemporary connectivity and genetic drift and we reconstructed the demographic history of these populations.ResultsThe pattern of spatial genetic structure resulted from the combination of hierarchical genetic clusters and isolation by distance (IBD). A significant decrease in genetic diversity and an increase of the frequencies of individual alleles likely due to allele surfing from the south to the north of the area were observed as expected after a postglacial sequential recolonization. Based on maximum likelihood analyses, the foundation of these populations was not linked to dramatic change in population size.Main conclusionOceanographic barriers to gene flow combined to the restricted dispersal of P. clavata likely maintain the hierarchical structure and the IBD pattern. We suggest that the latitudinal genetic gradient results from a northward “serial founder events” recolonization. By integrating patterns and processes, we bridge the gap between the evolutionary and the conservation biology of P. clavata, providing management guidelines, which will benefit the associated coralligenous biodiversity.

Topography explains the distribution of genetic diversity in one of the most fragile European hotspots

AbstractAimTo investigate factors that explain the spatial pattern of genetic diversity in three closely related species (Linaria glacialis, Linaria nevadensis and Chaenorhinum glareosum) endemic to a fragile high mountain ecosystem.LocationThe alpine belt of Sierra Nevada, Spain.MethodsWe analysed the spatial pattern of cpDNA diversity of the three species. To explain the distribution of genetic diversity, we investigated the effect of topographic features and the evolutionary history of the species (demography, habitat availability and colonization dynamics).ResultsGenetic diversity was heterogeneous across the landscape. We found moderate positive correlation values between genetic diversity indices of the two Linaria species. We also observed moderate negative correlation values between genetic diversity indices of C. glareosum and those of L. glacialis and L. nevadensis. Topographic variables correlated positively with genetic diversity of the Linaria species and negatively with genetic diversity of C. glareosum. Bayesian skyline plots (BSPs) displayed a shared demographic pattern with a population size stabilization/increase since the LGM (the last 21 kyr) in all three species. Discrete phylogeographical analyses showed similar patterns of westward diffusion for L. nevadensis and C. glareosum. Species distribution models pointed to similar range dynamics in all three species, with a reduction in range size since the LGM.Main conclusionsDifferent dispersal abilities, demographic trends and colonization patterns can hardly explain the differences in spatial patterns of genetic diversity between the Linaria species and C. glareosum. In contrast, topographic features seem to be an important factor to explain the distribution of genetic diversity in the alpine belt of Sierra Nevada. We point to a relevant role of microniche partitioning in determining patterns of genetic diversity distribution in alpine Mediterranean ecosystems. Furthermore, we highlight the role of microhabitat heterogeneity in the maintenance of distinct lineages, species and genetic diversity in high mountain biodiversity hotspots.

Landscape genomics provides evidence of climate-associated genetic variation in Mexican populations of Quercus rugosa.

Local adaptation is a critical evolutionary process that allows plants to grow better in their local compared to non‐native habitat and results in species‐wide geographic patterns of adaptive genetic variation. For forest tree species with a long generation time, this spatial genetic heterogeneity can shape the ability of trees to respond to rapid climate change. Here, we identify genomic variation that may confer local environmental adaptations and then predict the extent of adaptive mismatch under future climate as a tool for forest restoration or management of the widely distributed high‐elevation oak species Quercus rugosa in Mexico. Using genotyping by sequencing, we identified 5,354 single nucleotide polymorphisms (SNPs) genotyped from 103 individuals across 17 sites in the Trans‐Mexican Volcanic Belt, and, after controlling for neutral genetic structure, we detected 74 F ST outlier SNPs and 97 SNPs associated with climate variation. Then, we deployed a nonlinear multivariate model, Gradient Forests, to map turnover in allele frequencies along environmental gradients and predict areas most sensitive to climate change. We found that spatial patterns of genetic variation were most strongly associated with precipitation seasonality and geographic distance. We identified regions of contemporary genetic and climatic similarities and predicted regions where future populations of Q. rugosa might be at risk due to high expected rate of climate change. Our findings provide preliminary details for future management strategies of Q. rugosa in Mexico and also illustrate how a landscape genomic approach can provide a useful tool for conservation and resource management strategies.

Habitat Fragmentation Reduces Genetic Diversity and Connectivity of the Mexican Spotted Owl: A Simulation Study Using Empirical Resistance Models.

We evaluated how differences between two empirical resistance models for the same geographic area affected predictions of gene flow processes and genetic diversity for the Mexican spotted owl (Strix occidentalis lucida). The two resistance models represented the landscape under low- and high-fragmentation parameters. Under low fragmentation, the landscape had larger but highly concentrated habitat patches, whereas under high fragmentation, the landscape had smaller habitat patches that scattered across a broader area. Overall habitat amount differed little between resistance models. We tested eight scenarios reflecting a factorial design of three factors: resistance model (low vs. high fragmentation), isolation hypothesis (isolation-by-distance, IBD, vs. isolation-by-resistance, IBR), and dispersal limit of species (200 km vs. 300 km). Higher dispersal limit generally had a positive but small influence on genetic diversity. Genetic distance increased with both geographic distance and landscape resistance, but landscape resistance displayed a stronger influence. Connectivity was positively related to genetic diversity under IBR but was less important under IBD. Fragmentation had a strong negative influence on the spatial patterns of genetic diversity and effective population size (Ns). Despite habitats being more concentrated and less widely distributed, the low-fragmentation landscape had greater genetic diversity than the high-fragmentation landscape, suggesting that highly concentrated but larger habitat patches may provide a genetic refuge for the Mexican spotted owl.

Regional and local patterns of genetic variation and structure in yellow-necked mice - the roles of geographic distance, population abundance, and winter severity.

The goal of this study, conducted in seven large woodlands and three areas with small woodlots in northeastern Poland in 2004–2008, was to infer genetic structure in yellow‐necked mouse Apodemus flavicollis population and to evaluate the roles of environmental and population ecology variables in shaping the spatial pattern of genetic variation using 768 samples genotyped at 13 microsatellite loci. Genetic variation was very high in all studied regions. The primal genetic subdivision was observed between the northern and the southern parts of the study area, which harbored two major clusters and the intermediate area of highly admixed individuals. The probability of assignment of individual mice to the northern cluster increased significantly with lower temperatures of January and July and declined in regions with higher proportion of deciduous and mixed forests. Despite the detected structure, genetic differentiation among regions was very low. Fine‐scale structure was shaped by the population density, whereas higher level structure was mainly shaped by geographic distance. Genetic similarity indices were highly influenced by mouse abundance (which positively correlated with the share of deciduous forests in the studied regions) and exhibited the greatest change between 0 and 1 km in the forests, 0 and 5 km in small woodlots. Isolation by distance pattern, calculated among regions, was highly significant but such relationship between genetic and geographic distance was much weaker, and held the linearity at very fine scale (~1.5 km), when analyses were conducted at individual level.

Invoking adaptation to decipher the genetic legacy of past climate change.

Persistence of natural populations during periods of climate change is likely to depend on migration (range shifts) or adaptation. These responses were traditionally considered discrete processes and conceptually divided into the realms of ecology and evolution. In a milestone paper, Davis and Shaw (2001) Science 292:673 argued that the interplay of adaptation and migration was central to biotic responses to Quaternary climate, but since then there has been no synthesis of efforts made to set up this research program. Here we review some of the salient findings from molecular genetic studies assessing ecological and evolutionary responses to Quaternary climate change. These studies have revolutionized our understanding of population processes associated with past species migration. However, knowledge remains limited about the role of natural selection for local adaptation of populations to Quaternary environmental fluctuations and associated range shifts, and for the footprints this might have left on extant populations. Next-generation sequencing technologies, high-resolution paleoclimate analyses, and advances in population genetic theory offer an unprecedented opportunity to test hypotheses about adaptation through time. Recent population genomics studies have greatly improved our understanding of the role of contemporary adaptation to local environments in shaping spatial patterns of genetic diversity across modern-day landscapes. Advances in this burgeoning field provide important conceptual and methodological bases to decipher the historical role of natural selection and assess adaptation to past environmental variation. We suggest that a process called "temporal conditional neutrality" has taken place: some alleles favored in glacial environments become selectively neutral in modern-day conditions, whereas some alleles that had been neutral during glacial periods become under selection in modern environments. Building on this view, we present a new integrative framework for addressing the interplay of demographic and adaptive evolutionary responses to Quaternary climate dynamics, the research agenda initially envisioned by Davis and Shaw (2001) Science 292:673.

Revisiting the “Centre Hypotheses” of the Indo‐West Pacific: Idiosyncratic genetic diversity of nine reef species offers weak support for the Coral Triangle as a centre of genetic biodiversity

AbstractAimSeveral biogeographic hypotheses seek to explain the extreme species diversity of the coral reefs in the Coral Triangle (CT) of the Indo‐West Pacific (IWP). Here, we investigate processes implicit to the Centre of Origin, Centre of Overlap and Centre of Survival hypotheses. We focus on intraspecific genetic data, where intraspecific diversity can provide information regarding directionality of past dispersal (gene flow) and changes in population size such as expansion from refugia.LocationIndian Ocean, Coral Triangle and West Pacific Ocean.MethodsWe use existing mitochondrial DNA data augmented with new samples from the Philippines, thus filling in key sampling gaps from previous studies, to examine nine reef species. We first test if genetic diversity peaks in the CT. We then estimate gene flow between regions—CT vs. Indian Ocean (IO) and CT vs. Pacific Ocean (PO)—to assess concordance with predicted directions of movement (i.e. into the CT or out of the CT). Finally, we investigate the historical demography of these species and evaluate population expansions to ascertain whether genetic diversity is correlated with putative Pleistocene refugia.ResultsGenetic diversity in the CT was not greater than genetic diversity in the IO and PO, and we found highly variable spatial patterns of genetic diversity when species were evaluated individually. Gene flow estimates indicated very limited exchange between regions, consistent with peripheral isolation and/or signals of historical divergence. For some species, however, there were signals of movement out of the CT. While there was no significant correlation between genetic diversity and putative refugia, genetic patterns suggested that some populations have experienced population expansions. The locations of expansion were also variable across species. Overall, there was no signal of concordant processes shaping intraspecific genetic diversity.Main conclusionsGenetic patterns across species were highly variable with some species lending weak support for processes aligned with each of the “Centre” of diversity hypotheses. But overall there was no compelling evidence for uniform processes underlying all species, which possibly indicate that processes underlying the different “Centre” have not strongly affected IWP species within the recent past.

Population genomic analysis suggests strong influence of river network on spatial distribution of genetic variation in invasive saltcedar across the southwestern United States.

Understanding the complex influences of landscape and anthropogenic elements that shape the population genetic structure of invasive species provides insight into patterns of colonization and spread. The application of landscape genomics techniques to these questions may offer detailed, previously undocumented insights into factors influencing species invasions. We investigated the spatial pattern of genetic variation and the influences of landscape factors on population similarity in an invasive riparian shrub, saltcedar (Tamarix L.) by analysing 1,997 genomewide SNP markers for 259 individuals from 25 populations collected throughout the southwestern United States. Our results revealed a broad-scale spatial genetic differentiation of saltcedar populations between the Colorado and Rio Grande river basins and identified potential barriers to population similarity along both river systems. River pathways most strongly contributed to population similarity. In contrast, low temperature and dams likely served as barriers to population similarity. We hypothesize that large-scale geographic patterns in genetic diversity resulted from a combination of early introductions from distinct populations, the subsequent influence of natural selection, dispersal barriers and founder effects during range expansion.

Climatic suitability, isolation by distance and river resistance explain genetic variation in a Brazilian whiptail lizard.

Spatial patterns of genetic variation can help understand how environmental factors either permit or restrict gene flow and create opportunities for regional adaptations. Organisms from harsh environments such as the Brazilian semiarid Caatinga biome may reveal how severe climate conditions may affect patterns of genetic variation. Herein we combine information from mitochondrial DNA with physical and environmental features to study the association between different aspects of the Caatinga landscape and spatial genetic variation in the whiptail lizard Ameivula ocellifera. We investigated which of the climatic, environmental, geographical and/or historical components best predict: (1) the spatial distribution of genetic diversity, and (2) the genetic differentiation among populations. We found that genetic variation in A. ocellifera has been influenced mainly by temperature variability, which modulates connectivity among populations. Past climate conditions were important for shaping current genetic diversity, suggesting a time lag in genetic responses. Population structure in A. ocellifera was best explained by both isolation by distance and isolation by resistance (main rivers). Our findings indicate that both physical and climatic features are important for explaining the observed patterns of genetic variation across the xeric Caatinga biome.

Refining the biogeographical scenario of the land snail Cornu aspersum aspersum: Natural spatial expansion and human-mediated dispersal in the Mediterranean basin

The land snail Cornu aspersum aspersum, native to the Mediterranean region, has been the subject of several anatomical and molecular studies leading to recognize two divergent lineages, named “East” and “West” according to their geographical distribution in North Africa. The first biogeographical scenario proposed the role of Oligocene paleogeographic events and Quaternary glacial refugia to explain spatial patterns of genetic variation. The aim of this study was to refine this scenario using molecular and morphometric data from 169 populations sampled across Mediterranean islands and continents. The two previously described lineages no longer correspond to distinct biogeographical entities. Phylogenetic relationships reveal the existence of seven clades, do not support the Tyrrhenian vicariance hypothesis, and suggest that C. a. aspersum most likely originates from North Africa. We found two contrasted patterns with the seven clades defining spatially well-structured populations in the southern Mediterranean whereas one clade is distributed across the basin. High genetic diversities and rates of endemism in North Africa support the role of this region for the diversification of C. a. aspersum. In referring to divergence times previously estimated, we suggest allopatric differentiation due to geological changes of the Atlas system and multiple refugial areas during Pleistocene glaciations. The new biogeographical scenario implies an initial range expansion from North Africa to the Iberian Peninsula and the peri-Tyrrhenian regions through land bridges connections during the Messinian Salinity Crisis and Pleistocene glaciations. Historical events appear to have also structured morphometric variation but recent dispersal events favored the emergence of secondary contacts between clades. Southern Mediterranean clades are limited to their initial distribution and populations of the recent clade would have rapidly recolonized the whole Mediterranean in the Holocene due to greater adaptive potential and the influence of human transportations.

Admixture between released and wild game birds: a changing genetic landscape in European mallards (Anas platyrhynchos)

Disruption of naturally evolved spatial patterns of genetic variation and local adaptations is a growing concern in wildlife management and conservation. During the last decade, releases of native taxa with potentially non-native genotypes have received increased attention. This has mostly concerned conservation programs, but releases are also widely carried out to boost harvest opportunities. The mallard, Anas platyrhynchos, is one of few terrestrial migratory vertebrates subjected to large-scale releases for hunting purposes. It is the most numerous and widespread duck in the world, yet each year more than three million farmed mallard ducklings are released into the wild in the European Union alone to increase the harvestable population. This study aimed to determine the genetic effects of such large-scale releases of a native species, specifically if wild and released farmed mallards differ genetically among subpopulations in Europe, if there are signs of admixture between the two groups, if the genetic structure of the wild mallard population has changed since large-scale releases began in the 1970s, and if the current data matches global patterns across the Northern hemisphere. We used Bayesian clustering (Structure software) and Discriminant Analysis of Principal Components (DAPC) to analyze the genetic structure of historical and present-day wild (n = 171 and n = 209, respectively) as well as farmed (n = 211) mallards from six European countries as inferred by 360 single-nucleotide polymorphisms (SNPs). Both methods showed a clear genetic differentiation between wild and farmed mallards. Admixed individuals were found in the present-day wild population, implicating introgression of farmed genotypes into wild mallards despite low survival among released farmed mallards. Such cryptic introgression would alter the genetic composition of wild populations and may have unknown long-term consequences for conservation.