skip to main content


The NSF Public Access Repository (NSF-PAR) system and access will be unavailable from 11:00 PM ET on Thursday, May 23 until 2:00 AM ET on Friday, May 24 due to maintenance. We apologize for the inconvenience.

Title: Population genetic structure and demographic history of the lone star tick, Amblyomma americanum (Ixodida: Ixodidae): New evidence supporting old records

Range expansions are a potential outcome of changes in habitat suitability, which commonly result as a consequence of climate change. Hypotheses on such changes in the geographic distribution of a certain species can be evaluated using population genetic structure and demography. In this study we explore the population genetic structure, genetic variability, demographic history of, and habitat suitability forAmblyomma americanum, a North American tick species that is a known vector of several pathogenic microorganisms. We used a double digestion restriction site‐associated DNA sequencing technique (dd‐RAD seq) and discovered 8,181 independent single nucleotide polymorphisms (SNPs) in 189 ticks from across the geographic range of the species. Genetic diversity was low, particularly when considering the broad geographic range of this species. The edge populations were less diverse than populations belonging to the historic range, possibly indicative of a range expansion, but this hypothesis was not statistically supported by a test based on genetic data. Nonetheless, moderate levels of population structure and substructure were detected between geographic regions. For New England, demographic and species distribution models support a scenario whereA. americanumwas present in more northern locations in the past, underwent a bottleneck, and subsequently recovered. These results are consistent with a hypothesis that this species is re‐establishing in this area, rather than one focused on range expansion from the south. This hypothesis is consistent with old records describing the presence ofA. americanumin the northeastern US in the early colonial period.

more » « less
Author(s) / Creator(s):
 ;  ;  ;  
Publisher / Repository:
Date Published:
Journal Name:
Molecular Ecology
Page Range / eLocation ID:
p. 2810-2823
Medium: X
Sponsoring Org:
National Science Foundation
More Like this
  1. Abstract Aim

    The drivers of genetic diversity in Amazonia, the most species‐rich set of ecosystems on Earth, are still incompletely understood. Species from distinct Amazonian ecosystems have unique biogeographic histories that will reflect regional landscape and climatic drivers of genetic diversity. We studied bird species from patchy Amazonian white‐sand ecosystems (WSE) to evaluate the occurrence of shared biogeographic patterns to better understand the complex environmental and landscape history of Amazonia and its biodiversity.


    Northern South America; Amazonia.




    We sequenced Ultra‐conserved Elements (UCEs) from 177 samples of seven bird species associated with WSE that have overlapping ranges. We used the SNP matrices and sequence data to estimate genetic structure and migration surfaces using ‘conStruct’ andeems, performed model‐selection to obtain the most probable demographic histories on ‘PipeMasterand implemented analyses of shared demography withecoevolity.


    Shallow genetic structure patterns varied among species. The Amazon river was the only barrier shared among them. Population structure dates to no more than 450,000 years ago. Nine geographically structured populations showed signals of population size changes and eight of these occur in Northern Amazonia. Population expansion was inferred at two distinct times: ~100,000 and ~ 50,000 years ago. The timing of co‐expanding populations is consistent with differences in habitat preference, as species that prefer dense scrubby to forested vegetation expanded more recently compared to species that prefer open vegetation.

    Main conclusions

    WSE species responded in concert to environmental and landscape changes that occurred in the relatively recent past. Population expansions were likely driven by the genesis of new WSE patches and a return to wetter conditions after glacial periods. Pleistocene climatic cycles affected the distribution and dynamics of open vegetation habitats in Amazonia, especially in the Northern region, driving genetic diversity and demographic patterns of its associated biota.

    more » « less
  2. Abstract

    Populus tremuloidesis the widest‐ranging tree species in North America and an ecologically important component of mesic forest ecosystems displaced by the Pleistocene glaciations. Using phylogeographic analyses of genome‐wide SNPs (34,796 SNPs, 183 individuals) and ecological niche modeling, we inferred population structure, ploidy levels, admixture, and Pleistocene range dynamics ofP. tremuloides, and tested several historical biogeographical hypotheses. We found three genetic lineages located mainly in coastal–Cascades (cluster 1), east‐slope Cascades–Sierra Nevadas–Northern Rockies (cluster 2), and U.S. Rocky Mountains through southern Canadian (cluster 3) regions of theP. tremuloidesrange, with tree graph relationships of the form ((cluster 1, cluster 2), cluster 3). Populations consisted mainly of diploids (86%) but also small numbers of triploids (12%) and tetraploids (1%), and ploidy did not adversely affect our genetic inferences. The main vector of admixture was from cluster 3 into cluster 2, with the admixture zone trending northwest through the Rocky Mountains along a recognized phenotypic cline (Utah to Idaho). Clusters 1 and 2 provided strong support for the “stable‐edge hypothesis” that unglaciated southwestern populations persisted in situ since the last glaciation. By contrast, despite a lack of clinal genetic variation, cluster 3 exhibited “trailing‐edge” dynamics from niche suitability predictions signifying complete northward postglacial expansion. Results were also consistent with the “inland dispersal hypothesis” predicting postglacial assembly of Pacific Northwestern forest ecosystems, but rejected the hypothesis that Pacific‐coastal populations were colonized during outburst flooding from glacial Lake Missoula. Overall, congruent patterns between our phylogeographic and ecological niche modeling results and fossil pollen data demonstrate complex mixtures of stable‐edge, refugial locations, and postglacial expansion withinP. tremuloides. These findings confirm and refine previous genetic studies, while strongly supporting a distinct Pacific‐coastal genetic lineage of quaking aspen.

    more » « less
  3. Abstract

    The center‐periphery hypothesis predicts a decline in population performance toward the periphery of a species' range, reflecting an alteration of environmental conditions at range periphery. However, the rare demographic tests of this hypothesis failed to disentangle the role of geography from that of ecological niche and are biased toward temperate regions. We hypothesized that, because species are expected to experience optimal abiotic conditions at their climatic niche center, (1) central populations will have better demographic growth, survival, and fertility than peripheral populations. As a result, (2) central populations are expected to have higher growth rates than peripheral populations. Peripheral populations are expected to decline, thus limiting species range expansion beyond these boundaries. Because peripheral populations are expected to be in harsh environmental conditions, (3) population growth rate will be more sensitive to perturbation of survival‐growth rather than fertility in peripheral populations. Finally, we hypothesized that (4) soils properties will drive the variations in population growth rates for narrowly distributed species for which small scale ecological factors could outweigh landscape level drivers. To test these hypotheses, we studied the demography ofThunbergia atacorensis(Acanthaceae), a range‐limited herb in West Africa. We collected three years of demographic data to parameterize an integral projection model (IPM) and estimated population level demographic statistics. Demographic vital rates and population growth rates did not change significantly with distance from geographic or climatic center, contrary to predictions. However, populations at the center of the geographic range were demographically more resilient to perturbation than those at the periphery. Soil nitrogen was the main driver of population growth rate variation. The relative influence of survival‐growth on population growth rates exceeded that of fertility at the geographic range center while we observed the opposite pattern for climatic niche. Our study highlights the importance of local scale processes in shaping the dynamics and distribution of range‐limited species. Our findings also suggest that the distinction between geographic distribution and climatic niche is important for a robust demographic test of the center‐periphery hypothesis.

    more » « less
  4. Abstract Aim

    Patterns of genetic diversity within species’ ranges can reveal important insights into effects of past climate on species’ biogeography and current population dynamics. While numerous biogeographic hypotheses have been proposed to explain patterns of genetic diversity within species’ ranges, formal comparisons and rigorous statistical tests of these hypotheses remain rare. Here, we compared seven hypotheses for their abilities to describe the geographic pattern of two metrics of genetic diversity in balsam poplar (Populus balsamifera), a northern North American tree species.


    North America.


    Balsam poplar (Populus balsamiferaL.).


    We compared seven hypotheses, representing effects of past climate and current range position, for their ability to describe the geographic pattern of expected heterozygosity and per cent polymorphic loci across 85 populations of balsam poplar. We tested each hypothesis using spatial and non‐spatial least‐squares regression to assess the importance of spatial autocorrelation on model performance.


    We found that both expected heterozygosity and per cent polymorphic loci could best be explained by the current range position and genetic structure of populations within the contemporary range. Genetic diversity showed a clear gradient of being highest near the geographic and climatic range centre and lowest near range edges. Hypotheses accounting for the effects of past climate (e.g. past climatic suitability, distance from the southern edge), in contrast, had comparatively little support. Model ranks were similar among spatial and non‐spatial models, but residuals of all non‐spatial models were significantly autocorrelated, violating the assumption of independence in least‐squares regression.

    Main conclusions

    Our work adds strong support for the “Central‐Periphery Hypothesis” as providing a predictive framework for understanding the forces structuring genetic diversity across species’ ranges, and illustrates the value of applying a robust comparative model selection framework and accounting for spatial autocorrelation when comparing biogeographic models of genetic diversity.

    more » « less
  5. Abstract

    For many species, both local abundance and regional occupancy are highest near the centre of their geographic distributions. One hypothesis for this pattern is that niche suitability declines with increasing distance from a species geographic centre, such that populations near range margins are characterized by reduced density and increased patchiness. In these smaller edge populations, genetic drift is more powerful, leading to the loss of genetic diversity. This simple verbal model has been formalized as the central‐marginal hypothesis, which predicts that core populations should have greater genetic diversity than edge populations. Here, we tested the central‐marginal hypothesis using a genomic data set of 25 species‐level taxa of Australian scincid lizards in the generaCtenotusandLerista. A majority of taxa in our data set showed range‐wide patterns of genetic variation consistent with central‐marginal hypothesis, and eight of 25 taxa showed significantly greater genetic diversity in the centre of their range. We then explored biological, historical, and methodological factors that might predict which taxa support the central‐marginal hypothesis. We found that taxa with the strongest evidence for range expansion were the least likely to follow predictions of the central‐marginal hypothesis. The majority of these taxa had range expansions that originated at the range edge, which led to a gradient of decreasing genetic diversity from the range edge to the core, contrary to the central‐marginal hypothesis.

    more » « less