scholarly journals Sympatric and allopatric differentiation delineates population structure in free-living terrestrial bacteria

2019 ◽  
Author(s):  
Alexander B. Chase ◽  
Philip Arevalo ◽  
Eoin L. Brodie ◽  
Martin F. Polz ◽  
Ulas Karaoz ◽  
...  
Keyword(s):  
Gene Flow ◽  
Isolation By Distance ◽  
Biological Species ◽  
Microbial Populations ◽  
Marker Genes ◽  
Rrna Gene ◽  
Biological Species Concept ◽  
Free Living ◽  
Species Definition ◽  
Isolation By Environment

ABSTRACTIn free-living bacteria and archaea, the equivalent of the biological species concept does not exist, creating several barriers to the study of the processes contributing to microbial diversification. As such, microorganisms are often operationally defined using conserved marker genes (i.e., 16S rRNA gene) or whole-genome measurements (i.e., ANI) to interpret intra-specific processes. However, as in eukaryotes, investigations into microbial populations must consider the potential for interacting genotypes among individuals that are subjected to similar environmental selective pressures. Therefore, we isolated 26 strains within a single bacterial ecotype (equivalent to a eukaryotic species definition) from a common habitat (leaf litter) across a regional climate gradient and asked whether the genetic diversity in a free-living soil bacterium (Curtobacterium) was consistent with patterns of allopatric or sympatric differentiation. By examining patterns of gene flow, our results indicate that microbial populations are delineated by gene flow discontinuities and exhibit evidence for population-specific adaptation. We conclude that the genetic structure within this bacterium is due to both adaptation within localized microenvironments (isolation-by-environment) as well as dispersal limitation between geographic locations (isolation-by-distance).

scholarly journals Maintenance of Sympatric and Allopatric Populations in Free-Living Terrestrial Bacteria

mBio ◽  
2019 ◽  
Vol 10 (5) ◽  
Author(s):  
Alexander B. Chase ◽  
Philip Arevalo ◽  
Eoin L. Brodie ◽  
Martin F. Polz ◽  
Ulas Karaoz ◽  
...  
Keyword(s):  
Population Structure ◽  
Gene Flow ◽  
Isolation By Distance ◽  
Dispersal Limitation ◽  
Genomic Diversity ◽  
Gene Content ◽  
Microbial Populations ◽  
Free Living ◽  
Phylogenetic Structure ◽  
Isolation By Environment

ABSTRACT For free-living bacteria and archaea, the equivalent of the biological species concept does not exist, creating several obstacles to the study of the processes contributing to microbial diversification. These obstacles are particularly high in soil, where high bacterial diversity inhibits the study of closely related genotypes and therefore the factors structuring microbial populations. Here, we isolated strains within a single Curtobacterium ecotype from surface soil (leaf litter) across a regional climate gradient and investigated the phylogenetic structure, recombination, and flexible gene content of this genomic diversity to infer patterns of gene flow. Our results indicate that microbial populations are delineated by gene flow discontinuities, with distinct populations cooccurring at multiple sites. Bacterial population structure was further delineated by genomic features allowing for the identification of candidate genes possibly contributing to local adaptation. These results suggest that the genetic structure within this bacterium is maintained both by ecological specialization in localized microenvironments (isolation by environment) and by dispersal limitation between geographic locations (isolation by distance). IMPORTANCE Due to the promiscuous exchange of genetic material and asexual reproduction, delineating microbial species (and, by extension, populations) remains challenging. Because of this, the vast majority of microbial studies assessing population structure often compare divergent strains from disparate environments under varied selective pressures. Here, we investigated the population structure within a single bacterial ecotype, a unit equivalent to a eukaryotic species, defined as highly clustered genotypic and phenotypic strains with the same ecological niche. Using a combination of genomic and computational analyses, we assessed the phylogenetic structure, extent of recombination, and flexible gene content of this genomic diversity to infer patterns of gene flow. To our knowledge, this study is the first to do so for a dominant soil bacterium. Our results indicate that bacterial soil populations, similarly to those in other environments, are structured by gene flow discontinuities and exhibit distributional patterns consistent with both isolation by distance and isolation by environment. Thus, both dispersal limitation and local environments contribute to the divergence among closely related soil bacteria as observed in macroorganisms.

scholarly journals Genomic Adaptations to Salinity Resist Gene Flow in the Evolution of Floridian Watersnakes

2020 ◽  
Author(s):  
Rhett M Rautsaw ◽  
Tristan D Schramer ◽  
Rachel Acuña ◽  
Lindsay N Arick ◽  
Mark DiMeo ◽  
...  
Keyword(s):  
Gene Flow ◽  
Isolation By Distance ◽  
Species Boundaries ◽  
Selective Pressures ◽  
Nerodia Fasciata ◽  
Isolation By Environment ◽  
Significant Gene ◽  
Candidate Loci ◽  
Osmoregulatory Function ◽  
Ecological Differences

Abstract The migration-selection balance often governs the evolution of lineages, and speciation with gene flow is now considered common across the tree of life. Ecological speciation is a process that can facilitate divergence despite gene flow due to strong selective pressures caused by ecological differences; however, the exact traits under selection are often unknown. The transition from freshwater to saltwater habitats provides strong selection targeting traits with osmoregulatory function. Several lineages of North American watersnakes (Nerodia spp.) are known to occur in saltwater habitat and represent a useful system for studying speciation by providing an opportunity to investigate gene flow and evaluate how species boundaries are maintained or degraded. We use double digest restriction-site associated DNA sequencing to characterize the migration-selection balance and test for evidence of ecological divergence within the Nerodia fasciata-clarkii complex in Florida. We find evidence of high intraspecific gene flow with a pattern of isolation-by-distance underlying subspecific lineages. However, we identify genetic structure indicative of reduced gene flow between inland and coastal lineages suggesting divergence due to isolation-by-environment. This pattern is consistent with observed environmental differences where the amount of admixture decreases with increased salinity. Furthermore, we identify significantly enriched terms related to osmoregulatory function among a set of candidate loci, including several genes that have been previously implicated in adaptation to salinity stress. Collectively, our results demonstrate that ecological differences, likely driven by salinity, cause strong divergent selection which promotes divergence in the N. fasciata-clarkii complex despite significant gene flow.

scholarly journals Evaluation of genetic isolation within an island flora reveals unusually widespread local adaptation and supports sympatric speciation

2014 ◽  
Vol 369 (1648) ◽  
pp. 20130342 ◽  
Author(s):  
Alexander S. T. Papadopulos ◽  
Maria Kaye ◽  
Céline Devaux ◽  
Helen Hipperson ◽  
Jackie Lighten ◽  
...  
Keyword(s):  
Gene Flow ◽  
Local Adaptation ◽  
Plant Species ◽  
Environmental Gradients ◽  
Isolation By Distance ◽  
Model Averaging ◽  
Dispersal Limitation ◽  
Population Divergence ◽  
Environmental Pressures ◽  
Isolation By Environment

It is now recognized that speciation can proceed even when divergent natural selection is opposed by gene flow. Understanding the extent to which environmental gradients and geographical distance can limit gene flow within species can shed light on the relative roles of selection and dispersal limitation during the early stages of population divergence and speciation. On the remote Lord Howe Island (Australia), ecological speciation with gene flow is thought to have taken place in several plant genera. The aim of this study was to establish the contributions of isolation by environment (IBE) and isolation by community (IBC) to the genetic structure of 19 plant species, from a number of distantly related families, which have been subjected to similar environmental pressures over comparable time scales. We applied an individual-based, multivariate, model averaging approach to quantify IBE and IBC, while controlling for isolation by distance (IBD). Our analyses demonstrated that all species experienced some degree of ecologically driven isolation, whereas only 12 of 19 species were subjected to IBD. The prevalence of IBE within these plant species indicates that divergent selection in plants frequently produces local adaptation and supports hypotheses that ecological divergence can drive speciation in sympatry.

Genetic diversity of the lichen-forming alga, Diplosphaera chodatii, in North America and Europe

2013 ◽  
Vol 45 (6) ◽  
pp. 799-813 ◽  
Author(s):  
Kyle M. FONTAINE ◽  
Elfie STOCKER-WÖRGÖTTER ◽  
Tom BOOTH ◽  
Michele D. PIERCEY-NORMORE
Keyword(s):  
Genetic Diversity ◽  
Population Structure ◽  
Gene Flow ◽  
North America ◽  
Isolation By Distance ◽  
Water Level Fluctuations ◽  
Free Living ◽  
Climatic Zones ◽  
Algal Symbiont ◽  
Low Levels

AbstractDermatocarpon luridum is a subaquatic lichen which is distributed within temperate climatic zones around the world. It colonizes rock substrata along the shoreline of lakes and rivers of watersheds that regularly experience water level fluctuations. The mycobiont produces perithecia with small, simple spores that are thought to be wind dispersed. The photobiont, Diplosphaera chodatii, occurs both free-living and lichenized but little is known about its distribution and dispersal. The goal of this study was to compare the population structure of the photobiont from lakes and rivers in central North America with those of Europe. Specimens were collected in Manitoba, Canada and Austria. Population structure of the algal symbiont was assessed using the internal transcribed spacer (ITS) of ribosomal DNA (rDNA) and actin gene sequences. Results showed that genetic diversity and gene flow was high within local populations, but gene flow was low between continental populations. Low levels of gene flow between the most distant populations support the isolation-by-distance theory. The photobiont on both continents is also reported to be the photobiont for other lichen species contributing to photobiont availability for D. luridum.

scholarly journals Fine scale population structure and extensive gene flow within an Eastern Nearctic snake complex (Pituophis melanoleucus)

2021 ◽  
Author(s):  
Zachary L Nikolakis ◽  
Richard Orton ◽  
Brian I Crother
Keyword(s):  
Gene Flow ◽  
Isolation By Distance ◽  
Genomic Structure ◽  
Genomic Variation ◽  
Genomic Diversity ◽  
Evolutionary Relationships ◽  
Population Genomic ◽  
Isolation By Environment ◽  
Pituophis Melanoleucus ◽  
Lineage Divergence

Understanding the processes and mechanisms that promote lineage divergence is a central goal in evolutionary biology. For instance, studies investigating the spatial distribution of genomic variation often highlight biogeographic barriers underpinning geographic isolation, as well as patterns of isolation by environment and isolation by distance that can also lead to lineage divergence. However, the patterns and processes that shape genomic variation and drive lineage divergence may be taxa-specific, even across closely related taxa co-occurring within the same biogeographic region. Here, we use molecular data in the form of ultra-conserved elements (UCEs) to infer the evolutionary relationships and population genomic structure of the Eastern Pinesnake complex (Pituophis melanoleucus) – a polytypic wide-ranging species that occupies much of the Eastern Nearctic. In addition to inferring evolutionary relationships, population genomic structure, and gene flow, we also test relationships between genomic diversity and putative barriers to dispersal, environmental variation, and geographic distance. We present results that reveal shallow population genomic structure and ongoing gene flow, despite an extensive geographic range that transcends geographic features found to reduce gene flow among many taxa, including other squamate reptiles within the Eastern Nearctic. Further, our results indicate that the observed genomic diversity is spatially distributed as a pattern of isolation by distance and suggest that the current subspecific taxonomy do not adhere to independent lineages, but rather, show a significant amount of admixture across the entire P. melanoleucus range.

scholarly journals Endosperm-based hybridization barriers explain the pattern of gene flow between Arabidopsis lyrata and Arabidopsis arenosa in Central Europe

2017 ◽  
Vol 114 (6) ◽  
pp. E1027-E1035 ◽  
Author(s):  
Clément Lafon-Placette ◽  
Ida M. Johannessen ◽  
Karina S. Hornslien ◽  
Mohammad F. Ali ◽  
Katrine N. Bjerkan ◽  
...  
Keyword(s):  
Gene Flow ◽  
Central Europe ◽  
Diploid Species ◽  
Biological Species ◽  
Biological Species Concept ◽  
Interspecific Gene Flow ◽  
Seed Formation ◽  
Arabidopsis Lyrata ◽  
Hybrid Seeds ◽  
Arabidopsis Arenosa

Based on the biological species concept, two species are considered distinct if reproductive barriers prevent gene flow between them. In Central Europe, the diploid species Arabidopsis lyrata and Arabidopsis arenosa are genetically isolated, thus fitting this concept as “good species.” Nonetheless, interspecific gene flow involving their tetraploid forms has been described. The reasons for this ploidy-dependent reproductive isolation remain unknown. Here, we show that hybridization between diploid A. lyrata and A. arenosa causes mainly inviable seed formation, revealing a strong postzygotic reproductive barrier separating these two species. Although viability of hybrid seeds was impaired in both directions of hybridization, the cause for seed arrest differed. Hybridization of A. lyrata seed parents with A. arenosa pollen donors resulted in failure of endosperm cellularization, whereas the endosperm of reciprocal hybrids cellularized precociously. Endosperm cellularization failure in both hybridization directions is likely causal for the embryo arrest. Importantly, natural tetraploid A. lyrata was able to form viable hybrid seeds with diploid and tetraploid A. arenosa, associated with the reestablishment of normal endosperm cellularization. Conversely, the defects of hybrid seeds between tetraploid A. arenosa and diploid A. lyrata were aggravated. According to these results, we hypothesize that a tetraploidization event in A. lyrata allowed the production of viable hybrid seeds with A. arenosa, enabling gene flow between the two species.

scholarly journals Genes and speciation: is it time to abandon the biological species concept?

2019 ◽  
Vol 7 (8) ◽  
pp. 1387-1397 ◽  
Author(s):  
Xinfeng Wang ◽  
Ziwen He ◽  
Suhua Shi ◽  
Chung-I Wu
Keyword(s):  
Gene Flow ◽  
Species Concept ◽  
Biological Species ◽  
Biological Species Concept ◽  
Geographical Isolation ◽  
Fundamental Nature ◽  
Genomic Analyses ◽  
Number Of Genes ◽  
The Common ◽  
Common Perception

Abstract The biological species concept (BSC) is the cornerstone of neo-Darwinian thinking. In BSC, species do not exchange genes either during or after speciation. However, as gene flow during speciation is increasingly being reported in a substantial literature, it seems time to reassess the revered, but often doubted, BSC. Contrary to the common perception, BSC should expect substantial gene flow at the onset of speciation, not least because geographical isolation develops gradually. Although BSC does not stipulate how speciation begins, it does require a sustained period of isolation for speciation to complete its course. Evidence against BSC must demonstrate that the observed gene flow does not merely occur at the onset of speciation but continues until its completion. Importantly, recent genomic analyses cannot reject this more realistic version of BSC, although future analyses may still prove it wrong. The ultimate acceptance or rejection of BSC is not merely about a historical debate; rather, it is about the fundamental nature of species – are species (and, hence, divergent adaptations) driven by a relatively small number of genes, or by thousands of them? Many levels of biology, ranging from taxonomy to biodiversity, depend on this resolution.

scholarly journals An overview of speciation and species limits in birds

The Auk ◽  
2021 ◽  
Author(s):  
Kevin Winker
Keyword(s):  
Gene Flow ◽  
Integrative Taxonomy ◽  
Difficult Problem ◽  
Biological Species ◽  
Biological Species Concept ◽  
Species Limits ◽  
Main Driver ◽  
Data Signal ◽  
Clear Line ◽  
Silver Bullet

Abstract Accurately determining avian species limits has been a challenge and a work in progress for most of a century. It is a fascinating but difficult problem. Under the biological species concept, only lineages that remain essentially independent when they are in sympatry are clearly species. Otherwise, there is no clear line yet found that marks when a pair of diverging lineages (e.g., in allopatry) become different enough to warrant full biological species status. Also, with more data, species limits often require reevaluation. The process of divergence and speciation is itself very complex and is the focus of intense research. Translating what we understand of that process into taxonomic names can be challenging. A series of issues are important. Single-locus criteria are unlikely to be convincing. Genetic independence is not a species limits requirement, but the degree of independence (gene flow) needs to be considered when there is opportunity for gene flow and independence is not complete. Time-based species (limits determined by time of separation) are unsatisfactory, though integrating time more effectively into our datasets is warranted. We need to disentangle data signal due to neutral processes vs. selection and prioritize the latter as the main driver of speciation. Assortative mating is also not likely to be an adequate criterion for determining species limits. Hybridization and gene flow are more important than ever, and there is a condition not being treated evenly in taxonomy: evolutionary trysts of 2 or more lineages stuck together through gene flow just short of speciation over long periods. Comparative methods that use what occurs between good species in contact to infer species limits among allopatric forms remain the gold standard, but they can be inaccurate and controversial. Species-level taxonomy in birds is likely to remain unsettled for some time. While the study of avian speciation has never been more exciting and dynamic, there is no silver bullet for species delimitation, nor is it likely that there will ever be one. Careful work using integrative taxonomy in a comparative framework is the most promising way forward.

scholarly journals Comparative phylogeography reveals how a barrier filters and structures taxa in North American warm deserts

2020 ◽  
Author(s):  
Kaiya L. Provost ◽  
Edward A. Myers ◽  
Brian Tilston Smith
Keyword(s):  
Gene Flow ◽  
Model Selection ◽  
Statistical Power ◽  
Isolation By Distance ◽  
Species Traits ◽  
Phylogeographic Structure ◽  
Spatially Explicit ◽  
Isolation With Migration ◽  
Demographic Models ◽  
Isolation By Environment

AbstractThe study of biogeographic barriers have been instrumental in understanding the evolution and distribution of taxa. Now with the increased availability of empirical datasets, it is possible to infer emergent patterns from communities by synthesizing how barriers filter and structure populations across species. We assemble phylogeographic data for a barrier and perform spatially-explicit simulations to quantify temporal and spatial patterns of divergence, the influence of species traits on these patterns, and understand the statistical power of differentiating alternative diversification modes. We incorporate published datasets to examine taxa around the Cochise Filter Barrier, separating the Sonoran and Chihuahuan deserts of North America, to synthesize phylogeographic structuring across the community with respect to organismal functional traits. We then use a simulation and machine learning pipeline to assess the power of phylogeographic model selection. Taxa distributed across the Cochise Filter Barrier show heterogeneous responses to the barrier in levels of gene flow, phylogeographic structure, divergence timing, barrier width, and divergence mechanism. These responses vary concordantly with locomotor and thermoregulatory traits. Many taxa show a Pleistocene population genetic break, often with introgression after divergence. Allopatric isolation and isolation-by-environment are the primary mechanisms purported to structure taxa. Simulations reveal that in spatially-explicit isolation-with-migration models across the barrier, age of divergence, presence of gene flow, and presence of isolation-by-distance can confound the interpretation of evolutionary history and model selection by producing easily-confusable results. By synthesizing phylogeographic data for the Cochise Filter Barrier we show a pattern where barriers interact with species traits to differentiate taxa in communities over millions of years. Identifying the modes of differentiation across the barriers for these taxa remains challenging because commonly invoked demographic models may not be identifiable across a range of likely parameter space.

scholarly journals Genetic differentiation and restricted gene flow in rice landraces from Yunnan, China: effects of isolation-by-distance and isolation-by-environment

2020 ◽  
Author(s):  
Di Cui ◽  
Cuifeng Tang ◽  
Hongfeng Lu ◽  
Jinmei Li ◽  
Xiaoding Ma ◽  
...  
Keyword(s):  
Gene Flow ◽  
Genetic Differentiation ◽  
Target Genes ◽  
Isolation By Distance ◽  
Local Environment ◽  
Adaptive Divergence ◽  
Restricted Gene Flow ◽  
Geographical Isolation ◽  
Rice Landraces ◽  
Isolation By Environment

Abstract Background Understanding and identifying the factors responsible for genetic differentiation is of fundamental importance for efficient utilization and conservation of traditional rice landraces. In this study, we examined the spatial genetic differentiation of 594 individuals sampled from 28 locations in Yunnan Province, China, covering a wide geographic distribution and diverse growing conditions. All 594 accessions were studied using ten unlinked target genes and 48 microsatellite loci, and the representative 108 accessions from the whole collection were sampled for resequencing. Results The genetic diversity of rice landraces was quite different geographically and exhibited a geographical decline from south to north in Yunnan, China. Population structure revealed that the rice landraces could be clearly differentiated into japonica and indica groups, respectively. In each group, the rice accessions could be further differentiated corresponded to their geographic locations, including three subgroups from northern, southern and middle locations. We found more obvious internal geographic structure in the japonica group than in the indica group. In the japonica group, we found that genetic and phenotypic differentiation were strongly related to geographical distance, suggesting a pattern of isolation by distance (IBD); this relationship remained highly significant when we controlled for environmental effects, where the likelihood of gene flow is inversely proportional to the distance between locations. Moreover, the gene flow also followed patterns of isolation by environment (IBE) whereby gene flow rates are higher in similar environments. We detected 314 and 216 regions had been differentially selected between Jap-N and Jap-S, Ind-N and Ind-S, respectively, and thus referred to as selection signatures for different geographic subgroups. We also observed a number of significant and interesting associations between loci and environmental factors, which implies adaptation to local environment. Conclusion Our findings highlight the influence of geographical isolation and environmental heterogeneity on the pattern of the gene flow, and demonstrate that both geographical isolation and environment drives adaptive divergence play dominant roles in the genetic differentiation of the rice landraces in Yunnan, China as a result of limited dispersal.

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