The effects of riverine impoundment on genetic structure and gene flow in two stream fishes in the Mobile River basin

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Does long‐distance pollen dispersal preclude inbreeding in tropical trees? Fragmentation genetics of Dysoxylum malabaricum in an agro‐forest landscape

Tropical trees often display long‐distance pollen dispersal, even in highly fragmented landscapes. Understanding how patterns of spatial isolation influence pollen dispersal and interact with background patterns of fine‐scale spatial genetic structure (FSGS) is critical for evaluating the genetic consequences of habitat fragmentation. In the endangered tropical timber tree Dysoxylum malabaricum (Meliaceae), we apply eleven microsatellite markers with paternity and parentage analysis to directly estimate historic gene flow and contemporary pollen dispersal across a large area (216 km²) in a highly fragmented agro‐forest landscape. A comparison of genetic diversity and genetic structure in adult and juvenile life stages indicates an increase in differentiation and FSGS over time. Paternity analysis and parentage analysis demonstrate high genetic connectivity across the landscape by pollen dispersal. A comparison between mother trees in forest patches with low and high densities of adult trees shows that the frequency of short‐distance mating increases, as does average kinship among mates in low‐density stands. This indicates that there are potentially negative genetic consequences of low population density associated with forest fragmentation. Single isolated trees, in contrast, frequently receive heterogeneous pollen from distances exceeding 5 km. We discuss the processes leading to the observed patterns of pollen dispersal and the implications of this for conservation management of D. malabaricum and tropical trees more generally.     

Discordant patterns of genetic and phenotypic differentiation in five grasshopper species codistributed across a microreserve network

Conservation plans can be greatly improved when information on the evolutionary and demographic consequences of habitat fragmentation is available for several codistributed species. Here, we study spatial patterns of phenotypic and genetic variation among five grasshopper species that are codistributed across a network of microreserves but show remarkable differences in dispersal‐related morphology (body size and wing length), degree of habitat specialization and extent of fragmentation of their respective habitats in the study region. In particular, we tested the hypothesis that species with preferences for highly fragmented microhabitats show stronger genetic and phenotypic structure than codistributed generalist taxa inhabiting a continuous matrix of suitable habitat. We also hypothesized a higher resemblance of spatial patterns of genetic and phenotypic variability among species that have experienced a higher degree of habitat fragmentation due to their more similar responses to the parallel large‐scale destruction of their natural habitats. In partial agreement with our first hypothesis, we found that genetic structure, but not phenotypic differentiation, was higher in species linked to highly fragmented habitats. We did not find support for congruent patterns of phenotypic and genetic variability among any studied species, indicating that they show idiosyncratic evolutionary trajectories and distinctive demographic responses to habitat fragmentation across a common landscape. This suggests that conservation practices in networks of protected areas require detailed ecological and evolutionary information on target species to focus management efforts on those taxa that are more sensitive to the effects of habitat fragmentation.     

Genetic effects of habitat fragmentation and population isolation on Etheostoma raneyi (Percidae)

The use of genetic methods to quantify the effects of anthropogenic habitat fragmentation on population structure has become increasingly common. However, in today’s highly fragmented habitats, researchers have sometimes concluded that populations are currently genetically isolated due to habitat fragmentation without testing the possibility that populations were genetically isolated before European settlement. Etheostoma raneyi is a benthic headwater fish restricted to river drainages in northern Mississippi, USA, that has a suite of adaptive traits that correlate with poor dispersal ability. Aquatic habitat within this area has been extensively modified, primarily by flood-control projects, and populations in headwater streams have possibly become genetically isolated from one another. We used microsatellite markers to quantify genetic structure as well as contemporary and historical gene flow across the range of the species. Results indicated that genetically distinct populations exist in each headwater stream analyzed, current gene flow rates are lower than historical rates, most genetic variation is partitioned among populations, and populations in the Yocona River drainage show lower levels of genetic diversity than populations in the Tallahatchie River drainage and other Etheostoma species. All populations have negative F_IS scores, of which roughly half are significant relative to Hardy–Weinberg expectations, perhaps due to small population sizes. We conclude that anthropogenic habitat alteration and fragmentation has had a profoundly negative impact on the species by isolating E. raneyi within headwater stream reaches. Further research is needed to inform conservation strategies, but populations in the Yocona River drainage are in dire need of management action. Carefully planned human-mediated dispersal and habitat restoration should be explored as management options across the range of the species.     

Mitochondrial DNA and allozymes reveal high dispersal abilities and historical movement across drainage boundaries in two species of freshwater fishes from inland rivers in Queensland, Australia

This study used allozymes and mtDNA variation to test that: 1) populations of two fish species, Nematolosa erebi and Retropinna semoni , in lowland rivers in central Australia were highly connected within drainages, 2) populations in different drainages were highly differentiated and 3) there was evidence of historical connections between two major lowland drainages in inland Australia. Levels of genetic differentiation among populations within drainages were low, but still statistically significant, indicating that populations were not as highly connected as had been predicted. Populations from the Murray–Darling and the Lake Eyre drainages were highly differentiated, indicating no contemporary dispersal across drainage boundaries. Both species showed evidence of historical connections between the two drainage basins, although estimates of the time that these last occurred differed between the two species. Nematolosa erebi populations from the two drainages were estimated to have been separated c . 150 000 years ago, whereas populations of R. semoni , were estimated to have been separated c . 1.5 million years ago.     

Phylogeographic patterning in a freshwater crab species (Decapoda: Potamonautidae: Potamonautes) reveals the signature of historical climatic oscillations

Aim The phylogeographic relationships among populations of the common Cape River crab, Potamonautes perlatus, are examined to investigate whether the contemporary population genetic structure is congruent with the hypothesized hydrographic evolution of drainage systems established during the Pliocene, or whether it reflects an older Miocene climatic amelioration. Location 139 samples of P. perlatus were collected from 31 populations distributed among the five major perennial drainage systems and a number of smaller catchments in the Western and Eastern Cape, South Africa. Methods Phylogeographic analysis using parsimony, maximum likelihood, minimum evolution and Bayesian inferences was employed for the 16S rRNA mtDNA gene region, while bootstrapping and posterior probabilities were used to assess the robustness of clades. In addition, nested clade analysis was performed in an attempt to disentangle the contemporary and historical factors that have sculpted genealogical relationships among conspecific populations of P. perlatus. Results Phylogenetic topologies were congruent irrespective of the evolutionary method employed. Two highly distinct reciprocally monophyletic clades characterized by marked levels of corrected sequence divergence were present, with no shared haplotypes between the two major phylogroups. Phylogroup one comprises the populations of the westward‐flowing drainages (mainly the Berg and Olifants drainages), and phylogroup two comprises all of the southward‐flowing drainages and can further be divided into two subclades – one containing the Breede River populations, and the other containing the Gamtoos and Gourits drainage systems. The nested clade analysis demonstrated restricted gene flow and long‐distance dispersal for a number of higher clade levels. The higher‐level groups and results for the total cladogram suggest either fragmentation or isolation by distance. Main conclusions Freshwater crabs are generally highly philopatric, and dispersal, although not common, has occurred historically. The westward‐flowing drainages (Berg, Olifants, Eerste, Liesbeeck and Tokai) are isolated from the southward‐flowing drainages by the Cape Fold Mountains, while the southward‐flowing drainages have a number of tributaries that extend into the low‐lying regions, allowing for gene flow between these three major drainages systems (Breede, Gamtoos and Gourits). Among the westward‐flowing drainages, a more intensive sampling regime is required to understand evolutionary relationships. Our molecular results suggest that the observed patterns pre‐date the formation of contemporary hydrographic patterns in the Cape. This suggests that an older Late Miocene event has severely impacted the contemporary population structure in this species, as recent Pliocene hydrographic boundaries do not correspond to the phylogeographic pattern observed. Conservation efforts for aquatic taxa should clearly be directed at the catchments, in an attempt to conserve biological diversity.     

Diversification in a biodiversity hot spot: landscape correlates of phylogeographic patterns in the African spotted reed frog

The Eastern Afromontane Biodiversity Hotspot is known for microendemism and exceptional population genetic structure. The region's landscape heterogeneity is thought to limit gene flow between fragmented populations and create opportunities for regional adaptation, but the processes involved are poorly understood. Using a combination of phylogeographic analyses and circuit theory, I investigate how characteristics of landscape heterogeneity including regional distributions of slope, rivers and streams, habitat and hydrological basins (drainages) impact genetic distance among populations of the endemic spotted reed frog (Hyperolius substriatus), identifying corridors of connectivity as well as barriers to dispersal. Results show that genetic distance among populations is most strongly correlated to regional and local hydrologic structure and the distribution of suitable habitat corridors, not isolation by distance. Contrary to expectations, phylogeographic structure is not coincident with the two montane systems, but instead corresponds to the split between the region's two major hydrological basins (Zambezi and East Central Coastal). This results in a paraphyletic relationship for the Malawian Highlands populations with respect to the Eastern Arc Mountains and implies that the northern Malawian Highlands are the diversity centre for H. substriatus. Although the Malawian Highlands collectively hold the greatest genetic diversity, individual populations have lower diversity than their Eastern Arc counterparts, with an overall pattern of decreasing population diversity from north to south. Through the study of intraspecific differentiation across a mosaic of ecosystem and geographic heterogeneity, we gain insight into the processes of diversification and a broader understanding of the role of landscape in evolution.     

DOES FISH ECOLOGY PREDICT DISPERSAL ACROSS A RIVER DRAINAGE DIVIDE?

Obligate freshwater taxa are frequently distributed among catchments isolated by marine and terrestrial barriers. Such distributions can arise through vicariant changes in drainage geometry, or dispersal via intermittent freshwater connections. We employed two adjacent rivers in southern New Zealand to test for interdrainage dispersal while controlling for historical drainage geometry, and analyzed four ecologically distinct freshwater-limited fish taxa to assess any relationship with habitat preference. Individuals from the Mararoa and Oreti catchments (n >100 per species) were sequenced for a minimum of 1297 bp of mitochondrial DNA (cytochrome b and control region). Phylogeographic relationships were consistent with ecological expectations of interdrainage dispersal capability, with the two obligate riverine taxa each exhibiting reciprocal monophyly between catchments, whereas the two facultative swamp dwellers revealed paraphyletic relationships, one of which shared a haplotype between catchments. Statistical phylogeography, accommodating taxon-specific mutation rates and the known age of the last major riverine connection between these catchments, rejected complete isolation of populations for one of the swamp dwellers. Therefore, dispersal across a young (145-240 kyr) drainage divide is inferred for one species, and can be predicted to some extent by species ecology. Moreover, our study highlights the importance of historical drainage geometry when assessing the causes of contemporary genetic structuring in freshwater taxa.     

Do rivers influence fine-scale population genetic structure of tigers in the Sundarbans?

Global tiger Panthera tigris populations mostly survive within the geographically fragmented forest patches, thereby limited genetic exchange between isolated populations. Assessing the genetic status of these populations can reveal the effects of dispersal barriers and provide critical insights to guide future conservation actions. Using non-invasively collected biological samples, we investigated fine-scale genetic structure of tigers in the Sundarbans mangrove forests intersected by the complex river systems, and which holds one of the largest global tiger populations. We genotyped 52 tiger samples at 10 polymorphic microsatellite loci, and sequenced 33 of them for a total of 1263 base-pairs at four mitochondrial gene fragments. Microsatellite analyses exhibit a signature of fine-scale genetic structure, which might have been the consequence of limited tiger dispersal due to wide rivers across the Sundarbans. Similarly, mitochondrial data show a historic pattern of population isolation that might be due to wider rivers across the entire Sundarbans shared by Bangladesh and India. Given the intrinsic nature of the mangrove habitat embedded with numerous rivers, increased commercial traffic and human activities may further impede tiger dispersal across wide rivers, escalating further genetic isolation of the Sundarbans tigers.     

The influence of contemporary and historic landscape features on the genetic structure of the sand dune endemic,Cirsium pitcheri (Asteraceae)

Narrow endemics are at risk from climate change because of their restricted habitat preferences, lower colonization ability and dispersal distances. Landscape genetics combines new tools and analyses that allow us to test how both past and present landscape features have facilitated or hindered previous range expansion and local migration patterns, and thereby identifying potential limitations to future range shifts. We have compared current and historic habitat corridors in Cirsium pitcheri, an endemic of the linear dune ecosystem of the Great Lakes, to determine the relative contributions of contemporary migration and post-glacial range expansion on genetic structure. We used seven microsatellite loci to characterize the genetic structure for 24 populations of Cirsium pitcheri, spanning the center to periphery of the range. We tested genetic distance against different measures of geographic distance and landscape permeability, based on contemporary and historic landscape features. We found moderate genetic structure (Fst=0.14), and a north–south pattern to the distribution of genetic diversity and inbreeding, with northern populations having the highest diversity and lowest levels of inbreeding. High allelic diversity, small average pairwise distances and mixed genetic clusters identified in Structure suggest that populations in the center of the range represent the point of entry to the Lake Michigan and a refugium of diversity for this species. A strong association between genetic distances and lake-level changes suggests that historic lake fluctuations best explain the broad geographic patterns, and sandy habitat best explains local patterns of movement.     

Reconciling patterns of genetic variation with stream structure, earth history and biology in the Australian freshwater fish Craterocephalus stercusmuscarum (Atherinidae)

We examined the consequences of barriers, stream architecture and putative dispersal capability on levels of genetic differentiation among populations of the freshwater fish Craterocephalus stercusmuscarum. Seven polymorphic allozyme loci and sequences of a 498-bp fragment of the ATPase 6 mitochondrial DNA (mtDNA) gene were used to assess patterns of genetic variation among 16 populations from upland and lowland streams of five drainages in northern Queensland, Australia. Concordant patterns at both genetic markers revealed that there were significant levels of genetic subdivision among all populations, while an analysis of molecular variation showed that the distribution of genetic diversity was not consistent with contemporary drainage structure. There were reciprocally monophyletic mtDNA clades and fixed or large frequency differences at allozyme loci either side of instream barriers such as waterfalls. This implied barriers were effective in restricting gene flow between upland and lowland populations separated by waterfalls. However, there were two genetically distinct groups in upland areas, even within the same subcatchment, as well as high levels of genetic subdivision among lowland populations, suggesting barriers alone do not explain the patterns of genetic diversity. The data revealed a complex phylogeographic pattern, which we interpreted to be the result of one or more invasion events of independent lineages to different sections of each drainage, possibly mediated by well documented geomorphological changes. Our results highlight the importance of earth structure and history in shaping population genetic structure in stream organisms where dispersal capability may be limited, and reveal that the contemporary structure of drainages is not necessarily a good indicator of genetic relationships among populations.     

Predicting the effects of climate change on population connectivity and genetic diversity of an imperiled freshwater mussel,Cumberlandia monodonta (Bivalvia: Margaritiferidae), in riverine systems

In the face of global climate change, organisms may respond to temperature increases by shifting their ranges poleward or to higher altitudes. However, the direction of range shifts in riverine systems is less clear. Because rivers are dendritic networks, there is only one dispersal route from any given location to another. Thus, range shifts are only possible if branches are connected by suitable habitat, and stream‐dwelling organisms can disperse through these branches. We used Cumberlandia monodonta (Bivalvia: Unionoida: Margaritiferidae) as a model species to investigate the effects of climate change on population connectivity because a majority of contemporary populations are panmictic. We combined ecological niche models (ENMs) with population genetic simulations to investigate the effects of climate change on population connectivity and genetic diversity of C. monodonta. The ENMs were constructed using bioclimatic and landscape data to project shifts in suitable habitat under future climate scenarios. We then used forward‐time simulations to project potential changes in genetic diversity and population connectivity based on these range shifts. ENM results under current conditions indicated long stretches of highly suitable habitat in rivers where C. monodonta persists; populations in the upper Mississippi River remain connected by suitable habitat that does not impede gene flow. Future climate scenarios projected northward and headwater‐ward range contraction and drastic declines in habitat suitability for most extant populations throughout the Mississippi River Basin. Simulations indicated that climate change would greatly reduce genetic diversity and connectivity across populations. Results suggest that a single, large population of C. monodonta will become further fragmented into smaller populations, each of which will be isolated and begin to differentiate genetically. Because C. monodonta is a widely distributed species and purely aquatic, our results suggest that persistence and connectivity of stream‐dwelling organisms will be significantly altered in response to future climate change.     

Variation in fine‐scale genetic structure and local dispersal patterns between peripheral populations of a South American passerine bird

The distribution of suitable habitat influences natal and breeding dispersal at small spatial scales, resulting in strong microgeographic genetic structure. Although environmental variation can promote interpopulation differences in dispersal behavior and local spatial patterns, the effects of distinct ecological conditions on within‐species variation in dispersal strategies and in fine‐scale genetic structure remain poorly understood. We studied local dispersal and fine‐scale genetic structure in the thorn‐tailed rayadito (Aphrastura spinicauda), a South American bird that breeds along a wide latitudinal gradient. We combine capture‐mark‐recapture data from eight breeding seasons and molecular genetics to compare two peripheral populations with contrasting environments in Chile: Navarino Island, a continuous and low density habitat, and Fray Jorge National Park, a fragmented, densely populated and more stressful environment. Natal dispersal showed no sex bias in Navarino but was female‐biased in the more dense population in Fray Jorge. In the latter, male movements were restricted, and some birds seemed to skip breeding in their first year, suggesting habitat saturation. Breeding dispersal was limited in both populations, with males being more philopatric than females. Spatial genetic autocorrelation analyzes using 13 polymorphic microsatellite loci confirmed the observed dispersal patterns: a fine‐scale genetic structure was only detectable for males in Fray Jorge for distances up to 450 m. Furthermore, two‐dimensional autocorrelation analyzes and estimates of genetic relatedness indicated that related males tended to be spatially clustered in this population. Our study shows evidence for context‐dependent variation in natal dispersal and corresponding local genetic structure in peripheral populations of this bird. It seems likely that the costs of dispersal are higher in the fragmented and higher density environment in Fray Jorge, particularly for males. The observed differences in microgeographic genetic structure for rayaditos might reflect the genetic consequences of population‐specific responses to contrasting environmental pressures near the range limits of its distribution.     

Species delimitation in a range-restricted group of cascudinhos (Loricariidae: Epactionotus) supports morphological and genetic differentiation across coastal rivers of southern Brazil

Epactionotus species are known for inhabiting the rocky-bottom stretches of fast-flowing rivers in a limited geographic area along the Atlantic coast of southern Brazil. These species are endemic to single coastal river drainages (two neighbouring drainages for Epactionotus bilineatus) isolated from each other by the coastal lacustrine environments or the Atlantic Ocean. E. bilineatus is from the Maquiné and Três Forquilhas River basins, both tributaries of the Tramandaí River system, whereas E. itaimbezinho is endemic to the Mampituba River drainage and Epactionotus gracilis to the Araranguá River drainage. Recent fieldwork in the Atlantic coastal drainages of southern Brazil revealed new populations in the Urussanga, Tubarão, d'Una and Biguaçu River drainages. Iterative species delimitation using molecular data (cytochrome c oxidase subunit I) and morphology (morphometrics and meristics) was applied to evaluate species recognition of isolated populations. With regard to new data, the genus was re-diagnosed, the status of Epactionotus species/populations was re-evaluated, formerly described species were supported and population structure was recognized. As for the newly discovered populations, both morphological and molecular data strongly support the population from the Biguaçu River drainage, in Santa Catarina State, as a new species. Molecular data revealed strong per-basin population structure, which may be related to species habitat specificity and low or no dispersal among drainages.     

Small fish,large river: Surprisingly minimal genetic structure in a dispersal‐limited,habitat specialist fish

Genetic connectivity is expected to be lower in species with limited dispersal ability and a high degree of habitat specialization (intrinsic factors). Also, gene flow is predicted to be limited by habitat conditions such as physical barriers and geographic distance (extrinsic factors). We investigated the effects of distance, intervening pools, and rapids on gene flow in a species, the Tuxedo Darter (Etheostoma lemniscatum), a habitat specialist that is presumed to be dispersal‐limited. We predicted that the interplay between these intrinsic and extrinsic factors would limit dispersal and lead to genetic structure even at the small spatial scale of the species range (a 38.6 km river reach). The simple linear distribution of E. lemniscatum allowed for an ideal test of how these factors acted on gene flow and allowed us to test expectations (e.g., isolation‐by‐distance) of linearly distributed species. Using 20 microsatellites from 163 individuals collected from 18 habitat patches, we observed low levels of genetic structure that were related to geographic distance and rapids, though these factors were not barriers to gene flow. Pools separating habitat patches did not contribute to any observed genetic structure. Overall, E. lemniscatum maintains gene flow across its range and is comprised of a single population. Due to the linear distribution of the species, a stepping‐stone model of dispersal best explains the maintenance of gene flow across its small range. In general, our observation of higher‐than‐expected connectivity likely stems from an adaptation to disperse due to temporally unstable and patchy habitat.     

Linear habitats and the nested clade analysis: an empirical evaluation of geographic versus river distances using an Ozark crayfish (Decapoda: Cambaridae)

Abstract .The nested clade analysis can be extremely useful in testing for an association between genetic variation and geography and in explaining these observed patterns in terms of historical or contemporary population processes. The strength of this method lies in its ability to test a variety of processes simultaneously under a rigorous statistical framework. Indeed, many recent studies have used the nested analysis in a wide range of terrestrial and aquatic taxa. However, it has been suggested that riverine, riparian, or coastal species may be better examined using river (or coastal) distances rather than the standard geographic (great circle) distances among populations. It is thought that the standard geographic distances may not adequately describe the actual distances involved between populations of species inhabiting these one-dimensional (riverine) habitats. Therefore, we analyzed population data from an Ozark crayfish, Orconectes luteus , to examine the effects on the results of a nested clade analysis using river distances. In most cases, the haplotypes detected in this crayfish were unique to a particular drainage or a group of neighboring drainages, indicating very little movement of individuals among drainages. Five major population groups were detected, corresponding to many of the major river drainages sampled in this study. The two types of distance analyses obtain similar results for higher-level (older) clades, but differ in many of the inferences made for lower-level (younger) clades. However, we suggest that the comparison of both types of analyses for riverine species may enhance the process of elucidating historical and contemporary population processes, especially in cases where the transfer of individuals among different drainages are involved.     

Aquatic insects in a sea of desert: population genetic structure is shaped by limited dispersal in a naturally fragmented landscape

Habitat requirements and landscape features can exert strong influences on the population structure of organisms. For aquatic organisms in particular, hydrologic requirements can dictate the extent of available habitat, and thus the degree of genetic connectivity among populations. We used a landscape genetics approach to evaluate hypotheses regarding the influence of landscape features on connectivity among populations of the giant water bug Abedus herberti (Hemiptera: Belostomatidae). Abedus herberti is restricted to naturally‐fragmented, perennial stream habitats in arid regions of North America. This species is exceptional because it is flightless at all life stages. Thus, we hypothesized a high degree of population genetic structure in A. herberti due to hydrologic constraints on habitat and low dispersal ability of the organism. A total of 617 individuals were sampled from 20 populations across southeastern Arizona, USA and genotyped at 10 microsatellite loci. We used a Bayesian clustering method to delineate genetic groups among populations. To determine which of six landscape variables (representing hypotheses of landscape‐level connectivity) has the strongest association with genetic connectivity in A. herberti, we used information‐theoretic model selection. Strong population structure was evident among A. herberti populations, even at small spatial scales. At a larger scale, A. herberti populations were hierarchically structured across the study region, with groups of related populations generally occurring in the same mountain range, rather than in the same major watershed. Surprisingly, stream network connectivity was not important for explaining among‐population patterns. Only the Curvature landscape variable was identified as having an association with genetic connectivity in A. herberti. The Curvature variable hypothesizes that gene flow tends to occur where local topography is concave, such as within stream drainages and dry gullies. Thus, our results suggest that population connectivity may depend on the shape of local overland topography rather than direct connectivity within stream drainage networks.     

Fine-scale spatial genetic structure and gene flow in a small, fragmented population of Sinojackia rehderiana (Styracaceae), an endangered tree species endemic to China

Populations of Sinojackia rehderiana are highly threatened and have small and scattered distribution due to habitat fragmentation and human activities. Understanding changes in genetic diversity, the fine-scale spatial genetic structure (SGS) at different life stages and gene flow of S. rehderiana is critical for developing successful conservation strategies for fragmented populations of this endangered species. In this study, 208 adults, 114 juveniles and 136 seedlings in a 50 × 100-m transect within an old-growth forest were mapped and genotyped using eight microsatellite makers to investigate the genetic diversity and SGS of this species. No significant differences in genetic diversity among different life-history stages were found. However, a significant heterozygote deficiency in adults and seedlings may result from substantial biparental inbreeding. Significant fine-scale spatial structure was found in different life-history stages within 19 m, suggesting that seed dispersal mainly occurred near a mother tree. Both historical and contemporary estimates of gene flow (13.06 and 16.77 m) indicated short-distance gene dispersal in isolated populations of S. rehderiana. The consistent spatial structure revealed in different life stages is most likely the result of limited gene flow. Our results have important implications for conservation of extant populations of S. rehderiana. Measures for promoting pollen flow should be taken for in situ conservation. The presence of a SGS in fragmented populations implies that seeds for ex situ conservation should be collected from trees at least 19-m apart to reduce genetic similarity between neighbouring individuals.     

Habitat fragmentation influences genetic diversity and differentiation: Fine‐scale population structure of Cercis canadensis (eastern redbud)

Forest fragmentation may negatively affect plants through reduced genetic diversity and increased population structure due to habitat isolation, decreased population size, and disturbance of pollen‐seed dispersal mechanisms. However, in the case of tree species, effective pollen‐seed dispersal, mating system, and ecological dynamics may help the species overcome the negative effect of forest fragmentation. A fine‐scale population genetics study can shed light on the postfragmentation genetic diversity and structure of a species. Here, we present the genetic diversity and population structure of Cercis canadensis L. (eastern redbud) wild populations on a fine scale within fragmented areas centered around the borders of Georgia–Tennessee, USA. We hypothesized high genetic diversity among the collections of C. canadensis distributed across smaller geographical ranges. Fifteen microsatellite loci were used to genotype 172 individuals from 18 unmanaged and naturally occurring collection sites. Our results indicated presence of population structure, overall high genetic diversity (H_E = 0.63, H_O = 0.34), and moderate genetic differentiation (F_ST = 0.14) among the collection sites. Two major genetic clusters within the smaller geographical distribution were revealed by STRUCTURE. Our data suggest that native C. canadensis populations in the fragmented area around the Georgia–Tennessee border were able to maintain high levels of genetic diversity, despite the presence of considerable spatial genetic structure. As habitat isolation may negatively affect gene flow of outcrossing species across time, consequences of habitat fragmentation should be regularly monitored for this and other forest species. This study also has important implications for habitat management efforts and future breeding programs.