Low Levels Of Genetic Structure Research Articles

Genomic diversity of major tree species in the Eurasian relict forests of northern Iran

We investigated population genetics of the native tree species Acer velutinum Boiss., Fagus orientalis Lipsky, and Quercus castaneifolia C.A. Mey. in the Hyrcanian forests of northern Iran and also F. orientalis populations in the Euro-Siberian and Colchic subregions of northern Türkiye. We used the double-digest RADseq method and genotyped 90 populations and 1,589 individuals across the distribution range of the species. Genome-wide SNPs from 28 populations of A. velutinum, 32 populations of F. orientalis, and 30 Q. castaneifolia revealed higher genetic differentiation among A. velutinum populations than among F. orientalis and Q. castaneifolia. The global FST value was lowest for F. orientalis populations and highest for A. velutinum populations, while the global FIS value was negative for A. velutinum. Demographic history analysis revealed a bottleneck during the last glacial period (11,500—115,000 Kya) for the A. velutinum populations with reduced effective population size (Ne). All three species show multiple bottlenecks and reduced Ne during the Quaternary. Pronounced genetic divergence among A. velutinum populations in the Hyrcanian forests compared to the other two species suggests cryptic speciation. Conversely, F. orientalis and Q. castaneifolia populations showed low levels of genetic structure, suggesting that species-specific factors, such as pollen production and pollination efficiency, may have influenced the genetic patterns within these species in similar environments. The nucleotide diversity of F. orientalis populations in Iran is negatively correlated with altitude (p = 0.046). In contrast, A. velutinum populations show a significant correlation between nucleotide diversity and longitude (p = 0.008). Furthermore, the F. orientalis populations from Türkiye showed a distinct west–east genetic structure and were highly diverged from the Iranian F. orientalis populations.

Low population genetic structure is consistent with high habitat connectivity in a commercially important fish species (Lutjanus jocu)

The level of habitat availability influences genetic divergence among populations and the genetic diversity within populations. In the marine environment, near-shore species are among the most sensitive to habitat changes. Knowledge of how historical environmental change affected habitat availability and genetic variation can be applied to the development of proactive management strategies of exploited species. Here, we modeled the contemporary and historical distribution of Lutjanus jocu in Brazil. We describe patterns of genomic diversity to better understand how climatic cycles might correlate with the species demographic history and current genetic structure. We show that during the Last Glacial Maximum, there were ecological barriers that are absent today, possibly dividing the range of the species into three geographically separated areas of suitable habitat. Consistent with a historical reduction in habitat area, our analysis of demographic changes shows that L. jocu experienced a severe bottleneck followed by a population size expansion. We also found an absence of genetic structure and similar levels of genetic diversity throughout the sampled range of the species. Collectively, our results suggest that habitat availability changes have not obviously influenced contemporary levels of genetic divergence between populations. However, our demographic analyses suggest that the high sensitivity of this species to environmental change should be taken into consideration for management strategies. Furthermore, the general low levels of genetic structure and inference of high gene flow suggest that L. jocu likely constitutes a single stock in Brazilian waters and, therefore, requires coordinated legislation and management across its distribution.

Genetic structure of the threatened Gray Parrotfish (Sparisoma axillare) in the Southwestern Atlantic

Despite the marine environment being typified by a lack of obvious barriers to dispersal, levels of genetic divergence can arise in marine organisms from historical changes in habitat availability, current oceanographic regimes and anthropogenic factors. Here we describe the genetic structure of the Gray Parrotfish, Sparisoma axillare, and identify environmental variables associated with patterns of genetic divergence throughout most of its distribution in Brazil. The heavily exploited Gray Parrotfish is endemic to Brazil, and there is lack of data on population structure that is needed to support sustainable management. To address this shortfall we analyzed 5429 SNPs from individuals sampled in nine locations, ranging from tropical to subtropical reef systems and costal to oceanic environments with varying levels of protection. We found low levels of genetic structure along the coast, including the oceanic island of Fernando de Noronha, and that a combination of water depth, ocean currents and geographic distance were the major drivers explaining genetic divergence. We identified a distinct genetic population around Trindade Island, 1000 km from the coast, highlighting the conservation significance of this population. Colonization of this oceanic site probably occurred during the Pleistocene periods of lower sea levels, allowing this shallow water-dependent species to use the seamount chain as stepping stones to Trindade. Our data further suggest that two protected areas, Costa dos Corais and Fernando de Noronha, likely play an important role as larval sources for much of the species distribution.

Population and conservation genetics using RAD sequencing in four endemic conifers from South America

The emblematic conifer flora of Chile has experienced wide-scale habitat loss and fragmentation. Coupled with the complex topography of the region, this leads to a prediction of high levels of genetic drift and isolation resulting in strong population differentiation, and the potential for negative genetic consequences. To address the degree to which these predictions are realised, we conducted a comparative population genetic survey in four endemic conifers from South America, each of conservation concern and with a restricted distribution area. Between seven and ten populations per species were sampled, covering their entire natural distribution in Chile. We used restriction site-associated DNA markers (RAD-seq), with de novo assembly and optimisation, to accommodate the large and complex genome of conifers. The main finding was low levels of genetic structure in all four conifer species (FST = 0.017–0.062). This pattern was strikingly consistent across all four species. These results are likely due to the extreme longevity of individuals restricting divergence due to genetic drift. Thus, despite the currently highly fragmented ranges of these conifer species, we did not detect evidence for high levels of population differentiation and genetic isolation. This suggests some resilience to negative genetic impacts of habitat fragmentation, and the longevity of the individual trees may provide considerable buffering capacity. We recommend follow-up studies focusing on genotyping the seedling generation, to assess whether there is any evidence for the early warning of genetic isolation and/or elevated inbreeding in the currently established cohorts.

Genetic assessment reveals inbreeding, possible hybridization, and low levels of genetic structure in a declining goose population.

The population numbers of taiga bean goose (Anser fabalis fabalis) have halved during recent decades. Since this subspecies is hunted throughout most of its range, the decline is of management concern. Knowledge of the genetic population structure and diversity is important for guiding management and conservation efforts. Genetically unique subpopulations might be hunted to extinction if not managed separately, and any inbreeding depression or lack of genetic diversity may affect the ability to adapt to changing environments and increase extinction risk. We used microsatellite and mitochondrial DNA markers to study the genetic population structure and diversity among taiga bean geese breeding within the Central flyway management unit using non‐invasively collected feathers. We found some genetic structuring with the maternally inherited mitochondrial DNA between four geographic regions (ɸ ST = 0.11–0.20) but none with the nuclear microsatellite markers (all pairwise F ST‐values = 0.002–0.005). These results could be explained by female natal philopatry and male‐biased dispersal, which completely homogenizes the nuclear genome. Therefore, the population could be managed as a single unit. Genetic diversity was still at a moderate level (average H E = 0.69) and there were no signs of past population size reductions, although significantly positive inbreeding coefficients in all sampling sites (F IS = 0.05–0.10) and high relatedness values (r = 0.60–0.86) between some individuals could indicate inbreeding. In addition, there was evidence of either incomplete lineage sorting or introgression from the pink‐footed goose (Anser brachyrhynchus). The current population is not under threat by genetic impoverishment but monitoring in the future is desirable.

Genetic diversity and functional connectivity of a critically endangered salamander

AbstractHabitat loss and fragmentation are among the greatest threats to biodiversity since unsuitable habitat can be a significant barrier to gene flow, disrupting the connectivity of populations. In this scenario, landscape genetic approaches have been applied as invaluable tools to demonstrate how contemporary habitat alterations affect gene flow. In this study, we used nine microsatellite genotypes to determine the landscape features driving genetic diversity, structure, and functional connectivity among eleven sampling sites of a micro‐endemic, and critically endangered salamander (Pseudoeurycea robertsi) in the Nevado de Toluca Volcano. We also modelled the ecological niche of P. robertsi to assess the most important habitat features associated with its probability of presence and analyzed the corridor network to identify important core areas and corridors to keep the network connected, as well as points where the network connection is lost or tightened. Moreover, we performed a redundancy analysis to assess the effect of genetic distance and habitat resistances on gene flow. We found the highest levels of genetic diversity and connectivity at the northwestern part of the Volcano. Meson Viejo sampling site was of especial importance to overall network connectivity. We found low levels of migration rates and low levels of genetic structure. The most important habitat feature associated with probability of presence of Pseudoeurycea robertsi was the Abies forest, and two models were significant under the RDA analysis: Abies forest and Abies forest+Grasslands. We recommend prioritizing the areas of the northwestern and central parts of the volcano in conservation programs.Abstract in Spanish is available with online material.

Restricted dispersal in a sea of gene flow.

How far do marine larvae disperse in the ocean? Decades of population genetic studies have revealed generally low levels of genetic structure at large spatial scales (hundreds of kilometres). Yet this result, typically based on discrete sampling designs, does not necessarily imply extensive dispersal. Here, we adopt a continuous sampling strategy along 950 km of coast in the northwestern Mediterranean Sea to address this question in four species. In line with expectations, we observe weak genetic structure at a large spatial scale. Nevertheless, our continuous sampling strategy uncovers a pattern of isolation by distance at small spatial scales (few tens of kilometres) in two species. Individual-based simulations indicate that this signal is an expected signature of restricted dispersal. At the other extreme of the connectivity spectrum, two pairs of individuals that are closely related genetically were found more than 290 km apart, indicating long-distance dispersal. Such a combination of restricted dispersal with rare long-distance dispersal events is supported by a high-resolution biophysical model of larval dispersal in the study area, and we posit that it may be common in marine species. Our results bridge population genetic studies with direct dispersal studies and have implications for the design of marine reserve networks.

Revealing the biodiversity of Chilean birds through the COI barcode approach.

The mitochondrial cytochrome c oxidase subunit I (COI) gene is an effective molecular tool for the estimation of genetic variation and the identification of bird species. This molecular marker is used to differentiate among Chilean bird species by analyzing barcodes for 76 species (197 individuals), comprising 28 species with no previous barcode data and 48 species with sequences retrieved from the BOLD and GenBank databases. The DNA barcodes correctly identified 94.7% of the species analyzed (72 of 76 species). Mean intraspecific K2P distance was 0.3% (range 0–8.7%). Within the intraspecific divergence range, three species, Phrygilus gayi, Sephanoides sephanoides and Curaeus curaeus, showed relatively high intraspecific divergence (1.5–8.7%), possibly due to the presence of a species complex or geographic isolation of sub-populations. Mean interspecific K2P distance was 24.7% (range 1.3–43.5%). Consequently, the intraspecific K2P distance showed limited overlap with interspecific K2P distance. The mean intraspecific divergence in our study was similar to that found in temperate regions of South America (0.24%). However, it was approximately one order of magnitude lower than values reported for bird species in tropical regions of northern South America (1.8–2.13%). This result suggests that bird species from Chile show low levels of genetic structure and divergence. The small overlap between intra- and inter-specific distances implies that COI barcodes could be used as an effective tool to identify nearly all the Chilean bird species analyzed.

Genetic analyses reveal complex introduction histories for the invasive tree Acacia dealbata Link around the world

AbstractAimTo compare genetic diversity and structure between Acacia dealbata populations sampled across the species’ native range in Australia and from its non‐native ranges in Chile, Madagascar, New Zealand, Portugal, La Réunion island, South Africa and the United States, and to investigate the most likely introduction scenarios to non‐native ranges.LocationGlobal.TaxonAcacia dealbata, Fabaceae.MethodsOur dataset comprised 1615 samples representing 92 populations sampled in the species’ native and non‐native ranges. We employed a combination of genetic fingerprinting (microsatellite markers) and genetic modelling approaches. We calculated genetic diversity for each population and tested for genetic isolation by distance within each range. A combination of Bayesian assignment tests and multivariate ordination was applied to identify genetic structure among populations. Approximate Bayesian Computation (ABC) analyses were conducted to test different competing introduction scenarios for each non‐native range.ResultsThe majority of the species’ non‐native ranges was characterized by high genetic diversity and low levels of genetic structure. With regard to introduction histories, however, our results supported different introduction scenarios for different non‐native ranges. We did not find strong support for any of tested introduction scenarios for populations in Chile and Madagascar, but these likely originated from multiple introductions followed by admixture. Populations in New Zealand and La Réunion most likely originated directly from Tasmania, possibly through multiple introductions. Similar to previous findings for South African populations, no clear introduction history could be identified for populations in Portugal and the United States.Main conclusionsOur study shows that global introductions of A. dealbata were complex and one scenario does not fit the invasion history of the species in different regions. We discuss how this complexity needs to be considered when formulating strategies for the effective management of the species. Future research needs to help bridge persisting knowledge gaps are discussed.

New genomic resources for three exploited Mediterranean fishes

Extensive fishing has led to fish stock declines throughout the last decades. While clear stock identification is required for designing management schemes, stock delineation is problematic due to generally low levels of genetic structure in marine species. The development of genomic resources can help to solve this issue. Here, we present the first mitochondrial and nuclear draft genome assemblies of three economically important Mediterranean fishes, the white seabream, the striped red mullet, and the comber. The assemblies are between 613 and 785 Mbp long and contain between 27,222 and 32,375 predicted genes. They were used as references to map Restriction-site Associated DNA markers, which were developed with a single-digest approach. This approach provided between 15,710 and 21,101 Single Nucleotide Polymorphism markers per species. These genomic resources will allow uncovering subtle genetic structure, identifying stocks, assigning catches to populations and assessing connectivity. Furthermore, the annotated genomes will help to characterize adaptive divergence.

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.

Genetic diversity and structure of Hypochaeris catharinensis Cabrera (Asteraceae), an endemic species of Southern Brazil

The genus Hypochaeris was likely introduced into South America by a unique long-distance dispersal event from an ancestral Northwest African species, with subsequent radiation and diversification throughout the temperate zones of the continent. Hypochaeris catharinensis is an endemic species of Southern Brazil growing mostly in high-elevation grasslands with dry, rocky, and shallow soils. Here, we used nine microsatellite loci and a total sample of 441 individuals to assess genetic variability patterns within and among 13 H. catharinensis populations. The results showed a total of 215 alleles in the nine loci tested, the expected and observed heterozygosities average were 0.31 and 0.43, respectively, and allelic richness varied from 4.09 to 7.23. The levels of allelic richness showed that genetic variability is maintained by gene flow, thus ensuring that these populations are not genetically isolated. We found no correlation between geographic and genetic distances, with high levels of genetic variability within, rather than among, populations. The low levels of genetic structure in the species are consistent with recent evolutionary processes of the Hypochaeris genus in South America; after adaptive radiation and species differentiation, dispersal to similar habitats led to the establishment of new populations. Along with initial founder effects, the existence of gene flow explains the current genetic variation across the H. catharinensis populations. In addition, historic environmental changes reported for Southern Brazil may explain the current clustered distribution of the species.

Study of genetic diversity of Iranian indigenous buffalo populations using microsatellite markers

Indigenous buffalo breeds represent a unique genetic resource, and understanding their variability, population structure and breeding units is important for their sustainable conservation. In the present study the genetic structure of Iranian buffalo populations was analyzed using ten microsatellite markers. Two hundred hair samples were collected and DNA was extracted using modified salting out method. After Polymerase Chain Reaction (PCR), the PCR products were electrophoresed using 9% polyacrylamide gel. Fifty- nine alleles were observed for all the loci. The average number of alleles was 5.90 and the effective average number of alleles was 4.86. The high level of mean heterozygosity index between three populations indicate that the genetic diversity is high in within and between populations. The mean of polymorphism information content (PIC) value for all loci was 0.70. The FST value for the total loci was 0.01, indicating a very low level of genetic structure among populations. The genetic structure AMOVA analysis showed that about 3% of the total genetic variation was explained by population differences and 97 percent was corresponded to differences among individuals. The obtained results at the present study indicated that characterization of genetic diversity by employing molecular tools is a prerequisite in developing strategies for conservation and utilization of buffalo genetic resources.

Isolation on a remote island: genetic and morphological differentiation of a cosmopolitan odonate.

Although low levels of genetic structure are expected in highly widespread species, geographical and/or ecological factors can limit species distributions and promote population structure and morphological differentiation. In order to determine the effects of geographical isolation on population genetic structure and wing morphology, 281 individuals of the cosmopolitan odonate Pantala flavescens were collected from four continental (Central and South America) and five insular sites (Polynesian islands and the Maldives). COI sequences and eight microsatellite loci were used to characterize genetic diversity and genetic structure between and within locations. Linear and geometric morphometry were used to evaluate differences in the size and shape of wings. Genetic analysis showed a global genetic difference between the continental and insular sites. American locations did not show genetic structure, even in locations separated by a distance of 5000 km. Easter Island showed the lowest values of genetic diversity (mainly mitochondrial diversity) and the highest values of genetic differences compared to other insular and continental sites. Individuals from Easter Island showed smaller forewings, a different abdomen length to thorax length ratio, and a different configuration of anal loop in the hindwings. Thus, the greater isolation, smaller area, and young geological age seem to have determined the genetic and morphological differences in P. flavescens of Easter Island, where selection could promote a loss of migratory behavior and may improve other life history traits, such as reproduction. This work provides new insight into how microevolutionary processes operate in isolated populations of cosmopolitan species.

Multi‐gene phylogeny of the Hemileuca maia complex (Saturniidae) across North America suggests complex phylogeography and rapid ecological diversification

AbstractThe Hemileuca maia species complex occurs across the North American continent and consists of six named taxa, and several others that were recently synonymized. Taxa exhibit a wide span of adult flight periods, dramatic shifts in host‐plant use and occur in a range of habitats, all of which would suggest unrecognized diversity. We used one mitochondrial and three nuclear genes to generate 3900 bp per individual, including samples from every ecotype in the species group across the United States from New England to central Florida to California. We assessed phylogenetic relationships using both maximum likelihood and Bayesian phylogenetic methods. Results suggest very low levels of divergence across most of the continent and low levels of genetic structure – even between some recognized species that maintain clear ecological difference in sympatry. Our results suggest that meaningful and localized ecological divergence may occur in the absence of easily recognizable genetic divergence, due to either ongoing gene flow or the recent diversification in the group.

Genetic characterization of Brycon hilarii (Characiformes) populations within the Pantanal: Aspects of their conservation within a globally important neotropical wetland

Brycon hilarii, a characid species endemic to the Upper Paraguay hydrographic basin, is important to regional artisanal and sports fisheries. To develop effective strategies for conservation of this species in the face of potential environmental changes in the Pantanal region, we characterized genetic structuring within and among six B. hilarii collections based on variation at five microsatellite DNA markers. Within-population genetic variability was high, with 75 different alleles; mean average allelic richness per locus per sample location ranged from 6.06 to 7.99. Nei’s gene diversity (hs) varied among drainages from 0.66 (±0.2) to 0.69 (±0.2), with an average across the four genetically identified populations of 0.68 (±0.02). Analyses of Jost’s D EST and F ST-like indices, AMOVA, and Structure-based clustering analyses indicated that B. hilarii populations exhibit a low level of genetic structure, with some indications that the Taquari River population is somewhat distinct from others. Results of K-means analysis suggested little or no structuring, with weakly differentiated populations above and below the confluence of the Paraguay and Taquari rivers. Because B. hilarii populations in the Pantanal are linked by high levels of gene flow, habitat alterations that would interfere with gene flow may jeopardize the long-term persistance of the species.

High levels of male diploidy but low levels of genetic structure characterize Bombus vosnesenskii populations across the Western US

Patterns of genetic structure for some bee species suggest that gene flow may be limited across natural and human-created barriers and that local dispersal or natal site fidelity may be common. Interestingly, this past work has primarily focused on female bees, despite the fact that males may differ substantially in their dispersal processes. By examining genetic structure and diploidy in males, it is possible to gain insight into potential barriers to gene flow and drivers of inbreeding. In this study, we examine diploidy as well as regional and local spatial genetic structure using males of Bombus vosnesenskii, a stable bumble bee species found across western North America. Specifically, we investigate patterns of genetic structure in both island and mainland populations, across spatial scales, and over a range of natural and human-altered habitats. We document high levels of male diploidy, with significantly higher levels in mainland populations compared to island populations and increasing diploidy in areas with poor nesting habitat. Interestingly, we also find evidence of significant spatial genetic structure from 0 to 10 km and 0 to 5 km on island and mainland populations, respectively. Finally, we document low but significant genetic differentiation across the region (ΦST = 0.049). Overall, this work reveals the unique potential for biogeographic context and local habitat composition to drive male diploidy patterns in bumble bees.

Colony Foundation in an Oceanic Seabird

Seabirds are colonial vertebrates that despite their great potential for long-range dispersal and colonization are reluctant to establish in novel locations, often recruiting close to their natal colony. The foundation of colonies is therefore a rare event in most seabird species and little is known about the colonization process in this group. The Cory’s shearwater (Calonectris diomedea) is a pelagic seabird that has recently established three new colonies in Galicia (NE Atlantic) thus expanding its distribution range 500 km northwards. This study aimed to describe the establishment and early progress of the new Galician populations and to determine the genetic and morphometric characteristics of the individuals participating in these foundation events. Using 10 microsatellite loci, we tested the predictions supported by different seabird colonization models. Possibly three groups of non-breeders, adding up to around 200 birds, started visiting the Galician colonies in the mid 2000’s and some of them eventually laid eggs and reproduced, thus establishing new breeding colonies. The Galician populations showed a high genetic diversity and a frequency of private alleles similar to or even higher than some of the large historical populations. Most individuals were assigned to several Atlantic populations and a few (if any) to Mediterranean colonies. Our study suggests that a large and admixed population is settling in Galicia, in agreement with predictions from island metapopulation models of colonization. Multiple source colonies imply that some birds colonizing Galicia were dispersing from very distant colonies (> 1500 km). Long-distance colonizations undertaken by relatively large and admixed groups of colonizers can help to explain the low levels of genetic structure over vast areas that are characteristic of most oceanic seabird species.

A comparison of genetic structure in two low-dispersal crabs from the Wild Coast, South Africa

The Wild Coast in south-eastern South Africa is strongly influenced by the warm, southward-flowing Agulhas Current. This current has a significant impact on dispersal in the coastal biota of the region, and facilitates high levels of connectivity among populations. However, it is not known how the region's high-velocity hydrology affects genetic population structure in endemic estuarine species, populations of which are frequently isolated from the sea. Here, we compared genetic structure in two estuarine crabs of the family Hymenosomatidae. Both are presumed to have low dispersal potential, but they differ in terms of their life histories. Hymenosoma longicrure has abbreviated larval development and can complete its entire life cycle within estuaries, whereas Neorhynchoplax bovis is a direct developer that lacks planktonic larvae. Using DNA sequence data from the mitochondrial COI gene and the intron of the nuclear ANT gene, we found that levels of genetic structure differ considerably between the species. Depending on the genetic marker used, H. longicrure is genetically homogeneous (COI) or displays low levels of genetic structure and minor evidence of recruitment near natal sites (ANT). In contrast, connectivity in N. bovis is much lower, as this species has a unique combination of alleles at each site, indicating that recruitment is mostly local. These results support previous findings suggesting that even a short larval dispersal phase is sufficient to maintain high levels of connectivity and prevent genetic divergence among populations.

Life-stage differences in spatial genetic structure in an irruptive forest insect: implications for dispersal and spatial synchrony.

Dispersal determines the flux of individuals, energy and information and is therefore a key determinant of ecological and evolutionary dynamics. Yet, it remains difficult to quantify its importance relative to other factors. This is particularly true in cyclic populations in which demography, drift and dispersal contribute to spatio-temporal variability in genetic structure. Improved understanding of how dispersal influences spatial genetic structure is needed to disentangle the multiple processes that give rise to spatial synchrony in irruptive species. In this study, we examined spatial genetic structure in an economically important irruptive forest insect, the spruce budworm (Choristoneura fumiferana) to better characterize how dispersal, demography and ecological context interact to influence spatial synchrony in a localized outbreak. We characterized spatial variation in microsatellite allele frequencies using 231 individuals and seven geographic locations. We show that (i) gene flow among populations is likely very high (Fst ≈0); (ii) despite an overall low level of genetic structure, important differences exist between adult (moth) and juvenile (larvae) life stages; and (iii) the localized outbreak is the likely source of moths captured elsewhere in our study area. This study demonstrates the potential of using molecular methods to distinguish residents from migrants and for understanding how dispersal contributes to spatial synchronization. In irruptive populations, the strength of genetic structure depends on the timing of data collection (e.g. trough vs. peak), location and dispersal. Taking into account this ecological context allows us to make more general characterizations of how dispersal can affect spatial synchrony in irruptive populations.