Microgeographic Genetic Structure Research Articles

Heterogeneous microgeographic genetic structure of the common cockle (Cerastoderma edule) in the Northeast Atlantic Ocean: biogeographic barriers and environmental factors.

Knowledge of genetic structure at the finest level is essential for the conservation of genetic resources. Despite no visible barriers limiting gene flow, significant genetic structure has been shown in marine species. The common cockle (Cerastoderma edule) is a bivalve of great commercial and ecological value inhabiting the Northeast Atlantic Ocean. Previous population genomics studies demonstrated significant structure both across the Northeast Atlantic, but also within small geographic areas, highlighting the need to investigate fine-scale structuring. Here, we analysed two geographic areas that could represent opposite models of structure for the species: (1) the SW British Isles region, highly fragmented due to biogeographic barriers, and (2) Galicia (NW Spain), a putative homogeneous region. A total of 9250 SNPs genotyped by 2b-RAD on 599 individuals from 22 natural beds were used for the analysis. The entire SNP dataset mostly confirmed previous observations related to genetic diversity and differentiation; however, neutral and divergent SNP outlier datasets enabled disentangling physical barriers from abiotic environmental factors structuring both regions. While Galicia showed a homogeneous structure, the SW British Isles region was split into four reliable genetic regions related to oceanographic features and abiotic factors, such as sea surface salinity and temperature. The information gathered supports specific management policies of cockle resources in SW British and Galician regions also considering their particular socio-economic characteristics; further, these new data will be added to those recently reported in the Northeast Atlantic to define sustainable management actions across the whole distribution range of the species.

Inferring landscape factors driving microgeographic genetic structure of large-sized mountain ungulates: A case of Alashan red deer (Cervus elaphus alxaicus)

Gene flow is crucial for maintaining population genetic diversity and increasing the adaptive potential of species. Therefore, identifying factors affecting gene flow is important for conservation planning, especially for threatened mammals in complex mountain system. In this study, we conducted a microgeographic-scale landscape genetic analysis for the endangered and ecologically isolated Alashan red deer (Cervus elaphus alxaicus) for the first time. Using data from 329 individuals for 11 microsatellite loci, we assessed how the environmental factors in the Chinese Helan Mountains affect the gene flow of this species. Three genetic clusters were identified by both Bayesian (STRUCTURE and GENELAND) and non-Bayesian (DAPC) methods, revealing clear spatial genetic structure within a limited geographic range. Landscape genetic analysis based on causal modelling and linear mixed-effect modelling revealed that gene flow was primarily restricted by topographic factors, including topographic complexity, aspect, altitude and slope, rather than geographical distance or anthropogenic barriers. Based on these results, we identified two important corridors that can potentially facilitate gene flow within the complex landscape of the Helan Mountains. Our results highlight the role of natural landscape features on Alashan red deer's population genetic structure and functional connectivity. This differs from findings regarding red deer populations in other regions, indicating that landscape genetic patterns are not homogenous across the range of a species, but rather differ among specific landscapes.

Pooled whole genome sequencing of the endangered Banff Springs Snail, Physella johnsoni, reveals genetic separation to P. gyrina and cryptic micro-geographical genetic structure

Pooled whole genome sequencing of the endangered Banff Springs Snail, Physella johnsoni, reveals genetic separation to P. gyrina and cryptic micro-geographical genetic structure

The influence of anthropogenic habitat fragmentation on the genetic structure and diversity of the malaria vector Anopheles cruzii (Diptera: Culicidae)

Fragmentation of natural environments as a result of human interference has been associated with a decrease in species richness and increase in abundance of a few species that have adapted to these environments. The Brazilian Atlantic Forest, which has been undergoing an intense process of fragmentation and deforestation caused by human-made changes to the environment, is an important hotspot for malaria transmission. The main vector of simian and human malaria in this biome is the mosquito Anopheles cruzii. Anthropogenic processes reduce the availability of natural resources at the tree canopies, An. cruzii primary habitat. As a consequence, An. cruzii moves to the border of the Atlantic Forest nearing urban areas seeking resources, increasing their contact with humans in the process. We hypothesized that different levels of anthropogenic changes to the environment can be an important factor in driving the genetic structure and diversity in An. cruzii populations. Five different hypotheses using a cross-sectional and a longitudinal design were tested to assess genetic structure in sympatric An. cruzii populations and microevolutionary processes driving these populations. Single nucleotide polymorphisms were used to assess microgeographic genetic structure in An. cruzii populations in a low-endemicity area in the city of São Paulo, Brazil. Our results show an overall weak genetic structure among the populations, indicating a high gene flow system. However, our results also pointed to the presence of significant genetic structure between sympatric An. cruzii populations collected at ground and tree-canopy habitats in the urban environment and higher genetic variation in the ground-level population. This indicates that anthropogenic modifications leading to habitat fragmentation and a higher genetic diversity and structure in ground-level populations could be driving the behavior of An. cruzii, ultimately increasing its contact with humans. Understanding how anthropogenic changes in natural areas affect An. cruzii is essential for the development of more effective mosquito control strategies and, on a broader scale, for malaria-elimination efforts in the Brazilian Atlantic Forest.

Minimal genetic differentiation of the malaria vector Nyssorhynchus darlingi associated with forest cover level in Amazonian Brazil.

The relationship between deforestation and malaria in Amazonian Brazil is complex, and a deeper understanding of this relationship is required to inform effective control measures in this region. Here, we are particularly interested in characterizing the impact of land use and land cover change on the genetics of the major regional vector of malaria, Nyssorhynchus darlingi (Root). We used nextera-tagmented, Reductively Amplified DNA (nextRAD) genotyping-by-sequencing to genotype 164 Ny. darlingi collected from 16 collection sites with divergent forest cover levels in seven municipalities in four municipality groups that span the state of Amazonas in northwestern Amazonian Brazil: São Gabriel da Cachoeira, Presidente Figueiredo, four municipalities in the area around Cruzeiro do Sul, and Lábrea. Using a dataset of 5,561 Single Nucleotide Polymorphisms (SNPs), we investigated the genetic structure of these Ny. darlingi populations with a combination of model- and non-model-based analyses. We identified weak to moderate genetic differentiation among the four municipality groups. There was no evidence for microgeographic genetic structure of Ny. darlingi among forest cover levels within the municipality groups, indicating that there may be gene flow across areas of these municipalities with different degrees of deforestation. Additionally, we conducted an environmental association analysis using two outlier detection methods to determine whether individual SNPs were associated with forest cover level without affecting overall population genetic structure. We identified 14 outlier SNPs, and investigated functions associated with their proximal genes, which could be further characterized in future studies.

Retention of ancestral polymorphism in Culex nigripalpus (Diptera: Culicidae) from São Paulo, Brazil

Culex nigripalpus Theobald (Diptera: Culicidae) is a native species of Brazil that is well adapted to urban environments and found extensively in the city of São Paulo, Brazil. As a native species, it has been present in this region since long before the foundation of the city, but over time Cx. nigripalpus populations have been affected by man-made changes to the environment. We hypothesize that the populations analyzed in this study constituted a large Cx. nigripalpus population that separated into smaller populations as a result of increased levels of urbanization in the city, and that such high levels of urbanization would result in a genetic homogenization effect. We therefore investigated the microgeographic genetic structure and microevolutionary processes in Cx. nigripalpus populations from seven different locations in the city of São Paulo using a set of six microsatellite primers originally developed for Culex quinquefasciatus and Culex pipiens. Our results indicate that Cx. nigripalpus did not benefit from urbanization and is currently under selective pressures caused by anthropogenic changes and that populations from areas with higher levels of urbanization exhibited similar genetic patterns and low levels of polymorphism, contrasting with the more sylvatic SHA population. These findings are likely to contribute to a better understanding of how anthropogenic selective pressures are driving population genetics and, to some extent, the dynamics of Cx. nigripalpus populations. They should also help elucidate the effects that urbanization processes have on the ecology and behavior of these mosquito populations.

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.

Influence of landscape features on the microgeographic genetic structure of a resident songbird.

Landscape features influence individual dispersal and as a result can affect both gene flow and genetic variation within and between populations. The landscape of British Columbia, Canada, is already highly heterogeneous because of natural ecological and geological transitions, but disturbance from human-mediated processes has further fragmented continuous habitat, particularly in the central plateau region. In this study, we evaluated the effects of landscape heterogeneity on the genetic structure of a common resident songbird, the black-capped chickadee (Poecile atricapillus). Previous work revealed significant population structuring in British Columbia that could not be explained by physical barriers, so our aim was to assess the pattern of genetic structure at a microgeographic scale and determine the effect of different landscape features on genetic differentiation. A total of 399 individuals from 15 populations were genotyped for fourteen microsatellite loci revealing significant population structuring in this species. Individual- and population-based analyses revealed as many as nine genetic clusters with isolation in the north, the central plateau and the south. Moreover, a mixed modelling approach that accounted for non-independence of pairwise distance values revealed a significant effect of land cover and elevation resistance on genetic differentiation. These results suggest that barriers in the landscape influence dispersal which has led to the unexpectedly high levels of population isolation. Our study demonstrates the importance of incorporating landscape features when interpreting patterns of population differentiation. Despite taking a microgeographic approach, our results have opened up additional questions concerning the processes influencing dispersal and gene flow at the local scale.

Local habitat condition rather than geographic distance determines the genetic structure of Tamarix chinensis populations in Yellow River Delta, China

Understanding the extent of genetic diversity in natural populations and the correlations between population structure and environmental heterogeneity is of great importance to estimation of the potential for species to undergo rapid adaptive changes in response to environmental variation. In this study, we systematically sampled 800 individuals from 26 Tamarix chinensis populations distributed throughout the Yellow River Delta (YRD) region in China, where a mosaic of habitat patches varying in soil salinity was detected. By exploring the micro-geographic genetic structure using the co-dominant microsatellite (SSR) markers, we aimed to answer questions as to what extent the populations diverged and how habitat heterogeneity affected the local population structure and dynamics of T. chinensis in YRD. The results demonstrated a moderate high level of genetic diversity and a low level of genetic differentiation in T. chinensis populations. The genetic variation was mainly maintained within populations, with a weak but significant genetic differentiation being detected among populations (Fst = 0.053, P < 0.001). While the overall genetic diversity within populations decreased progressively along with the increasing soil salinity, the locus T1D12 exhibited an elevated variation among populations, with the allele 558 (T1D12) showing a significantly higher frequency in highly saline habitats than in other sites, suggesting that the ecological differences among patchy habitats led to weak but significant adaptive divergence among populations. Local habitat conditions rather than geographic distances determine the genetic structure of T. chinensis populations in YRD.

Temporal fine‐scale genetic variation in the zoonosis‐carrying long‐tailed pygmy rice rat in Patagonia, Argentina

AbstractFactors modeling the population genetic structure are of major importance when species involved in the transmission of zoonoses are considered. The long‐tailed pygmy rice rat or ‘colilargo’ Oligoryzomys longicaudatus is a highly vagile rodent species which acts as the reservoir of the Andes hantavirus in southern Argentina and Chile. To contribute to the knowledge of the processes determining the microgeographical genetic structure of populations of this species, we used 10 microsatellite loci to estimate the levels of polymorphism, individual relatedness and the population effective sizes through time and to explore if the effective size correlated with density. Individuals were sampled seasonally during a 25‐month period in a population from the Argentinean Patagonia located in the endemic area of Hantavirus Pulmonary Syndrome. We detected three genetic clusters. An important temporal change in cluster prevalence was detected after a population bottleneck. Individuals of the same period presented higher mean relatedness values than between consecutive periods. Density and effective population size estimations were correlated. All analyses performed in this study are in line with the conclusion that high levels of gene flow encompassing different habitats would be a major process producing fine‐scale temporal changes in the genetic composition of the studied population.

Persistence in the desert: ephemeral waterways and small‐scale gene flow in the desert spring amphipod, Wangiannachiltonia guzikae

Summary The springs of the Great Artesian Basin (GAB) in central Australia support a unique, but threatened ecosystem adapted to permanent fresh water in a desert landscape. Taxa within these springs are short‐range endemics, with very limited distributions, making them highly susceptible to extinction. Knowledge of fine‐scale dispersal mechanisms will help to develop conservation management plans that maintain gene flow and, by extension, the genetic diversity of these populations. This study focused on determining dispersal capabilities of the endemic GAB spring amphipod, Wangiannachiltonia guzikae, in an area &lt;4 km2. Using 11 microsatellite loci, 288 individuals were genotyped from 14 springs. Despite the very small area, low levels of gene flow and significant population differences were found among individual spring populations. Microgeographical genetic structure is clearly evident, and fine‐scale dispersal is significantly correlated with temporary waterways between springs. This pattern of genetic divergence supports the stream hierarchy model of population structure and suggests that connectivity between springs is critical for gene flow, although the precise mechanism of dispersal is still open for interpretation.

Social Hybridogenesis in the Clonal Ant Cataglyphis hispanica

With a few rare exceptions, the vast majority of animals reproduce sexually. Some species have, however, evolved alternative modes of reproduction by shifting from classical bisexuality to unorthodox reproductive systems, like parthenogenesis, gynogenesis, or hybridogenesis. Under hybridogenesis, both the maternal and paternal genomes are expressed in somatic tissues, whereas the germline is purely maternal. Recently, a form of hybridogenesis at the level of the society has been reported in some ants, where purebred females develop into reproductive queens and interlineage hybrids into sterile workers. Here, we report a unique case of social hybridogenesis in the desert ant Cataglyphis hispanica. Workers are produced exclusively from interbreeding between two distinct genetic lineages, whereas male and female sexuals are produced by asexual reproduction through parthenogenesis. As a consequence, all workers are pure hybridogens, and only maternal genes are perpetuated from one generation to the next. Thus, queens of C. hispanica use sexual reproduction for colony growth, whereas they reproduce asexually through parthenogenesis for germline production.

Temporal differentiation across a West-European Y-chromosomal cline: genealogy as a tool in human population genetics

The pattern of population genetic variation and allele frequencies within a species are unstable and are changing over time according to different evolutionary factors. For humans, it is possible to combine detailed patrilineal genealogical records with deep Y-chromosome (Y-chr) genotyping to disentangle signals of historical population genetic structures because of the exponential increase in genetic genealogical data. To test this approach, we studied the temporal pattern of the 'autochthonous' micro-geographical genetic structure in the region of Brabant in Belgium and the Netherlands (Northwest Europe). Genealogical data of 881 individuals from Northwest Europe were collected, from which 634 family trees showed a residence within Brabant for at least one generation. The Y-chr genetic variation of the 634 participants was investigated using 110 Y-SNPs and 38 Y-STRs and linked to particular locations within Brabant on specific time periods based on genealogical records. Significant temporal variation in the Y-chr distribution was detected through a north-south gradient in the frequencies distribution of sub-haplogroup R1b1b2a1 (R-U106), next to an opposite trend for R1b1b2a2g (R-U152). The gradient on R-U106 faded in time and even became totally invisible during the Industrial Revolution in the first half of the nineteenth century. Therefore, genealogical data for at least 200 years are required to study small-scale 'autochthonous' population structure in Western Europe.

Macro- and microgeographic genetic structure in an ant species with alternative reproductive tactics in sexuals

The genetic structure of social insect populations is influenced by their social organization and dispersal modes. The ant Hypoponera opacior shows diverse reproductive behaviours with regular cycles of outbreeding via winged sexuals and inbreeding via within-nest mating wingless sexuals that reproduce by budding. This unusual life cycle should be reflected in the genetic population structure, and we studied this on different scales using microsatellites. On a macrogeographic scale, populations were considerably structured and migration rates within the Chiricahuas were higher than those in between mountain ranges. On a local scale, our analyses revealed population viscosity through dependent colony foundation and a high genetic diversity with a multicolonial structure. The latter was also evident from recognition trials revealing consistent aggression between non-nestmates. Within-nest matings led to high inbreeding coefficients. Finally, the observed seasonal changes in relatedness can be explained by variation in queen number and differential dispersal of the two reproductive morphs.

Do common eiders nest in kin groups? Microgeographic genetic structure in a philopatric sea duck

We investigated local genetic associations among female Pacific common eiders (Somateria mollissima v-nigrum) nesting in a stochastic Arctic environment within two groups of barrier islands (Simpson Lagoon and Mikkelsen Bay) in the Beaufort Sea, Alaska. Nonrandom genetic associations were observed among nesting females using regional spatial autocorrelation analyses for distance classes up to 1000 m in Simpson Lagoon. Nearest-neighbour analyses identified clusters of genetically related females with positive lr values observed for 0-13% and 0-7% of the comparisons in Simpson Lagoon and Mikkelsen Bay, respectively, across years. These results indicate that a proportion of females are nesting in close proximity to more genetically related individuals, albeit at low frequency. Such kin groupings may form through active association between relatives or through natal philopatry and breeding site fidelity. Eiders nest in close association with driftwood, which is redistributed annually by seasonal storms. Yet, genetic associations were still observed. Microgeographic structure may thus be more attributable to kin association than natal philopatry and site fidelity. However, habitat availability may also influence the level of structure observed. Regional structure was present only within Simpson Lagoon and this island group includes at least three islands with sufficient driftwood for colonies, whereas only one island at Mikkelsen Bay has these features. A long-term demographic study is needed to understand more fully the mechanisms that lead to fine-scale genetic structure observed in common eiders breeding in the Beaufort Sea.

Seasonal Changes in the Genetic Structure of an Aphid-Ant Mutualism as Revealed Using Microsatellite Analysis of the AphidTuberculatus quercicolaand the AntFormica yessensis

The present study examined whether the mutualistic relationship between the aphid Tuberculatus quercicola (Matsumura) (Homoptera: Aphididae) and the attending ant Formica yessensis Forel (Hymenoptera: Formicidae) has had any mutual effects on the microgeographical genetic population structure of both partner species. The aphids and the attending ants were collected in June, August, and October 2004 from six trees of the Daimyo oak Quercus dentata Thunberg (Fagales: Fagaceae) and were genotyped using microsatellite loci. Significant genetic differentiation was detected among T. quercicola populations on the respective trees across seasons (an average of pairwise FST = 0.183). Similarly, significant genetic differentiation was found among populations of F. yessensis that attended aphid colonies on the respective host trees, though the averages of pairwise FST were lower (an average of pairwise FST = 0.070). An analysis of molecular variance and two-way ANOVA detected a significantly large genetic difference between spring and summer samples in F. yessensis but not in T. quercicola, indicating that changes in genetic composition occurred in the F. yessensis colony. In spite of a drastic seasonal change in the genetic difference in F. yessensis, principle coordinate analysis showed that the relative position among the six populations was maintained from spring to summer, suggesting that the tree where honeydew was available for a long time was occupied by F. yessensis over the same period and that the honeydew sources were inherited at the level of the ant colony. It is hypothesized that the suitability of host trees for the aphid T. quercicola may have an affect on the genetic structure of the attending ant F. yessensis. Within a colony of aphids, clonal diversity decreased significantly as the season progressed. The reduction in clonal diversity may be due to an increase in identical genotypes by parthenogenesis or selective pressure from host plant deterioration.

Microgeographical genetic structure of forest robusta coffee (Coffea canephora, Pierre), in Kibale National Park, Uganda

AbstractGenetic variation within and between forest coffee (Coffea canephora, Pierre) populations sampled from five localities in Kibale National Park was assessed using six polymorphic microsatellite loci. In a total sample of 113 individuals, 53 alleles were observed across all loci in all populations while the total number of alleles across loci in each population ranged from fifteen to forty‐five. Significant genetic differentiation was observed between all populations in all pair‐wise population comparisons (FST = 0.041–0.284; P &lt; 0.01). A high proportion of private alleles was observed across all loci with some of them occurring at relatively high frequencies. The implications of these findings for the future survival of the species are discussed.

Changes in Artificial Feeding Regulations Impact White-Tailed Deer Fine-Scale Spatial Genetic Structure

Human activities can change the spatial distribution of individuals within wildlife populations that in turn alters population allele frequencies and spatial genetic structure at fine scales. Artificial feeding is one such activity whose impact on wildlife physical condition, population dynamics, and transmission of disease has been well documented. To evaluate the impact of artificial feeding on the spatial distribution and social organization of white-tailed deer (Odocoileus virginianus) we estimated allele frequencies at 3 microsatellite loci for 2,177 hunter-harvested deer and characterized microgeographic genetic structure in 2 regions of the northeast lower peninsula of Michigan, USA, during and following cessation of artificial feeding. While artificial feeding was ongoing we observed no evidence of spatial genetic structure across either region. Spatial homogeneity of allele frequencies over such a large area was surprising given numerous studies that have documented spatial genetic structure in other deer populations, and it was likely a function of the aggregation of multiple kin-structured social groups (i.e., matrilines) at artificial feeding sites. Subsequently, when artificial feeding was banned, we found significant genetic differentiation among groups of deer in both regions. Detection of microgeographic genetic structure consistent with a pattern of isolation-by-distance following the ban on artificial feeding was likely the result of increased spatial segregation of social groups of related deer. Our results illustrate how analyses of the degree to which natural populations are spatially genetically structured can be used to infer the effects of human actions on wildlife movement patterns, breeding behaviors, and disease transmission that are difficult to determine using traditional methods.

Genetic structure of black abalone ( Haliotis cracherodii) populations in the California islands and central California coast: Impacts of larval dispersal and decimation from withering syndrome

The genetic structure of black abalone populations in the southern California Islands and central California coast was investigated by protein electrophoresis. Detailed sampling of San Nicolas Island (SN) permitted investigation of microgeographic genetic differentiation among local geomorphological features. In addition, temporal genetic differentiation was assessed by comparing juveniles and adults at three islands, San Miguel (SM), Santa Cruz (SC) and San Nicolas (SN). Mainland and island locations were genetically differentiated based on allele frequency differences and the presence of private alleles in some island populations. Although microgeographic genetic structure among sites on SN was weak and not statistically significant, heterozygosity varied among sites, with diversity decreasing from west to east. In addition, investigation among cohorts showed that adults were genetically differentiated among island locations, whereas no differences among juveniles were detected. Genetic differentiation between adult and juvenile abalones was detected at SC but not SM or SN. These data are generally consistent with local recruitment augmented by relatively more gene flow among island populations than among island and mainland populations, and possible selection acting on immigrant recruits.

Restricted effective queen dispersal at a microgeographic scale in polygynous populations of the ant Formica exsecta.

Ecological constraints on effective dispersal have been suggested to be a key factor influencing social evolution in animal societies as well as the shift from single queen colonies (monogyny) to multiple queen colonies (polygyny) in ants. However, little is known about the effective dispersal patterns of ant queens. Here we investigate the microgeographic genetic structure of mitochondrial haplotypes in polygynous populations of the ant Formica exsecta, both between pastures and among nests within pastures. An analysis of molecular variance revealed a very high genetic differentiation (phiST = 0.72) between pastures, indicating that queens rarely disperse successfully between pastures, despite the fact that pastures were sometimes as close as 1 km. Most of the pastures contained only a single haplotype, and haplotypes were frequently distinct between nearby pastures and even between groups of nests within the same pasture. In the three pastures that contained several haplotypes, haplotypes were not randomly distributed, the genetic differentiation between nests being phiST = 0.17, 0.52, and 0.69. This indicates that most queens are recruited within their parental colonies. However, a large proportion of nests contained more than one haplotype, demonstrating that colonies will sometimes accept foreign queens. The relatedness of mitochondrial genes among nestmates varied between 0.62 and 0.75 when relatedness was measured within each pasture and ranged between 0.72 and 1.0 when relatedness was assessed with all pastures as a reference population. Neighboring nests were more genetically similar than distant ones, and there was significant isolation by distance. This pattern may be due to new nests being formed by budding or by limited effective queen dispersal, probably on foot between neighboring nests. These results show that effective queen dispersal is extremely restricted even at a small geographical scale, a pattern consistent with the idea that ecological constraints are an important selective force leading to the evolution and maintenance of polygyny.