Genetic inference of orchid population dynamics on different-aged lava flows in Costa Rica

Colonization is a fundamental ecological process that is important for the persistence of species, particularly when a changing environment necessitates range shifts. Vacant habitats available for colonization often arise from landscape disturbance. Colonization and population expansion processes can be inferred by examining the levels and spatial distribution of genetic variation of plant populations with known disturbance histories. Samples (N = 690) of the terrestrial orchid, Epidendrum radicans, were collected from five lava flow sites on the slopes of Volcán Arenal in Costa Rica that last experienced major eruptions in 1968 and 1992. Individuals were also sampled (N = 188) from four regional populations. Samples were characterized using 15 nuclear genetic markers and analyzed using population genetics statistics. Genetic diversity within sites was moderate (H_e = 0.092–0.192). Contrary to expectation, diversity tended to be lower on the older lava flows (0.131 vs. 0.172) which may reflect their more sheltered topography that restricted pollen/seed immigration, and/or greater intra- and interspecific competition. Genetic diversity measures indicate that the lava flows were colonized by numerous individuals that likely originated from multiple sources while spatial genetic structure (SGS) statistics indicate that most recruitment in the study sites subsequent to colonization resulted from in situ reproduction and localized seed deposition. Younger sites had significantly greater SGS over larger distances which reflects fewer reproductive events, and less spatial and temporal overlap of seed shadows relative to the older sites. Clones were also generally smaller on the younger sites (≤3 m vs. ≤8 m).     

Hidden founder effects: small‐scale spatial genetic structure in recently established populations of the grassland specialist plant Anthyllis vulneraria

The long‐term establishment success of founder plant populations has been commonly assessed based on the measures of population genetic diversity and among population genetic differentiation, with founder populations expected to carry sufficient genetic diversity when population establishment is the result of many colonists from multiple source populations (the ‘migrant pool’ colonization model). Theory, however, predicts that, after initial colonization, rapid population expansion may result in a fast increase in the extent of spatial genetic structure (SGS), independent of extant genetic diversity. This SGS can reduce long‐term population viability by increasing inbreeding. Using 12 microsatellite markers, we inferred colonization patterns in four recent populations of the grassland specialist plant Anthyllis vulneraria and compared the extent of SGS between recently established and old populations. Assignment analyses of the individuals of recent population based on the genetic composition of nine adjacent putative source populations suggested the occurrence of the ‘migrant pool’ colonization model, further confirmed by high genetic diversity within and low genetic differentiation among recent populations. Population establishment, however, resulted in the build‐up of strong SGS, most likely as a result of spatially restricted recruitment of the progeny of initial colonists. Although reduced, significant SGS was nonetheless observed to persist in old populations. The presence of SGS was in all populations associated with elevated inbreeding coefficients, potentially affecting the long‐term viability of these populations. In conclusion, this study illustrates the importance of studying SGS next to population genetic diversity and differentiation to adequately infer colonization patterns and long‐term establishment success of plant species.     

Inferring recruitment history from spatial genetic structure within populations of the colonizing tree Albizia julibrissin (Fabaceae)

Comparative analyses of spatial genetic structure (SGS) among species, populations, or cohorts give insight into the genetic consequences of seed dispersal in plants. We analysed SGS of a weedy tree in populations with known and unknown recruitment histories to first establish patterns in populations with single vs. multiple founders, and then to infer possible recruitment scenarios in populations with unknown histories. We analysed SGS in six populations of the colonizing tree Albizia julibrissin Durazz. (Fabaceae) in Athens, Georgia. Study sites included two large populations with multiple, known founders, two small populations with a single, known founder, and two large populations with unknown recruitment histories. Eleven allozyme loci were used to genotype 1385 individuals. Insights about the effects of colonization history from the SGS analyses were obtained from correlograms and Sp statistics. Distinct differences in patterns of SGS were identified between populations with multiple founders vs. a single founder. We observed significant, positive SGS, which decayed with increasing distance in the populations with multiple colonists, but little to no SGS in populations founded by one colonist. Because relatedness among individuals is estimated relative to a local reference population, which usually consists of those individuals sampled in the study population, SGS in populations with high background relatedness, such as those with a single founder, may be obscured. We performed additional analyses using a regional reference population and, in populations with a single founder, detected significant, positive SGS at all distances, indicating that these populations consist of highly related descendants and receive little seed immigration. Subsequent analyses of SGS in size cohorts in the four large study populations showed significant SGS in both juveniles and adults, probably because of a relative lack of intraspecific demographic thinning. SGS in populations of this colonizing tree is pronounced and persistent and is determined by the number and relatedness of founding individuals and adjacent seed sources. Patterns of SGS in populations with known histories may be used to indirectly infer possible colonization scenarios for populations where it is unknown.     

THE RELATIVE IMPORTANCE OF HISTORICAL EVENTS AND GENE FLOW ON THE POPULATION STRUCTURE OF A MEDITERRANEAN RAGWORT,SENECIO GALLICUS (ASTERACEAE)

Comparisons of cytoplasmic and nuclear diversity within and among natural plant populations have the potential to distinguish the relative influences of seed and pollen dispersal on contemporary gene flow, or alternatively, may permit inferences of the colonization history of a species via seed. We examined patterns of cpDNA and allozyme variation in Senecio gallicus, a diploid, annual plant that occurs in both coastal and ruderal inland areas of the Iberian Peninsula and southern France. The species appears to have a strong propensity for long-distance seed dispersal. Five cpDNA haplotypes were found by RFLP analysis among a sample of 111 individuals derived from 11 populations. Differences in haplotype frequencies across populations were most evident with respect to a dramatic increase in the frequency of a derived haplotype from coastal to inland localities. The level of cpDNA differentiation among populations within the inland group (θ₀ = 0.07) was significantly less than that seen within the coastal group (θ₀ = 0.41). In contrast, for allozymes, no significant difference in population structure was evident between collections from coastal and inland habitats. At the rangewide geographic scale, there was only a very weak association between inferred levels of gene flow and geographic distance for cpDNA, and no such association was found for allozymes. It appears that while seed movement in the species might be sufficiently great to disturb the pattern of isolation by distance for cpDNA, it cannot fully account for the nearly randomized spatial structure at polymorphic allozyme loci. It is suggested that isolation of populations in Atlantic-Mediterranean coastal refugia during previous glacial maxima, and the effects of subsequent colonization events in inland areas, have had an important effect on molding the present genetic structure of the species.     

Spatial genetic structure in Milicia excelsa (Moraceae) indicates extensive gene dispersal in a low-density wind-pollinated tropical tree

In this study, we analysed spatial genetic structure (SGS) patterns and estimated dispersal distances in Milicia excelsa (Welw.) C.C. Berg (Moraceae), a threatened wind-pollinated dioecious African tree, with typically low density (∼10 adults/km²). Eight microsatellite markers were used to type 287 individuals in four Cameroonian populations characterized by different habitats and tree densities. Differentiation among populations was very low. Two populations in more open habitat did not display any correlation between genetic relatedness and spatial distance between individuals, whereas significant SGS was detected in two populations situated under continuous forest cover. SGS was weak with a maximum S _p-statistic of 0.006, a value in the lower quartile of SGS estimates for trees in the literature. Using a stepwise approach with Bayesian clustering methods, we demonstrated that SGS resulted from isolation by distance and not colonization by different gene pools. Indirect estimates of gene dispersal distances ranged from σ _g = 1 to 7.1 km, one order of magnitude higher than most estimates found in the literature for tropical tree species. This result can largely be explained by life-history traits of the species. Milicia excelsa exhibits a potentially wide-ranging wind-mediated pollen dispersal mechanism as well as very efficient seed dispersal mediated by large frugivorous bats. Estimations of gene flow suggested no major risk of inbreeding because of reduction in population density by exploitation. Different strategy of seed collection may be required for reforestation programmes among populations with different extent of SGS.     

Genetic inference of epiphytic orchid colonization; it may only take one

Colonization of vacant habitat is a fundamental ecological process that affects the ability of species to persist and undergo range modifications in continually shifting landscapes. Thus, understanding factors that affect and limit colonization has important ecological and conservation implications. Epiphytic orchids are increasingly threatened by various factors, including anthropogenic habitat disturbance. As cleared areas (e.g. pastures) are recolonized by suitable host trees, the establishment and genetic composition of epiphytic orchid populations are likely a function of their colonization patterns. We used genetic analyses to infer the prevailing colonization pattern of the epiphytic orchid, Brassavola nodosa. Samples from three populations (i.e. individuals within a tree) from each of five pastures in the dry forest of Costa Rica were genotyped with neutral nuclear and chloroplast markers. Spatial autocorrelation and hierarchical genetic structure analyses were used to assess the relatedness of individuals within populations, among populations within pastures and among populations in different pastures. The results showed significant relatedness within populations (mean r = 0.166) and significant but lower relatedness among populations within a pasture (mean r = 0.058). Our data suggest that colonization of available habitats is by few individuals with subsequent population expansion resulting from in situ reproduction, and that individuals within a tree are not a random sample of the regional seed pool. Furthermore, populations within a pasture were likely colonized by seeds produced by founders of a neighbouring population within that pasture. These results have important ramifications for understanding conservation measures needed for this species and other epiphytic orchids.     

Spatial genetic structure within size classes of the endangered tropical tree Guaiacum sanctum (Zygophyllaceae)

? Premise of the study: Patterns of spatial genetic structure (SGS) were analyzed within a population of the endangered tropical tree Guaiacum sanctum located in northwestern Costa Rica. Documentation of these patterns provides insights into the gene dispersal mechanisms that play a central role in the maintenance and structure of genetic diversity within plant populations. ? Methods: Allozyme analyses were used to examine SGS in Palo Verde National Park, Costa Rica. The SGS was compared among three plots and different age classes. ? Key results: High levels of genetic diversity were found overall with a pooled genetic diversity of H(e) = 0.302 (±0.02). Selfing was proposed as the proximate cause for significant levels of heterozygote deficiency observed across size classes and plots. An unexpected lack of SGS (r(j) < 0.02) was observed for all size classes, suggesting the mixing of seeds from several adults. A parent-pair parentage analysis indicated that at least 48% of the smaller individuals within a plot were produced by parents located at distances of at least 150 m. ? Conclusions: Populations of G. sanctum are established and maintained by bird-mediated, moderate- to long-distance seed dispersal, which results in a mixture of seeds from unrelated maternal individuals, effectively eliminating SGS. Proximity between individuals is, therefore, a poor predictor of family structure in this species. Long-distance seed dispersal, coupled with estimates of high genetic diversity, suggests that this endangered species has the potential for natural regeneration and restoration given the availability of suitable habitats.     

Fine-Scale Spatial Genetic Structure in Emmer Wheat and the Role of Population Range Position

The extent of spatial genetic structure (SGS) within plant populations depends on seed and pollen dispersal distance, breeding type, level of self-fertilization and effective plant density. Self-fertilizing species with gravity-dispersed seeds are expected to have both small effective population sizes and low pollen movement leading to high genetic structure. Higher SGS can be expected in more patchy and peripheral populations because of lower plant density and population sizes, and lower intensity of gene flow. We tested these predictions analyzing SGS in two core and two peripheral populations of predominantly self-fertilizing emmer wheat. Analysis of SGS with 11 nuclear microsatellites revealed (1) a negative linear relationship between kinship coefficients, calculated for pairs of individuals, and the logarithm of geographical distance between members of the pairs, in all studied populations; and (2) a significant autocorrelation for a distance up to 5 m (core populations) or 20 m (peripheral populations). Pollen flow, estimated from comparison of nuclear and chloroplast variation, was spatially limited, as was seed dispersal. Our results support a hypothesized relationship between SGS intensity and breeding system, the mode of seed dispersal and the population range position (core vs. periphery).     

Fine-scale spatial genetic structure of the distylous Primula veris in fragmented habitats

In Flanders (northern Belgium), the distylous self-incompatible perennial herb Primula veris is common, but mainly occurs in fragmented habitats. Distyly, which favours disassortative mating, is characterized in P. veris by two genetically determined floral morph types (pin or thrum). Using 18 polymorphic loci, we investigated fine-scale spatial genetic structure (SGS) and spatial distribution of the morphs within four populations from two regions that differ in degree of habitat fragmentation. We studied the contributions made by sexual reproduction and clonal propagation and compared the SGS patterns between pin and thrum morph types. Clonal growth was very restricted to a few individuals and to short distances. One population showed a non-random spatial distribution of the morphs. Pin and thrum individuals differed in SGS patterns at a small scale, suggesting intrapin biparental inbreeding, also related to high plant densities. This may be explained by partial self-compatibility of the pin morph combined with restricted seed dispersal and pollinator behaviour. There is an indication of more pronounced SGS when populations occur in highly fragmented habitats. From our findings, we may hypothesize disruption of the gene flow processes if these large populations evolve into patchworks of small remnants, but also a possible risk for long-term population survival if higher intrapin biparental inbreeding leads to inbreeding depression. Our study emphasizes the need for investigating the interactions between the heterostylous breeding system, population demographic and genetic structure for understanding population dynamics in fragmented habitats and for developing sustainable conservation strategies.     

Spatial genetic structure in continuous and fragmented populations of Pinus pinaster Aiton

Habitat fragmentation, i.e., the reduction of populations into small isolated remnants, is expected to increase spatial genetic structure (SGS) in plant populations through nonrandom mating, lower population densities and potential aggregation of reproductive individuals. We investigated the effects of population size reduction and genetic isolation on SGS in maritime pine ( Pinus pinaster Aiton) using a combined experimental and simulation approach. Maritime pine is a wind-pollinated conifer which has a scattered distribution in the Iberian Peninsula as a result of forest fires and habitat fragmentation. Five highly polymorphic nuclear microsatellites were genotyped in a total of 394 individuals from two population pairs from the Iberian Peninsula, formed by one continuous and one fragmented population each. In agreement with predictions, SGS was significant and stronger in fragments ( Sp  = 0.020 and Sp  = 0.026) than in continuous populations, where significant SGS was detected for one population only ( Sp  = 0.010). Simulations suggested that under fat-tailed dispersal, small population size is a stronger determinant of SGS than genetic isolation, while under normal dispersal, genetic isolation has a stronger effect. SGS was always stronger in real populations than in simulations, except if unrealistically narrow dispersal and/or high variance of reproductive success were modelled (even when accounting for potential overestimation of SGS in real populations as a result of short-distance sampling). This suggests that factors such as nonrandom mating or selection not considered in the simulations were additionally operating on SGS in Iberian maritime pine populations.     

Effects of zoochory on the spatial genetic structure of plant populations

Spatial genetic structure (SGS) of plants results from the nonrandom distribution of related individuals. SGS provides information on gene flow and spatial patterns of genetic diversity within populations. Seed dispersal creates the spatial template for plant distribution. Thus, in zoochorous plants, dispersal mode and disperser behaviour might have a strong impact on SGS. However, many studies only report the taxonomic group of seed dispersers, without further details. The recent increase in studies on SGS provides the opportunity to review findings and test for the influence of dispersal mode, taxonomic affiliation of dispersers and their behaviour. We compared the proportions of studies with SGS among groups and tested for differences in strength of SGS using Sp statistics. The presence of SGS differed among taxonomic groups, with reduced presence in plants dispersed by birds. Strength of SGS was instead significantly influenced by the behaviour of seed dispersal vectors, with higher SGS in plant species dispersed by animals with behavioural traits that result in short seed dispersal distances. We observed high variance in the strength of SGS in plants dispersed by animals that actively or passively accumulate seeds. Additionally, we found SGS was also affected by pollination and marker type used. Our study highlights the importance of vector behaviour on SGS even in the presence of variance created by other factors. Thus, more detailed information on the behaviour of seed dispersers would contribute to better understand which factors shape the spatial scale of gene flow in animal‐dispersed plant species.     

Effects of seed dispersal, adult tree and seedling density on the spatial genetic structure of regeneration at fine temporal and spatial scales

Several demographic factors can produce family structured patches within natural plant populations, particularly limited seed and pollen dispersal and small effective density. In this paper, we used computer simulations to examine how seed dispersal, density, and spatial distribution of adult trees and seedlings can explain the spatial genetic structure (SGS) of natural regeneration after a single reproductive event in a small population. We then illustrated the results of our simulations using genetic (isozymes and chloroplast microsatellites) and demographic experimental data from an Abies alba (silver fir) intensive study plot located in the Southern French Alps (Mont Ventoux). Simulations showed that the structuring effect of limited dispersal on seedling SGS can largely be counterbalanced by high effective density or a clumped spatial distribution of adult trees. In addition, the clumping of natural regeneration far from adult trees, which is common in temperate forest communities where gap dynamics are predominant, further decreases SGS intensity. Contrary to our simulation results, low adult tree density, aggregated spatial distribution of seedlings, and limited seed dispersal did not generate a significant SGS in our A. alba experimental plot. Although some level of long distance pollen and seed flow could explain this lack of SGS, our experimental data confirm the role of spatial aggregation (both in adult trees and in seedlings far from adult trees) in reducing SGS in natural populations.     

Contrasting spatial genetic structure in Annona crassiflora populations from fragmented and pristine savannas

In continuous populations, fine-scale genetic structure tends to be stronger in species with restricted pollen and seed dispersal. However, habitat fragmentation and disturbances can affect genetic diversity and spatial genetic structure due to disruption in ecological processes, such as plant reproduction and seed dispersal. In this study, we compared the genetic diversity and fine-scale spatial genetic structure (SGS) in two populations of Annona crassiflora (Annonaceae) in a pristine savanna Reserve (ESECAE) and in a fragmented disturbed savanna area (PABE), both in Cerrado biome in Central Brazil. The analyses were based on the polymorphism at 10 microsatellite loci. Our working hypothesis was that SGS is stronger and genetic diversity is lower in population at fragmented area (PABE) than at pristine area (ESECAE). Both populations presented high levels of polymorphism and genetic diversity and showed no sign of bottleneck for both Wilcoxon sign-rank test for heterozygosity excess (p > 0.05) and coalescent analyses (growth parameter g not different from zero), but population at fragmented area showed higher fixation index and stronger SGS. Besides, populations are significantly differentiated (F _ST = 0.239, R _ST = 0.483, p < 0.001 for both). Coalescent analyses showed high historical effective population sizes for both populations, high gene flow between ESECAE and PABE and recent time to most recent common ancestor (~37 k year BP). Our results suggest that despite the high genetic diversity, fragmentation and disturbance may have been affecting populations of this species increasing mating between closely related individuals leading to high fixation index and strong SGS.     

Spatiotemporal structure of genetic variation of a spreading plant metapopulation on dynamic riverbanks along the Meuse River

Long-distance seed dispersal is a crucial determinant of within-population genetic variability and among-population genetic differentiation in plant metapopulations undergoing recurrent local extinctions and (re-)colonization. We investigated the spatial and temporal structure of genetic variation in a metapopulation of Sisymbrium austriacum located along a dynamic river system using dominant AFLP markers. Data on riverbank dynamics and colonization history allowed separating populations based on their age (< or =5 vs >5 years old). Bayesian analysis of population genetic structure indicated that populations were significantly differentiated from each other, but Mantel tests revealed that there was no relationship between pairwise geographic and genetic distances, suggesting that long-distance seed dispersal partly determines spatial genetic structure. Recent populations were less differentiated from each other than old populations. Analysis of molecular variance (AMOVA) indicated that both spatial factors and population age significantly determined genetic diversity, the effects of age being more important than spatial location. Clustering analysis revealed five large clusters, which were related primarily to population age and to a minor extent to geographical location. Our results indicate that the recurrent formation and destruction of riverbank habitats following peak flow events have a large impact on genetic diversity of riparian plant species.     

Spatial genetic structure of the southeastern North American endemic, Ceratiola ericoides (Empetraceae)

Understanding the spatial distribution of genetic diversity (i.e., spatial genetic structure [SGS]) within plant populations can elucidate mechanisms of seed dispersal and patterns of recruitment that may play an important role in shaping the demography and spatial distribution of individuals in subsequent generations. Here we investigate the SGS of allozyme diversity in 2 populations of the southeastern North American endemic shrub, Ceratiola ericoides. The data suggest that the 2 populations have similar patterns of SGS at distances of 0-45 m that likely reflect the isolation by distance (IBD) model of seed dispersal. However, at distances >or=50 m, the pattern of SGS differs substantially between the 2 populations. Whereas one population continues to reflect the classical IBD pattern, the second population shows a marked increase in autocorrelation coefficient (r) values at 50-75 m. Furthermore, r values at these distances are as much as 33% higher than at 0-5 m where the highest r value would be predicted by IBD. A likely explanation is the differing frequencies of 2 fruit morphologies in these populations and the greater role that birds play in seed dispersal in the second population.     

The role of gene flow in shaping genetic structures of the subtropical conifer species Araucaria angustifolia

The morphological features of pollen and seed of Araucaria angustifolia have led to the proposal of limited gene dispersal for this species. We used nuclear microsatellite and AFLP markers to assess patterns of genetic variation in six natural populations at the intra- and inter-population level, and related our findings to gene dispersal in this species. Estimates of both fine-scale spatial genetic structure (SGS) and migration rate suggest relatively short-distance gene dispersal. However, gene dispersal differed among populations, and effects of more efficient dispersal within population were observed in at least one stand. In addition, even though some seed dispersal may be aggregated in this principally barochorous species, reasonable secondary seed dispersal, presumably facilitated by animals, and overlap of seed shadows within populations is suggested. Overall, no correlation was observed between levels of SGS and inbreeding, density or age structure, except that a higher level of SGS was revealed for the population with a higher number of juvenile individuals. A low estimate for the number of migrants per generation between two neighbouring populations implies limited gene flow. We expect that stepping-stone pollen flow may have contributed to low genetic differentiation among populations observed in a previous survey. Thus, strategies for maintenance of gene flow among remnant populations should be considered in order to avoid degrading effects of population fragmentation on the evolution of A. angustifolia.     

Variation in demographic and fine-scale genetic structure with population-history stage of <Emphasis Type="Italic">Hemerocallis taeanensis</Emphasis> (Liliaceae) across the landscape

Fine-scale genetic structure (FSGS) in plant populations is expected to be influenced by variation in demographic processes across space and over time. I chose Hemerocallis taeanensis (Liliaceae), a perennial herb with a rapid population turnover, to quantify how demographic structure and FSGS change with a population’s history (i.e., density). Nonaccumulative O-ring statistic and spatial autocorrelation analysis (kinship coefficient, F _ij ) were used to quantify spatial patterns of individuals and FSGS in four populations belonging to two population stages (expansion and maturation) in west-central Korea. The O-ring function revealed that significant aggregation of individuals occurs at short spatial scales during the earlier stage of population expansion, which reflects restricted seed dispersal around maternal individuals. However, this pattern disappears as the population density increases during population maturation, probably due to a high population density. Significant evidence of FSGS was found in two populations at the stage of population expansion (Sp, a statistic which describes the rate of decrease of pairwise kinship with distance, was 0.018 and 0.029). The results show that most seeds fall around maternal plants when initially established colonists proliferate at suitable microhabitats. In contrast to this, much lower Sp values (−0.003 and 0.004) were estimated for two populations at the stage of population maturation, which may result from the overlapping of seed shadows due to high adult density. All of these results demonstrate considerable variation in within-population demographic and genetic structures of H. taeanensis with respect to population temporal stage across the landscape.     

Life-history variation following habitat degradation associated with differing fine-scale spatial genetic structure in a rainforest cycad

Habitat degradation can result in drastic environmental changes potentially affecting the life-history of populations and aspects of the reproductive biology and the genetic structure within and among populations. Here, we explore how life-history differences between subpopulations from contrasting habitats may affect mating availability, which in turn will indirectly affect the strength of spatial genetic structure within populations of a tropical rainforest cycad (Zamia fairchildiana). Subpopulations exposed to higher light availability in degraded-forest habitats had male individuals that grew faster, reproduced earlier, and invested more in reproduction than in native-forest habitat subpopulations. These differences in life history resulted in degraded-habitat subpopulations showing a higher proportion of reproductive adults and greater mate availability in a reproductive season. Subpopulations in the degraded habitat showed weaker SGS, i.e., a smaller slope in the linear regression of genetic relatedness on linear distance. Environmentally induced changes in life history and subsequent changes in the strength of the SGS after habitat degradation may have important consequences for population viability and should be of concern in conservation.     

Spatial genetic analyses reveal strong genetic structure in two populations of the outcrossing tree fern Alsophila firma (Cyatheaceae)

The development of spatial genetic structure (SGS) in seed plants has been linked to several biological attributes of species, such as breeding system and life form. However, little is known about SGS in ferns, which together with lycopods are unique among land plants in having two free‐living life stages. We combined spatial aggregation statistics and spatial genetic autocorrelation analyses using five plastid microsatellites and one nuclear gene to investigate SGS in two populations of the outcrossing tree fern Alsophila firma (Cyatheaceae). We assessed how the observed patterns compare with those estimated for other ferns and seed plants. Populations of A. firma exhibited strong SGS, spatial clustering of individuals, substantial clonal diversity and no inbreeding. SGS in ferns appears to be higher than in most seed plants analysed to date. Contrary to our expectations, an outcrossing breeding system, wind dispersal and an arborescent life form did not translate into weak or no SGS. In ferns, SGS is probably being affected by the life cycle with two free‐living life stages. The reproductive biology of ferns appears to be more complex than previously thought. This implies that SGS in ferns is affected by some factors that cannot be inferred from the study of flowering plants. © 2015 The Linnean Society of London, Botanical Journal of the Linnean Society, 2015, 177 , 439–449.     

Spatial structure of genetic variation and primary succession in the pioneer tree species Antirhea borbonica on La Réunion

In habitats where colonization and extinction are recurrent, the distribution of gene frequencies among patches of suitable habitat may reflect the age structure of different populations. In this study, we quantify population genetic structure for a pioneer tree species, Antirhea borbonica, in a chrono-sequence of primary succession on the lava flows of the Piton de La Fournaise volcano (La Réunion). Using microsatellite loci and amplified fragment length polymorphism (AFLP) markers, we quantified genetic variation within and among populations for early- and late-succession populations in a landscape where extinction and recolonization are recurrent (the ‘Grand Brûlé’) and for late-succession populations in a more stable landscape. This study produced three main results. First, we detected no evidence that founder events increase genetic differentiation among colonizing populations; F_ST values among early- and among late-succession populations were similar. Second, we found no evidence for isolation by distance; genetic distance was not correlated with spatial distance within and among populations. Third, F_IS values are consistently high in all populations, despite the fact that A. borbonica populations are functionally close to dioecy and thus expected to have an outcrossing mating system. Multiple colonization events from different sources may limit differentiation among young populations and spatial isolation may enhance differentiation among late-succession populations. Ecological processes acting during colonization may create the conditions for spatial aggregation within pioneer populations, and thus contribute to the high F_IS values.