INTRAPOPULATIONAL VARIATION IN POLLEN-MEDIATED GENE FLOW IN PLANTAGO LANCEOLATA L.

The extent of genetic variation in pollen-mediated gene flow distance was measured for a population of the wind-pollinated herb, Plantago lanceolata. Twenty-nine genotypes were collected from a natural population, clonally replicated, and grown to reproductive maturity in a greenhouse. Relative gene flow distances were measured for each replicate and genotype in a wind tunnel. Approximately five percent of the total variation in gene flow distance was attributable to variation among genotypes. Most of the remaining variation was attributable to differences between the early part of the growing season, when most of the flowering occurs, and the remainder of the season, when flowering is sparse. The rankings of the genotypes' gene flow distances showed significant concordance between the early-season measures and measures from later in the summer. There was no correlation between the average inflorescence height for a plant and the average gene flow distance for that plant. For analysis of pollen characteristics suspected as causal factors of variation in gene flow distance, the variation in gene flow distance was divided into two components: that due to environmental (seasonal) differences, and that due to the differences among genotypes within runs. Variation in both the buoyant properties of the pollen grains and their adhesiveness was significantly partially correlated with environmental variation in gene flow distance, while only the buoyant properties of the pollen grains were significantly partially correlated with among-genotype variation in gene flow distances.     

Parentage versus two-generation analyses for estimating pollen-mediated gene flow in plant populations

Assessment of contemporary pollen-mediated gene flow in plants is important for various aspects of plant population biology, genetic conservation and breeding. Here, through simulations we compare the two alternative approaches for measuring pollen-mediated gene flow: (i) the NEIGHBORHOOD model--a representative of parentage analyses, and (ii) the recently developed TWOGENER analysis of pollen pool structure. We investigate their properties in estimating the effective number of pollen parents (N(ep)) and the mean pollen dispersal distance (delta). We demonstrate that both methods provide very congruent estimates of N(ep) and delta, when the methods' assumptions considering the shape of pollen dispersal curve and the mating system follow those used in data simulations, although the NEIGHBORHOOD model exhibits generally lower variances of the estimates. The violations of the assumptions, especially increased selfing or long-distance pollen dispersal, affect the two methods to a different degree; however, they are still capable to provide comparable estimates of N(ep). The NEIGHBORHOOD model inherently allows to estimate both self-fertilization and outcrossing due to the long-distance pollen dispersal; however, the TWOGENER method is particularly sensitive to inflated selfing levels, which in turn may confound and suppress the effects of distant pollen movement. As a solution we demonstrate that in case of TWOGENER it is possible to extract the fraction of intraclass correlation that results from outcrossing only, which seems to be very relevant for measuring pollen-mediated gene flow. The two approaches differ in estimation precision and experimental efforts but they seem to be complementary depending on the main research focus and type of a population studied.     

Extensive pollen flow in a natural fragmented population of Patagonian cypress Austrocedrus chilensis

Habitat fragmentation might significantly affect mating and pollen dispersal patterns in plant populations, contributing to the decline of remnant populations. However, wind-pollinated species are able to disperse pollen at longer distances after opening of the canopy. Our objectives were to characterize the mating system parameters and to estimate the average distance of effective pollen dispersal in the wind-pollinated conifer Austrocedrus chilensis. We sampled 19 “mother trees,” 200 progeny, and 81 additional adults (both male and female), in a fragmented population at the Argentinean Patagonian steppe. We registered the spatial positions of individuals and genotyped all samples with five microsatellite markers. We found a high genetic diversity, a moderated rate of biparental inbreeding (t _m? ??t _s?=?0.105), and a complete absence of correlated paternity (r _p?=??0.015). The effective number of pollen donors contributing to a single mother (N _ep) was 13.9. Applying TWOGENER, we estimated a low but significant differentiation among the inferred pollen pools (Φ_FT?=?0.036, p?=?0.001) and a very large average pollen dispersal distance (d?=?1,032.3 m). The leptokurtic distribution (b?=?0.18) presumes a potential for even larger dispersal distances. The high genetic diversity, the mating patterns, and the extensive pollen dispersal presume that habitat fragmentation did not have a negative impact on pollen movement in this population of A. chilensis. Genetic connectivity among fragmented populations scattered in the Patagonian region is possible, and we stress the need of management policies at the landscape level.     

Mechanisms of pollen deposition by insect pollinators

Summary Studies of pollen dispersal in insect-pollinated plants have often documented highly leptokurtic patterns of pollen deposition that can increase the likelihood of long-distance mating. To examine potential causes of highly leptokurtic deposition, we introduce four functions that arise when (1) the duration of pollinator visits to pollen sources is limited, (2) the rate of pollen deposition varies randomly among pollinators and/or among visits, (3) the rate of pollen deposition changes monotonically over time or (4) pollen is carried in layers or compartments on the pollinator's body that differ in deposition rate. Maximum likelihood techniques were used to fit deposition functions to data obtained from honey bees (Apis mellifera L.) visiting mustard plants (Brassica campestris L.) that contained a marker gene. Each of the alternative leptokurtic functions fit the experimental data better than a simple exponential function and the best-fit function predicted a mean pollen dispersal distance more than three times greater than the exponential. We argue that studies of pollen deposition need to test a broader range of deposition models to assess outcrossing distance in plant populations accurately.     

EFFICACY OF WIND POLLINATION: POLLEN LOAD SIZE AND NATURAL MICROGAMETOPHYTE POPULATIONS IN WIND-POLLINATED STABEROHA BANKSII (RESTIONACEAE)

A detailed examination was made of the numbers of pollen grains and microgametophytes occurring on stigmas of a dioecious, wind-pollinated species, Staberoha banksii. The number of pollen grains per pistil ranged from 0 to over 100; and the mean pollen load size per pistil per plant varied from 4.4 to 28. Distance to the fourth nearest male plant was used as a measure of local pollen availability and accounted for 63.5% of the between-plant variation in pollen load size. In contrast, the number of microgametophytes did not show a leptokurtic decline with increasing distance from a pollen source. On average there were 5.19 microgametophytes per ovule, which is higher than all comparable data for insect-pollinated taxa, and 66% of the stigmas had two or more microgametophytes. Given that there is a single ovule per flower, there is considerable potential for gametophytic competition. Seedset was very high, averaging 93%, and showed no significant decline in isolated female plants. We suggest that the quantity of pollen is not limiting fertilization in this population and that wind may therefore be highly effective as a pollen vector.     

Biparental inbreeding depression,genetic relatedness and progeny vigour in a wind-pollinated treeline species in Argentina

Spatially restricted gene flow and resulting spatial genetic structure are generally considered as being the primary controlling factors in the dynamics of biparental inbreeding depression in a wide range of plant species. However, wind-pollinated angiosperm trees have not been studied adequately in this respect. The present study analyses the relationships among parental genetic similarity, outcrossing distances, progeny vigour and mortality in Polylepis australis (Rosaceae), a wind-pollinated treeline species endemic to Argentina. We investigated whether spatial genetic structuring occurs in anthropogenically fragmented P. australis woodlands of the Córdoba Mountains. We also performed a controlled crossing experiment using pollen collected from different distances. Genetic variability (using RAPD-PCR), vigour (N-metabolism capacity) and mortality of the resulting progeny were contrasted with progeny from unmanipulated flowers. We found a continuous decrease in parental genetic similarity with spatial distance among mates and an increase in N-metabolism capacity in the progeny produced from pollen at increasing distances, as well as a very high mortality of seedlings resulting from short-distance crosses. Additionally, our results suggest that there is still fragment connectivity in P. australis through long-distance pollen-mediated gene flow.     

Novel polymorphic microsatellite loci and patterns of pollen-mediated gene flow in an ex situ population of <Emphasis Type="Italic">Eurycorymbus cavaleriei</Emphasis> (Sapindaceae) as revealed by categorical paternity analysis

Understanding the patterns of contemporary, pollen-mediated gene flow is of great importance for designing appropriate conservation strategies. In this study, ten novel polymorphic microsatellite loci were isolated for the rare dioecious tree, Eurycorymbus cavaleriei, and the patterns of pollen dispersal were investigated in an ex situ conserved population. A combination of microsatellite markers with high-collective exclusion power (0.932) was used to assign paternity to 240 seeds collected from eight maternal trees. The average effective pollen dispersal distance (δ) was 292.6 m and the frequency distribution of pollen movement suggested extensive pollen movement in the population. The effective pollen donors per maternal tree (N _ep) ranged from 5 to 10, and the most isolated maternal trees were observed with the largest number of N _ep = 10. Although a trend of near-neighbor mating was found in seven of eight maternal trees, no significant correlations were detected between the average effective pollen dispersal distance (δ) and the geographic distances (d1 and d2) between maternal and male trees. The increased average effective distance of pollen dispersal and number of N _ep for isolated maternal trees might be a compound consequence of low density and long-distance flight of pollinators of this species. The conservation implications of these results are discussed.     

Limited seed dispersal shapes fine-scale spatial genetic structure in a Neotropical dioecious large-seeded palm

Seed and pollen dispersal contribute to gene flow and shape the genetic patterns of plants over fine spatial scales. We inferred fine-scale spatial genetic structure (FSGS) and estimated realized dispersal distances in Phytelephas aequatorialis, a Neotropical dioecious large-seeded palm. We aimed to explore how seed and pollen dispersal shape this genetic pattern in a focal population. For this purpose, we genotyped 138 seedlings and 99 adults with 20 newly developed microsatellite markers. We tested if rodent-mediated seed dispersal has a stronger influence than insect-mediated pollen dispersal in shaping FSGS. We also tested if pollen dispersal was influenced by the density of male palms around mother palms in order to further explore this ecological process in large-seeded plants. Rodent-mediated dispersal of these large seeds occurred mostly over short distances (mean 34.76 ± 34.06 m) while pollen dispersal distances were two times higher (mean 67.91 ± 38.29 m). The spatial extent of FSGS up to 35 m and the fact that seed dispersal did not increase the distance at which male alleles disperse suggest that spatially limited seed dispersal is the main factor shaping FSGS and contributes only marginally to gene flow within the population. Pollen dispersal distances depended on the density of male palms, decreasing when individuals show a clumped distribution and increasing when they are scattered. Our results show that limited seed dispersal mediated by rodents shapes FSGS in P. aequatorialis, while more extensive pollen dispersal accounts for a larger contribution to gene flow and may maintain high genetic diversity. Abstract in Spanish is available with online material.     

Patterns of pollen dispersal in a small population of the Canarian endemic palm (Phoenix canariensis)

The genetic diversity of small populations is greatly influenced by local dispersal patterns and genetic connectivity among populations, with pollen dispersal being the major component of gene flow in many plants species. Patterns of pollen dispersal, mating system parameters and spatial genetic structure were investigated in a small isolated population of the emblematic palm Phoenix canariensis in Gran Canaria island (Canary Islands). All adult palms present in the study population (n=182), as well as 616 seeds collected from 22 female palms, were mapped and genotyped at 8 microsatellite loci. Mating system analysis revealed an average of 5.8 effective pollen donors (N_ep) per female. There was strong variation in correlated paternity rates across maternal progenies (ranging from null to 0.9) that could not be explained by the location and density of local males around focal females. Paternity analysis revealed a mean effective pollen dispersal distance of ∼71 m, with ∼70% of effective pollen originating from a distance of <75 m, and 90% from <200 m. A spatially explicit mating model indicated a leptokurtic pollen dispersal kernel, significant pollen immigration (12%) from external palm groves and a directional pollen dispersal pattern that seems consistent with local altitudinal air movement. No evidence of inbreeding or genetic diversity erosion was found, but spatial genetic structure was detected in the small palm population. Overall, the results suggest substantial pollen dispersal over the studied population, genetic connectivity among different palm groves and some resilience to neutral genetic erosion and subsequently to fragmentation.     

ESTIMATION OF GENETIC NEIGHBORHOOD PARAMETERS FROM POLLEN AND SEED DISPERSAL IN THE MARINE ANGIOSPERM ZOSTERA MARINA L.

The relative importance of random genetic drift and local adaptation in causing population substructuring in plant species remains an important empirical question. Here I estimate the effective size of the genetic neighborhood, N_b, as a means of evaluating the likely role of genetic drift in creating genetic differentiation within a population of a marine plant, Zostera marina L. (eelgrass). Calculations of effective neighborhood size are based on field estimates of pollen and seed-dispersal distributions, an electrophoretic estimate of the mating system using open-pollinated progeny arrays, and determination of the effective density of reproductive individuals in the population. Neighborhood area calculated from the parent-offspring dispersal variances was equal to N_a = 524 m²; variance in the seed-dispersal distribution contributes more than twice as much as variance in pollen dispersal to N_a. Including an outcrossing rate slightly different from random, estimated neighborhood size for Z. marina is N_b = 6255. This estimate is one of the largest reported for plants or animals and indicates that genetic drift in small neighborhoods is highly unlikely to cause genetic substructuring in the study population. High gene-flow levels provided by the marine environment appear to prevent genetic isolation by distance among eelgrass patches, but the importance of drift through founder events in this population characterized by high patch turnover cannot be discounted and is the subject of ongoing study.     

UNDERSTANDING THE POPULATION GENETIC STRUCTURE OF GLEDITSIA TRIACANTHOS L.: THE SCALE AND PATTERN OF POLLEN GENE FLOW

Spatial and temporal patterns of gene flow determine the extent to which populations can differentiate from one another as a result of natural selection or genetic drift. In this study, we investigated pollen-mediated gene flow in two eastern Kansas populations of the subdioecious tree species, Gleditsia triacanthos L. (Leguminosae), or honeylocust. In 2 yr at each site, we used paternity-exclusion analysis to estimate the proportion of seeds sired by immigrant pollen. We also used a single-parent and parent-pair exclusion analysis on naturally established seedlings and saplings to estimate gene flow into one site over a 12-yr period and into the second site over a 22-yr period. Results of both analyses showed high minimum estimates of pollen gene flow into each site (17%–30%). In each population, we found significantly less gene flow in years of high fruit production than in years of low fruit production, but in one population, we observed little variation in gene-flow rates among age classes of seedlings and saplings. The level of pollen gene flow showed weak negative dependence on the relative isolation distances of the maternal trees sampled (140–240 m at one site vs. 85–120 m at the second site), and gene-flow estimates from naturally established juveniles were very similar at the two sites. Within populations, a multiple regression model showed that maximum-likelihood estimates of male fertility were negatively associated with distances between mates and positively associated with male size as measured by stem diameter. In neither population, however, did the regression explain more than 16% of the total variation in male fertilities.     

Patterns of pollen dispersal in a small population of Pinus sylvestris L. revealed by total-exclusion paternity analysis

Patterns of pollen dispersal were investigated in a small, isolated, relict population of Pinus sylvestris L., consisting of 36 trees. A total-exclusion battery comprising four chloroplast and two nuclear microsatellites (theoretical paternity exclusion probability EP=0.996) was used to assign paternity to 813 seeds, collected from 34 trees in the stand. Long-distance pollen immigration accounted for 4.3% of observed matings. Self-fertilization rate was very high (0.25), compared with typical values in more widespread populations of the species. The average effective pollen dispersal distance within the stand was 48 m (or 83 m excluding selfs). Half of effective pollen was dispersed within 11 m, and 7% beyond 200 m. A strong correlation was found between the distance to the closest tree and the mean mating-distance calculated for single-tree progenies. The effective pollen dispersal distribution showed a leptokurtic shape, with a large and significant departure from that expected under uniform dispersal. A maximum-likelihood procedure was used to fit an individual pollen dispersal distance probability density function (dispersal kernel). The estimated kernel indicated fairly leptokurtic dispersal (shape parameter b=0.67), with an average pollen dispersal distance of 135 m, and 50% of pollen dispersed beyond 30 m. A marked directionality pattern of pollen dispersal was found, mainly caused by the uneven distribution of trees, coupled with restricted dispersal and unequal male success. Overall, results show that the number and distribution of potential pollen donors in small populations may strongly influence the patterns of effective pollen dispersal.     

Efficient long-distance gene flow into an isolated relict oak stand

Geographically isolated and small populations outside a species' central distribution range are likely to be of major importance to a species' ability to quickly adjust its distribution range to global change dynamics. Gene flow from the outside plays a pivotal role in the fate of these marginal populations. It has been proposed that spatial fragmentation and perceived geographic isolation do not necessarily reflect a loss of genetic connectivity in tree species. However, the spatial limits of long-distance gene flow, as well as its magnitude and impact, are still generally unknown. In the present study, we analyzed long-distance pollen-mediated gene flow into an isolated relict stand consisting of 7 individuals of Quercus robur L. based on a total sample of 177 trees and 9 microsatellite loci. We show that pollen-mediated gene flow across more than 80 km in this wind-pollinated tree species contributed at least 35% of all successful pollinations in the investigated isolated and small oak stand at the eastern limit of the species' distribution. The observed pollen immigration shaped the genetic diversity of acorn progenies in the stand and might explain the comparably high genetic diversity in the persisting adult population.     

Paternity analysis of pollen-mediated gene flow for Fraxinus excelsior L. in a chronically fragmented landscape

Paternity analysis based on microsatellite marker genotyping was used to infer contemporary genetic connectivity by pollen of three population remnants of the wind-pollinated, wind-dispersed tree Fraxinus excelsior, in a deforested Scottish landscape. By deterministically accounting for genotyping error and comparing a range of assignment methods, individual-based paternity assignments were used to derive population-level estimates of gene flow. Pollen immigration into a 300 ha landscape represents between 43 and 68% of effective pollination, mostly depending on assignment method. Individual male reproductive success is unequal, with 31 of 48 trees fertilizing one seed or more, but only three trees fertilizing more than ten seeds. Spatial analysis suggests a fat-tailed pollen dispersal curve with 85% of detected pollination occurring within 100 m, and 15% spreading between 300 and 1900 m from the source. Identification of immigrating pollen sourced from two neighbouring remnants indicates further effective dispersal at 2900 m. Pollen exchange among remnants is driven by population size rather than geographic distance, with larger remnants acting predominantly as pollen donors, and smaller remnants as pollen recipients. Enhanced wind dispersal of pollen in a barren landscape ensures that the seed produced within the catchment includes genetic material from a wide geographic area. However, gene flow estimates based on analysis of non-dispersed seeds were shown to underestimate realized gene immigration into the remnants by a factor of two suggesting that predictive landscape conservation requires integrated estimates of post-recruitment gene flow occurring via both pollen and seed.     

Extensive long-distance pollen dispersal in a fragmented landscape maintains genetic diversity in white spruce

Conifers are among the most genetically diverse plants but show the lowest levels of genetic differentiation, even among geographically distant populations. High gene flow among populations may be one of the most important factors in maintaining these genetic patterns. Here, we provide empirical evidence for extensive pollen-mediated gene dispersal between natural stands of a widespread northern temperate/boreal conifer, Picea glauca. We used 6 polymorphic allozyme loci to quantify the proportion of seeds sired by pollen originating from different sources in a landscape fragmented by agriculture in North Central Ontario, Canada. In 7 stands, a small proportion of seeds were sired by self-pollen or neighboring trees but 87.1% (+/-1.7% standard error [SE]) of seeds were sired by pollen from at least 250 to 3000 m away. In 4 single isolated trees, self-fertilization rates were low and more than 96% (+/-1.3% SE) of seeds were sired by immigrant pollen. The average minimum pollen dispersal distance in outcrossed matings was 619 m. These results provide strong evidence that extensive long-distance pollen dispersal plays a primary role in maintaining low genetic differentiation among natural populations of P. glauca and helps maintain genetic diversity and minimize inbreeding in small stands in a fragmented landscape.     

Spatial genetic structure and restricted gene flow in a functionally dioecious fig, <Emphasis Type="Italic">Ficus pumila</Emphasis> L. var. <Emphasis Type="Italic">pumila</Emphasis> (Moraceae)

The mutualism between fig plants and fig wasps has been recognized as one of the most specialized systems of symbiosis. Figs are pollinated by their highly specific pollinating fig wasps, and the pollinating fig wasps are raised within the syconia of figs. Recent studies indicated a difference between monoecious and dioecious figs in the dispersal range of pollinating wasps, which has potential consequences for gene flow. In this study, we detected the gene-flow pattern of the dioecious climbing fig, Ficus pumila L. var. pumila, at both local and regional scales. At the local scale, spatial autocorrelation analysis indicated strong genetic structure at short distances, a pattern of limited gene flow. This result was also supported by a high inbreeding coefficient (F _IS = 0.287) and significant substructuring (F _ST = 0.060; P < 0.001). Further analysis indicated that the effective gene dispersal range was 1,211 m, and the relative contribution of seed dispersal was smaller than that of pollen dispersal. The inferred effective range of pollen dispersal ranged from 989 to 1,712 m, while the effective seed dispersal range was less than 989 m. Lack of long-distance dispersal agents may explain the limited seed dispersal. The high density of receptive fig trees was the most likely explanation for limited pollen dispersal, and the position of syconia and relatively low wind speed beneath the canopy may contribute to this phenomenon. At the regional scale, significant negative correlations (kinship coefficient F _ij ranging from −0.038 to −0.071) existed in all comparisons between the studied population and other populations, and the assignment test grouped almost all individuals of the studied population into a distinct cluster. Asynchronous flowering on the regional scale, which provides a barrier for the pollinating wasps to fly from the studied population to the other populations, is probably responsible for the limited gene flow on the regional scale.     

Consequences of the pollination system on genetic structure and patterns of species distribution in the Andean genus Polylepis (Rosaceae): a comparative study

As a member of the Sanguisorbinae, taxonomically complicated Polylepis Ruiz & Pav. from the South American Andes is one of the few wind-pollinated genera of Rosaceae. Here, it is suggested that problems traditionally faced in species delimitation and phylogeny reconstruction may be in part due to a combination of weak reproductive barriers and the large distances that can be covered by genetic information as a result of wind pollination. The pattern of species distribution as well as molecular data (AFLP) of Polylepis were contrasted with those of an unrelated, insect-pollinated genus of similar species number and distribution, Minthostachys (Griseb.) Spach (Lamiaceae). In the present case, extensive pollen-mediated gene flow may explain the homogenization of genetic variability over larger distances and a lower number of species restricted to individual countries in the wind-pollinated genus Polylepis, but more examples will have to be studied before arriving at final conclusions.     

Positive effects of flower abundance and synchronous flowering on pollination success,and pollinia dispersal in rewardless Changnienia amoena (Orchidaceae)

Pollination success and pollen dispersal in natural populations depend on the spatial‐temporal variation of flower abundance. For plants that lack rewards for pollinators, pollination success is predicted to be negatively related to flower density and flowering synchrony. We investigated the relationships between pollination success and flower abundance and flowering synchrony, and estimated pollinia dispersal distance in a rewardless species, Changnienia amoena (Orchidaceae). The results obtained in the present study revealed that male pollination success was negatively influenced by population size but was positively affected by population density, whereas female pollination success was independent of both population size and density. Phenotypic analysis suggested that highly synchronous flowering was advantageous through total pollination success, which is in contrast to previous studies. These results indicate that pollination facilitation rather than competition for pollinator visits occurs in this rewardless plant. The median distance of pollinia dispersal was 11.5 m (mean distance = 17.5 m), which is comparable to that of other rewardless plants but longer than for rewarding plants. However, pollen transfer occured mainly within populations; pollen import was a rare event. Restricted gene flow by pollinia and seeds probably explains the previous population genetic reporting a high degree of genetic differentiation between populations. © 2010 The Linnean Society of London, Biological Journal of the Linnean Society, 2010, 99 , 477–488.     

Historical and contemporary mating patterns in remnant populations of the forest tree Fraxinus excelsior L

Genetic variation at microsatellite markers was used to quantify genetic structure and mating behavior in a severely fragmented population of the wind-pollinated, wind-dispersed temperate tree Fraxinus excelsior in a deforested catchment in Scotland. Remnants maintain high levels of genetic diversity, comparable with those reported for continuous populations in southeastern Europe, and show low interpopulation differentiation (E = 0.080), indicating that historical gene exchange has not been limited (Nm = 3.48). We estimated from seeds collected from all trees producing fruits in three of five remnants that F. excelsior is predominantly outcrossing (t(m) = 0.971 +/- 0.028). Use of a neighborhood model approach to describe the relative contribution of local and long-distance pollen dispersal indicates that pollen gene flow into each of the three remnants is extensive (46-95%) and pollen dispersal has two components. The first is very localized and restricted to tens of meters around the mother trees. The second is a long-distance component with dispersal occurring over several kilometers. Effective dispersal distances, accounting for the distance and directionality to mother trees of sampled pollen donors, average 328 m and are greater than values reported for a continuous population. These results suggest that the opening of the landscape facilitates airborne pollen movement and may alleviate the expected detrimental genetic effects of fragmentation.     

Simple sequence repeats provide a direct estimate of pollen-mediated gene dispersal in the tropical tree Gliricidia sepium

An understanding of the processes that determine the observed patterns of genetic variation in natural plant populations is an important factor in the management of biodiversity. Pollen-mediated gene dispersal is recognized as a major determinant of population genetic structure. Here, the utility of simple sequence repeat (SSR) analysis was investigated for the measurement of pollen-mediated gene transfer by paternity exclusion in a restricted, fragmented and endangered population of the insect-pollinated tropical leguminous tree Gliricidia sepium located in Guatemala. Data at a single SSR locus, which revealed six allelic variants, were employed to generate minimum distance curves of pollen dispersal. Combined data from all six alleles indicated that a minimum of 1.8% of transfer events occurred over a distance of greater than 75 m. However, this value represents an underestimate because of the exclusion approach employed for analysis. Considering the four rarest alleles in the population only (combined frequency = 0.196), which provides a less biased indicator of gene transfer, a minimum of 6.1% of pollen movements could be attributed to greater than 75 m. One extreme example of gene transfer of over 275 m was recorded. Estimates of pollen transfer suggest a homogenizing effect on genetic structure over the spatial scale of the study population and provide an important indicator for the genetic management of natural and exotic stands of G. sepium . This study provides the first example of SSR analysis being employed to estimate directly pollen movement in a natural stand of any tree species.