Two-generation analysis of pollen flow across a landscape. III. Impact of adult population structure

The rate and distance of instantaneous pollen flow in a population are parameters of considerable current interest for plant population geneticists and conservation biologists. We have recently developed an estimator (phi ft) of differentiation between the inferred pollen clouds that fertilize several females, sampled within a single population. We have shown that there is a simple relation between phi ft and the average pollen dispersal distance (delta) for the case of a population with no geographic structure. Though forest trees usually show considerable pollen flow, assuming an absence of spatially distributed genetic structure is not always wise. Here, we develop analytical theory for the relation between phi ft and delta, for the case where the probability of Identity by Descent (IBD) for two individuals decreases with the physical distance between them. This analytical theory allows us to provide an effective method for estimating pollen dispersal distance in a population with adult genetic structure. Using real examples, we show that estimation errors can be large if genetic structure is not taken into account, so it is wise to evaluate adult genetic structure simultaneously with estimation of phi ft for the pollen clouds. We show that the results are only moderately affected by changes in the decay function, a result of some importance since no completely established theory is available for this function.     

A new method of estimating the pollen dispersal curve independently of effective density

We introduce a novel indirect method of estimating the pollen dispersal curve from mother-offspring genotypic data. Unlike an earlier indirect approach (TwoGener), this method is based on a normalized measure of correlated paternity between female pairs whose expectation does not explicitly depend on the unknown effective male population density (d(e)). We investigate the statistical properties of the new method, by comparison with those of TwoGener, considering the sensitivity to reductions of d(e), relative to census density, resulting from unequal male fecundity and asynchronous flowering. Our main results are: (i) it is possible to obtain reliable estimates of the average distance of pollen dispersal, delta, from indirect methods, even under nonuniform male fecundity and variable flowering phenology; (ii) the new method yields more accurate and more precise delta-estimates than TwoGener under a wide range of sampling and flowering scenarios; and (iii) TwoGener can be used to obtain approximate d(e) estimates, if needed for other purposes. Our results also show that accurately estimating the shape of the tail of the pollen dispersal function by means of indirect methods remains a very difficult challenge.     

gener: a server‐based analysis of pollen pool structure

The server‐based program gener performs the two‐generation analysis of pollen flow for data consisting of mother/offspring arrays using genetic markers. The gener program decomposes the genetic variance sampled by maternal individuals within and among pollen pool components of genetic variance and is accessible from http://dyerlab.bio.vcu.edu . These estimates are used to construct the test statistic, Φ_ft , whose significance is tested via permutation. The Φ_ft statistic can subsequently be used to quantify genetic effective pollen donor population size (N_ep), effective mating area and dispersal distance. Furthermore, the gener program can calculate Φ_ft values for all pairs of substrata within the data set.     

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.     

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.     

Pollen movement in declining populations of California Valley oak,Quercus lobata: where have all the fathers gone?

The fragmented populations and reduced population densities that result from human disturbance are issues of growing importance in evolutionary and conservation biology. A key issue is whether remnant individuals become reproductively isolated. California Valley oak (Quercus lobata) is a widely distributed, endemic species in California, increasingly jeopardized by anthropogenic changes in biota and land use. We studied pollen movement in a savannah population of Valley oak at Sedgwick Reserve, Santa Barbara County, to estimate effective number of pollen donors (Nep) and average distance of effective pollen movement (delta). Using twogener, our recently developed hybrid model of paternity and genetic structure treatments that analyses maternal and progeny multilocus genotypes, we found that current Nep = 3.68 individuals. Based on an average adult density of d= 1.19 stems/ha, we assumed a bivariate normal distribution to model current average pollen dispersal distance (delta) and estimated delta= 64.8 m. We then deployed our parameter estimates in spatially explicit models of the Sedgwick population to evaluate the extent to which Nep may have changed, as a consequence of progressive stand thinning between 1944 and 1999. Assuming that pollen dispersal distance has not changed, we estimate Nep was 4.57 individuals in 1944, when stand density was 1.48. Both estimates indicate fewer effective fathers than one might expect for wind-pollinated species and fewer than observed elsewhere. The results presented here provide a basis for further refinements on modelling pollen movement. If the trends continue, then ongoing demographic attrition could further reduce neighbourhood size in Valley oak resulting in increased risk of reproductive failure and genetic isolation.     

Understanding the effects of isolation on seed and pollen flow,spatial genetic structure and effective population size of the dioecious tropical tree species <Emphasis Type="Italic">Myracrodruon urundeuva</Emphasis>

This study examines the levels of gene flow, the distance and the patterns of pollen and seed dispersal, the intra-population spatial genetic structure (SGS) and the effective population size of a spatially isolated Myracrodruon urundeuva population using five microsatellite loci. The study was carried out in the Paulo de Faria Ecological Station, São Paulo State, Brazil and included the sampling and mapping of 467 adult-trees and 149 juveniles. Open-pollinated seeds (514) from 29 seed-trees were also sampled and genotyped. Significant SGS was detected in both adult (S _p  = 0.0269) and juveniles trees (S _p  = 0.0246), indicating short-distance seed dispersal. Using maternity analysis, all juveniles had the mother-tree assigned within the stand. A father-tree within the stand was also assigned for 97.3% of the juveniles and 98.4% of offspring. The average pollen dispersal distance measured in juveniles \( \left( {\hat{\delta } = 1 3 8\pm 1 6 9 {\text{ m}},{\text{ mean}} \pm {\text{SD}}} \right) \) and offspring \( \left( {\hat{\delta } = 2 5 2\pm 20 4 {\text{ m}}} \right) \) were higher than the average seed dispersal distance measured in juveniles \( \left( {\hat{\delta } = 1 2 4\pm 1 50{\text{ m}}} \right) \). About 70% of the pollen from juveniles and 51% from offspring traveled less than 200 m and, 72% of the seeds traveled less than 50 m. The effective population size of the studied sample indicates that the 467 adult-trees and 145 juveniles correspond respectively to 335 and 63 individuals that are neither inbred nor relatives. The results are discussed in relation to their impact on seed collection practices and genetic conservation.     

Evaluating the influence of different aspects of habitat fragmentation on mating patterns and pollen dispersal in the bird-pollinated Banksia sphaerocarpa var. caesia

Habitat fragmentation can significantly affect mating and pollen dispersal patterns in plant populations, although the differential effects of the various aspects of fragmentation are poorly understood. In this study, we used eight microsatellite loci to investigate the effect of fragmentation on the mating system and pollen dispersal within one large and eight small population remnants of Banksia sphaerocarpa var. caesia, a bird-pollinated shrub in the southern agricultural region of Western Australia. The large population had a much larger neighbourhood size and lower selfing rate, maternal pollen pool differentiation and within-plot mean pollen dispersal distance than the small populations. Outcrossing was consistently high and ranged from 85.7% ± 2.6 to 98.5% ± 0.9, and mating patterns suggested nearest-neighbour pollination. Pollen immigration into small populations ranged from 2.8% ± 1.8 to 16.5% ± 3.2. Using the small populations, we tested for correlations between various fragmentation variables and mating system and pollen dispersal parameters. We found significant negative linear relationships between population isolation and outcrossing rate; population shape and neighbourhood size; and conspecific density and mean pollen dispersal distance. There were significant positive linear relationships between population shape and pollen pool differentiation and between population size and number of different fathers per seed crop. Our results suggest that birds may use a series of fragmented populations as a vegetation corridor while foraging across the landscape and that population connectivity is a critical determinant of pollinator visitation. Our results also suggest that the effect of a linear population shape on the mating system and pollen dispersal is routinely underestimated.     

Two-generation analysis of pollen flow across a landscape. I. Male gamete heterogeneity among females

Gene flow is a key factor in the spatial genetic structure in spatially distributed species. Evolutionary biologists interested in microevolutionary processess and conservation biologists interested in the impact of landscape change require a method that measures the real time process of gene movement. We present a novel two-generation (parent-offspring) approach to the study of genetic structure (TwoGener) that allows us to quantify heterogeneity among the male gamete pools sampled by maternal trees scattered across the landscape and to estimate mean pollination distance and effective neighborhood size. First, we describe the model's elements: genetic distance matrices to estimate intergametic distances, molecular analysis of variance to determine whether pollen profiles differ among mothers, and optimal sampling considerations. Second, we evaluate the model's effectiveness by simulating spatially distributed populations. Spatial heterogeneity in male gametes can be estimated by phiFT, a male gametic analogue of Wright's F(ST) and an inverse function of mean pollination distance. We illustrate TwoGener in cases where the male gamete can be categorically or ambiguously determined. This approach does not require the high level of genetic resolution needed by parentage analysis, but the ambiguous case is vulnerable to bias in the absence of adequate genetic resolution. Finally, we apply TwoGener to an empirical study of Quercus alba in Missouri Ozark forests. We find that phiFT = 0.06, translating into about eight effective pollen donors per female and an effective pollination neighborhood as a circle of radius about 17 m. Effective pollen movement in Q. alba is more restricted than previously realized, even though pollen is capable of moving large distances. This case study illustrates that, with a modest investment in field survey and laboratory analysis, the TwoGener approach permits inferences about landscape-level gene movements.     

Short-distance pollen dispersal for an outcrossed, wind-pollinated southern beech (Nothofagus nervosa (Phil.) Dim. et Mil.)

The mating system (outcrossing, selfing, and biparental inbreeding) and the extent of pollen flow are two of the most important genetic features that determine the genetic structure of plant populations, and both are crucial for the design of conservation strategies. The objectives here were to estimate mating system parameters and to fit the pollen dispersal kernel for the southern beech, Nothofagus nervosa. We sampled 25 mothers and 372 progeny from two stands in the Tromen Lake region of Argentina. We registered spatial positions of the maternal trees, and genotyped mothers and offspring for five simple sequence repeat markers. We estimated single-locus (t _s?=?0.95) and multilocus (t _m?=?0.99) outcrossing rates and biparental inbreeding (t _m-t _s?=?0.04). The species is strongly outcrossing, but correlated paternity within maternal sibships (r _p?=?0.10) indicates that each maternal parent is sampling a different and restricted array of pollen donors. We used two protocols (twogener and kindist) to fit an exponential power dispersal kernel to the structure of pollen clouds sampled by different mothers. The estimated effective number of pollen donors contributing to a single mother was N _ep?=?9.9. The twogener and kindist analyses yielded slightly different estimates, but both indicated short average distances for pollen dispersal (<35?m), indicating that the dispersal kernel was strongly leptokurtic (???=?0.36). While short-distance pollen dispersal predominates, there remains a nontrivial probability of long-distance dispersal. The results are discussed in the context of ongoing conservation and management programs.     

Measuring the genetic structure of the pollen pool as the probability of paternal identity

Contemporary pollen flow in forest plant species is measured by the probability of paternal identity (PPI) for two randomly sampled offspring, drawn from a single female, and contrasting that with PPI for two random offspring, drawn from different females. Two different estimation strategies have emerged: (a) an indirect approach, using the 'genetic structure' of the pollen received by different mothers and (b) a direct approach, based on parentage analysis. The indirect strategy is somewhat limited by the assumptions, but is widely useful. The direct approach is most appropriate where a large majority of the true fathers can be identified exactly, which is sometimes possible with high-resolution SSR markers. Using the parentage approach, we develop estimates of PPI, showing that the obvious estimates are severely biased, and providing an unbiased alternative. We then illustrate the methods with SSR data from a 36-tree isolated population of Pinus sylvestris from the Meseta region of Spain, for which categorical paternity assignment was available for over 95% of offspring. For all the females combined, we estimate that PPI=0.0425, indicating uneven male reproductive contributions. Different (but overlapping) arrays of males pollinate different females, and for the average female, PPI=0.317, indicating substantial 'pollen structure' for the population. We also relate the direct measures of PPI to those available from indirect approaches, and show that they are generally comparable.     

Fluorescent dye particles as pollen analogues for measuring pollen dispersal in an insect-pollinated forest herb

In flowering plants, pollen dispersal is often the major contributing component to gene flow, hence a key parameter in conservation genetics and population biology. A cost-effective method to assess pollen dispersal consists of monitoring the dispersal of fluorescent dyes used as pollen analogues. However, few comparisons between dye dispersal and realized pollen dispersal have been performed to validate the method. We investigated pollen dispersal in two small populations of the insect-pollinated herb Primula elatior from urban forest fragments using direct (paternity analyses based on microsatellite DNA markers) and indirect (fluorescent dyes) methods. We compared these methods using two approaches, testing for the difference between the distance distributions of observed dispersal events and estimating parameters of a dispersal model, and related these results to dye dispersal patterns in three large populations. Dye and realized (based on paternity inference) pollen dispersal showed exponential decay distributions, with 74.2?C94.8% of the depositions occurring at <50?m and a few longer distance dispersal events (up to 151?m). No significant difference in curve shape was found between dye and realized pollen dispersal distributions. The best-fitting parameters characterizing the dye dispersal model were consistent with those obtained for realized pollen dispersal. Hence, the fluorescent dye method may be considered as reliable to infer realized pollen dispersal for forest herbs such as P. elatior. However, our simulations reveal that large sample sizes are needed to detect moderate differences between dye and realized pollen dispersal patterns because the estimation of dispersal parameters suffers low precision.     

Estimating seed and pollen movement in a monoecious plant: a hierarchical Bayesian approach integrating genetic and ecological data

The scale of seed and pollen movement in plants has a critical influence on population dynamics and interspecific interactions, as well as on their capacity to respond to environmental change through migration or local adaptation. However, dispersal can be challenging to quantify. Here, we present a Bayesian model that integrates genetic and ecological data to simultaneously estimate effective seed and pollen dispersal parameters and the parentage of sampled seedlings. This model is the first developed for monoecious plants that accounts for genotyping error and treats dispersal from within and beyond a plot in a fully consistent manner. The flexible Bayesian framework allows the incorporation of a variety of ecological variables, including individual variation in seed production, as well as multiple sources of uncertainty. We illustrate the method using data from a mixed population of red oak (Quercus rubra, Q. velutina, Q. falcata) in the NC piedmont. For simulated test data sets, the model successfully recovered the simulated dispersal parameters and pedigrees. Pollen dispersal in the example population was extensive, with an average father-mother distance of 178 m. Estimated seed dispersal distances at the piedmont site were substantially longer than previous estimates based on seed-trap data (average 128 m vs. 9.3 m), suggesting that, under some circumstances, oaks may be less dispersal-limited than is commonly thought, with a greater potential for range shifts in response to climate change.     

Pollen Dispersal Between Isolated Trees in the Brazilian Savannah: A Case Study of the Neotropical Tree Hymenaea stigonocarpa

This case study examines the pollen dispersal distance, pollen dispersal patterns and intra‐family genetic structure for isolated trees in pastures of the bat‐pollinated Neotropical tree species Hymenaea stigonocarpa using six microsatellite loci and parentage analysis. The sampling included 28 grouped trees (referred to as the population) and six isolated trees in pastureland along a highway in Mato Grosso do Sul State, Brazil. From the population, we sampled 137 seeds from 12 seed‐trees, and from the isolated trees, we sampled 34 seeds from two seed‐trees. The results showed that pollen was dispersed over long distances (reaching 7353 m) and therefore the spatially isolated trees were not reproductively isolated. The pollen immigration rate in the population was also high (31%). Isolated trees presented a higher selfing rate (s=26%) than trees in the population (s=12%), suggesting that the spatial isolation of the trees increased selfing. However, selfing was responsible for only 30 percent of the inbreeding in offspring and mating among relatives was 70 percent. In the population, excluding selfing, ca 72 percent of the pollen was dispersed over distances <1000 m (average: 860 m). For the two isolated seed‐trees, excluding selfing, the average pollen dispersal distance was 5229 m. The results demonstrate that although pollen can be dispersed over long distances for H. stigonocarpa isolated trees, a high percentage of pollen comes from the same tree (selfing) and mating was correlated. Consequently, seeds must be collected from a large number of seed‐trees for conservation purposes.     

A method to determine the mean pollen dispersal of individual plants growing within a large pollen source

Pollen dispersal has been recently focused on as a major issue in the risk assessment of transgenic crop plants. The shape of the pollen dispersal of individual plants is hard to determine since a very large number of plants must be monitored in order to track rare longdistance dispersal events. Conversely, studies using large plots as a pollen source provide a pollen distribution that depends on the shape of the source plot. We report here on a method based on the use of Fourier transforms by which the pollen dispersal of a single, average individual can be obtained from data using large plots as pollen sources, thus allowing the estimation of the probability of long-distance dispersal for single plants. This method is subsequently implemented on simulated data to test its susceptibility to random noise and edge effects. Its conditions of application and value for use in ecological studies, in particular risk assessment of the deliberate release of transgenic plants, are discussed.     

Short-distance pollen dispersal and high self-pollination in a bat-pollinated neotropical tree

We investigated pollen dispersal and breeding structure in the tropical tree species Caryocar brasiliense Camb. (Caryocaraceae), using genetic data from ten microsatellite loci. All adult trees (101) within a patch of 8.3 ha were sampled, and from these adults 18 open-pollinated maternal progeny arrays were analyzed (280 seeds from 265 fruits). Most fruits presented only one seed (median equal to 1.000) and mean number of ripened seeds per fruit was 1.053 (SD?=?0.828). Our results showed that C. brasiliense presents a mixed-mating system, with 11.4% of self-pollination, multilocus outcrossing rate of t _m?=?0.891?±?0.025, and high probability of full-sibship within progeny arrays (r _p?=?0.135?±?0.032). Outcrossing rate and self-pollination varied significantly among mother trees. We could detect a maximum pollen dispersal distance of ～500 m and a mean pollen dispersal distance of ～132 m. However, most pollination events (80%) occurred at distances less than 200 m. Our results also indicated that pollen dispersal is restricted to a neighborhood of 5.4 ha with rare events of immigration (～1% N _e m?=?0.35). C. brasiliense also presents a significant but weak spatial genetic structure (Sp?=?0.0116), and extension of pollen dispersal distance was greater than seed dispersal (N _b?=?86.20 m). These results are most likely due to the foraging behavior of pollinators that may have limited flight range. The highly within-population synchronous flowering, high population density, and clumped distribution reinforce pollinator behavior and affect residence time leading to a short-distance pollen dispersal.     

Temporal variation in pollen dispersal and breeding structure in a bee-pollinated Neotropical tree

Variation among flowering seasons in the time of flowering, synchrony and length of flowering, and fluctuations in the abundance of pollinators may cause a variation in pollen dispersal distance. In this study, we analyzed the temporal variation in pollen dispersal and breeding structure in the Neotropical tree species Tabebuia aurea (Bignoniaceae) and evaluated pollen dispersal between a population inside the reserve and a patch of isolated individuals on the edge of the reserve, and tested the hypothesis that isolated individuals are sinking for pollen. All adult trees (260) within a population of 40 ha and 9 isolated individuals on the edge of the reserve were sampled, and from these adults, 21 open-pollinated progeny arrays were analyzed in 2 flowering seasons (309 seeds in 2004 and 328 in 2005). Genetic analyses were based on the polymorphism at 10 microsatellite loci. A high proportion of self-pollination found in both flowering seasons indicated a mixed-mating system. The mean pollen dispersal distance differed significantly between the two flowering seasons (307.78 m in 2004 and 396.26 m in 2005). Maximum pollen dispersal was 2608 m, but most pollination events (65%) occurred at distances <300 m. Our results also showed that isolated individuals are sinking for pollen, with high pollen flow between the population inside the reserve and individuals on the edge. These results are most likely due to the large pollinator species, which can potentially fly long distances, and also due to temporal variation in individual fecundity and contribution to pollen dispersal.     

Some physical properties of teosinte (Zea mays subsp. parviglumis) pollen

In parts of the world where teosinte and maize are grown in close proximity, there is concern about gene flow between them. Pollen is the primary vehicle for gene flow. Quantifying the biophysical properties of pollen, such as its settling speed and dehydration rate, is important for evaluating outcrossing potential. These properties were measured for teosinte (Zea mays subsp. parviglumis) pollen. Pollen was found to have an average settling speed of 0.165 m s(-1), which agrees well with theoretical values based on the size of the pollen grains. The conductance of the pollen wall for water was derived from the time rate of change of pollen grain size and gave an average conductance of 3.42x10(-4) m s(-1). Water potential, psi, of teosinte pollen was determined at various values of relative water content (dry-weight basis), theta, by using a thermocouple psychrometer and by allowing samples of pollen to come to vapour equilibrium with various saturated salt solutions. Non-linear regression analysis of the data yielded psi (MPa) = -4.13 theta(-1.23) (r2=0.77). Results for conductance and psi were incorporated into a model equation for the rate of water loss from pollen grains, which yielded results that agreed well (r2=0.96) with observations of water loss from pollen grains in air. The data reported here are important building blocks in a model of teosinte pollen movement and should be helpful in establishing the main factors influencing the degree and the direction of pollination between teosinte populations and between maize and teosinte.     

Mating patterns of black oak Quercus velutina (Fagaceae) in a Missouri oak-hickory forest

Wind-pollinated forest trees usually have high outcrossing rates, but allogamy does not necessarily translate into high pollen movement. The goal of this study was to determine the outcrossing rates, pollen pool genetic structure, and the size of the effective pollination neighborhood in a population of black oak, Quercus velutina, in a Missouri oak-hickory forest. Based on 6 allozyme loci, 12 maternal trees, and 439 progenies sampled along a transect of 1300 m, we found complete outcrossing (t(m) = 1.000, P < 0.001) and small amounts of biparental inbreeding. Using a TwoGener analysis of the pollen gene pool, we found significant structure across maternal plants (Phi(FT) = 0.078, P < 0.001), which when corrected for adult inbreeding translates into Phi(FT) = 0.066 that corresponds to an effective number of pollen donors of 7.5 individuals. Assuming a bivariate normal distribution and an adult density of 16.25 trees ha(-1), we estimated that the effective pollination neighborhood area had a radius of 41.9 m. Even assuming that our estimates may be conservative, these findings join a growing body of evidence that suggest that the local neighborhood of wind-pollinated forest tree populations may be relatively small creating opportunities for local selection and genetic drift.     

Estimating seed vs. pollen dispersal from spatial genetic structure in the common ash

Spatial genetic structure was analysed with five highly polymorphic microsatellite loci in a Romanian population of common ash (Fraxinus excelsior L.), a wind-pollinated and wind-dispersed tree species occurring in mixed deciduous forests over almost all of Europe. Contributions of seed and pollen dispersal to total gene flow were investigated by analysing the pattern of decrease in kinship coefficients among pairs of individuals with geographical distance and comparing it with simulation results. Plots of kinship against the logarithm of distance were decomposed into a slope and a shape component. Simulations showed that the slope is informative about the global level of gene flow, in agreement with theoretical expectations, whereas the shape component was correlated with the relative importance of seed vs. pollen dispersal. Hence, our results indicate that insights into the relative contributions of seed and pollen dispersal to overall gene flow can be gained from details of the pattern of spatial genetic structure at biparentally inherited loci. In common ash, the slope provided an estimate of total gene dispersal in terms of Wright's neighbourhood size of Nb = 519 individuals. No precise estimate of seed vs. pollen flow could be obtained from the shape because of the stochasticity inherent to the data, but the parameter combinations that best fitted the data indicated restricted seed flow, sigmas pound 14 m, and moderate pollen flow, 70 m pound sigmap pound 140 m.