Patterns of pollen flow in a dense population of the insect-pollinated canopy tree species Castanopsis sieboldii

Insect pollinations of tree species with high-density populations have rarely been studied. Since the density of adults can affect effective pollen dispersal, short-distance pollination, even by insects, may frequently occur in high-density populations. To test this prediction, we investigated pollination patterns in a high-density population of the insect-pollinated canopy tree species Castanopsis sieboldii by paternity analysis using genotypes at 8 microsatellite loci of 145 adult trees and 439 seeds from 11 seed parents in a 4-ha plot. We then explored their genetic effects on the population by calculating other population genetics parameters. Although C. sieboldii has high potential for long-distance dispersal of pollen (as indicated by a fat-tailed dispersal kernel), the cumulative pollination at the local scale was spatially limited and strongly dependent on the distance between parents due to the high density of adults. Genetic diversity estimates for pollen pools accepted by each seed parent converged on a maximum as the effective number of pollen parents increased. The genetic diversity of pollen pool bulked over all the seed parents from inside the plot did not differ from that of the total pollen pools. Therefore, although pollen flow from distant pollen parents may help to maintain the genetic diversity of offspring, pollen parents neighboring seed parents may be the main contributors to the genetic diversity of the offspring at the seed stage.     

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.     

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.     

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.     

Estimating anisotropic pollen dispersal: a case study in Quercus lobata

The pollen dispersal distribution is an important element of the neighbourhood size of plant populations. Most methods aimed at estimating the dispersal curve assume that pollen dispersal is isotropic, but evidence indicates that this assumption does not hold for many plant species, particularly wind-pollinated species subject to prevailing winds during the pollination season. We propose here a method of detecting anisotropy of pollen dispersal and of gauging its intensity, based on the estimation of the differentiation of maternal pollen clouds (TWOGENER extraction), assuming that pollen dispersal is bivariate and normally distributed. We applied the new method to a case study in Quercus lobata, detecting only a modest level of anisotropy in pollen dispersal in a direction roughly similar to the prevailing wind direction. Finally, we conducted a simulation to explore the conditions under which anisotropy can be detected with this method, and we show that while anisotropy is detectable, in principle, it requires a large volume of data.     

Using genetic markers to estimate the pollen dispersal curve

Pollen dispersal is a critical process that shapes genetic diversity in natural populations of plants. Estimating the pollen dispersal curve can provide insight into the evolutionary dynamics of populations and is essential background for making predictions about changes induced by perturbations. Specifically, we would like to know whether the dispersal curve is exponential, thin-tailed (decreasing faster than exponential), or fat-tailed (decreasing slower than the exponential). In the latter case, rare events of long-distance dispersal will be much more likely. Here we generalize the previously developed TWOGENER method, assuming that the pollen dispersal curve belongs to particular one- or two-parameter families of dispersal curves and estimating simultaneously the parameters of the dispersal curve and the effective density of reproducing individuals in the population. We tested this method on simulated data, using an exponential power distribution, under thin-tailed, exponential and fat-tailed conditions. We find that even if our estimates show some bias and large mean squared error (MSE), we are able to estimate correctly the general trend of the curve - thin-tailed or fat-tailed - and the effective density. Moreover, the mean distance of dispersal can be correctly estimated with low bias and MSE, even if another family of dispersal curve is used for the estimation. Finally, we consider three case studies based on forest tree species. We find that dispersal is fat-tailed in all cases, and that the effective density estimated by our model is below the measured density in two of the cases. This latter result may reflect the difficulty of estimating two parameters, or it may be a biological consequence of variance in reproductive success of males in the population. Both the simulated and empirical findings demonstrate the strong potential of TWOGENER for evaluating the shape of the dispersal curve and the effective density of the population (d(e)).     

Pollen pool heterogeneity in shortleaf pine, Pinus echinata Mill

Pollen is the dominant vector of gamete exchange for most temperate tree species. Because pollen movement influences the creation, maintenance and erosion of genetic structure in adult populations, it is important to understand what factors influence the process of pollen movement. Isolation by distance in pollen donor populations can create highly structured pollen polls by increased sampling of local fathers. Extrinsic factors, such as the intervening vegetative structure and local pollen donor densities, can also influence the genetic composition of local pollen pools. Using paternally inherited chloroplast microsatellite markers, we examined the structure and diversity of pollen pools in Pinus echinata Mill. in southern Missouri, USA. Our analysis is based on a multivariate AMOVA analysis of stands ( approximately 1 ha; six per region) nested within regions (approximately 800 ha; four each). Significant multilocus structure of the pollen pool within regions (phiSR = 0.095), but not among regions (phiRT = 0.010), indicates that pollen movement is relatively restricted. Furthermore, the significant correlation between pairwise genetic and physical distances (Mantel correlation; rho = 0.32) provided support for the isolation by distance hypothesis. Our results indicated that availability of pollen donors did not affect diversity of the pollen pool, measured by the number of unique multilocus genotypes at each stand. However, pollen pool diversity was negatively associated with vegetative structure, measured as total forest tree density. Our findings indicated that on-going pollen movement within continuous forest is relatively restricted as a result of both isolation by distance and vegetative structure.     

Extensive pollen immigration and no evidence of disrupted mating patterns or reproduction in a highly fragmented holm oak stand

Aims Forest fragmentation and reduced tree population densities can potentially have negative impacts on mating patterns, offspring genetic diversity and reproductive performance. The aim of the present study is to test these hypotheses comparing an extremely fragmented, low tree density (~0.02 trees/ha) holm oak (Quercus ilex L.) stand from Central Spain with a nearby high tree density stand (~50 trees/ha).Methods We genotyped adult trees and seeds from the low-density stand (436 seeds from 15 families) and the high-density stand (404 seeds from 11 families) using nine microsatellite markers. With these data, we performed paternity analyses, determined pollen flow, mating patterns and pollen pool structure, and estimated progeny genetic diversity in both stands. We also studied seed set and production and performed a pollen supplementation experiment to determine whether reduced tree density has limited foreign pollen availability.Important findings We have found extensive pollen immigration (>75%) into the low tree density stand and Monte Carlo simulations revealed that pollen moves larger distances than expected from null models of random dispersal. Mating patterns and differentiation of pollen pools were similar in the high-density stand and the low-density stand but we found higher inter-annual differentiation of pollen pools in the former. Progeny genetic diversity and self-fertilization rates did not differ between the low-density stand and the high-density stand. Seed set rates were significantly lower in the low-density stand than in the high-density stand and experimental cross-pollen supplementation evidenced that foreign pollen availability is indeed a limiting factor in the former. However, seed crops did not differ between the low-density stand and the high-density stand, indicating that limitation of foreign pollen is not likely to be of great concern in terms of reduced seed production and potential recruitment. Poor forest regeneration due to other ecological and human factors is probably a more important threat for the persistence of fragmented and low tree density stands than reduced pollen flow and only extremely small and isolated tree populations would be expected to suffer severe loss of genetic diversity in the long term.     

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.     

Founder events as determinants of within-island and among-island genetic structure of Daphnia metapopulations

The genetic structure of metapopulations offers insights into the genetic consequences of local extinction and recolonization. We studied allozyme variation in rock pool metapopulations of two species of waterfleas (Daphnia) with the aim to understand how these dynamics influence genetic differentiation. We screened 138 populations of D. magna and 65 populations of D. longispina from an area in the archipelago of southern Finland. The pools from which they were sampled are separated by distances between 1.5 and 4710 m and located on a total of 38 islands. The genetic population structure of the two species was strikingly similar, consistent with their similar metapopulation ecology. The mean F(PT) value (differentiation among pools with respect to the total metapopulation) was 0.55 and a hierarchical analysis showed that genetic differentiation was strong (>0.25) among pools within islands as well as among whole islands. Within islands, pairwise genetic differentiation increased with geographic distance, indicating isolation by distance due to spatially limited dispersal. Previous studies have shown strong founder events occurring during colonization in our metapopulation. We suggest that the genetic population structure in the studied metapopulations is largely explained by three consequences of these founder events: (i) strong drift during colonization, (ii) local inbreeding, which results in hybrid vigour and increased effective migration rates after subsequent immigration, and (iii) effects of selection through hitchhiking of neutral genes with linked loci under selection.     

Interannual genetic heterogeneity of pollen pools accepted by Quercus salicina individuals

Since flowering often varies among years in wind-pollinated woody species, the genetic composition of pollen pools accepted by seed parents can differ between years. The interannual heterogeneity of pollen flow may be important for maintaining genetic diversity within populations because it can increase genetic variation within populations and the effective sizes of the populations. In this study we examined heterogeneity, using paternity analysis and analysis of molecular variance, in the genetic composition of pollen pools among different reproductive years for six Quercus salicina seed parents in an 11.56-ha plot in a temperate old-growth evergreen broadleaved forest. The genotypes at seven microsatellite loci were determined for 111 adult trees and 777 offspring of the six seed parents in 2-5 reproductive years. Genetic differentiation of pollen pools among different reproductive years for each seed parent was significant over all seed parents and for each of four seed parents that were analysed for more than 2 years, but not for either of the other two seed parents (analysed for 2 years). For both the pollen pools originating from inside the plot and those originating from outside it, genetic differentiation among different reproductive years for each seed parent was significant over all seed parents. However, among-year genetic differentiation in the pollen pools originating from within the plot was detected for all four of the seed parents that were analysed for more than 2 years, but for only one of the four in the pools originating from outside the plot. Genetic diversity (estimated as allelic richness and gene diversity) was higher for pollen pools over all reproductive years than for pollen pools in single years. These results indicate that the year-to-year genetic variation of pollen pools increases genetic diversity in offspring and is strongly affected by the variation in pollen parents within the plot because of their high pollination contributions. The high year-to-year variation in pollen parents within the plot and overall supports the hypothesis that the offspring produced across years represent a larger genetic neighbourhood.     

Fire‐induced population reduction and landscape opening increases gene flow via pollen dispersal in Pinus halepensis

Population reduction and disturbances may alter dispersal, mating patterns and gene flow. Rather than taking the common approach of comparing different populations or sites, here we studied gene flow via wind‐mediated effective pollen dispersal on the same plant individuals before and after a fire‐induced population drop, in a natural stand of Pinus halepensis. The fire killed 96% of the pine trees in the stand and cleared the vegetation in the area. Thirteen trees survived in two groups separated by ~80 m, and seven of these trees had serotinous (closed) prefire cones that did not open despite the fire. We analysed pollen from closed pre and postfire cones using microsatellites. The two groups of surviving trees were highly genetically differentiated, and the pollen they produced also showed strong among‐group differentiation and very high kinship both before and after the fire, indicating limited and very local pollen dispersal. The pollen not produced by the survivors also showed significant prefire spatial genetic structure and high kinship, indicating mainly within‐population origin and limited gene flow from outside, but became spatially homogeneous with random kinship after the fire. We suggest that postfire gene flow via wind‐mediated pollen dispersal increased by two putative mechanisms: (i) a drastic reduction in local pollen production due to population thinning, effectively increasing pollen immigration through reduced dilution effect; (ii) an increase in wind speeds in the vegetation‐free postfire landscape. This research shows that dispersal can alleviate negative genetic effects of population size reduction and that disturbances might enhance gene flow, rather than reduce it.     

Genetic diversity,spatial genetic structure and realised seed and pollen dispersal of Himatanthus drasticus (Apocynaceae) in the Brazilian savanna

We assessed the pollen and seed dispersal patterns, genetic diversity, inbreeding and spatial genetic structure of Himatanthus drasticus (Apocynaceae), a tree native to the Brazilian Savanna (Cerrado) that is heavily exploited for its medicinal latex. The study was conducted in the Araripe National Forest, Ceará State, Brazil. Within a one-hectare plot, samples were collected from all adult trees, adult trees located in the immediate vicinity of the plot, and seedlings. All sampled individuals were mapped and genotyped using microsatellite markers. High levels of polymorphism and significant levels of inbreeding were found, which indicates that self-fertilisation and mating among relatives occur in this population. Both the adults and seedlings had significant spatial genetic structure up to ~40 m and our results confirmed the occurrence of isolation by distance. Pollen and seeds were dispersed over short distances and immigration of pollen and seeds into the plot was estimated at 13 and 9 %, respectively. Taking into consideration the degree of inbreeding, relatedness, intrapopulation spatial genetic structure and pollen dispersal distance, we recommend collecting seeds from a large number of trees spaced at least 150 m apart to avoid collecting seeds from related individuals and an overlap of pollen pools among seed trees.     

Does long‐distance pollen dispersal preclude inbreeding in tropical trees? Fragmentation genetics of Dysoxylum malabaricum in an agro‐forest landscape

Tropical trees often display long‐distance pollen dispersal, even in highly fragmented landscapes. Understanding how patterns of spatial isolation influence pollen dispersal and interact with background patterns of fine‐scale spatial genetic structure (FSGS) is critical for evaluating the genetic consequences of habitat fragmentation. In the endangered tropical timber tree Dysoxylum malabaricum (Meliaceae), we apply eleven microsatellite markers with paternity and parentage analysis to directly estimate historic gene flow and contemporary pollen dispersal across a large area (216 km²) in a highly fragmented agro‐forest landscape. A comparison of genetic diversity and genetic structure in adult and juvenile life stages indicates an increase in differentiation and FSGS over time. Paternity analysis and parentage analysis demonstrate high genetic connectivity across the landscape by pollen dispersal. A comparison between mother trees in forest patches with low and high densities of adult trees shows that the frequency of short‐distance mating increases, as does average kinship among mates in low‐density stands. This indicates that there are potentially negative genetic consequences of low population density associated with forest fragmentation. Single isolated trees, in contrast, frequently receive heterogeneous pollen from distances exceeding 5 km. We discuss the processes leading to the observed patterns of pollen dispersal and the implications of this for conservation management of D. malabaricum and tropical trees more generally.     

Impact of selective logging on inbreeding and gene dispersal in an Amazonian tree population of Carapa guianensis Aubl

Selective logging may impact patterns of genetic diversity within populations of harvested forest tree species by increasing distances separating conspecific trees, and modifying physical and biotic features of the forest habitat. We measured levels of gene diversity, inbreeding, pollen dispersal and spatial genetic structure (SGS) of an Amazonian insect-pollinated Carapa guianensis population before and after commercial selective logging. Similar levels of gene diversity and allelic richness were found before and after logging in both the adult and the seed generations. Pre- and post-harvest outcrossing rates were high, and not significantly different from one another. We found no significant levels of biparental inbreeding either before or after logging. Low levels of pollen pool differentiation were found, and the pre- vs. post-harvest difference was not significant. Pollen dispersal distance estimates averaged between 75 m and 265 m before logging, and between 76 m and 268 m after logging, depending on the value of tree density and the dispersal model used. There were weak and similar levels of differentiation of allele frequencies in the adults and in the pollen pool, before and after logging occurred, as well as weak and similar pre- and post-harvest levels of SGS among adult trees. The large neighbourhood sizes estimated suggest high historical levels of gene flow. Overall our results indicate that there is no clear short-term genetic impact of selective logging on this population of C. guianensis.     

Species diversity and population density affect genetic structure and gene dispersal in a subtropical understory shrub

Aims The dispersal of pollen and seeds is spatially restricted and may vary among plant populations because of varying biotic interactions, population histories or abiotic conditions. Because gene dispersal is spatially restricted, it will eventually result in the development of spatial genetic structure (SGS), which in turn can allow insights into gene dispersal processes. Here, we assessed the effect of habitat characteristics like population density and community structure on small-scale SGS and estimate historical gene dispersal at different spatial scales.Methods In a set of 12 populations of the subtropical understory shrub Ardisia crenata, we assessed genetic variation at 7 microsatellite loci within and among populations. We investigated small-scale genetic structure with spatial genetic autocorrelation statistics and heterogeneity tests and estimated gene dispersal distances based on population differentiation and on within-population SGS. SGS was related to habitat characteristics by multiple regression.Important findings The populations showed high genetic diversity (H e = 0.64) within populations and rather strong genetic differentiation (F ′ ST = 0.208) among populations, following an isolation-by-distance pattern, which suggests that populations are in gene flow–drift equilibrium. Significant SGS was present within populations (mean Sp = 0.027). Population density and species diversity had a joint effect on SGS with low population density and high species diversity leading to stronger small-scale SGS. Estimates of historical gene dispersal from between-population differentiation and from within-population SGS resulted in similar values between 4.8 and 22.9 m. The results indicate that local-ranged pollen dispersal and inefficient long-distance seed dispersal, both affected by population density and species diversity, contributed to the genetic population structure of the species. We suggest that SGS in shrubs is more similar to that of herbs than to trees and that in communities with high species diversity gene flow is more restricted than at low species diversity. This may represent a process that retards the development of a positive species diversity–genetic diversity relationship.     

High paternal diversity in the self-incompatible herb Arabidopsis halleri despite clonal reproduction and spatially restricted pollen dispersal

The number of sires fertilizing a given dam is a key parameter of the mating system in species with spatially restricted offspring dispersal, since genetic relatedness among maternal sibs determines the intensity of sib competition. In flowering plants, the extent of multiple paternity is determined by factors such as floral biology, properties of the pollen vector, selfing rate, spatial organization of the population, and genetic compatibility between neighbours. To assess the extent of multiple paternity and identify ecological factors involved, we performed a detailed study of mating patterns in a small population of a self-incompatible clonal herb, Arabidopsis halleri . We mapped and genotyped 364 individuals and 256 of their offspring at 12 microsatellite loci and jointly analysed the level of multiple paternity, pollen and seed dispersal, and spatial genetic structure. We found very low levels of correlated paternity among sibs ( P _full-sib = 3.8%) indicating high multiple paternity. Our estimate of the outcrossing rate was 98.7%, suggesting functional self-incompatibility. The pollen dispersal distribution was significantly restricted (mean effective pollen dispersal distance: 4.42 m) but long-distance successful pollination occurred and immigrating pollen was at most 10% of all pollination events. Patterns of genetic structure indicated little extent of clonal reproduction, and a low but significant spatial genetic structure typical for a self-incompatible species. Overall, in spite of restricted pollen dispersal, the multiple paternity in this self-incompatible species was very high, a result that we interpret as a consequence of high plant density and high pollinator service in this population.     

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.     

The relative contributions of seed and pollen dispersal to gene flow and genetic diversity in seedlings of a tropical palm

Seed and pollen dispersal shape patterns of gene flow and genetic diversity in plants. Pollen is generally thought to travel longer distances than seeds, but seeds determine the ultimate location of gametes. Resolving how interactions between these two dispersal processes shape microevolutionary processes is a long‐standing research priority. We unambiguously isolated the separate and combined contributions of these two dispersal processes in seedlings of the animal‐dispersed palm Oenocarpus bataua to address two questions. First, what is the spatial extent of pollen versus seed movement in a system characterized by long‐distance seed dispersal? Second, how does seed dispersal mediate seedling genetic diversity? Despite evidence of frequent long‐distance seed dispersal, we found that pollen moves much further than seeds. Nonetheless, seed dispersal ultimately mediates genetic diversity and fine‐scale spatial genetic structure. Compared to undispersed seedlings, seedlings dispersed by vertebrates were characterized by higher female gametic and diploid seedling diversity and weaker fine‐scale spatial genetic structure for female gametes, male gametes and diploid seedlings. Interestingly, the diversity of maternal seed sources at seed deposition sites (N _em) was associated with higher effective number of pollen sources (N _ep), higher effective number of parents (N _e) and weaker spatial genetic structure, whereas seed dispersal distance had little impact on these or other parameters we measured. These findings highlight the importance maternal seed source diversity (N _em) at frugivore seed deposition sites in driving emergent patterns of fine‐scale genetic diversity and structure.     

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.