Fat-tailed gene flow in the dioecious canopy tree species Fraxinus mandshurica var. japonica revealed by microsatellites

Pollen flow, seed dispersal and individual reproductive success can be simultaneously estimated from the genotypes of adults and offspring using stochastic models. Using four polymorphic microsatellite loci, gene flow of the wind-pollinated and wind-seed-dispersed dioecious tree species, Fraxinus mandshurica var. japonica, was quantified in a riparian forest, in northern Japan. In a 10.5-ha plot, 74 female adults, 76 male adults and 292 current-year seedlings were mapped and genotyped, together with 200 seeds. To estimate dispersal kernels of pollen and seeds, we applied normal, exponential power, Weibull, bivariate t-distribution kernels, and two-component models consisting of two normal distribution functions, one with a small and one with a large variance. A two-component pollen flow model with a small contribution (26.1%) from short-distance dispersal (sigma = 7.2 m), and the rest from long-distance flow (sigma = 209.9 m), was chosen for the best-fitting model. The average distance that integrated pollen flows inside and outside the study plot was estimated to be 196.8 m. Tree size and flowering intensity affected reproduction, and there appeared to be critical values that distinguished reproductively successful and unsuccessful adults. In contrast, the gene flow model that estimated both pollen and seed dispersal from established seedlings resulted in extensive seed dispersal, and the expected spatial genetic structures did not satisfactorily fit with the observations, even for the selected model. Our results advanced small-scale individual-based parentage analysis for quantifying fat-tailed gene flow in wind-mediated species, but also clarified its limitations and suggested future possibilities for gene flow studies.     

A comment on García et al. (2005, 2007) and related papers on mating patterns and gene dispersal in Prunus mahaleb

In two studies on mating patterns and spatial components of pollen and seed dispersal of Prunus mahaleb based on parentage analysis, García et al. (2005, 2007) depicted their 196 focal trees as a spatially isolated population where all reproductive trees had been genotyped. Additional distributional data for P. mahaleb trees in their study area, however, revealed that García and colleagues’ depiction of their study system bears little resemblance to reality. The trees these authors studied did not form a discrete, geographically isolated population. Around 300 ungenotyped reproductive trees occurred within the 1.5‐km distributional gap to the nearest population proclaimed by García and colleagues. Since exhaustive sampling of potential parental genotypes is essential in parentage analyses, the occurrence of a large number of ungenotyped trees in the immediate neighbourhood of focal trees can severely affect the main conclusions of García et al. (2005, 2007) as well as of several related publications on gene dispersal and mating patterns of P. mahaleb conducted on the same trees and relying on the same false premises of spatial isolation and exhaustive sampling.     

Patterns and levels of gene flow in Rhododendron metternichii var. hondoense revealed by microsatellite analysis

Parentage analysis was conducted to elucidate the patterns and levels of gene flow in Rhododendron metternichii Sieb. et Zucc. var. hondoense Nakai in a 150 x 70 m quadrant in Hiroshima Prefecture, western Japan. The population of R. metternichii occurred as three subpopulations at the study site. Seventy seedlings were randomly collected from each of three 10 x 10 m plots (S1, S2, and S3) on the forest floor of each subpopulation (A1, A2, and A3). Almost all parents (93.8%) of the 70 seedlings were unambiguously identified by using 12 pairs of microsatellite markers. Within the quadrant, adult trees less than 5 m from the centre of the seedling bank (plots S1, S2, and S3) produced large numbers of seedlings. The effects of tree height and distance from the seedling bank on the relative fertilities of adult trees were highly variable among subpopulations because of the differences in population structure near the seedling bank: neither distance nor tree height had any significant effect in subpopulation A1; distance from the seedling bank had a significant effect in subpopulation A2; and tree height had a significant effect in subpopulation A3. Although gene flow within each subpopulation was highly restricted to less than 25 m and gene flow among the three subpopulations was extremely small (0-2%), long-distance gene flow from outside the quadrant reached 50%. This long-distance gene flow may be caused by a combination of topographical and vegetational heterogeneity, differences in flowering phenology, and genetic substructuring within subpopulations.     

Characterization of microsatellite loci in the African fig Ficus sycomorus L. (Moraceae)

Microsatellite loci were characterized in the African fig tree Ficus sycomorus in order to investigate patterns of pollination and gene flow in this species. The loci characterized included new loci isolated from F. sycomorus and a single locus originally developed in Ficus carica. In total 12 loci were polymorphic when tested in between eight and 79 Namibian F. sycomorus individuals. Three of the new F. sycomorus loci were found to be polymorphic in cultivars of the edible fig F. carica suggesting a selection of these loci will be useful for population studies in other fig species.     

Pollen and seed dispersal inferred from seedling genotypes: the Bayesian revolution has passed here too

Understanding precisely how plants disperse their seeds and pollen in their neighbourhood is a central question for both ecologists and evolutionary biologists because seed and pollen dispersal governs both the rate of spread of an expanding population and gene flow within and among populations. The concept of a 'dispersal kernel' has become extremely popular in dispersal ecology as a tool that summarizes how dispersal distributes individuals and genes in space and at a given scale. In this issue of Molecular Ecology, the study by Moran & Clark (2011) (M&C in the following) shows how genotypic and spatial data of established seedlings can be analysed in a Bayesian framework to estimate jointly the pollen and seed dispersal kernels and finally derive a parentage analysis from a full-probability approach. This approach applied to red oak shows important dispersal of seeds (138 m on average) and pollen (178 m on average). For seeds, this estimate contrasts with previous results from inverse modelling on seed trap data (9.3 m). This research gathers several methodological advances made in recent years in two research communities and could become a cornerstone for dispersal ecology.     

Gynodioecy in structured populations: understanding fine-scale sex ratio variation in Beta vulgaris ssp. maritima

Natural selection, random processes and gene flow are known to generate sex ratio variations among sexually polymorphic plant populations. In gynodioecious species, in which hermaphrodites and females coexist, the relative effect of these processes on the maintenance of sex polymorphism is still up for debate. The aim of this study was to document sex ratio and cytonuclear genetic variation at a very local scale in wind-pollinated gynodioecious Beta vulgaris ssp. maritima and attempt to elucidate which processes explained the observed variation. The study sites were characterized by geographically distinct patches of individuals and appeared to be dynamic entities, with recurrent establishment of distinct haplotypes through independent founder events. Along with substantial variation in sex ratio and unexpectedly low gene flow within study sites, our results showed a high genetic differentiation among a mosaic of genetically distinct demes, with isolation by distance or abrupt genetic discontinuities taking place within a few tens of metres. Overall, random founder events with restricted gene flow could be primary determinants of sex structure, by promoting the clumping of sex-determining genes. Such high levels of sex structure provide a landscape for differential selection acting on sex-determining genes, which could modify the conditions of maintenance of gynodioecy in structured populations.     

Realized gene flow within mixed stands of Quercus robur L. and Q. petraea (Matt.) L. revealed at the stage of naturally established seedling

The estimates of contemporary gene flow assessed based on naturally established seedlings provide information much needed for understanding the abilities of forest tree populations to persist under global changes through migration and/or adaptation facilitated by gene exchange among populations. Here, we investigated pollen‐ and seed‐mediated gene flow in two mixed‐oak forest stands (consisting of Quercus robur L. and Q. petraea [Matt.] Liebl.). The gene flow parameters were estimated based on microsatellite multilocus genotypes of seedlings and adults and their spatial locations within the sample plots using models that attempt to reconstruct the genealogy of the seedling cohorts. Pollen and seed dispersal were modelled using the standard seedling neighbourhood model and a modification—the 2‐component seedling neighbourhood model, with the later allowing separation of the dispersal process into local and long‐distance components. The 2‐component model fitted the data substantially better than the standard model and provided estimates of mean seed and pollen dispersal distances accounting for long‐distance propagule dispersal. The mean distance of effective pollen dispersal was found to be 298 and 463 m, depending on the stand, while the mean distance of effective seed dispersal was only 8.8 and 15.6 m, which is consistent with wind pollination and primarily seed dispersal by gravity in Quercus. Some differences observed between the two stands could be attributed to the differences in the stand structure of the adult populations and the existing understory vegetation. Such a mixture of relatively limited seed dispersal with occasional long distance gene flow seems to be an efficient strategy for colonizing new habitats with subsequent local adaptation, while maintaining genetic diversity within populations.     

基于DNA分子标记的花粉流动态分析

花粉介导的基因流是植物有性繁殖世代之间的桥梁, 花粉散布属性是植物繁殖生态学、保护生物学和进化生物学研究关注的焦点。随着DNA分子技术的发展, 花粉流分析所使用的分子标记(尤其是微卫星标记)逐步替代了早期物理标记, 基于最大似然法估计以及新兴的基于贝叶斯推断的父本指派算法的发展, 能有效地估计花粉流散布的方向、距离和强度等重要特征。花粉散布曲线由单一参数向多参数模型发展, 以更好地获得花粉散布特征的拟合效果, 双组分的复合模型利用相互独立的参数空间使得散布曲线在长距离和短距离形状上呈现更大的可塑性。这些革新的技术和方法被成功应用于植物性别表型、隔离种群和杂交物种间花粉流分析, 以探讨进化、生态和保护等多领域的基础理论问题。近年来, 高通量测序技术的发展将进一步加快以分子标记为基础的花粉流动态分析在更广泛的植物类群中运用。     

Slatkin’s Paradox: when direct observation and realized gene flow disagree. A case study in Ficus

In flying insects, there is frequently a lack of congruence between empirical estimates of local demographic parameters and the prediction that differentiation between populations should decrease with increasing dispersal, a puzzling phenomenon known as Slatkin’s Paradox. Here, we generalize Slatkin’s Paradox to other taxa, drawing from available information on dispersal to predict the relative importance of pollen vs. seed migration in structuring broad‐scale patterns of genetic variation in Ficus hirta, a dioecious fig whose pollen is dispersed by minute, species‐specific fig wasps and whose seeds are disseminated by a variety of vagile vertebrates (especially bats and birds). Local‐scale observational and genetic studies of dioecious understory figs suggest comparable rates of pollen and seed migration. In contrast, we found unusually low nuclear differentiation (F_ST = 0.037, R_ST = 0.074) and high chloroplast differentiation (G_ST = 0.729, N_ST = 0.798) among populations separated by up to 2850 km, leading us to reject the hypothesis of equal pollen and seed migration rates and to obtain an equilibrium estimate of the ratio of pollen to seed migration of r = 16.2–36.3. We reconcile this example of Slatkin’s Paradox with previously published data for dioecious figs and relate it to the picture of exceptionally long‐distance wasp‐mediated pollen dispersal that is emerging for large monoecious fig trees. More generally, we argue that Slatkin’s Paradox is a general phenomenon and suggest it may be common in plants and animals.     

Genetic structure in large, continuous mammal populations: the example of brown bears in northwestern Eurasia

Knowledge of population structure and genetic diversity and the spatio-temporal demographic processes affecting populations is crucial for effective wildlife preservation, yet these factors are still poorly understood for organisms with large continuous ranges. Available population genetic data reveal that widespread mammals have for the most part only been carefully studied at the local population scale, which is insufficient for understanding population processes at larger scales. Here, we provide data on population structure, genetic diversity and gene flow in a brown bear population inhabiting the large territory of northwestern Eurasia. Analysis of 17 microsatellite loci indicated significant population substructure, consisting of four genetic groups. While three genetic clusters were confined to small geographical areas-located in Estonia, southern Finland and Leningrad oblast, Russia-the fourth cluster spanned a very large area broadly falling between northern Finland and the Arkhangelsk and Kirov oblasts of Russia. Thus, the data indicate a complex pattern where a fraction of the population exhibits large-scale gene flow that is unparalleled by other wild mammals studied to date, while the remainder of the population appears to have been structured by a combination of demographic history and landscape barriers. These results based on nuclear data are generally in good agreement with evidence previously derived using mitochondrial markers, and taken together, these markers provide complementary information about female-specific and population-level processes. Moreover, this study conveys information about spatial processes occurring over multiple generations that cannot be readily gained using other approaches, e.g. telemetry.