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

Pollen production for 13 urban North American tree species: Allometric equations for tree trunk diameter and crown area

Estimates of airborne pollen concentrations at the urban scale would be useful for epidemiologists, land managers, and allergy sufferers. Mechanistic models could be well suited for this task, but their development will require data on pollen production across cities, including estimates of pollen production by individual trees. In this study, we developed predictive models for pollen production as a function of trunk size, canopy area, and height, which are commonly recorded in tree surveys or readily extracted from remote sensing data. Pollen production was estimated by measuring the number of flowers per tree, the number of anthers per flower, and the number of pollen grains per anther. Variability at each morphological scale was assessed using bootstrapping. Pollen production was estimated for the following species: Acer negundo, Acer platanoides, Acer rubrum, Acer saccharinum, Betula papyrifera, Gleditsia triacanthos, Juglans nigra, Morus alba, Platanus x acerfolia, Populus deltoides, Quercus palustris, Quercus rubra, and Ulmus americana. Basal area predicted pollen production with a mean R2 of 0.72 (range: 0.41 - 0.99), whereas canopy area predicted pollen production with a mean R2 of 0.76 (range: 0.50 - 0.99). These equations are applied to two tree datasets to estimate total municipal pollen production and the spatial distribution of street tree pollen production for the focal species. We present some of the first individual-tree based estimates of pollen production at the municipal scale; the observed spatial heterogeneity in pollen production is substantial and can feasibly be included in mechanistic models of airborne pollen at fine spatial scales.

Functional androdioecy in the ornamental shrub Osmanthus delavayi (Oleaceae)

Androdioecy is one of the rarest sexual systems among plants, characterized by males co-occurring with hermaphrodites. Osmanthus delavayi (Oleaceae), an ornamental shrub from southern China, is known to have both male and hermaphrodite individuals, but little is known regarding the breeding system of this species and whether it is functionally androdioecious, and how this potentially evolved. In this study, we explore the characteristics of the breeding system of O. delavayi through the study of phenology, sex ratio, floral organ morphology, pollen number, stigma receptivity, artificial pollination, pollinators, and gene flow within and between populations, while also discussing the evolution and maintenance of androdioecy within the genus. The proportion of males was less than 0.5 and the out-crossing index (OCI) was 5. Morphological androdioecy was observed, with hermaphrodite flowers having fertile pistils, while male flowers had degenerated pistils. Males and hermaphrodites both had large amounts of small and fertile pollen grains, although the pollen number of males was ca. 1.21 × more than that of hermaphrodites, and pollen was generally smaller. Self-pollination was found to produce a much lower fruit set than outcrossing under natural conditions. Gene flow between males and hermaphrodites within a population was greater (1.007) than that between populations (0.753). All these results indicate that O. delavayi is functionally androdioecious, which may be an intermediate state in the evolutionary transition from hermaphroditism to dioecy.

Contrasting patterns of pollen and seed flow influence the spatial genetic structure of sweet vernal grass (Anthoxanthum odoratum) populations

The spatial genetic structure of plant populations is determined by a combination of gene flow, genetic drift, and natural selection. Gene flow in most plants can result from either seed or pollen dispersal, but detailed investigations of pollen and seed flow among populations that have diverged following local adaptation are lacking. In this study, we compared pollen and seed flow among 10 populations of sweet vernal grass (Anthoxanthum odoratum) on the Park Grass Experiment. Overall, estimates of genetic differentiation that were based on chloroplast DNA (cpDNA) and, which therefore resulted primarily from seed flow, were lower (average F(ST) = 0.058) than previously published estimates that were based on nuclear DNA (average F(ST) = 0.095). Unlike nuclear DNA, cpDNA showed no pattern of isolation by adaptation; cpDNA differentiation was, however, inversely correlated with the number of additions (nutrients and lime) that each plot had received. We suggest that natural selection is restricting pollen flow among plots, whereas nutrient additions are increasing seed flow and genetic diversity by facilitating the successful germination and growth of immigrant seeds. This study highlights the importance of considering all potential gene flow mechanisms when investigating determinants of spatial genetic structure, and cautions against the widespread assumption that pollen flow is more important than seed flow for population connectivity in wind-pollinated species.

Fine-scale genetic structure within plots of Polygala reinii (Polygalaceae) having an ant-dispersal seed

Fine-scale genetic structure within a population was analyzed for the myrmecochorous forest perennial Polygala reinii (Polygalaceae) using allozyme loci. In the analysis, two sampling plots were established to cover the isolated patchy distribution within the study population. Size and spatial structure were also examined for the plots to assess their interaction with the genetic structuring. Allozyme analysis based on 13 putative loci encoding 10 enzyme systems showed high genetic variation and low values of fixation indices at the two plots. Spatial autocorrelation analysis based on the multilocus coancestry coefficient (f ( ij )) revealed significant genetic structuring in both plots, suggesting limited gene-, especially seed-, dispersal within the population. The spatial structure within the plots, assessed by O-ring statistics, was characterized by the occurrence of spatial clustering of individuals within a few meters. In particular, the range of the spatial clustering roughly corresponded to that of the genetic structuring. While the size structure did not significantly differ between the plots, these results indicate that the fine-scale genetic structure reflects the formation of spatial clustering of related individuals within the population, partly due to the restricted ant-mediated seed dispersal in P. reinii.

Clonality and spatial genetic structure in Populus x canescens and its sympatric backcross parent P. alba in a Central European hybrid zone

Spatial genetic structure (SGS) holds the key to understanding the role of clonality in hybrid persistence, but multilocus SGS in hybrid zones has rarely been quantified. Here, the aim was to fill this gap for natural hybrids between two diploid, ecologically divergent European tree species with mixed sexual/asexual reproduction, Populus alba and P. tremula. Nuclear microsatellites were used to quantify clonality, SGS, and historical gene dispersal distances in up to 407 trees from an extensive Central European hybrid zone including three subpopulation replicates. The focus was on P. x canescens and its backcross parent P. alba, as these two genotypic classes co-occur and interact directly. Sexual recombination in both taxa was more prominent than previously thought, but P. x canescens hybrids tended to build larger clones extending over larger areas than P. alba. The 3.4 times stronger SGS in the P. x canescens genet population was best explained by a combination of interspecific gene flow, assortative mating, and increased clonality in hybrids. Clonality potentially contributes to the maintenance of hybrid zones of P. alba and P. tremula in time and space. Both clonality and SGS need to be taken into account explicitly when designing population genomics studies of locus-specific effects in hybrid zones.

Pollen‐mediated gene dispersal within continuous and fragmented populations of a forest understorey species,Trillium cuneatum

Pollen movement plays a critical role in the distribution of genetic variation within and among plant populations. Direct measures of pollen movement in the large, continuous populations that characterize many herbaceous plant species are often technically difficult and biologically unreliable. Here, we studied contemporary pollen movement in four large populations of Trillium cuneatum. Three populations, located in the Georgia Piedmont, are exposed to strong anthropogenic disturbances, while the fourth population, located in the Southern Appalachian Mountains, is relatively undisturbed. Using the recently developed TwoGener analysis, we extracted estimates of the effective number of pollen donors (N(ep)), effective mating neighbourhood size (A(ep)) and the average distance of pollen movement (delta) for each population. We extended the TwoGener method by developing inference on the paternal gametic contribution to the embryo in situations where offspring genotypes are inferred from seeds and elaiosomes of species with bisporic megagametogenesis. Our estimates indicate that maternal plants do not sample pollen randomly from a global pool; rather, pollen movement in all four populations is highly restricted. Although the effective number of pollen donors per maternal plant is low (1.22-1.66) and pollen movement is highly localized in all populations, N(ep) in the disturbed Piedmont populations is higher and there is more pollen movement than in the mountains. The distance pollen moves is greater in disturbed sites and fragmented populations, possibly due to edge effects in Trillium habitats.

Half-sib family structure of Fagus crenata saplings in an old-growth beech-dwarf bamboo forest

Half-sib family structure of Fagus crenata saplings was examined in an old-growth beech-dwarf bamboo (Sasa spp.) forest using microsatellite genotypes in parentage analysis to identify the half-sib families in two 50 x 50 m plots: one with 36 adults, 641 saplings and no Sasa cover, the other with 21 adults, 61 saplings and Sasa cover. For large proportions of the saplings (44.6% and 75.4%, respectively) both of their parents were found within the same plot, indicating that pollination events frequently involved short-range pollen dispersal, probably because of the high density of adults in the study population. Although almost all of the adults had half-sib families, the number of offspring in the families was highly variable. In the plot with no Sasa cover, the variation in the number of offspring was significantly explained by the size of parents, i.e. the reproductive success is higher for large adults than for small adults. The half-sibs were aggregately distributed around their parents and the distribution overlapped among different half-sib families, which may be due to the limited seed dispersal and overlapping seed shadows of this species. As expected, there was weak genetic structure in the plot. By contrast, in the plot with Sasa cover, the half-sibs were distributed sparsely with a low density, and the degree of genetic structure was very weak. The difference in the half-sib family structure and genetic structure among saplings presumably reflects the difference in density that should be affected by regeneration dynamics associated with environmental conditions.

Mating patterns, pollen dispersal, and the ecological maternal neighbourhood in a Prunus mahaleb L. population

Gender polymorphism, plant-animal interactions, and environmental heterogeneity are the three important sources of variation in mating system and pollen dispersal patterns. We used progeny arrays and paternity analysis to assess the effects of gender type and density level on variation in mating patterns within a highly isolated population of Prunus mahaleb, a gynodioecious species. All the adult trees in the population were sampled and located. The direct estimate of long-distance insect-mediated pollination events was low (< 10%). Gender expression deeply influenced the mating system, decreasing the outcrossing rates (t(m)) and the pollen pool diversity in hermaphrodite trees. Long intermate distances (> 250 m) were significantly more frequent among female mother trees. Variation in local tree density also affected pollen pool diversity and intermate distance, with a higher effective number of fathers (k(e)) and longer intermate distances for female trees in low-density patches. A canonical correlation analysis showed significant correlations between mating variables and the maternal ecological neighbourhood. Only the first canonical variable was significant and explained 78% of variation. Outcrossing rates tended to decrease, and the relatedness among the fathers tended to increase, when mother trees grew in dense patches with high cover of other woody species and taller vegetation away from the pine forest edge. We highlight the relevance of considering maternal ecological neighbourhood effects on mating system and gene flow studies as maternal trees act simultaneously as receptors of pollen and as sources of the seeds to be dispersed.

Variation in pollen dispersal between years with different pollination conditions in a tropical emergent tree

We examined differences in pollen dispersal efficiency between 2 years in terms of both spatial dispersal range and genetic relatedness of pollen in a tropical emergent tree, Dipterocarpus tempehes. The species was pollinated by the giant honeybee (Apis dorsata) in a year of intensive community-level mass-flowering or general flowering (1996), but by several species of moths in a year of less-intensive general flowering (1998). We carried out paternity analysis based on six DNA microsatellite markers on a total of 277 mature trees forming four spatially distinct subpopulations in a 70 ha area, and 147 and 188 2-year-old seedlings originating from seeds produced in 1996 and 1998 (cohorts 96 and 98, respectively). Outcrossing rates (0.93 and 0.96 for cohorts 96 and 98, respectively) did not differ between years. Mean dispersal distances (222 and 192 m) were not significantly different between the 2 years but marginally more biased to long distance in 1996. The mean relatedness among cross-pollinated seedlings sharing the same mothers in cohort 96 was lower than that in cohort 98. This can be attributed to the two facts that the proportion of intersubpopulations pollen flow among cross-pollination events was marginally higher in cohort 96 (44%) than in cohort 98 (33%), and that mature trees within the same subpopulations are genetically more related to each other than those between different subpopulations. We conclude that D. tempehes maintained effective pollen dispersal in terms of outcrossing rate and pollen dispersal distance in spite of the large difference in foraging characteristics between two types of pollinators. In terms of pollen relatedness, however, a slight difference was suggested between years in the level of biparental inbreeding.

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.

Long-distance pollen flow and tolerance to selfing in a neotropical tree species

Outcrossing rates, pollen dispersal and male mating success were assessed in Dicorynia guianensis Amshoff, a neotropical tree endemic to the Guiana shield. All adult trees within a continuous area of 40 ha (n = 157) were mapped, and were genotyped with six microsatellite loci. In addition, progenies were genotyped from 22 mature trees. At the population level, the species was mostly outcrossing (tm = 0.89) but there was marked variation among individuals. One tree exhibited mixed mating, confirming earlier results obtained with isozymes that D. guianensis can tolerate selfing. A Bayesian extension of the fractional paternity method was used for paternity analysis, and was compared with the neighbourhood method used widely for forest trees. Both methods indicated that pollen dispersal was only weakly related to distance between trees within the study area, and that the majority (62%) of pollen came from outside the study stand. Using maximum likelihood, male potential population size was estimated to be 1119, corresponding to a neighbourhood size of 560 hectares. Male mating success was, however, related to the diameter of the stem and to flowering intensity assessed visually. The mating behaviour of D. guianensis is a combination of long-distance pollen flow and occasional selfing. The species can still reproduce when it is extremely rare, either by selfing or by dispersing pollen at long distances. These results, together with the observation that male mating success was correlated with the size of the trees, could be implemented in management procedures aiming at regenerating the species.

Spatial structure and genetic diversity of two tropical tree species with contrasting breeding systems and different ploidy levels

Analyses of the spatial distribution pattern, spatial genetic structure and of genetic diversity were carried out in two tropical tree species with contrasting breeding systems and different ploidy levels using a 50-ha demographic plot in a lowland dipterocarp forest in Peninsular Malaysia. Shorea leprosula is a diploid and predominantly outcrossed species, whereas S. ovalis ssp. sericea is an autotetraploid species with apomictic mode of reproduction. Genetic diversity parameters estimated for S. leprosula using microsatellite were consistently higher than using allozyme. In comparisons with S. leprosula and other tropical tree species, S. ovalis ssp. sericea also displayed relatively high levels of genetic diversity. This might be explained by the lower pressure of genetic drift due to tetrasomic inheritance, and for autotetraploids each locus can accommodate up to four different alleles and this allows maintenance of more alleles at individual loci. The observed high levels of genetic diversity in S. ovalis ssp. sericea can also be due to a random retention of more heterogeneous individuals in the past, and the apomictic mode of reproduction might be an evolutionary strategy, which allows the species to maintain high levels of genetic diversity. The spatial distribution pattern analyses of both species showed significant levels of aggregation at small and medium but random distribution at the big diameter-class. The decrease in magnitude of spatial aggregation from small- to large-diameter classes might be due to compensatory mortality during recruitment and survival under competitive thinning process. Spatial genetic structure analyses for both species revealed significant spatial genetic structure for short distances in all the three diameter-classes. The magnitude of spatial genetic structure in both species was observed to be decreasing from smaller- to larger-diameter classes. The high spatial genetic structuring observed in S. ovalis ssp. sericea at the small-diameter class is due primarily to limited seed dispersal and apomictic mode of reproduction. The similar observation in S. leprosula, however, can be explained by limited seed and pollen dispersal, which supports further the fact that the species is pollinated by weak fliers, mainly of Thrips and Megalurothrips in the lowland dipterocarp forest.

Spatial genetic structure in populations of the terrestrial orchid Cephalanthera longibracteata (Orchidaceae)

Orchid seeds are unusual for being the smallest among flowering plants. These dust-like seeds are wind-borne and, thus, would seem to have the potential for long-distance dispersal (a common perception); this perception has led to a prediction of near-random spatial genetic structure within orchid populations. Mathematical models (e.g., simple ballistic model) for wind-dispersed seeds and wind-tunnel experiments, in contrast, indicate that most seeds of orchids should fall close to the maternal plant (<6 m), supporting a prediction of significant fine-scale genetic structure within populations. In reality we do not know much about seed dispersion in orchids. To determine which of these two predictions is more appropriate, Wright's F statistics and spatial autocorrelation analysis were used to examine the genetic structure within two adult populations of the terrestrial orchid Cephalanthera longibracteata (Orchidaceae) in southern Korea. In results comparable to those of other self-compatible, mixed-mating plant species, C. longibracteata populations exhibited low levels of genetic diversity (mean H(e) = 0.036) and a significant excess of homozygosity (mean F(IS) = 0.330), consistent with substantial inbreeding via selfing and/or mating among close relatives in a spatially structured population. Spatial autocorrelation analysis revealed significant positive genetic correlations among plants located <10 m, with relatedness at <3 m comparable to that expected for half sibs and first cousins. This genetic structure supports the prediction that the majority of seed dispersal occurs over distances of less than 10 m and is responsible for generating substantial overlap in seed shadows within C. longibracteata populations.

Reproductive uncertainty and the relative competitiveness of simultaneous hermaphroditism versus dioecy

Hermaphroditism is typically associated with a sedentary existence, whereas dioecy is associated with mobility. This pattern is reflected within flowering plants, as dioecious species commonly possess traits that promote high dispersal. We investigated these associations with three population dynamics models (an individual-based simulation and two mathematical models, one deterministic and the other stochastic) that allowed us to examine competition for space between a hermaphroditic and dioecious species from different perspectives. The competing species are identical in every way but their sexual system. Separation of the sexes increases the variances of pollen import and seed dispersal for the dioecious species. These variances propagate through subsequent reproductive processes and ultimately reduce mean recruitment as a result of nonlinear averaging (Jensen's inequality). A dioecious species could overcome this disadvantage simply by producing more gametes than hermaphrodites; however, in line with the association with mobility, selection on dioecious species should also favor traits that reduce reproductive uncertainty, such as extensive dispersal.

A multiyear estimate of the effective pollen donor pool for Albizia julibrissin

Studies of pollen movement in plant populations are often limited to a single reproductive event, despite concerns about the adequacy of single-year measures for perennial organisms. In this study, we estimate the effective number of pollen donors per tree from a multiyear study of Albizia julibrissin Durazz (mimosa, Fabaceae), an outcrossing, insect-pollinated tree. We determined 40 seedling genotypes for each of 15 seed trees during 4 successive years. A molecular analysis of variance of the pollen gametes fertilizing the sampled seeds was used to partition variation in pollen pools among seed trees, among years, and within single tree-year collections. Using these variance components, we demonstrate significant male gametic variability among years for individual trees. However, results indicate that yearly variation in the 'global pollen pool', averaged over all 15 seed trees for these 4 years, is effectively zero. We estimate the effective number of pollen donors for a single mimosa tree (N(ep)) to be 2.87. Single season analyses yield N(ep) approximately 2.05, which is 40% less than the value of N(ep) estimated from 4 years of data. We discuss optimal sampling for future studies designed to estimate N(ep). Studies should include more trees, each sampled over at least a few years, with fewer seeds per tree per year than are needed for a traditional parentage study.

Complex patterns of mating revealed in a Eucalyptus regnans seed orchard using allozyme markers and the neighbourhood model

The neighbourhood model apportions offspring of individual mother plants to self-fertilization, outcrossing to males within a circumscribed area around the mother plant (the neighbourhood), and outcrossing to males outside the neighbourhood. Formerly the model was applied only to haploid pollen gametes in the offspring of conifers, but is extended so that it can be used with genotypic data from diploid offspring of both angiosperms and gymnosperms. In addition, it is shown that the mating parameters can be estimated without independent estimates of allele frequencies in the pollen pools outside the neighbourhood; thus the model might be applied effectively to natural populations exposed to unknown external pollen sources. Parameters of the neighbourhood mating model were estimated for a 10-year-old seed orchard population of the insect-pollinated tree, Eucalyptus regnans, in southeast Australia, which contained a mixture of two geographical provenances (Victoria and Tasmania). The mating patterns revealed were complex. Crosses between trees of the same provenance occurred three times more often than crosses between trees of different provenances. Levels of self-fertilization and patterns of mating within neighbourhoods were influenced by provenance origin, crop fecundity and orchard position (central vs. edge) of mother trees. Gene dispersal, however, was extensive, with approximately 50% of effective pollen gametes coming from males more than 40 m away from mother trees (average distance between neighbouring trees was 7.4 m). Thus, insect pollinators are efficient promoters of cross-fertilization in this orchard, with the result that the effective number of males mating with each female is large.

Fine-scale genetic structure in populations ofQuercus variabilis(Fagaceae) from southern Korea

Quercus variabilis Blume (Fagaceae) is a deciduous broad-leaved tree, and an important forest element among the hillsides of southern Korea. To date, there are contrasting results with respect to fine-scale spatial genetic structure among adults in populations of several oak species; some studies have shown evidence of significant within-population spatial genetic structure, while others found weak or little evidence of fine-scale genetic structuring within populations. We used allozyme loci, Wright's F statistics, and multilocus spatial autocorrelation statistics to examine the distribution of genetic diversity and the spatial genetic structure within three undisturbed populations at a landscape level (72.5 ha, 500 × 1450 m) on Dolsan Island, South Korea. The spatial autocorrelation analyses of adults showed little evidence of fine-scale genetic structuring within populations, which could be due to random mortality among related seedlings, resulting in extensive thinning within maternal half-sib groups. Alternatively, low genetic differentiation between adjacent populations (mean FST= 0.023) and little within-population spatial genetic structure suggest probable secondary acorn movement by animals. Our results are very similar to those observed in three populations of Quercus acutissima at a landscape level (15 ha, 250 × 600 m) on Oenaro Island, South Korea. Together, these studies describe relatively subtle differences in genetic structure among adjacent populations of oaks on southern islands in Korea.Key words: allozymes, Quercus variabilis, Fagaceae, landscape level, multiple populations, spatial genetic structure.

Relatedness structure in Rhododendron metternichii var. hondoense revealed by microsatellite analysis

The relatedness structure of Rhododendron metternichii Sieb. et Zucc. var. hondoense Nakai was analysed in a 150 x 70-m quadrat in Hiroshima Prefecture, western Japan. The population of R. metternichii occurred as three subpopulations at the study site (A1-A3). Pairwise relatedness based on microsatellite genotypes at eight loci and Mantel tests revealed a hierarchical structure of relatedness within and among subpopulations: (i) relatedness between individuals within 10 m of one another was significantly positive; (ii) relatedness between individuals in the same subpopulation was significantly positive, but negative between individuals in distant subpopulations; and (iii) relatedness was not significantly different from zero among neighbouring subpopulations. In detail, however, relatedness within each subpopulation was significantly positive in subpopulation A1, relatively weak but significantly positive in subpopulation A2, and not significantly different from zero in subpopulation A3. Relatedness within each subpopulation was inversely related to correlations between interindividual distance and relatedness. The aggregation of related individuals at short distances from one another may lead to decreasing relatedness within subpopulations as a whole. Moreover, negative correlations between interindividual distance and relatedness corresponded to high flowering densities at less than 10-m distance, implying that high flowering densities reduce pollinator foraging distance and lead to stronger genetic structure within subpopulations. Small individuals (< 2.0 m in height) showed stronger genetic structure compared with that of large individuals (> or = 2.0 m in height). The different relatedness structure within and among subpopulations may be caused by various degrees of gene flow affected by distribution patterns of individuals and population density.

Fine-scale genetic structure in Pinus clausa (Pinaceae) populations: effects of disturbance history

Spatial autocorrelation analyses of 12 allozyme loci were used to compare genetic structure within populations of two varieties of Pinus clausa. P. clausa var. immuginata populations tend to be uneven-aged, with continuous recruitment in small gaps created by wind damage, whereas P. clausa var. clausa populations are more even-aged, with recruitment postdating periodic canopy fires. Three var. immuginata populations and three matched pairs of var. clausa populations, including both a mature and a nearby recently burned population, were examined. Aggregation of multilocus genotypes at small distances was evident in all young var. clausa populations. Little inbreeding was apparent among juveniles or adults in these populations; their genetic structure is likely to have resulted from limited seed dispersal. Genotypes were not significantly spatially structured in nearby matched mature populations. Genetic structure was less evident in var. immuginata populations. Aggregated genotypes were only apparent in the population where patches included juveniles of similar ages; dense juvenile clumps in the other two var. immuginata populations comprised a variety of ages. Interannual variability in allele frequencies of surviving seedlings may account for the absence of genetic structure in these populations.

Two-generation analysis of pollen flow across a landscape. II. Relation between phi(ft), pollen dispersal and interfemale distance

We study the behavior of Phi(ft), a recently introduced estimator of instantaneous pollen flow, which is basically the intraclass correlation of inferred pollen cloud genetic frequencies among a sample of females drawn from a single population. Using standard theories of identity by descent and spatial processes, we show that Phi(ft) depends on the average distance of pollen dispersal (delta) and on the average distance between sampled mothers (x(1)). Provided that mothers are sampled far enough apart (x(1) > 5delta), Phi(ft) becomes independent of x(1) and is then inversely proportional to the square of delta. Provided that this condition is fulfilled, delta is directly estimable from Phi(ft). Even when x(1) < 5delta, estimation can easily be achieved via numerical evaluation. We show that the relation between Phi(ft) and delta is only modestly affected by the shape of the distribution function, a result of importance, since this shape is generally unknown. We also study the impact of adult density within the population on Phi(ft), showing that to achieve the correct inference of delta from Phi(ft) it must be taken into account, but that it has no effect on the distance at which mothers must be sampled.

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.

Paternity and gene dispersal in limber pine (Pinus flexilis James)

This study provides empirical information on intrapopulation gene dispersal via pollen, the size of genetic 'neighbourhoods', and interpopulation gene flow in a long-lived conifer, limber pine (Pinus flexilis). We used allozyme loci for a paternity analysis of 518 seeds produced in an isolated population of limber pine located in north-eastern Colorado, U.S.A., separated by 2 km from the nearest conspecific trees and nearly 100 km from populations in the Rocky Mountains. We also used indirect techniques (FST analyses) to estimate gene flow rates among subdivisions of the study population and among five widely separated populations. Within the main study population limber pine exhibited a polymorphism level of 50%, observed heterozygosity of 0.159, and 2.36 alleles per polymorphic locus. Mountain populations were slightly more variable. The main study population showed significant differentiation in allozymes among neighbouring subpopulations. The mean FST was 0.031 and the gene flow rate among subpopulations was estimated as 7.8 migrants per generation. Among widely separated populations the mean FST was 0.035 and the gene flow rate was estimated as 6.9 migrants per generation. The paternity analysis indicated a best estimate of 6.5% pollen immigration (minimum 1.1%) from populations 2 km to 100+ km away. For 4% of the seeds examined, paternity could be ascribed to a single tree in the study population. Fractional paternity and likelihood methods were used to estimate pollen dispersal distances for the remainder of the seeds. Mean pollen dispersal distance was estimated at 140 m using the fractional method, similar to results from the other techniques. This compares with a mean distance of 172 m between potential mates. These results suggest near-panmictic pollen dispersal over this population, which covers about 15 ha. The observed allozyme differences and surprisingly low estimates of among-subpopulation gene flow are ascribed to a probable restriction of gene dispersal by seeds.

Performance of non-motile male gametes in the sea: analysis of paternity and fertilization success in a natural population of a red seaweed, Gracilaria gracilis

In haploid–diploid red seaweeds, the dispersal of male gametes is presumed limited due to their lack of flagella. It has been suggested that this group suffers from sperm limitation and, consequently, that fertilization is relatively inefficient. Fertilization in most floridean rhodophytes results in the formation a cystocarp, a swelling on the haploid female thallus housing the diploid zygote and its thousands of diploid daughter spores. To study the performance of non-motile male gametes in the sea, we evaluated both female and male fertilization success in a natural population of the red marine alga Gracilaria gracilis . Female fertilization success, estimated by cystocarp yield per unit female thallus, was evaluated with respect to the availability of male gametes. Male fertilization success, estimated by the individual contribution of different males to zygotes, was assessed by paternity analyses on 350 cystocarps produced in one reproductive season using two microsatellite loci. The results show that cystocarp yield is not sperm limited and that the large variation in male fertilization success cannot be solely explained by the distance travelled by the male gamete to find a mate. Taken together, the results suggest that, not only is fertilization efficient, but that male–male competition and/or female choice may play a role in shaping population mating patterns.