Genetic structure of montane isolates of Pinus sylvestris L. in a Mediterranean refugial area

Aim This work investigates the population genetic effects of periodic altitudinal migrations and interstadial fragmentation episodes in long‐term Scots pine (Pinus sylvestris L.) populations at a regional scale. Location The study focuses on Scots pine populations in the northern Meseta and peripheral mountain chains, central and north‐western Iberian Peninsula. The ample macrofossil record in the area shows that this 60,000‐km² region represent a glacial refugium for Scots pine. The species occupied large areas on the Meseta plains during glacial cold stages, but it has periodically sheltered at high elevation in the surrounding mountain chains during warm episodes, conforming to a fragmented pattern similar to its present‐day distribution. Methods We perform a fine‐scale chloroplast microsatellite (cpSSR) survey to assess the genetic structure of 13 montane Scots pine isolates in the northern Meseta (total N = 322 individuals). Using a hierarchical analysis of molecular variance (amova ), we test the hypothesis of genetic isolation among disjunct mountain areas. We use a standard coalescence model to estimate genealogical relationship among populations, investigating the potential role of the regional relief as a factor influencing historic gene exchange among Scots pine populations. Results Population haplotypic diversity was high among Scots pine populations (H_e = 0.978), greater than values reported for other more thermophilic pine species in the Iberian Peninsula. The amova revealed low (but significant) differentiation among populations (Φ_ST = 0.031, P = 0.010), showed that the disjoint montane distribution could not account for the genetic divergence among areas (Φ_CT = 0.012, P = 0.253), and that there was non‐trivial subdivision among populations within the same mountain region (Φ_SC = 0.021, P = 0.012). The genealogical relationships among populations showed that Scots pine isolates growing on disjoint mountain blocks, but on slopes flowing to the same basin, were genetically closer than populations growing on different slopes of the same mountain chain, flowing to different basins. Main conclusions The observed genetic structure for Scots pine is consistent with its population history, inferred from the palaeobotanical record, with vertical migrations throughout climatic pulses and with the drainage basins and large long‐term population sizes connecting different mountain blocks during the cooler glacial periods. Overall, the results suggest that, despite periodic interstadial fragmentation episodes, Scots pine biology provides for the long‐term maintenance of high within‐population and low among‐population genetic diversity at neutral genetic markers.     

Small-scale spatial genetic structure in the Central African rainforest tree species Aucoumea klaineana: a stepwise approach to infer the impact of limited gene dispersal, population history and habitat fragmentation

Under the isolation-by-distance model, the strength of spatial genetic structure (SGS) depends on seed and pollen dispersal and genetic drift, which in turn depends on local demographic structure. SGS can also be influenced by historical events such as admixture of differentiated gene pools. We analysed the fine-scale SGS in six populations of a pioneer tree species endemic to Central Africa, Aucoumea klaineana. To infer the impacts of limited gene dispersal, population history and habitat fragmentation on isolation by distance, we followed a stepwise approach consisting of a Bayesian clustering method to detect differentiated gene pools followed by the analysis of kinship-distance curves. Interestingly, despite considerable variation in density, the five populations situated under continuous forest cover displayed very similar extent of SGS. This is likely due to an increase in dispersal distance with decreased tree density. Admixture between two gene pools was detected in one of these five populations creating a distinctive pattern of SGS. In the last population sampled in open habitat, the genetic diversity was in the same range as in the other populations despite a recent habitat fragmentation. This result may due to the increase of gene dispersal compensating the effect of the disturbance as suggested by the reduced extent of SGS estimated in this population. Thus, in A. klaineana, the balance between drift and dispersal may facilitate the maintenance of genetic diversity. Finally, from the strength of the SGS and population density, an indirect estimate of gene dispersal distances was obtained for one site: the quadratic mean parent-offspring distance, sigma(g), ranged between 210 m and 570 m.     

Fragmentation can increase spatial genetic structure without decreasing pollen-mediated gene flow in a wind-pollinated tree

Fragmentation reduces population sizes, increases isolation between habitats and can result in restricted dispersal of pollen and seeds. Given that diploid seed dispersal contributes more to shaping fine-scale spatial genetic structure (SGS) than haploid pollen flow, we tested whether fine-scale SGS can be sensitive to fragmentation even if extensive pollen dispersal is maintained. Castanopsis sclerophylla (Lindley & Paxton) Schottky (Fagaceae), a wind-pollinated and gravity seed-dispersed tree, was studied in an area of southeast China where its populations have been fragmented to varying extents by human activity. Using different age classes of trees in areas subject to varying extents of fragmentation, we found no significant difference in genetic diversity between prefragmentation vs. postfragmentation C. sclerophylla subpopulations. Genetic differentiation among postfragmentation subpopulations was also only slightly lower than among prefragmentation subpopulations. In the most fragmented habitat, selfing rates were significantly higher than zero in prefragmentation, but not postfragmentation, cohorts. These results suggest that fragmentation had not decreased gene flow among these populations and that pollen flow remains extensive. However, significantly greater fine-scale SGS was found in postfragmentation subpopulations in the most fragmented habitat, but not in less fragmented habitats. This alteration in SGS reflected more restricted seed dispersal, induced by changes in the physical environments and the prevention of secondary seed dispersal by rodents. An increase in SGS can therefore result from more restricted seed dispersal, even in the face of extensive pollen flow, making it a sensitive indicator of the negative consequences of population fragmentation.     

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.     

Spatial genetic structure in Milicia excelsa (Moraceae) indicates extensive gene dispersal in a low-density wind-pollinated tropical tree

In this study, we analysed spatial genetic structure (SGS) patterns and estimated dispersal distances in Milicia excelsa (Welw.) C.C. Berg (Moraceae), a threatened wind-pollinated dioecious African tree, with typically low density (∼10 adults/km²). Eight microsatellite markers were used to type 287 individuals in four Cameroonian populations characterized by different habitats and tree densities. Differentiation among populations was very low. Two populations in more open habitat did not display any correlation between genetic relatedness and spatial distance between individuals, whereas significant SGS was detected in two populations situated under continuous forest cover. SGS was weak with a maximum S _p-statistic of 0.006, a value in the lower quartile of SGS estimates for trees in the literature. Using a stepwise approach with Bayesian clustering methods, we demonstrated that SGS resulted from isolation by distance and not colonization by different gene pools. Indirect estimates of gene dispersal distances ranged from σ _g = 1 to 7.1 km, one order of magnitude higher than most estimates found in the literature for tropical tree species. This result can largely be explained by life-history traits of the species. Milicia excelsa exhibits a potentially wide-ranging wind-mediated pollen dispersal mechanism as well as very efficient seed dispersal mediated by large frugivorous bats. Estimations of gene flow suggested no major risk of inbreeding because of reduction in population density by exploitation. Different strategy of seed collection may be required for reforestation programmes among populations with different extent of SGS.     

Contrasting spatial genetic structure in Annona crassiflora populations from fragmented and pristine savannas

In continuous populations, fine-scale genetic structure tends to be stronger in species with restricted pollen and seed dispersal. However, habitat fragmentation and disturbances can affect genetic diversity and spatial genetic structure due to disruption in ecological processes, such as plant reproduction and seed dispersal. In this study, we compared the genetic diversity and fine-scale spatial genetic structure (SGS) in two populations of Annona crassiflora (Annonaceae) in a pristine savanna Reserve (ESECAE) and in a fragmented disturbed savanna area (PABE), both in Cerrado biome in Central Brazil. The analyses were based on the polymorphism at 10 microsatellite loci. Our working hypothesis was that SGS is stronger and genetic diversity is lower in population at fragmented area (PABE) than at pristine area (ESECAE). Both populations presented high levels of polymorphism and genetic diversity and showed no sign of bottleneck for both Wilcoxon sign-rank test for heterozygosity excess (p > 0.05) and coalescent analyses (growth parameter g not different from zero), but population at fragmented area showed higher fixation index and stronger SGS. Besides, populations are significantly differentiated (F _ST = 0.239, R _ST = 0.483, p < 0.001 for both). Coalescent analyses showed high historical effective population sizes for both populations, high gene flow between ESECAE and PABE and recent time to most recent common ancestor (~37 k year BP). Our results suggest that despite the high genetic diversity, fragmentation and disturbance may have been affecting populations of this species increasing mating between closely related individuals leading to high fixation index and strong SGS.     

Spatio-temporal genetic variation in sympatric and allopatric Mediterranean Cicada species (Hemiptera, Cicadidae)

Cicada barbara (Stål) and Cicada orni L. are two Mediterranean cicadas, very similar in morphology, that produce distinct acoustic mating signals and that have partially overlapping distribution ranges in the Iberian Peninsula, occurring in sympatry in several locations. In the present study, six microsatellite loci were analysed in C. barbara , four of which were also analysed in C. orni . Geographical and temporal genetic variation in these species was studied. No evidence of hybridization was found, enabling us to infer that the isolating barriers between these species are efficient. Partitioning of geographic variation in each species, revealed the following patterns: Iberian Peninsula and Northwestern African populations of C. barbara showed higher differentiation between than within each region, supporting C. barbara subspecific taxonomy ( C. barbara lusitanica in the Iberian Peninsula and C. barbara barbara in Northwestern Africa) and highlighting isolation coincident with the presence of physical barriers to gene-flow; differentiation between populations of C. orni from both sides of the Pyrenees was very low, and this mountain range may not constitute a significant barrier for the dispersal of this species; Greek populations of C. orni were found to be highly differentiated from Western European populations; and no pattern of isolation-by-distance was found in either species within the Iberian Peninsula.  © 2009 The Linnean Society of London, Biological Journal of the Linnean Society , 2009, 96 , 249–265.     

Geographical isolation versus dispersal: Relictual alpine grasshoppers support a model of interglacial diversification with limited hybridization

Alpine biotas are paradigmatic of the countervailing roles of geographical isolation and dispersal during diversification. In temperate regions, repeated distributional shifts driven by Pleistocene climatic oscillations produced both recurrent pulses of population fragmentation and opportunities for gene flow during range expansions. Here, we test whether a model of divergence in isolation vs. with gene flow is more likely in the diversification of flightless alpine grasshoppers of the genus Podisma from the Iberian Peninsula. The answer to this question can also provide key insights about the pace of evolution. Specifically, if the data fit a divergence in isolation model, this suggests rapid evolution of reproductive isolation. Genomic data confirm a Pleistocene origin of the species complex, and multiple analytical approaches revealed limited asymmetric historical hybridization between two taxa. Genomic-based demographic reconstructions, spatial patterns of genetic structure and range shifts inferred from palaeodistribution modelling suggest severe range contraction accompanied by declines in effective population sizes during interglacials (i.e., contemporary populations confined to sky islands are relicts) and expansions during the coldest stages of the Pleistocene in each taxon. Although limited hybridization during secondary contact leads to phylogenetic uncertainty if gene flow is not accommodated when estimating evolutionary relationships, all species exhibit strong genetic cohesiveness. Our study lends support to the notion that the accumulation of incipient differences during periods of isolation were sufficient to lead to lineage persistence, but also that the demographic changes, dispersal constraints and spatial distribution of the sky islands themselves mediated species diversification in temperate alpine biotas.     

Postglacial dispersal of the European rabbit (Oryctolagus cuniculus) on the Iberian peninsula reconstructed from nested clade and mismatch analyses of mitochondrial DNA genetic variation

Nested clade analysis was applied to cytochrome b restriction site data previously obtained on 20 natural populations of the European rabbit across the Iberian Peninsula to test the hypothesis of postglacial dispersal from two main refugia, one in the northeast and the other in the southwest. Apart from historical fragmentation that resulted in geographic discontinuity of two distinct mitochondrial DNA (mtDNA) clades A and B, patterns of haplotype genetic variability have been shaped mostly by restricted gene flow via isolation by distance. The distribution of tip versus interior haplotypes suggests that dispersal occurred from both the southwestern and northeastern groups. Dispersal from the southwest had a north and northwest direction, whereas from the northeast it had mostly a western and southern orientation, with subsequent overlap in a southeastern-northwestern axis across the Iberian Peninsula. The analysis of the pairwise mismatch distribution of a 179-181-bp fragment of the mtDNA control region, for seven of those populations, further supports the idea that major patterns of dispersal were in the direction of central Iberia. Additionally, rabbit populations do not show signs of any significant loss of genetic diversity in the recent past, implying that they maintained large population sizes and structure throughout the ice ages. This is congruent with the fact that the Iberian Peninsula was itself a glacial refugium during Quaternary ice ages. Nonetheless, climatic oscillations of this period, although certainly milder than in northern Europe, were sufficient to affect the range distributions of Iberian organisms.     

Within-population spatial genetic structure in four naturally fragmented species of a neotropical inselberg radiation, Alcantarea imperialis, A. geniculata, A. glaziouana and A. regina (Bromeliaceae)

Studies of organisms on 'terrestrial islands' can improve our understanding of two unresolved issues in evolutionary genetics: the likely long-term effects of habitat fragmentation and the genetic underpinnings of continental species radiations in island-like terrestrial habitats. We have addressed both issues for four closely related plant species of the adaptive radiation Bromeliaceae, Alcantarea imperialis, A. geniculata, A. regina and A. glaziouana. All four are adapted to ancient, isolated inselberg rock outcrops in the Brazilian Atlantic rainforest and are thus long-term fragmented by nature. We used eight nuclear microsatellites to study within-population spatial genetic structure (SGS) and historical gene dispersal in nine populations of these species. Within-population SGS reflected known between-species differences in mating systems. The strongest SGS observed in A. glaziouana (Sp=0.947) was stronger than literature estimates available for plants. Analysis of short- and long-distance components of SGS identified biparental inbreeding, selfing and restricted seed dispersal as main determinants of SGS, with restricted pollen dispersal by bats contributing in some localities. The ability of Alcantarea spp. to colonize isolated inselbergs probably stems from their flexible mating systems and an ability to tolerate inbreeding. Short-ranging gene dispersal (average sigma=7-27 m) is consistent with a loss of dispersal power in terrestrial island habitats. Population subdivision associated with sympatric colour morphs in A. imperialis is accompanied by between-morph differences in pollen and seed dispersal. Our results indicate a high potential for divergence with gene flow in inselberg bromeliads and they provide base-line data about the long-term effects of fragmentation in plants.     

Spatial genetic structure in wild cherry (Prunus avium L.): II. Effect of density and clonal propagation on spatial genetic structure based on simulation studies

We conducted simulations to disentangle the effect of density and clonal propagation on spatial genetic structure and genetic diversity parameters in Prunus avium. In a previous paper, we observed stronger family structure in populations exhibiting high density and high clonal propagation, whereas one low-density and low clonal propagation population showed negative spatial autocorrelation. We tried to understand these results by simulating 200 years of growth, mating and dispersal with the model Eco-Gene, using two levels of density and three levels of clonal propagation in one high-density population (Spargründe) and one low-density population (Chorin). We used allele frequencies from eight microsatellite loci to generate the populations used for simulations. In order to detect effects of the initial structure on the results, we also ran the simulations starting from the real data. We observed positive effect of clonal propagation on the strength of SGS, while high densities exhibited a negative impact. Genetic diversity was maintained at high densities, while pollen dispersal was shorter. Heterozygosity increased at higher clonal propagation rates, although genetic diversity (accumulated number of genotypes) was lower. We discuss the results according to mating processes and potential management scenario.     

鹅掌楸贵州烂木山居群的微卫星遗传多样性及空间遗传结构

濒危植物鹅掌楸(Liriodendron chinense)目前仅零散分布于我国亚热带及越南北部地区, 残存居群生境片断化较为严重。研究濒危植物片断化居群的遗传多样性及小尺度空间遗传结构(spatial genetic structure)有助于了解物种的生态进化过程以及制定相关的保育策略。本研究采用13对微卫星引物, 对鹅掌楸的1个片断化居群进行了遗传多样性及空间遗传结构的研究, 旨在揭示生境片断化条件下鹅掌楸的遗传多样性及基因流状况。研究结果表明: 鹅掌楸烂木山居群内不同生境斑块及不同年龄阶段植株的遗传多样性水平差异不显著(P>0.05), 居群内存在寨内和山林2个遗传分化明显的亚居群。烂木山居群个体在200 m以内呈现显著的空间遗传结构, 而2个亚居群内的个体仅在20 m的距离范围内存在微弱或不显著的空间遗传结构。鹅掌楸的空间遗传结构强度较低(Sp = 0.0090), 且寨内亚居群的空间遗传结构强度(Sp = 0.0067)要高于山林亚居群(Sp = 0.0053)。鹅掌楸以异交为主, 种子较轻且具翅, 借助风力传播, 在一定程度上降低了空间遗传结构的强度。此外, 居群内个体密度及生境特征也对鹅掌楸的空间遗传结构产生了一定影响。该居群出现显著的杂合子缺失, 近交系数(F_IS)为0.099 (P < 0.01), 表明生境片断化的遗传效应正逐渐显现。因此, 对鹅掌楸的就地保护应注意维护与强化生境的连续性, 促进基因交流。迁地保护时, 取样距离应不小于20 m, 以涵盖足够多的遗传变异。     

Analysis of spatial genetic structure in an expanding Pinus halepensis population reveals development of fine-scale genetic clustering over time

We analysed the change of spatial genetic structure (SGS) of reproductive individuals over time in an expanding Pinus halepensis population. To our knowledge, this is the first empirical study to analyse the temporal component of SGS by following the dynamics of successive cohorts of the same population over time, rather than analysing different age cohorts at a single time. SGS is influenced by various factors including restricted gene dispersal, microenvironmental selection, mating patterns and the spatial pattern of reproductive individuals. Several factors that affect SGS are expected to vary over time and as adult density increases. Using air photo analysis, tree-ring dating and molecular marker analysis we reconstructed the spread of reproductive individuals over 30 years beginning from five initial individuals. In the early stages, genotypes were distributed randomly in space. Over time and with increasing density, fine-scale (< 20 m) SGS developed and the magnitude of genetic clustering increased. The SGS was strongly affected by the initial spatial distribution and genetic variation of the founding individuals. The development of SGS may be explained by fine-scale environmental heterogeneity and possibly microenvironmental selection. Inbreeding and variation in reproductive success may have enhanced SGS magnitude over time.     

The role of gene flow in shaping genetic structures of the subtropical conifer species Araucaria angustifolia

The morphological features of pollen and seed of Araucaria angustifolia have led to the proposal of limited gene dispersal for this species. We used nuclear microsatellite and AFLP markers to assess patterns of genetic variation in six natural populations at the intra- and inter-population level, and related our findings to gene dispersal in this species. Estimates of both fine-scale spatial genetic structure (SGS) and migration rate suggest relatively short-distance gene dispersal. However, gene dispersal differed among populations, and effects of more efficient dispersal within population were observed in at least one stand. In addition, even though some seed dispersal may be aggregated in this principally barochorous species, reasonable secondary seed dispersal, presumably facilitated by animals, and overlap of seed shadows within populations is suggested. Overall, no correlation was observed between levels of SGS and inbreeding, density or age structure, except that a higher level of SGS was revealed for the population with a higher number of juvenile individuals. A low estimate for the number of migrants per generation between two neighbouring populations implies limited gene flow. We expect that stepping-stone pollen flow may have contributed to low genetic differentiation among populations observed in a previous survey. Thus, strategies for maintenance of gene flow among remnant populations should be considered in order to avoid degrading effects of population fragmentation on the evolution of A. angustifolia.     

Mating system and pollen gene flow in Mediterranean maritime pine

Mating systems define the mode of gene transmission across generations, helping to determine the amount and distribution of genetic variation within and among populations of plant species. A hierarchical analysis of Mediterranean maritime pine mating system (61 mother trees from 24 plots, clustered in three populations) was used to identify factors affecting mating patterns and to fit pollen dispersal kernels. Levels of ovule and seed abortion, multi- and single-locus outcrossing rates and correlated paternity were estimated from progeny arrays and correlated with ecological stand variables and biometric tree measures. Pollen dispersal kernels were fitted using TwoGener and KinDist indirect methods and simulations were carried out to identify relevant factors affecting correlated paternity. Maritime pine showed high outcrossing rates (t(m) and t(s) approximately 0.96) and relatively low levels of correlated paternity [an r(p) of 0.018 (Ritland's estimate) or 0.048 (Hardy's estimate)], although higher than in other anemophilous tree species. Mating system parameters had high variation at the single-tree level (99-100%) but no stand or population effect was detected. At the single-tree level, outcrossing rates were correlated with tree (diameter and height) and crown size. In addition, correlated paternity showed a significant negative correlation with tree height, height to crown base and height to the largest crown width, probably reflecting the importance of the trees' 'ecological neighborhoods'. Indirectly estimated pollen dispersal kernels were very leptokurtic (exponential-power distributions with beta<0.5), with mean dispersal distances from 78.4 to 174.4 m. Fitted dispersal kernels will be useful in building explicit simulation models that include dispersal functions, and which will contribute to current conservation and management programs for maritime pine. Nevertheless, the numerical simulations showed that restricted dispersal, male fertility and phenological overlap could only partially explain the observed levels of correlated paternity; so other factors may also be relevant for the management of this valuable forest tree species.     

Genetic variation in central and peripheral populations of Excoecaria agallocha from Indo-West Pacific

Genetic diversity and differentiation were studied in Excoecaria agallocha L., a mangrove species growing on the coastlines of the Indo-West Pacific region. Twenty natural populations of E. agallocha were sampled from the Malay Peninsula (central population), and in Southern China, North Australia, Sri Lanka and Southern Japan (peripheral populations). Our results showed that central populations from Malay Peninsula possessed significantly higher genetic diversity than the peripheral ones (P < 0.05). Analysis of molecular variation (AMOVA) revealed that genetic variability was partitioned at 22.9% among regions, 23.6% among populations within regions, and 53.5% within populations. Genetic differentiation (G_ST = 0.300) among the six central populations was stronger than those from peripheral regions. Populations from North Australia clustered closely together in the dendrogram and were distinct from the rest of the populations. Those from Southern Japan, Southern China and Sri Lanka also clustered closely together, respectively. However, populations from Malay Peninsula did not cluster by region. The east coast populations of Malay Peninsula (including Pasir population) were more genetically similar to the populations from Southern China than those from the west coastline of Malay Peninsula. Our study suggests that ocean currents, land barriers, limited dispersal ability of seeds, and founder effect may play important roles in the distribution of genetic diversity in E. agallocha.     

Effects of seed dispersal, adult tree and seedling density on the spatial genetic structure of regeneration at fine temporal and spatial scales

Several demographic factors can produce family structured patches within natural plant populations, particularly limited seed and pollen dispersal and small effective density. In this paper, we used computer simulations to examine how seed dispersal, density, and spatial distribution of adult trees and seedlings can explain the spatial genetic structure (SGS) of natural regeneration after a single reproductive event in a small population. We then illustrated the results of our simulations using genetic (isozymes and chloroplast microsatellites) and demographic experimental data from an Abies alba (silver fir) intensive study plot located in the Southern French Alps (Mont Ventoux). Simulations showed that the structuring effect of limited dispersal on seedling SGS can largely be counterbalanced by high effective density or a clumped spatial distribution of adult trees. In addition, the clumping of natural regeneration far from adult trees, which is common in temperate forest communities where gap dynamics are predominant, further decreases SGS intensity. Contrary to our simulation results, low adult tree density, aggregated spatial distribution of seedlings, and limited seed dispersal did not generate a significant SGS in our A. alba experimental plot. Although some level of long distance pollen and seed flow could explain this lack of SGS, our experimental data confirm the role of spatial aggregation (both in adult trees and in seedlings far from adult trees) in reducing SGS in natural populations.     

Spatial genetic structure of the southeastern North American endemic, Ceratiola ericoides (Empetraceae)

Understanding the spatial distribution of genetic diversity (i.e., spatial genetic structure [SGS]) within plant populations can elucidate mechanisms of seed dispersal and patterns of recruitment that may play an important role in shaping the demography and spatial distribution of individuals in subsequent generations. Here we investigate the SGS of allozyme diversity in 2 populations of the southeastern North American endemic shrub, Ceratiola ericoides. The data suggest that the 2 populations have similar patterns of SGS at distances of 0-45 m that likely reflect the isolation by distance (IBD) model of seed dispersal. However, at distances >or=50 m, the pattern of SGS differs substantially between the 2 populations. Whereas one population continues to reflect the classical IBD pattern, the second population shows a marked increase in autocorrelation coefficient (r) values at 50-75 m. Furthermore, r values at these distances are as much as 33% higher than at 0-5 m where the highest r value would be predicted by IBD. A likely explanation is the differing frequencies of 2 fruit morphologies in these populations and the greater role that birds play in seed dispersal in the second population.     

Cryptic differences in dispersal lead to differential sensitivity to habitat fragmentation in two bumblebee species

Habitat loss has led to fragmentation of populations of many invertebrates, but social hymenopterans may be particularly sensitive to habitat fragmentation due to their low effective population sizes. The impacts of fragmentation depend strongly on dispersal abilities, but these are difficult to quantify. Here, we quantify and compare dispersal abilities of two bumblebee species, Bombus muscorum and Bombus jonellus , in a model island system. We use microsatellites to investigate population genetic structuring, dispersal and spatial patterns in genetic diversity. Populations of both species showed significant structuring, and isolation by distance, but this was markedly greater in B. muscorum (θ = 0.13) than in B. jonellus (θ = 0.034). This difference could reflect a higher effective population size in B. jonellus compared to B. muscorum , but this is not consistent with the observed abundance of the two species. We argue that it is more likely that B. jonellus has a higher propensity to disperse than B. muscorum . This will influence their relative susceptibility to habitat fragmentation and may in part explain differential declines of mainland populations of these and other bumblebee species.