Morph-specific differences in reproductive success in the distylous Primula veris in a context of habitat fragmentation

Heterostylous self-incompatible plant species are particularly sensitive to habitat fragmentation and to disruption of pollination processes because of the need of intermorph cross-pollination for producing seeds. Heterostyly is characterized by sexual polymorphism through the occurrence of two (distyly) or three (tristyly) morph types that differ in floral traits (style length and anther position). We examined whether the long-styled (pin) and short-styled (thrum) morph types show differences in reproductive components and responses to habitat fragmentation in the distylous, self-incompatible perennial herb Primula veris. We documented reproductive components for pin and thrum individuals and their relationships with population size, plant density and morph ratio (pin frequency), in nine populations from Flanders (northern Belgium) located in fragmented habitats of the intensively used agricultural landscape. Seed abortion increased in small populations as a result of inbreeding depression. Fruit set increased with plant density. Seed set was positively related to pin proportion. Seed set was higher for pin than thrum in small populations, but lower in large populations. Two hypotheses can be considered to explain these morph-specific differences: a pollen transfer asymmetry, and a reproductive advantage for the partially self-compatible pin morph. Morph types appear to respond differently to habitat fragmentation constraints. A floral morph type showing partial self-compatibility may be favored in populations under pollination failure, because it can increase reproductive success and mating opportunities through intramorph crosses.     

Biased morph ratios and skewed mating success contribute to loss of genetic diversity in the distylous Pulmonaria officinalis

Background and Aims

In heterostylous plant species, skewed morph ratios are not uncommon and may arise from a range of factors. Despite the recognized importance of skewed morph ratios on overall reproductive success within populations, little is known about the impact of skewed morph ratios on population genetic diversity and differentiation in heterostylous species. This study specifically aimed to clarify the effect of population size and morph bias on population genetic diversity and differentiation in the temperate forest herb Pulmonaria officinalis. This species is characterized by a distylous breeding system and shows morph-specific differences in reproductive success.

Methods

Genetic diversity was determined for 27 P. officinalis populations in northern Belgium by using eight recently developed microsatellite markers. Multiple regressions were used to assess the relationship between genetic diversity, morph bias and population size, and F_ST-values were calculated for short- and long-styled morphs separately to study genetic differentiation as a function of morph type.

Key Results

For all genetic measures used, morph bias was more important in explaining patterns of genetic diversity than population size, and in all cases patterns of population genetic diversity followed a quadratic function, which showed a symmetrical decrease in genetic diversity with increasing morph bias. However, probably due to the reproductive advantage of L-morphs relative to S-morphs, maximum genetic diversity was found in populations showing an excess of L-morphs (60·7 % L-morph). On the other hand, no significant difference in pairwise genetic distances between populations was observed between L- (0·107) and S-morphs (0·106).

Conclusions

Our results indicate that significant deviations from equal morph ratios not only affect plant reproductive success but also population genetic diversity of heterostylous plant species. Hence, when defining conservation measures for populations of heterostylous plant species, morph ratios should be considered as an important trait affecting their long-term population viability.     

Genetic diversity within and between remnant populations of the endangered calcareous grassland plant <Emphasis Type="Italic">Globularia bisnagarica</Emphasis> L.

Changes in agricultural production methods over the last century have caused a massive reduction and fragmentation of the area of European semi-natural grasslands. It remains unclear whether small and isolated grassland fragments can support viable plant populations in a sustainable way. In our study area in southern Belgium, the extent of calcareous grasslands was reduced from c. 650 ha in 1775 to less than 30 ha in 2004. We used AFLP markers to quantify the effects of present and historical grassland fragmentation on the genetic structure of 27 populations of the rare perennial plant species Globularia bisnagarica. Given the mixed breeding system of the species and the relatively small area of the studied system, the populations were characterized by high genetic differentiation (F _st range: 0.42–0.48; Φ_st=0.53). A Mantel test revealed significant isolation by distance of the populations. Average within population genetic diversity, measured as expected heterozygosity or gene diversity, was low (H _j =0.081) and was negatively related to population isolation. This suggests more gene flow into less isolated populations. Population size and local habitat characteristics did not significantly influence population genetic diversity. Both, high selfing rates in G. bisnagarica and a population genetic response to habitat fragmentation may explain our findings. Finally, a clear geographical clustering was observed, with cluster membership partially explainable by historical grassland connectivity. If populations indeed started to differentiate after fragmentation, this process was not (yet) strong enough to erase the genetic similarity between fragments that historically belonged to the same large grassland fragment.     

Disruption of the distylous syndrome in Primula veris

Background and Aims Distyly is a floral polymorphism characterized by the presence of two discrete morphs with reciprocal positioning of anthers and stigmas in flowers on different plants within the same population. Although reciprocal herkogamy and associated floral traits are generally thought to be discrete and strict polymorphisms, little is known about variation in floral traits related to the distylous syndrome within and among populations of a single species. In this study, variation in floral morphology and reciprocal positioning of the sexual organs in the distylous Primula veris (cowslip) is quantified.Methods Data were collected in ten populations occurring in two contrasting habitat types (grasslands and forests), and for each population the average level of reciprocity was assessed, the strength of the self-incompatibility system was determined, and seed production under natural conditions was quantified.Results In grassland populations, flowers showed clear distyly with low and symmetric reciprocity indices at both the lower and upper level. In forests, P. veris produced larger flowers that showed strong deviations in stigma–anther separation, especially in the L-morph. This deviation was mainly driven by variation in stigma height, resulting in high and asymmetric reciprocity indices and the occurrence of several short-styled homostylous plants. Self-incompatibility was, however, strict in both habitats, and morph ratios did not differ significantly from isoplethy. The observed shift in reciprocity in forest populations was associated with a significant reduction in seed production in the L-morph.Conclusions The results indicate that populations of P. veris show habitat-specific variation in flower morphology. Deviations from perfect reciprocal positioning of stigmas and anthers also translate into reduced seed production, suggesting that small changes in sexual organ reciprocity can have far-reaching ecological and evolutionary implications.     

Unravelling the stylar polymorphism in Melochia (Malvaceae): reciprocity and ancillary characters

Heterostyly is a genetic polymorphism in which plant populations possess two (distyly) or three (tristyly) morphs with flowers differing reciprocally in stigma and anther height. Sex organ deployment has been described as being highly variable among and within species of several distylous taxa belonging to different taxonomic groups. However, the number of studies considering within‐species disparities is still limited. For a better overview of the existing amount of variation that can occur within and between heterostylous species, we sampled 46 populations of six Melochia spp., a style‐polymorphic genus in Cuba. We characterized the floral morphology in all populations and described a set of ancillary characters per species. All of these Melochia spp. are distylous, except for the monomorphic M. nodiflora. The S‐morph produces fewer, larger pollen than L‐morphs, and has verrucose ornamentation. The L‐morph produces reticulate pollen and has larger stigmatic papillae than the S‐morph. The monomorphic M. nodiflora shows ancillary characteristics that are similar to the L‐morph individuals in the related species. As expected, there are differences in ancillary characters among species and also dissimilarities in reciprocity among and within species of Melochia. Our results highlight the importance of considering intraspecific variation in the morphometric characterization of heterostylous taxa. © 2014 The Linnean Society of London, Botanical Journal of the Linnean Society, 2014, 176 , 147–158.     

Species and genetic diversity are not congruent in fragmented dry grasslands

Biological diversity comprises both species diversity (SD) and genetic diversity (GD), and it has been postulated that both levels of diversity depend on similar mechanisms. Species‐genetic diversity correlations (SGDC) are therefore supposed to be generally positive. However, in contrast to theory, empirical data are contradictory. Furthermore, there is a pronounced lack of multispecies studies including also the ecological factors potentially driving species and genetic diversity. We analyzed the relationship between the species diversity of dry grasslands and the genetic diversity of several dry grassland plant species, therefore, in the context of habitat fragmentation and habitat conditions. Our study revealed a lack of correlation between species and genetic diversity. We demonstrated previously that SD mainly depends on habitat conditions (vegetation height and cover of litter), whereas GD is significantly affected by habitat fragmentation (distance to the nearest dry grassland in 1830 and connectivity in 2013). This seems to be the main reason why SD and GD are not congruent in fragmented grasslands. Our results support, hence, the observation that positive SGDCs can mainly be found in natural, island‐like study systems in equilibrium and at similar levels of heterogeneity. In fragmented dry grassland ecosystems, which differ in heterogeneity, this state of equilibrium may not have been reached mitigating the positive relationship between SD and GD. From our study, it can be concluded that in fragmented dry grasslands, the protection of SD does not necessarily ensure the conservation of GD.     

异质生境对二型花柱植物雪地报春花性状分化的影响

该研究以生长在新疆北部的二型花柱植物雪地报春(Primula nivalis)为研究材料,对7个低海拔(1 657~2 013m)森林居群和7个高海拔(2 400~2 700m)草原居群的花型比例进行调查,对花部特征进行检测,并对长、短花柱植株在不同海拔居群的花性状分化进行了比较研究,以揭示该类群植物的演化与环境的关系。结果表明:(1)在高海拔草原居群中长花柱植株比例高于短花柱植株。(2)比较草原和森林生态系统中的雪地报春居群,长花柱花形态特征间存在显著差异,但短花柱花形态特征间的差异不显著。(3)高海拔草原居群中长花柱花的花冠筒长度、雄蕊长度、雌雄蕊距离、雌雄异位程度以及雌雄蕊空间位置是导致花性状分化的主要因素。(4)在草原居群中,长花柱花的雄蕊、雌雄异位程度及花冠口至雄蕊距离的变化减少了长短花柱花间的互补式对应性。研究认为,在异质生境中长花柱花性状的变化是导致该物种长短花柱花性状分化的主要因素。     

Population genetics and fitness in fragmented populations of the dioecious and endangered <Emphasis Type="Italic">Silene otites</Emphasis> (Caryophyllaceae)

Population fragmentation is often correlated with loss of genetic diversity and reduced fitness. Obligate out-crossing (dioecy) is expected to enhance genetic diversity, reduce genetic differentiation, and avoid inbreeding depression through frequent gene flow. However, in highly fragmented populations dioecy has only diminishing effects upon genetic structure as pollination limitations (e.g. flight distance of pollinators) most often restrict inter-population gene flow in insect pollinated species. In fragmented dry grasslands in northeastern Germany, we analysed genetic structure, fitness, and habitat quality of the endangered dioecious Silene otites (Caryophyllaceae). Using AFLP markers, a high level of differentiation among ten populations was found (F _st = 0.36), while the intra-population genetic diversities (H _E = 0.165–0.240) were similar as compared to hermaphroditic species. There was neither a correlation between geographic and genetic distance nor between genetic diversity and population size, which indicates reduced gene flow among populations and random genetic drift. Plant size was positively correlated with genetic diversity. Seed set and number of juveniles were positively related to population size. Higher total coverage resulted in reduced plant fitness, and the number of juveniles was negatively correlated to cryptogam cover. Additionally, we found a sex ratio bias towards more male plants in larger populations. Overall, our results indicate that on a regional geographic scale dioecy does not necessarily prevent genetic erosion in the case of habitat fragmentation, especially in the absence of long distance seed and pollen dispersal capacity.     

Rapid genetic restoration of a keystone species exhibiting delayed demographic response

Genetic founder effects are often expected when animals colonize restored habitat in fragmented landscapes, but empirical data on genetic responses to restoration are limited. We examined the genetic response of banner‐tailed kangaroo rats (Dipodomys spectabilis) to landscape‐scale grassland restoration in the Chihuahuan Desert of New Mexico, USA. Dipodomys spectabilis is a grassland specialist and keystone species. At sites treated with herbicide to remove shrubs, colonization by D. spectabilis is slow and populations persist at low density for ≥10 years (≥6 generations). Persistence at low density and low gene flow may cause strong founder effects. We compared genetic structure of D. spectabilis populations between treated sites and remnant grasslands, and we examined how the genetic response to restoration depended on treatment age, area, and connectivity to source populations. Allelic richness and heterozygosity were similar between treated sites and remnant grasslands. Allelic richness at treated sites was greatest early in the restoration trajectory, and genetic divergence did not differ between recently colonized and established populations. These results indicated that founder effects during colonization of treated sites were weak or absent. Moreover, our results suggested founder effects were not mitigated by treatment area or connectivity. Dispersal is negatively density‐dependent in D. spectabilis, and we hypothesize that high gene flow may occur early in the restoration trajectory when density is low. Our study shows genetic diversity can be recovered more rapidly than demographic components of populations after habitat restoration and that founder effects are not inevitable for animals colonizing restored habitat in fragmented landscapes.     

Loss of floral polymorphism in heterostylous Luculia pinceana (Rubiaceae): a molecular phylogeographic perspective

Both deterministic and stochastic forces determine the representation and frequency of floral morphs in heterostylous plant populations. Phylogeographic analysis of molecular variation can provide information on the role of historical factors, including founder events, in affecting population morph structure. Here, we investigate geographical patterns of floral morph variation in a distylous shrub Luculia pinceana (Rubiaceae) by examining the relations between floral polymorphism and molecular (cpDNA and microsatellite) variation in 25 populations sampled throughout the distribution of the species in southwest China and adjacent countries. In 19 of the 25 populations, the frequency of floral morphs was not significantly different from the expected 1:1 ratio. The remaining populations were either L‐morph biased (2) or monomorphic (4) for this form and were morphologically differentiated from the remaining populations in several floral traits, that is, corolla tube length, sex organ position and stigma‐anther separation. Phylogeographic analysis supports the hypothesis that L. pinceana was initially split into west‐central and eastern lineages in the Early Pleistocene (～1.982 Mya). A centrally located lineage composed of morph‐biased and monomorphic populations appears to have been subsequently derived from the west‐central lineage, perhaps by a founder event after the last glacial maximum. Hypotheses to explain why these populations have not returned to equilibrium morph frequencies are considered.     

Sex ratio rather than population size affects genetic diversity in Antennaria dioica

• Habitat fragmentation and small population size can lead to genetic erosion in threatened plant populations. Classical theory implies that dioecy can counteract genetic erosion as it decreases the magnitude of inbreeding and genetic drift due to obligate outcrossing. However, in small populations, sex ratios may be strongly male‐ or female‐biased, leading to substantial reductions in effective population size. This may theoretically result in a unimodal relationship between sex ratios and genetic diversity; yet, empirical studies on this relationship are scarce.
• Using AFLP markers, we studied genetic diversity, structure and differentiation in 14 highly fragmented Antennaria dioica populations from the Central European lowlands. Our analyses focused on the relationship between sex ratio, population size and genetic diversity.
• Although most populations were small (mean: 35.5 patches), genetic diversity was moderately high. We found evidence for isolation‐by‐distance, but overall differentiation of the populations was rather weak. Females dominated 11 populations, which overall resulted in a slightly female‐biased sex ratio (61.5%). There was no significant relationship between population size and genetic diversity. The proportion of females was not unimodally but positively linearly related to genetic diversity.
• The high genetic diversity and low genetic differentiation suggest that A. dioica has been widely distributed in the Central European lowlands in the past, while fragmentation occurred only in the last decades. Sex ratio has more immediate consequences on genetic diversity than population size. An increasing proportion of females can increase genetic diversity in dioecious plants, probably due to a higher amount of sexual reproduction.

     

Negative effects of habitat degradation and fragmentation on the declining grassland plant Trifolium montanum

Changes in land use have resulted in a strong decline in the plant diversity of nutrient-poor grasslands, but little is known about the combined effects of habitat degradation and fragmentation on populations of individual species. We studied these effects on stage structure, recruitment, reproduction and offspring fitness in populations of the declining perennial grassland plant Trifolium montanum in central Germany. Density and survival probability of juvenile plants decreased with light competition, measured as leaf area index (LAI) above T. montanum plants, resulting in aged populations with few juvenile plants at unmanaged sites with higher LAI. Reproduction of T. montanum was not related to LAI, but increased strongly with local density, suggesting pollinator limitation in fragmented populations with a low density of flowering plants. In the common garden, the survival of sown offspring increased with mean seed size, whereas seed production of offspring decreased with isolation, and in strong contrast to previous studies, also decreased with size and density of the population of origin. This could be due to increased inbreeding because of pollination between closely related neighbouring plants in dense and large populations. Our results indicate that both habitat degradation and fragmentation have negative effects on populations of T. montanum, but affect different phases of the life cycle. In the short term, the effects of habitat degradation are more important than those of fragmentation, and populations of T. montanum are primarily threatened by an increase in light competition in unmanaged sites, which rapidly affects the dynamics of the populations. The observed opposite effects of habitat fragmentation on reproduction and offspring fitness indicate that the effects of population size, density and isolation on plant fitness and population viability may be complex.     

Genetic Diversity and Spatial Genetic Structure of the Grassland Perennial Saxifraga granulata along Two River Systems

Due to changes in land use, the natural habitats of an increasing number of plant species have become more and more fragmented. In landscapes that consist of patches of suitable habitat, the frequency and extent of long-distance seed dispersal can be expected to be an important factor determining local genetic diversity and regional population structure of the remaining populations. In plant species that are restricted to riparian habitats, rivers can be expected to have a strong impact on the dynamics and spatial genetic structure of populations as they may enable long-distance seed dispersal and thus maintain gene flow between fragmented populations. In this study, we used polymorphic microsatellite markers to investigate the genetic diversity and the spatial genetic structure of 28 populations of Saxifraga granulata along two rivers in central Belgium. We hypothesized that rivers might be essential for gene flow among increasingly isolated populations of this species. Genetic diversity was high (H_S = 0.68), which to a certain extent can be explained by the octoploid nature of S. granulata in the study area. Populations along the Dijle and Demer rivers were also highly differentiated (G” _ST = 0.269 and 0.164 and D _EST = 0.190 and 0.124, respectively) and showed significant isolation-by-distance, indicating moderate levels of gene flow primarily between populations that are geographically close to each other. Along the river Demer population genetic diversity was higher upstream than downstream, suggesting that seed dispersal via the water was not the primary mode of dispersal. Overall, these results indicate that despite increasing fragmentation populations along both rivers were highly genetically diverse. The high ploidy level and longevity of S. granulata have most likely buffered negative effects of fragmentation on genetic diversity and the spatial genetic structure of populations in riparian grasslands.     

The effects of riverine impoundment on genetic structure and gene flow in two stream fishes in the Mobile River basin

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High genetic diversity and low structure in an endemic long-lived tree,Yucca capensis (Asparagaceae)

• Endemic species distributed in fragmented habitats are highly vulnerable to extinction because they may have low genetic diversity. However, some life-history traits can mitigate the effect of genetic drift on populations. We analysed the level and distribution of genetic variation and ancestral population size of Yucca capensis, a long-lived endemic plant of the Baja California Peninsula, Mexico. Its populations are scattered across a habitat that is suffering accelerated transformation.
• We used six nuclear microsatellites to genotype 224 individuals from 17 locations across the entire species' geographic range. We estimated polymorphisms, heterozygosity and genetic structure. We also evaluated the ancestral and recent effective size and time since the population started to change.
• We found high heterozygosity, high polymorphism and low differentiation among locations, suggesting a panmictic population across the range. We also detected a large ancestral effective population size, which suffered a strong reduction in the Mid-Holocene.
• Despite changes in environmental conditions caused by habitat modification, the high diversity and low differentiation in Y. capensis may result from its large ancestral effective size and life-history traits, such as plant longevity, clonal growth and mating system, which reduce the rate of loss of genetic variation. However, the dependence on a specialist pollinator that displays short flight range can reduce gene flow among the plant populations and could, shortly, lead them into an extinction vortex.

     

Limited structure and widespread diversity suggest potential buffers to genetic erosion in a threatened grassland shrub Pimelea spinescens (Thymelaeaceae)

Pimelea spinescens is a critically endangered species of the temperate grasslands of southeastern Australia. Two subspecies are recognised. Subspecies, P. spinescens subsp. spinescens, formerly common and widespread, is found in isolated remnants of previously extensive grasslands. The second subspecies, subsp. pubiflora, is thought to have been historically rare with only two geographically-isolated extant populations. The grassland communities exist now as fragmented remnants representing <1 % of their extent prior to European settlement in the early 1800s. Conservation management strategies for species in these critically endangered ecosystems rely on an understanding of genetic diversity and population structure to ensure long-term evolutionary potential. We used chloroplast DNA (cpDNA) and microsatellite markers to examine the population genetic structure of both subspecies. Analysis of cpDNA revealed 14 haplotypes with high divergence between the single haplotype found in subsp. pubiflora and most remaining haplotypes restricted to subsp. spinescens. Microsatellites also indicated high genetic differentiation between subspecies but little evidence of sub-structuring within either subspecies. Results suggest that seed dispersal has not been as limited as previously thought. In this fragmented habitat, a lack of genetic structure suggests buffers to genetic erosion, with current patterns reflecting genetic diversity prior to fragmentation. Plant longevity and the presence of seed banks may contribute to the maintenance of these patterns, resulting in a lag between fragmentation and genetic erosion. Whilst factors such as longevity and seed banks may be preserving historic genetic diversity, management is required to ensure the maintenance of this diversity into the future.     

Microsatellite variation and structure of 28 populations of the common wetland plant, Lychnis flos-cuculi L., in a fragmented landscape

Habitat fragmentation is known to cause genetic differentiation between small populations of rare species and decrease genetic variation within such populations. However, common species with recently fragmented populations have rarely been studied in this context. We investigated genetic variation and its relationship to population size and geographical isolation of populations of the common plant species, Lychnis flos-cuculi L., in fragmented fen grasslands. We analysed 467 plants from 28 L. flos-cuculi populations of different sizes (60 000-54 000 flowering individuals) in northeastern Switzerland using seven polymorphic microsatellite loci. Genetic differentiation between populations is small (F(ST) = 0.022; amova; P < 0.001), suggesting that gene flow among populations is still high or that habitat fragmentation is too recent to result in pronounced differentiation. Observed heterozygosity (H(O) = 0.44) significantly deviates from Hardy-Weinberg equilibrium, and within-population inbreeding coefficient F(IS) is high (0.30-0.59), indicating a mixed mating breeding system with substantial inbreeding in L. flos-cuculi. Gene diversity is the only measure of genetic variation which decreased with decreasing population size (R = 0.42; P < 0.05). While our results do not indicate pronounced effects of habitat fragmentation on genetic variation in the still common L. flos-cuculi, the lower gene diversity of smaller populations suggests that the species is not entirely unaffected.     

Extinction debt in a common grassland species: immediate and delayed responses of plant and population fitness

Changes in landscape structure and environmental conditions due to habitat fragmentation can have significant effects on plant populations. Decreasing genetic diversity and changing population structure can reduce plant fitness and influence the long-term persistence of populations. Dry calcareous grasslands in Estonia have witnessed a large decline in area within the last 80 years, but due to extinction debt, the species richness in these grasslands has not yet responded to this decline. In these calcareous grasslands, we studied genetic diversity, phenotypic performance and population characteristics of a common habitat-specialist grass, Briza media. A decrease in genetic diversity was associated with a decrease in plant reproductive output. In addition, we found that some fitness components of B. media showed a delayed response to landscape changes. Specifically, plant height and germination success were related to historical rather than to current landscape parameters, indicating a time-lagged response of plant performance to habitat fragmentation. Dependence on historical landscape structure may thus result in a future decline in population fitness even if habitat loss and fragmentation no longer continue. The documented effect of current environmental conditions, however, shows that fitness-related traits are already slowly adapting to the changing conditions. Our results indicate that even common habitat-specialist species can be susceptible to landscape changes and be threatened by decreased population performance in the future.     

Evidence for demographic bottlenecks and limited gene flow leading to low genetic diversity in a rare thistle

European calcareous grasslands have decreased dramatically in area and number during the last two centuries. As a result, many populations of calcareous grassland species are confined to small and isolated fragments, where their long-term survival is to some extent uncertain. Recently, several restoration projects have been initiated to enlarge the current grassland area in order to maintain the exceptionally high species richness. However, from a genetic point of view, the success of these restoration measures is not necessarily guaranteed, as strong historical decreases in population size and limited gene flow may have led to low genetic diversity through genetic bottlenecks and drift. In this study, we investigated genetic diversity and structure of 16 populations of the calcareous grassland specialist Cirsium acaule in a severely fragmented landscape in south-western Belgium. The overall distribution of this species in the study area was significantly and positively related to patch area, suggesting that small patches do not allow survival of this species. Both allelic richness and genetic diversity were significantly and positively related to population size. Estimation of observed and expected gene diversity provided evidence for population bottlenecks in the history of not less than 31% of all sampled populations. Reconstruction of the historical land use showed that patch area decline in populations that went through a recent bottleneck was significantly larger than that in populations that showed no evidence of a bottleneck. Assignment analyses showed low migration rates, suggesting that replenishment of lost alleles through gene flow is highly unlikely. Overall, our results indicate that in the absence of gene flow strong decreases in calcareous grassland area may have long-lasting effects on genetic diversity of plant populations and may hamper the success of restoration projects that simply aim at restoring initial habitat conditions or enlarging population fragments, as indicated by the fact that none of the recently restored areas has been occupied by C. acaule.     

Fine-scale genetic structure of the common Primula elatior (Primulaceae) at an early stage of population fragmentation

Many rare species are threatened by habitat fragmentation; however, less is known about effects of fragmentation on common species, despite their potential role in ecosystem productivity and functioning. We identified key factors and processes influencing gene flow in a large population of Primula elatior, a common distylous perennial herb, at an early stage of the fragmentation process, i.e., when fragmentation is taking place. Using 19 allozyme loci, we investigated genetic variation and fine-scale spatial genetic structure (SGS) at seedling and adult life stages in relation to fragmentation history (recent bottlenecks), selection, clonal propagation, sexual reproduction (seed and pollen dispersal, distyly), and patchy structure (patch size, plant density, and morph ratio). The main factors contributing to the strong SGS are seed and (to a lesser extent) pollen dispersal, through a spatial Wahlund effect and biparental inbreeding. Significant differences in allele frequencies between seedlings and adults indicate a temporal Wahlund effect. Patch plant density and biased morph ratio also affect the genetic patterns. Our results show that if P. elatior populations evolve into patchworks of small, isolated remnants, genetic erosion, reduced gene flow, and increased inbreeding can be expected, suggesting that such common plant species might require large population sizes to remain viable.