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Dynamics of plant dispersal-related traits in fragmented european habitats: consequences for species survival and landscape management

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Centaurea corymbosa (Asteraceae) is endemic to a small area (< or = 3 km2), and < 500 individuals reproduce in any given year. Levels of gene flow among populations and seed and pollen dispersal data indicated very low dispersal capacity. To analyse the genetic basis of dispersal capacity in this species, a QTL analysis is being conducted. Seeds have been collected on 198 plants issued from full-sib hybrids between Centaurea maculosa x Centaurea corymbosa cross. Measurements have been performed in fall 2003 on 10 seeds per plant for their size (pappus and achene length, width, surface, volume and their ratio achene to pappus) and weight. Several other measurements from germination to reproductive stage have also been performed. The same plants have been analysed by AFLP, and are being analysed with microsatellite markers. A genetic map will be composed, onto which the QTLs for dispersal-related traits will me placed.
In this study we related dispersal related traits of the wind dispersed species Inula conyza and Cirsium dissectum to the spatial isolation of the populations. It was expected that spatial isolation would result in selection pressure against dispersal capacity. Reduced dispersal capacity was observed for other wind dispersed species on real islands and it could be explained by the degree of isolation of the islands. We therefore wanted to test the hypothesis for species in fragmented habitat on land, being islands of suitable habitats in a sea of unsuitable land. 11 Inula populations and 8 Cirsium populations were sampled and dispersal related traits were measured. Strong correlations were found between drop time of seeds and seed weight for both species. The average drop time of Inula and Cirsium seeds was 0.29 m.s-1 and 0.34 m.s-1 respectively. The highest average drop time for an Inula population was 0.35 m.s-1 and for a Cirsium population 0.37 m.s-1, the lowest 0.25 m.s-1 and 0.31 m.s-1 respectively. Drop times differed only slightly between populations. We could not find a correlation between the average drop times and the isolation or size of populations of either species. We also analysed the variance of drop times of Inula seeds. The largest variations in droptime were found within individual plants, not within populations or between populations. We related the variance of the drop times of the populations with spatial isolation and population size, but relations were not found. In the studied populations the selection pressure of isolation on dispersal capacity could not be showed. We conclude that differences in drop time of seeds are not caused by isolation or population size. We discuss why we could not find an effect of isolation or population size and which other factors might be more important in determination of dispersal related traits. The large variation within individual plants indicates that resource allocation within the plants may play an important role. The genetic basis of dispersal traits and the role of environmental influences are discussed.
Fragmentation is a difficult process to study because: (i) it can act at different spatial scales (at the scale of the species range, the region, or that of a patchy population); (ii) it may affect different landscapes characteristics (distance between patches, size of patches, density of individuals within patches); and (iii) it has consequences on both the demographic functioning of metapopulations and their genetic structure. Matters of scales are rarely acknowledged explicitly in theoretical studies of dispersal. The evolutionary consequences of different modes of fragmentation and how they may combine has been extremely rarely studied from a theoretical point-of-view. Most analytical models address separately the genetic and demographic consequences of fragmentation, while simulation studies allow the investigation of their combined effects with the drawback that they often lack clear insights on the mechanisms generating patterns. Finally, fragmentation and its consequences for the evolution of dispersal cannot be studied in isolation from its consequences for the evolution of other traits or more generally for the genetic make-up of metapopulations. A book chapter written by members of P4, to appear in the new "metapopulation" volume edited by Ilkka Hanski and Oscar Gaggiotti, summarizes the state of the art concerning the evolution of life histories in fragmented landscapes and their limitations. Our efforts in this work-package have been devoted to fill in, in part, such theoretical gaps by focusing on: Elements: (i) Integration of realistic demography and genetic structure in analytical evolutionary models; (ii) Comparison of analytical and simulation results for the co-evolution of dispersal with other life history traits; (iii) Integration of evolutionary aspects of dispersal traits in existing spatially explicit, individual-based simulation models; (iv) Development of threshold values of landscape parameters for sustainable plant populations; (v) Evaluation of different landscape scenarios for plant survival. The studies show that evolutionary consequences of fragmentation are scale and pattern-dependant In addition, the theoretical studies have illustrated how evolution of seed dispersal is tightly linked to the evolution of other plant traits (such as pollen dispersal, seed dormancy, adaptation to local climatic or soil conditions, mating systems, inbreeding depression). Fragmentation results in changes in a set of traits larger than those affecting dispersal in space, which in turn affect the selection pressures on dispersal. Yet, it is difficult at this stage to identify clear syndromes that would specifically evolve in response to fragmentation, in part because fragmentation is a complex process with many dimensions and the evolutionary consequences of fragmentation are themselves very variable. It is however this set of coevolving character that we need to consider if we want to evaluate the demographic impact of evolutionary change caused by fragmentation. The series of theoretical studies represent a significant step in this direction, even though they unravel the very complex nature of such evolutionary interactions.
The distribution of genetic diversity in Mycelis muralis was investigated at a local scale in Sweden using microsatellite markers to infer seed dispersal ability from genetic patterns. A total of 24 populations were sampled in Sweden, among which 18 were located on a 375 square km area around Gävle, and six up to 500 km northwards. All these populations showed very high selfing rate (0.93 < s < 1) confirming that gene flow in M. muralis is mainly by seed. An analysis of the spatial distribution of identical nine-locus genotypes across populations revealed that ten different genotypes were common to between two to six populations each. These identical genotypes present in different populations could not have been created by recombination among other genotypes found in the resident populations (probability estimates based on allelic frequencies within populations: P < 10-4). These results thus provided evidence of seed migration between distantly separated populations, separated by up to 20km. One implication of these results is that variation conditioning the ability for long distance gene flow by seeds still exists in M. muralis populations, at least for habitats with the degree and type of fragmentation typical of this area of Sweden. Fine scale analyses of multilocus genotype distributions within three of these Swedish populations further revealed that the dispersal distances of each single genotypes can range from a few cm (distribution patterns consistent with sibs establishing near mother plants) to a few hundred meters within single sites. Moreover, the common genotypes that were present in different populations also showed aggregated distributions within populations, with related individuals distributed on local patches. These latter results suggest that "good dispersers" genotypes can disperse at both long (inter-populations) and short (intra-populations) distances. In regard to these results, we are thus unable to conclude that, for M. muralis, either short or long distance dispersal abilities have become predominant through selective processes.
Seed dispersal is a complex process in which several genetic and environmental factors play a role. Amongst others, factors like the presence and size of pappus, achene weight, plant height and wind conditions determine together the average distance that a seed will disperse. Presence and size of a pappus may affect the dispersal distance of a seeds as seeds with larger pappus have a higher terminal velocity and as a result may float longer in the air. Also, a larger pappus may improve the adherence to, e.g., the fur of animals that function as dispersal vector. Plant height can strongly affect dispersal distance, as taller plants release the seeds higher up in the air, where currents are stronger and therefore seeds can travel further. Under specific conditions selective pressure may act upon dispersal-related traits. This seems for instance to be the case for island populations, were selection can occur relatively fast, within a few generations (Cody and Overton 1996). A prerequisite for selection to take place is the presence of genetic variation. Also important is the number of genes involved in expression of the dispersal-related trait, as it can be expected that traits that are determined by only a few major genes, can evolve very fast. As a model system, we have analyze the molecular basis of two characters related to dispersal capacity by mapping the major gene(s) involved through QTL (Quantitative Trait Loci) analysis in Chichorium. A segregating population was be constructed between C. endivia and C. calvum and offspring was characterized for pappus length and plant height to determine the number of QTLs involved in these two different dispersal-related traits. QTLs involved in pappus length C. endivia has a pappus of more than 0.5 mm (which is up to 25% of the achene length), while C. calvum has (virtually) no pappus. In the F2 population a whole range of different pappus lengths was observed, indicating that pappus length is a quantitative character. To our surprise we detected only two QTLs for pappus length, which together explained 62% of the variation in pappus size (Table 1). Both loci have a strong additive effect when present in homozygote state. Also it turns out that the locus on linkage group 10 is a dominant characteristic. Although the rudimentary pappus of C. endivia is unlikely to be involved in wind dispersal of the seed, this study shows that the presence or absence of a pappus may be governed by just a few QTLs and perhaps only a few genes. This means that it is conceivable that selection can takes place in a relatively short period of time (in the order of a few generations in a small population). QTLs involved in plant height The C. endivia parent was substantially larger than C. calvum (Table 2). The F2 plants were on average larger than both parents, indicating a heterosis effect. In the F2 again a whole spectrum of different plant lengths was found and five QTLs were detected, clearly indicating the polygenic nature of this character. These results are in line with observations of others (tomaat Fulton et al 1997 TAG). Alleles having a positive effect on plant height were found in C. endivia as well as in C. calvum. In four of the five loci dominance was in the positive direction. Among the five pairs of QTL loci, pair-wise interactions were observed. All these data indicate that plant height is a complex trait, in which several genes are involved; therefore selection in either direction will most likely take much more time than selection for pappus size.
For Mycelis muralis, two measures of dispersal ability were analyzed: pappus to achene length ratio (PALR) and pappus radius to achene weight ratio (PRAWR). An initial evaluation of the variability for dispersal ability in the whole data-set (more than 30 populations), including all regions (countries) sampled, shows significant differences among countries (regions), populations within regions and individual plants within populations. The variability associated with individual plants was similar for both dispersal traits. The variability among regions within Europe was higher for PALR than for PRAWR. Mean values for dispersal ability (both PRAWR and PALR) were consistently higher for plants in Sweden and lower for Spain, with the Netherlands showing intermediate values. Although the results from common environment conditions do not suggest that such differences have a genetic basis, they are consistent with the patterns of postglacial recolonization obtained from the analysis of the distribution of genetic variability throughout Europe. Northward recolonization from postglacial refugia could be expected to be carried out by genotypes with higher dispersal abilities. Differences in dispersal ability between plants within populations have been previously shown not to be related to differences in resource allocation (plant size or total number of seeds produced per plant). However, differences among populations seem to be related to fragmentation or connectivity. Connectivity within a given landscape can be estimated as the ratio between mean patch size and the mean distance between nearest neighbour patches: connectivity increases with increasing patch size and/or decreasing with increasing distances among patches. Dispersal ability (PRAWR) seems to be higher in highly connected landscapes and lower in highly fragmented landscapes (low connectivity). Since most of the observed variability seems to be environmentally induced, such differences at a global scale must be mainly interpreted in terms of habitat quality. The patterns of distribution of phenotypic variability were further analysed at a local scale, within each of the regions considered. The results obtained highlight the different role of that ecological (habitat) and/or evolutionary forces may have in determining dispersal ability in different landscapes and regions. Overall, the variability attributed to populations and individuals within populations, as compared to within-individual variability, was much higher in Spain (66.7 % and 77.3 for PRAWR and PALR, respectively) than in The Netherlands (56.7 % and 53.9 %) and Sweden (37% and 57.3%).
Most Mycelis muralis populations in The Netherlands did show both low levels of microsatellite variation and a significant deficit of heterozygotes, suggesting that the species is predominantly selfing. As even low levels of outcrossing might be sufficient to in the end create a high level of genotypic variation through recombination, we were very interested in estimating outcrossing rates in natural populations. However, as many populations are mainly comprised of one or only a few multi-locus genotypes (MLG’s) and consequently most individuals share the same alleles, it was impossible to estimate outcrossing rates directly as most outcrossing events will be cryptic and undetectable by the microsatellite markers. For the experiments we used the population of Winschoten, an isolated population (nearest population at 30-40 km) in the north of the Netherlands. Microsatellite analysis of 17 plants for 6 polymorphic loci revealed that this population is mainly comprised of 1 multi-locus genotype (MLG) that was homozygous at all examined loci. To nevertheless estimate the frequency of possible cryptic outcrossing events within this population, 'experimental' plants were placed in this population with MLG's that differed for at least 4 microsatellite loci from the one found in Winschoten. These ‘experimental’ plants were also homozygous at all loci. In this situation, the seeds that are produced by the ‘experimental’ plants, that are the result of an outcrossing event, will produce seedlings with a MLG that is heterozygous for at least these 4 microsatellite loci. Determining the percentage of 'heterozygous' offspring produced by these 'experimental' plants will give insight into the level of outcrossing in natural M. muralis populations. We placed 57 plants with 'different' MLG’s in population Winschoten on 10 different days spread over 2 months (June 15th – August 15th). Individual 'experimental' plants were placed in this population for one day during which insect visitation was recorded for (15 minutes time intervals) and insects-visited flowerheads were marked. At the end of the day, 'experimental' plants were returned to the greenhouse to allow seed set. For all experimental plants all the seeds from the insect-visited flowerheads were sampled and germinated to produce seedlings. The same was done for seeds from all, visited and non-visited, flowerheads of a subset of the experimental plants that flowered during the same experimental period. These seedlings were grown in the greenhouse and analyzed for 2 diagnostic microsatellite loci. The results indicate that approximately 34% of the flowerheads of the experimental plants produce at least one outcrossed seed, and that approximately 15% of all the seeds produced by the experimental plants were the result of an outcrossing event. Thus there seems to be ample opportunities for redistribution and randomisation of genetic variation within populations.
Higher densities reduce the dispersal distances in plants Habitat fragmentation is shown to be a major threat for many plant species. If patches of suitable habitat become small and the there living populations as well, then these populations are more affected by environmental and demographic stochasticity and can suffer from inbreeding depression. In case of genetic variation in dispersal distances, in large patches larger dispersal distances will evolve. This is already shown in natural populations. In stationary landscapes, species should evolve an evolutionary stable state. Here in this study we model this system and test whether patch size is the only landscape related factor influencing the evolution of dispersal distances or whether population size (determined by habitat quality) also matters. In order to study the effect of patch size and habitat quality in a general context and to avoid structural uncertainties we used two different models and five different dispersal curves covering a wide range of realistic possibilities. The models we used are two spatially explicit, individual-based simulation models, one designed for a selfing, the other for an outcrossing species. Each individual contains a genotype that is linked to the dispersal distance. Seed establishment can only happen in open spots of bare ground. As expected, we found that patch size has a huge influence on the evolution of dispersal distances. But also population size matters. Surprisingly, in higher population sizes the dispersal distances decreased. To our knowledge, this effect never has been found before and, in the first moment, is against the intuition, but the reason is clear: in high densities the distance to the next open spot for germination is shorter than in large populations. So there is a selection towards shorter dispersal distances in denser populations. Furthermore, we found that the variability between the individuals is larger in larger patches. Both results are relevant to conservation. Even if the single seeds in higher densities disperse shorter, this effect can be overcompensated by higher seed numbers in these populations. We tested whether this effects gets overcompensated or not and found, this is dependent on the dispersal curves, which usually are not known. We derived the following conclusions for conservation: It is important to have large patches, first to get larger mean dispersal distances and second to have a larger variation between the individuals, which again has two advantages: (1) to keep few very long distance dispersers and (2) in a changing environment populations with a high variation can faster adapt and therefore survive more likely. Finally it is important to keep in mind that an increase of a certain degree in population density does not automatically mean that the colonization potential increase over the same degree. Especially after several years or decades there can be a significant reduction in the dispersal distances with the consequences of a decline in the colonization potential.
Evolution of plant dispersal has been studied quite extensively both theoretically and ecologically. Little is known about the evolutionary genetics of dispersal. We determined levels of genetic variation for dispersal related traits (e.g. seed and pappus size) within and among populations of the wind-dispersed plant species Mycelis muralis (Asteraceae) in a greenhouse experiment. By comparing the level and structure of genetic variation for dispersal ability with that for neutral traits (microsatellites), we assessed the possibility that the observed phenotypic variation in dispersal ability among natural populations was due to natural selection. Parent-offspring regressions revealed significant heritabilities for all examined dispersal related traits within one population (Balma). Almost no genetic variation for dispersal related traits was observed in all other examined M. muralis populations. Most studied populations, except Balma, showed low levels of genetic and genotypic variation for microsatellite loci. Significant genetic differences in dispersal related traits were observed among populations in the greenhouse. These differences were not correlated with the observed phenotypic variation for dispersal ability among these populations under field conditions. Some European populations of M. muralis harbour sufficient levels of genetic variation for dispersal related traits in order for dispersal ability to evolve under natural selection pressures. Founder events, by reducing within population genetic diversity, prevent evolution of dispersal in most populations. Environmental effects may be the most important factor determining the phenotypic differences in dispersal related traits between individuals both within and among natural M. muralis populations.
The relationship between habitat fragmentation and dispersal related traits has been studied in three different species of composites differing in life-history traits: Mycelis muralis, Crepis triasii and Leontodon taraxacoides. In Spain M. muralis behaves as a self-compatible and polycarpic perennial and it is found in humid valleys and ravines. C. triasii is an allogamous polycarpic species endemic from the Balearic Islands inhabiting rocky places. L. taraxacoides is an allogamous annual plant with seed dimorphism colonizing abandoned fields. The relationship between seed dispersal ability and fragmentation has been analysed at different spatial scales in the three species (different populations in different regions or islands). The results indicate that fragmentation has different effects according to the species and spatial scale considered. Although in M. muralis traits such as pappus and seed size show significant within-population temporal variability, probably associated to resource (water) availability, dispersal ability (pappus to achene size ratio) seems to be highly constant, at the population and individual level, and independent of resource variability. Comparisons among landscapes with marked differences in fragmentation show that dispersal ability decreases with increasing isolation. Although the analysis of genetic variation for dispersal traits suggest that such differences are purely environmental, the selection against dispersal in highly fragmented landscapes cannot be ruled out, since Spanish populations harbour significant levels of genetic variability for dispersal traits. The individual variability in dispersal traits in L. taraxacoides, both the ratio between seeds with pappus and without pappus and pappus size, is clearly related to individual patterns of resource allocation (plant size). However, after accounting for differences in plant size, dispersal ability at the population level seems to depend on the disturbance regime and landscape structure. Since significant additive genetic variability for dispersal-related traits has been found in other heterocarpic species with identical life-history traits and inhabiting the same kind of habitat (e.g., Crepis sancta), such differences might be interpreted as the result of selection for dispersal traits. Although significant differences in dispersal ability have been found among populations of C. triasii, such differences does not seem to be related to habitat fragmentation or genetic differentiation among populations, and should thus be considered as purely environmental. However, ample genetic variability does seem to exist for other traits, particularly those related with water-use efficiency and water stress avoidance. The analyses and comparisons performed also suggest that such differences are the result of local adaptations both between islands and within islands. The results obtained are remarkable from the point of view of the management policies aiming at the conservation of genetic resources and long-term viability of the high diversity of endemic species in the Balearic Islands. They clearly point to the need of including some additional areas under protection covering a wider geographical area. This is particularly relevant since the plans for spatial planning implemented by the actual local government (Govern de les Illes Balears) aim at substantially reducing already existing protected areas.
The distribution of genetic diversity in Mycelis muralis, or wall lettuce, was investigated at a European scale using microsatellite markers to infer historical and contemporary forces from genetic patterns. Mycelis muralis has the potential for long-distance seed dispersal by wind, is mainly self-pollinated, and has patchily distributed populations, some of which may show turnover. Populations were sampled in southern Europe (Spain and France), The Netherlands, and Sweden. At this within region scale, contemporary evolutionary forces (selfing and metapopulation dynamics) are responsible for high differentiation between populations but, contrary to expectation, levels of within-population diversity were not low. The latter result suggests that seed dispersal efficiently counteracts population turnover and maintains genetic diversity within populations. At the European scale, northern regions showed lower allelic richness and higher selfing rates than populations from southern Europe. In light of post-glacial colonization hypotheses, these results suggest that rare alleles may have been lost during recolonization northwards, and that selfing may have been selected owing to the reproductive assurance that it confers to long distance dispersers. Our results further suggest that mutation has contributed to genetic differentiation between southern and northern Europe, and that Sweden may have been colonized by dispersers originating from at least two different refugia.

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