Effects of population size on genetic diversity and seed production in the rare Dictamnus albus(Rutaceae) in central Germany

Tracking population genetic signatures of local extinction with herbarium specimens

BACKGROUND AND AIMS

Habitat degradation and landscape fragmentation dramatically lower population sizes of rare plant species. Decreasing population sizes may, in turn, negatively affect genetic diversity and reproductive fitness, which can ultimately lead to local extinction of populations. Although such extinction vortex dynamics have been postulated in theory and modelling for decades, empirical evidence from local extinctions of plant populations is scarce. In particular, comparisons between current vs. historical genetic diversity and differentiation are lacking despite their potential to guide conservation management.

METHODS

We studied the population genetic signatures of the local extinction of Biscutella laevigata subsp. gracilis populations in Central Germany. We used microsatellites to genotype individuals from 15 current populations, one ex situ population, and 81 herbarium samples from five extant and 22 extinct populations. In the current populations, we recorded population size and fitness proxies, collected seeds for a germination trial and conducted a vegetation survey. The latter served as a surrogate for habitat conditions to study how habitat dissimilarity affects functional connectivity among the current populations.

KEY RESULTS

Bayesian clustering revealed similar gene pool distribution in current and historical samples but also indicated that a distinct genetic cluster was significantly associated with extinction probability. Gene flow was affected by both the spatial distance and floristic composition of population sites, highlighting the potential of floristic composition as a powerful predictor of functional connectivity which may promote decision-making for reintroduction measures. For an extinct population, we found a negative relationship between sampling year and heterozygosity. Inbreeding negatively affected germination.

CONCLUSIONS

Our study illustrates the usefulness of historical DNA to study extinction vortices in threatened species. Our novel combination of classical population genetics together with data from herbarium specimens, an ex situ population and a germination trial underlines the need for genetic rescue measures to prevent extinction of B. laevigata in Central Germany.

What factors shape genetic diversity in cetaceans?

Understanding what factors drive patterns of genetic diversity is a central aspect of many biological questions, ranging from the inference of historical demography to assessing the evolutionary potential of a species. However, as a larger number of datasets have become available, it is becoming clear that the relationship between the characteristics of a species and its genetic diversity is more complex than previously assumed. This may be particularly true for cetaceans, due to their relatively long lifespans, long generation times, complex social structures, and extensive ranges. In this study, we used microsatellite and mitochondrial DNA data from a systematic literature review to produce estimates of diversity for both markers across 42 cetacean species. Factors relating to demography, distribution, classification, biology, and behavior were then tested using phylogenetic methods and linear models to assess their relative influence on the genetic diversity of both marker types. The results show that while relative nuclear diversity is correlated with population size, mitochondrial diversity is not. This is particularly relevant given the widespread use of mitochondrial DNA to infer historical demography. Instead, mitochondrial diversity was mostly influenced by the range and social structure of the species. In addition to population size, habitat type (neritic vs. oceanic) had a significant correlation with relative nuclear diversity. Combined, these results show that many often-unconsidered factors are likely influencing patterns of genetic diversity in cetaceans, with implications regarding how to interpret, and what can be inferred from, existing patterns of diversity.

Pollen sleuthing for terrestrial plant surveys: Locating plant populations by exploiting pollen movement

PREMISE OF THE STUDY

We present an innovative technique for sampling, identifying, and locating plant populations that release pollen, without extensive ground surveys. This method (1) samples pollen at random locations within the target species' habitat, (2) detects species' presence using morphological pollen analysis, and (3) uses kriging to predict likely locations of populations to focus future search efforts.

METHODS

To demonstrate, we applied the pollen sleuthing system to search for artificially constructed populations of Brassica rapa in an old field. Population size varied from 0-100 flowers labeled with artificial pollen (paint pellets). After characterizing the landscape, we pan-trapped 2762 potential insect vectors from random locations across the field and washed particulate matter from their bodies to assess artificial pollen abundance with a microscope.

RESULTS

Population size greatly influenced artificial pollen detection success; following random pollen trap sampling and interpolation, ground surveys would be best focused on identified areas with high pollen density and low variation in pollen density. Sampling sites most successfully detected artificial pollen when they were located at higher elevations, near showy flowering plants that were not grasses.

DISCUSSION

Detection of nascent populations using the proposed system is possible but accuracy will depend on local environmental factors (e.g., wind, elevation). Conservation and invasive species control programs may be improved by using this approach.

Seasonal and annual variations in the pollination efficiency of a pollinator community of Dictamnus albus L

The interplay between insect and plant traits outlines the patterns of pollen transfer and the subsequent plant reproductive fitness. We studied the factors that affect the pollination efficiency of a pollinator community of Dictamnus albus L. by evaluating insect behaviour and morphological characteristics in relation to flowering phenology. In order to extrapolate the pollinator importance of single taxa and of the whole pollinator guild, we calculated an index distinguishing between potential (PPI) and realized (RPI) pollinator importance. Although the pollinator species spectrum appeared rather constant, we found high intra- and inter-annual variability of pollinator frequency and importance within the insect community. Flower visitation rate strictly depended on insect abundance and on the overlap between their flying period and flower blooming. All the pollinators visited flowers from the bottom to the top of the racemes, excluding intra-plant geitonogamous pollination, and most of them showed high pollen fidelity. Only medium large-sized bees could contact the upward bending stiles while feeding on nectar, highlighting a specialisation of the plant towards bigger pollinators. Moreover, we found evidence of functional specialisation, since all pollinators were restricted to a single taxonomic group (order: Hymenoptera; superfamily: Apoidea). Both the PPI and RPI indices indicate Habropoda tarsata as the most important pollinator of D. albus. Following hand cross-pollination experiments we revealed the presence of pollination limitation in 1 of the 3 years of field study. We discuss this result in relation to flowering abundance and to possible mismatches of phenological periods between plants and insects.

Estructura genética y germinación de semillas en poblaciones portuguesas de

Cheirolophus uliginosus es una especie amenazada endémica de la costa atlántica de la península ibérica, donde ocupa unas pocas y reducidas localidades. En nuestro estudio, analizamos los patrones de variación de los haplotipos de ADN cloroplástico y el éxito reproductivo —capacidad germinativa— en siete poblaciones portuguesas de diferente tamaño. El éxito reproductivo de Ch. uliginosus se ha examinado en relación con la estructura genética y el tamaño de sus poblaciones. Los resultados indican una variabilidad intrapoblacional muy baja para los marcadores cloroplásticos utilizados. Nuestro estudio muestra una tasa de germinación significativamente reducida en las poblaciones pequeñas (< 50 individuos) respecto a aquellas de tamaño mediano (50-250 individuos) o grande (> 250 individuos). Para explicar este fenómeno, se deben tomar en consideración las limitaciones ecológicas y el aislamiento genético. Por otro lado, en las poblaciones de Ch. uliginosus de mayor tamaño (> 250 individuos) se ha observado una incidencia más acusada de la depredación de semillas antes de su dispersión, lo cual podría estar afectando a su respuesta reproductiva. Finalmente, las poblaciones más pequeñas —que presentan un reducido éxito reproductivo— contienen los haplotipos más distantes evolutivamente y su conservación debería ser, por tanto, prioritaria.

Habitat fragmentation and recent bottlenecks influence genetic diversity and differentiation of the Central European halophyte Suaeda maritima (Chenopodiaceae)

PREMISE OF THE STUDY

Central European salt habitats are mainly restricted to the maritime coast but scattered occurrences can also be found inland. In inland habitats, human activities have caused losses and reductions in the size of natural salt sites but have also created new anthropogenic habitats around potash mining dumps colonized by halophytic species in the last 30 yr. We aimed to investigate the effects of bottlenecks, isolation, and ongoing habitat fragmentation on the genetic variation of a species commonly growing in these special habitats.

METHODS

We used 10 microsatellite markers to compare genetic diversity and differentiation of 31 populations of Suaeda maritima (Chenopodiaceae) from Central European coasts and inland habitats. Two approaches were applied to analyze the tetraploid data based on allele frequencies directly derived from microsatellite data and from transformed binary data.

KEY RESULTS

In comparison to the coastal populations from the North Sea and the English Channel, significantly reduced genetic variation and increased between-population differentiation was revealed for populations from the German inland and the Baltic Sea coast. Genetic structure analyses clearly separated coastal and inland populations.

CONCLUSIONS

Our results indicate that gene flow is restricted among populations from inland salt sites and the Baltic Sea coast, presumably due to their isolation, small sizes, genetic bottlenecks and/or founder events. Patterns of allele distribution indicate some occasional genetic exchange among habitat types in the past. Anthropogenic salt sites may facilitate gene flow among inland salt habitats preventing endangered inland halophyte populations from genetic erosion.

Demographic loss, genetic structure and the conservation implications for Indian tigers

India is home to approximately 60 per cent of the world's remaining wild tigers, a species that has declined in the last few centuries to occupy less than 7 per cent of its former geographical range. While Indian tiger numbers have somewhat stabilized in recent years, they remain low and populations are highly fragmented. Therefore, the application of evidence-based demographic and genetic management to enhance the remaining populations is a priority. In this context, and using genetic data from historical and modern tigers, we investigated anthropogenic impacts on genetic variation in Indian tigers using mitochondrial and nuclear genetic markers. We found a very high number of historical mitochondrial DNA variants, 93 per cent of which are not detected in modern populations. Population differentiation was higher in modern tigers. Simulations incorporating historical data support population decline, and suggest high population structure in extant populations. Decreased connectivity and habitat loss as a result of ongoing fragmentation in the Indian subcontinent has therefore resulted in a loss of genetic variants and increased genetic differentiation among tiger populations. These results highlight that anthropogenic fragmentation and species-specific demographic processes can interact to alter the partitioning of genetic variation over very short time scales. We conclude that ongoing strategies to maximize the size of some tiger populations, at the expense of losing others, is an inadequate conservation strategy, as it could result in a loss of genetic diversity that may be of adaptive significance for this emblematic species.

Ex situ cultivation affects genetic structure and diversity in arable plants

Worldwide, botanical gardens cultivate around 80,000 taxa, corresponding to approximately one-quarter of all vascular plants. Most cultivated taxa are, however, held in a small number of collections, and mostly only in small populations. Lack of genetic exchange and stochastic processes in small populations make them susceptible to detrimental genetic effects, which should be most severe in annual species, as sowing cycles are often short. In order to assess whether ex situ cultivation affects genetic diversity of annuals, five annual arable species with similar breeding systems were assessed with 42 in situ populations being compared to 20 ex situ populations using a random amplified polymorphic DNA (RAPD) analysis approach. Population sizes tended to be lower under ex situ cultivation and levels of genetic diversity also tended to be lower in four of the five species, with differences being significant in only two. Ex situ populations showed incomplete representation of alleles found in the wild. The duration of cultivation did not indicate any effect on genetic diversity. This implies that cultivation strategies resulted in different genetic structures in the garden populations. Although not unequivocally pronounced, differences nonetheless imply that conservation strategies in the involved gardens may need improvement. One option is cold storage of seeds, a practice that is not currently followed in the studied ex situ collections. This may reflect that the respective gardens focus on displaying living plant populations.

Pollinator directionality as a response to nectar gradient: promoting outcrossing while avoiding geitonogamy

Plants with multiple flowers could be prone to autonomous self-pollination and insect-mediated geitonogamy, but physiological and ecological features have evolved preventing costs related to autogamy. We studied the rare perennial herb Dictamnus albus as a model plant, with the aim of describing the plant-pollinator system from both plant and pollinator perspectives and analysing features that promote outcrossing in an entomophilous species. The breeding system and reproductive success of D. albus were investigated in experimental and natural conditions, showing that it is potentially self-compatible, but only intra-inflorescence insect-mediated selfing is possible. Nectar analysis showed gender-biased production towards the female phase, which follows the male phase, and during flowering, full blooming is found in flowers at the bottom of the raceme. Among a wide spectrum of insect visitors, three genera (Bombus, Apis, Megachile) were found to be principal pollinators. A study of insect behaviour showed a tendency towards bottom-to-top flights for the most important pollinators Bombus spp. and Apis mellifera: upward movements on the racemes could be explained by foraging behaviour, from more to less rewarding flowers. In accordance with the 'declining reward hypothesis', bumblebees and honeybees leave the plant when gain of reward is low, after which few flowers are visited, reducing the chance of self-pollen transfer among flowers. Intra-flower self-pollination is prevented in D. albus by protandry and herkogamy, while the nectar-induced sequential pattern of pollinator visits avoids geitonogamy and tends to maximise pollen export, promoting outcrossing. All these features for preventing selfing benefit plant fitness and population genetic structure.

Genetic population structure, fitness variation and the importance of population history in remnant populations of the endangered plant Silene chlorantha (Willd.) Ehrh. (Caryophyllaceae)

Habitat fragmentation can lead to a decline of genetic diversity, a potential risk for the survival of natural populations. Fragmented populations can become highly differentiated due to reduced gene flow and genetic drift. A decline in number of individuals can result in lower reproductive fitness due to inbreeding effects. We investigated genetic variation within and between 11 populations of the rare and endangered plant Silene chlorantha in northeastern Germany to support conservation strategies. Genetic diversity was evaluated using AFLP techniques and the results were correlated to fitness traits. Fitness evaluation in nature and in a common garden approach was conducted. Our analysis revealed population differentiation was high and within population genetic diversity was intermediate. A clear population structure was supported by a Bayesian approach, AMOVA and neighbour-joining analysis. No correlation between genetic and geographic distance was found. Our results indicate that patterns of population differentiation were mainly caused by temporal and/or spatial isolation and genetic drift. The fitness evaluation revealed that pollinator limitation and habitat quality seem, at present, to be more important to reproductive fitness than genetic diversity by itself. Populations of S. chlorantha with low genetic diversity have the potential to increase in individual number if habitat conditions improve. This was detected in a single large population in the investigation area, which was formerly affected by bottleneck effects.

Low genetic variability and strong differentiation among isolated populations of the rare steppe grass Stipa capillata L. in Central Europe

Stipa capillata L. (Poaceae) is a rare grassland species in Central Europe that is thought to have once been widespread in post-glacial times. Such relict species are expected to show low genetic diversity within populations and high genetic differentiation between populations due to bottlenecks, long-term isolation and ongoing habitat fragmentation. These patterns should be particularly pronounced in selfing species. We analysed patterns of random amplified polymorphic DNA (RAPD) variation in the facultatively cleistogamous S. capillata to examine whether genetic diversity is associated with population size, and to draw initial conclusions on the migration history of this species in Central Europe. We analysed 31 S. capillata populations distributed in northeastern, central and western Germany, Switzerland and Slovakia. Estimates of genetic diversity at the population level were low and not related to population size. Among all populations, extraordinarily high levels of genetic differentiation (amova: phi(ST) = 0.86; Bayesian analysis: theta(B) = 0.758) and isolation-by-distance were detected. Hierarchical amova indicated that most of the variability was partitioned among geographic regions (59%), or among populations between regions when the genetically distinct Slovakian populations were excluded. These findings are supported by results of a multivariate ordination analysis. We also found two different groups in an UPGMA cluster analysis: one that contained the populations from Slovakia, and the other that combined the populations from Germany and Switzerland. Our findings imply that S. capillata is indeed a relict species that experienced strong bottlenecks in Central Europe, enhanced by isolation and selfing. Most likely, populations in Slovakia were not the main genetic source for the post-glacial colonization of Central Europe.

Genetic Variation in Past and Current Landscapes: Conservation Implications Based on Six Endemic Florida Scrub Plants

If genetic variation is often positively correlated with population sizes and the presence of nearby populations and suitable habitats, landscape proxies could inform conservation decisions without genetic analyses. For six Florida scrub endemic plants (Dicerandra frutescens, Eryngium cuneifolium, Hypericum cumulicola, Liatris ohlingerae, Nolina brittoniana, and Warea carteri), we relate two measures of genetic variation, expected heterozygosity and alleles per polymorphic locus (APL), to population size and landscape variables. Presettlement areas were estimated based on soil preferences and GIS soils maps. Four species showed no genetic patterns related to population or landscape factors. The other two species showed significant but inconsistent patterns. ForLiatris ohlingerae, APL was negatively related to population density and weakly, positively related to remaining presettlement habitat within 32 km. ForNolina brittoniana, APL increased with population size. The rather weak effects of population area/size and both past and current landscape structures suggest that genetic variation needs to be directly measured and not inferred for conservation planning.

Genetic structure of coastal and inland populations of the annual halophyte Suaeda maritima (L.) dumort. in Central Europe, inferred from amplified fragment length polymorphism markers

Naturally occurring inland salt habitats are highly threatened due to increasing fragmentation and area reduction, while the surroundings of former potash mining dumps have experienced a massive invasion by halophytes over the last 20 years. We reconstructed colonisation patterns of these purely anthropogenic inland salt sites using molecular markers in the obligate halophyte Suaeda maritima (L.) dumort. (Chenopodiaceae), a typical plant in such areas. In the present study, 120 individual plants from 40 coastal and inland populations in Central Europe were subjected to AFLP analysis with nine primer combinations. A total of 243 AFLP band positions were scored as presence/absence characters. Genetic diversity values were not significantly different in populations from natural and anthropogenic inland salt sites as compared to coastal habitats. Results from principal coordinate analysis, neighbour-joining analysis and analysis of molecular variance (amova) all indicated that most of the genetic variation is preserved within populations, while genetic differentiation among populations is comparatively low. We conclude that S. maritima has repeatedly and independently colonised the surroundings of former potash mining dumps in Central Germany. However, the absence of founder effects and the lack of phylogeographic structure prevented us from identifying putative donor populations.

Fitness and genetic variation of Viola calaminaria, an endemic metallophyte: implications of population structure and history

We investigated variations in genetic diversity and plant fitness in a rare endemic metallophyte of calamine soils, Viola calaminaria, in relation to population size, population connectivity and population history in order to evaluate and discuss potential conservation strategies for the species. Mean population genetic diversity (H(s) = 0.25) of V. calaminaria was similar to endemic non-metallophyte taxa. Twenty-one per cent of the genetic variation was partitioned among populations and a low (9%) but significant differentiation was found among geographical regions. Our results did not support the hypothesis that the acquisition of metal tolerance may result in reduced genetic diversity, and suggested that strict metallophytes do not exhibit higher inter-population differentiation resulting from scattered habitats. There were no relationships between population genetic diversity and population size. Significant correlations were found between plant fitness and (i) population size and (ii) connectivity index. Recently-founded populations exhibited the same level of genetic diversity as ancient populations and also possessed higher plant fitness. There was no indication of strong founder effects in recently-established populations. The results suggest that the creation of habitats through human activities could provide new opportunities for conservation of this species.

Genetic rescue in an isolated metapopulation of a naturally fragmented plant species, Parnassia palustris

The loss of habitat of naturally fragmented species may result in isolated metapopulations: small groups of populations that are still connected by gene flow but have become isolated from other metapopulations. Genetic isolation may result in genetic differentiation between metapopulations and lowered genetic diversity within the metapopulation. Gene input from outside the metapopulation can hence be expected to enhance crosscompatibility and seed viability. Simultaneously, due to genetic drift, inbreeding, and pollen limitation in the small populations, fitness is likely to increase with population size within a metapopulation. I tested these hypotheses in three isolated metapopulations of the endangered plant species Parnassia palustris L. in a study area of 15 x 0.5 km along the coast of western Belgium and northwestern France. Seed and fruit weight, measured in the 27 populations distributed over the three metapopulations, significantly differed among the metapopulations, which suggests genetic differentiation. These fitness variables also increased with population size within two metapopulations. With a pollination experiment, I investigated whether genetic rescue with pollen from another metapopulation enhanced seed set. Outside-metapopulation crosses led to a significantly higher seed set than within-metapopulation crosses, and this effect was more pronounced in small populations. This means pollen from outside the metapopulation was more compatible than pollen from within the metapopulation, due to a lowered genetic diversity within the metapopulation. The seed set of naturally pollinated flowers was at least equal to that of hand-pollinated flowers, which can be explained by a compensation effect of pollen quantity and donor diversity for pollen quality. One can assume that the loss of genetic diversity at the level of the metapopulation affected loci not just related to cross compatibility. Reconnection of metapopulations by stepping stones may be necessary to prevent further genetic erosion and assure the viability of the Parnassia populations in the study area over the long term.