Assembly of the Arctic flora: Highly parallel and recurrent patterns in sedges (Carex).

PREMISE OF THE STUDY: Understanding the origin of ecosystems and their changes through time is important. Two mutually contrasting types of grasslands existed in the Arctic: dry- and cold-adapted grasslands of the Pleistocene dominated by Poaceae species, and presently dominating graminoid grasslands composed of sedges and rushes. We studied the taxon recruitment of the Arctic flora for Carex, the most species-rich and widespread genus of the Arctic. In this study we explore the possible geographical and altitudinal origins of the species, their ecological provenance in terms of soil moisture and light requirements, and salt tolerance. METHODS: We addressed these questions in a phylogenetic context using the latest megaphylogeny of Carex comprising almost all Arctic species and about half of the genus' total species diversity. Ecological data were extracted from the literature and analyzed for each clade comprising Arctic species. KEY RESULTS: Arctic Carex species were observed in 48 independent lineages. Almost all areas north of the meridional zone of the Northern Hemisphere may have served as sources of Arctic lineages. Source areas are unrelated to the distribution within the Arctic. Arctic species evolved in lowland and high mountain clades; mostly in wet, rarely in dry adapted clades that occur principally in open conditions. Salt tolerant Arctic species occur in five clades. CONCLUSIONS: Many independent lineages of different geographical areas and ecological backgrounds provided species for the northernmost ecosystem; clear main sources were not discernible. Carex shows the whole dynamic of ecosystem assembly from a seemingly simple immigration of preadapted species, evolution in geographical distant areas, to species radiations in the North.

High mountain origin, phylogenetics, evolution, and niche conservatism of arctic lineages in the hemiparasitic genus Pedicularis (Orobanchaceae).

The origin of the arctic flora covering the northernmost treeless areas is still poorly understood. Arctic plants may have evolved in situ or immigrated from the adjacent ecosystems. Frequently arctic species have disjunctive distributions between the Arctic and high mountain systems of the temperate zone. This pattern may result from long distance dispersal or from glacial plant migrations and extinctions of intermediate populations. The hemiparasitic genus Pedicularis is represented in the Arctic by c. 28 taxa and ranks among the six most species-rich vascular plant genera of this region. In this study, we test the hypothesis that these lineages evolved from predecessors occurring in northern temperate mountain ranges, many of which are current centers of diversity for the genus. We generated a nuclear ribosomal and chloroplast DNA phylogeny including almost all of the arctic taxa and nearly half of the genus as a whole. The arctic taxa of Pedicularis evolved 12-14 times independently and are mostly nested in lineages that otherwise occur in the high mountains of Eurasia and North America. It appears that only three arctic lineages arose from the present-day center of diversity of the genus, in the Hengduan Mountains and Himalayas. Two lineages are probably of lowland origin. Arctic taxa of Pedicularis show considerable niche conservatism with respect to soil moisture and grow predominantly in moist to wet soils. The studied characteristics of ecology, morphology, and chromosome numbers of arctic Pedicularis show a heterogeneous pattern of evolution. The directions of morphological changes among the arctic lineages show opposing trends. Arctic taxa are chiefly diploid, the few tetraploid chromosome numbers of the genus were recorded only for arctic taxa. Five arctic Pedicularis are annuals or biennials, life forms otherwise rare in the Arctic. Other genera of the Orobanchaceae consist also of an elevated number of short-lived species, thus hemiparasitism may favor this life form in the Arctic.

Rapid and recent world-wide diversification of bluegrasses (Poa, Poaceae) and related genera.

Rapid species diversifications provide fascinating insight into the development of biodiversity in time and space. Most biological radiations studied to date, for example that of cichlid fishes or Andean lupines, are confined to isolated geographical areas like lakes, islands or island-like regions. Using DNA sequence data of the ribosomal internal transcribed spacer (ITS) for many species of the Poa alliance, a group comprising about 775 C3 grass species, revealed rapid and parallel diversifications in various parts of the world. Some of these radiations are restricted to isolated areas like the Andes, whereas others are typical of the lowlands of mainly the northern hemisphere. These radiations thus are not restricted to island-like areas and are seemingly actively ongoing. The ages of the diversifying clades are estimated to be 2.5-0.23 million years (Myr). Conservative diversification rates in the Poa alliance amount to 0.89-3.14 species per Myr, thus are in the order of, or even exceeding, other instances of well-known radiations. The grass radiations of the mainly cold-adapted Poa alliance coincide with the Late Tertiary global cooling, which resulted in the retreat of forests and the subsequent formation of cold-adapted grasslands especially in the northern, but also in parts of the southern hemisphere. The cold tolerance, suggested to be one of the ecological key innovations, may have been acquired during the early diversification of the subfamily Pooideae, but became significant millions of years later during the Pliocene/Pleistocene radiation of the Poa alliance.

Not across the North Pole: plant migration in the Arctic.

• The vascular plant flora of 66 arctic islands was studied to determine whether the islands have been occupied by random long-distance dispersal (LDD) or in a highly structured northward migration pattern via intervening islands as stepping-stones. • A maximum parsimonious migration model minimizing dispersal distances of 1256 vascular plant taxa was calculated in the framework of network analysis. • Plant dispersal is not stochastic in the Arctic at the global scale. Inferred mean dispersal distances of the plants occurring on arctic islands are c. 580 km (median 460 km). A LDD across the North Pole could not be inferred in the model and species may be recruited mainly from the nearest mainland or islands. At smaller scales, among adjacent islands, dispersal of vascular plants may be incomplete. Arctic islands do not yet appearto be saturated with species. • The results suggest that changes in biodiversity in Arctic islands can be more easily predicted at the global scale than at the local scale. Because islands are not yet saturated with species, new colonizations may not necessarily be linked to climate change.

SOURCES OF THE ARCTIC FLORA: ORIGINS OF ARCTIC SPECIES IN RANUNCULUS AND RELATED GENERA.

The arctic biome is a relatively young ecosystem with ~2300 species of vascular plants. We studied the genus Ranunculus as an example of the origin and evolution of the arctic flora. For this purpose we used molecular phylogenetic and clock analyses based on evaluation of nuclear ITS and chloroplast matK-trnK DNA sequences in 194 taxa of Ranunculus and closely related genera. Taxa occurring in the Arctic arose form seven phylogenetic lineages of Ranunculus and also in the genera Coptidium and Halerpestes. Two clades of Ranunculus are species-rich in the Arctic, i.e., Ranunculus sect. Ranunculus and R. sect. Auricomus (both from R. subg. Ranunculus), but this is due to a number of arctic "microtaxa" morphologically barely separate from R. acris in the former clade and the widely agamospermic species complex of R. auricomus in the latter. Lineages with species adapted to wetlands or aquatic habitats are significant groups represented in the arctic flora (R. subg. Ranunculus sectt. Flammula and Hecatonia/Xanthobatrachium, R. subg. Batrachium, genus Coptidium) but show no clear signs of radiation in the Arctic or the northern boreal zone, except for sectt. Hecatonia/Xanthobatrachium, with R. hyperboreus and R. sceleratus subsp. reptabundus. Astonishingly few of the otherwise numerous lineages of Ranunculus with distributions in the higher mountain systems of Eurasia and North America have acted as "founding sources" for the arctic flora. The only clear example is that of the arctic-alpine R. glacialis and the Beringian R. chamissonis from the lineage of subg. R. sectt. Aconitifolii/Crymodes, although there might be others in sect. Auricomus not recovered in the current molecular data. Lineages that gave rise to arctic taxa diverged from each other from the early Miocene (R. glacialis/R. chamissonis, Coptidium, lineages in Halerpestes) and continued at an even rate throughout the Tertiary. There are no signs that the intense climate changes of the late Pliocene and the Quaternary substantially accelerated or impeded diversification in Ranunculus. Only the crown group split of R. acris and its relatives is clearly of Quaternary age. A detailed comparison concerning morphology, karyology, and life form excludes fundamental differences between taxa of Ranunculus in the Arctic and their respective closest relatives in regions south of it. Ecological traits, e.g., preferences for dry or moist soils or growth in open and sheltered conditions, also do not differ between arctic and nonarctic̣ taxa. Migration into the Arctic thus started from different phylogenetic lineages and at different times, without development of obvious special traits in the adaptation to arctic environments. This recurrent pattern in Ranunculus differs from that seen in other arctic genera, e.g., Artemisia, in which special traits of adaptation to arctic environments are found. In Ranunculus, the origin of the open arctic biome primarily favored range expansions of taxa/species already adapted to wet habitats in cold areas and depending on rapid dispersal.

Conclusions about niche expansion in introduced Impatiens walleriana populations depend on method of analysis.

Determining the degree to which climate niches are conserved across plant species' native and introduced ranges is valuable to developing successful strategies to limit the introduction and spread of invasive plants, and also has important ecological and evolutionary implications. Here, we test whether climate niches differ between native and introduced populations of Impatiens walleriana, globally one of the most popular horticultural species. We use approaches based on both raw climate data associated with occurrence points and ecological niche models (ENMs) developed with Maxent. We include comparisons of climate niche breadth in both geographic and environmental spaces, taking into account differences in available habitats between the distributional areas. We find significant differences in climate envelopes between native and introduced populations when comparing raw climate variables, with introduced populations appearing to expand into wetter and cooler climates. However, analyses controlling for differences in available habitat in each region do not indicate expansion of climate niches. We therefore cannot reject the hypothesis that observed differences in climate envelopes reflect only the limited environments available within the species' native range in East Africa. Our results suggest that models built from only native range occurrence data will not provide an accurate prediction of the potential for invasiveness if applied to areas containing a greater range of environmental combinations, and that tests of niche expansion may overestimate shifts in climate niches if they do not control carefully for environmental differences between distributional areas.

Floral visitation and reproductive traits of Stamenoid petals, a naturally occurring floral homeotic variant of Capsella bursa-pastoris (Brassicaceae).

Homeotic changes played a considerable role during the evolution of flowers, but how floral homeotic mutants initially survive in nature has remained enigmatic. To better understand the evolutionary potential of floral homeotic mutants, we established as a model system Stamenoid petals (Spe), a natural variant of Capsella bursa-pastoris (Brassicaceae). In the flowers of Spe plants, petals are transformed into stamens, whereas all other floral organs are unaffected. In contrast with most other homeotic mutants, the Spe variant occurs in relatively stable populations in the wild. In order to determine how the profound change in floral architecture influences plant performance in the wild, we performed common garden experiments running over 3 years. Here, we show that Spe and wild-type plants attract the same assemblage of floral visitors: mainly hoverflies, wild bees and thrips. However, floral visitation is about twice as frequent in wild-type plants as in Spe plants. Nevertheless, the numbers of seeds per fruit were about the same in both variants. Wild-type plants produced more flowers, fruits and seeds per plant than Spe plants, whereas the germination capacity of Spe seeds was higher than that of the wild-type. Determination of volatile composition revealed monoterpenes and 3,4-dimethylbenzaldehyde, which were detected only in wild-type flowers, presumably because they are produced only by petals. Our data indicate that the similar fitness of Spe and wild-type C. bursa-pastoris in the field results from complex compensation between plant architecture and germination capacity. In contrast, flower structure and floral visitation are only of minor importance, possibly because C. bursa-pastoris is mainly self-pollinating.

Taxon recruitment of the arctic flora: an analysis of phylogenies.

The Arctic is the endpoint of many climatic gradients and is presently occupied by c. 2200 vascular plant species. Glaciation started in the Middle Eocene but a significant expansion of the Greenland ice shield occurred only c. 3.2 million yr ago, leading to the expansion of the treeless circumpolar arctic tundra. Available molecular phylogenetic studies were evaluated for 148 of 374 genera occurring in the Arctic to determine the relative roles of their independent origins and their diversification in the development of the contemporary arctic flora. The number of arctic species paralleled the total number of species in a genus. Multiple arctic species within a genus originated mostly independently of each other and from different lineages. Minor radiations occurred in only a few genera and major radiations were absent. Mostly parallel evolution of arctic taxa from nonarctic ancestors, supposedly of different ages of origin, scarcity of radiations and rarity of endemics are main features of the arctic flora.

Effects of preconditioning and temperature during germination of 73 natural accessions of Arabidopsis thaliana.

BACKGROUND AND AIMS: Germination and establishment of seeds are complex traits affected by a wide range of internal and external influences. The effects of parental temperature preconditioning and temperature during germination on germination and establishment of Arabidopsis thaliana were examined. METHODS: Seeds from parental plants grown at 14 and at 22 degrees C were screened for germination (protrusion of radicle) and establishment (greening of cotyledons) at three different temperatures (10, 18 and 26 degrees C). Seventy-three accessions from across the entire distribution range of A. thaliana were included. KEY RESULTS: Multifactorial analyses of variances revealed significant differences in the effects of genotypes, preconditioning, temperature treatment, and their interactions on duration of germination and establishment. Reaction norms showed an enormous range of plasticity among the preconditioning and different germination temperatures. Correlations of percentage total germination and establishment after 38 d with the geographical origin of accessions were only significant for 14 degrees C preconditioning but not for 22 degrees C preconditioning. Correlations with temperature and precipitation on the origin of the accessions were mainly found at the lower germination temperatures (10 and 18 degrees C) and were absent at higher germination temperatures (26 degrees C). CONCLUSIONS: Overall, the data show huge variation of germination and establishment among natural accessions of A. thaliana and might serve as a valuable source for further germination and plasticity studies.

Evidence for a large-scale population structure of Arabidopsis thaliana from genome-wide single nucleotide polymorphism markers.

Population-based methods for the genetic mapping of adaptive traits and the analysis of natural selection require that the population structure and demographic history of a species are taken into account. We characterized geographic patterns of genetic variation in the model plant Arabidopsis thaliana by genotyping 115 genome-wide single nucleotide polymorphism (SNP) markers in 351 accessions from the whole species range using a matrix-assisted laser desorption/ionization time-of-flight assay, and by sequencing of nine unlinked short genomic regions in a subset of 64 accessions. The observed frequency distribution of SNPs is not consistent with a constant-size neutral model of sequence polymorphism due to an excess of rare polymorphisms. There is evidence for a significant population structure as indicated by differences in genetic diversity between geographic regions. Accessions from Central Asia have a low level of polymorphism and an increased level of genome-wide linkage disequilibrium (LD) relative to accessions from the Iberian Peninsula and Central Europe. Cluster analysis with the structure program grouped Eurasian accessions into K = 6 clusters. Accessions from the Iberian Peninsula and from Central Asia constitute distinct populations, whereas Central and Eastern European accessions represent admixed populations in which genomes were reshuffled by historical recombination events. These patterns likely result from a rapid postglacial recolonization of Eurasia from glacial refugial populations. Our analyses suggest that mapping populations for association or LD mapping should be chosen from regional rather than a species-wide sample or identified genetically as sets of individuals with similar average genetic distances.

Evolution of the realized climatic niche in the genus Arabidopsis (Brassicaceae).

The evolution of the realized climatic niche in the genus Arabidopsis was studied using an almost complete phylogenetic tree based on DNA sequences of the ribosomal internal transcribed spacers. The realized climatic niche (climate space) was determined by the intersections of the distribution ranges of the taxa with climate data and is presented in temperature/precipitation diagrams. A positive correlation exists between the climate spaces of the taxa and their range sizes. The diagrams revealed a core climate; that is, a climate space in which all taxa co-exist. This core climate is almost identical to the most parsimonious reconstruction of the genus' ancestral climate space and may be considered an ancestral state of these characters. Mapping the evolutionary changes occurring in the realized climatic space on the phylogenetic tree from the core climate proved to be the most parsimonious procedure. The character complex is homoplastic; that is, many parallel evolutionary events have occurred in the subclades. With the exception of A. thaliana, which is sister to the other species of the genus and occupies a very large climate space, the late-diverged taxa of the other subclades experienced great evolutionary changes whereas the realized climate space of the taxa that diverged earlier resembles the core climate. The latter also show some parallel contractions in the climate space. It is hypothesized that the diversification of Arabidopsis may have started from small to midsized ranges in a temperate climate.

Analysis of molecular data of Arabidopsis thaliana (L.) Heynh. (Brassicaceae) with Geographical Information Systems (GIS).

A Geographical Information System (GIS) is used to analyse allelic information of 13 sequenced loci of natural populations of Arabidopsis thaliana and to identify geographical structures. GIS provides tools for visualization and analysis of geographical population structures using molecular data. The geographical distribution of the number of variable positions in the alignments, the distribution of recombinant sequence blocks, and the distribution of a newly defined measure, the differentiation index, are studied. The differentiation index is introduced to measure the sequence divergence among individual plants sampled from various geographical localities. The numbers of variable positions and the differentiation index are also used for a metadata analysis covering about 26 kb of the genome. This analysis reveals, for the first time, differences in DNA sequence structures of geographically different populations of A. thaliana. The broadly defined west Mediterranean region consists of accessions with the highest numbers of polymorphic positions followed by the west European region. The GIS technology Kriging is used to define Arabidopsis specific diversity zones in Europe. The highest genetic variability is observed along the Atlantic coast from the western Iberian Peninsula to southern Great Britain, while lowest variability is found in central Europe.