Arctic Alpine Species

Arctic-alpine distribution ranges comprise the arctic as well as the alpine belt of more Southern mountain ranges. Typically, arctic-alpine distributions are large and often encompass the entire Northern Hemisphere (circumpolar distribution); more restricted ranges are, e.g., amphi-Atlantic or amphi-Pacific (Abbott and Brochmann 2003). The last few years have seen increasing interest in phylogeographic studies of arctic-alpine higher plants, which are by now the doubtlessly best studied group. In the following, we focus on the relationships between the disjunct distribution areas of arctic-alpine taxa in the EMS and their main distribution area in the Arctic.

A close relationship between the central and Northern European populations of a species appears intuitively self-evident and has often been suggested by classical biogeographers (e.g., Vierhapper 1918; Holdhaus 1954; de Lattin 1967; Ozenda 1988). Good examples for such a connection are Ranunculus glacialis (Schönswetter et al. 2003), Arabis alpina (Ehrich et al. 2007), and Veronica alpina (Albach et al. 2006), which all colonized the North Atlantic region from source populations in the Alps. Dryas octopetala likely colonized Northern Europe from source populations located between the Scandinavian ice shield and the Alps (Skrede et al. 2006). In contrast, the Scandinavian populations formed monophyletic groups in Minuartia biflora (Schönswetter et al. 2006b) and Carex atrofusca (Schönswetter et al. 2006a), indicating that Northern European and Alpine populations have been separated for a certain period of time. In Carex bigelowii (Schönswetter et al. 2008), the single Alpine (meta-)population has strong links to the phylogeographical group dominant in the Hercynic mountains, the Tatras, Scandinavia, Iceland, and Scotland. The Hercynic populations were genetically variable, while only a few clones were detected in the Alps, indicating a strong founder effect. Whereas the lack of phylo-geographic structure within the four detected phylogeographic groups of Juncus biglumis did not allow tracing the closest relatives for Alpine and Scandinavian populations (Schönswetter et al. 2007), Ranunculus pygmaeus populations from the two latter areas have their closest relatives on the Taymyr Peninsula in Northern Siberia and in the Urals, respectively, (Schönswetter et al. 2006b). In Comastoma tenellum (Schönswetter et al. 2004b), two unrelated lineages were found in the Alps. The by far most common one was endemic to the Alps, while the second, encountered only in two Eastern Alpine populations, exhibited unresolved relationships to Carpathian and Scandinavian populations. Thus, this species has apparently colonized the Alps at least twice.

Similar arctic-alpine distribution patterns like in plants are also observed in many animal species. Holdhaus (1954) listed several dozens of invertebrate species with this distribution pattern in Europe in his seminal monograph on the legacy of the Pleistocene glaciations preserved in contemporary animal distributions. However, while most molecular studies are from plants, mitochondrial sequence variation has been studied in not more than three arthropod taxa of this disjunction type until now.

An illustrative example is provided by wolf spiders of the Pardosa saltuaria group (Muster and Berendonk 2006). This is a complex of six closely related species (Wunderlich 1984; Marusik et al. 1996) whose taxonomic status needs revision in the light of the recent genetic findings. The species group is patchily distributed from the Western Palearctic over Siberia to the Altai (Fig. 3a). Focussing on the European range, Muster and Berendonk (2006) found three clades of deep mito-chondrial divergence, i.e., a Pyrenean clade, a Balkan clade, and a "Northern clade." The latter included Scandinavia, the Alps, the Carpathians, the Giant Mountains, and the Bohemian Forest. Recent results from a nuclear rDNA marker (spanning ITS1, 5.8S rDNA, ITS2) generally confirmed this pattern of genetic divergence (Muster et al. unpublished). A detailed population genetic analysis using the information of both loci (mitochondrial and nuclear) in a coalescent-based Bayesian Markov chain Monte Carlo (MCMC) framework (Hey and Nielsen 2007) revealed a pattern of extensive but differential late glacial gene flow among Northern clade areas (Muster et al. 2009). The inferred gene flow was mainly unidirectional with a prevailing direction from the North to the South.

Fig. 3 European distribution areas and sampling localities of three arctic-alpine invertebrates: the wolf spiders of the Pardosa saltuaria group (a) redrawn from Muster and Berendonk (2006), the ground beetle Nebria rufescens (b) modified from Holdhaus and Lindroth (1939), and the butterfly Erebia pandrose (c) modified from Varga (1971). The white dots indicate the sampling locations. Abbreviations: A Alps; B Balkans; G Giant Mountains; P Pyrenees; S Scandinavia. In Fig. 3d the geographic extension of major mitochondrial clades is shown, together with the Kimura 2 parameter distance (in %) between clades and within the Northern clade. Solid line Pardosa saltuaria group, chain line N. rufescens, dotted line Erebia pandrose

Fig. 3 European distribution areas and sampling localities of three arctic-alpine invertebrates: the wolf spiders of the Pardosa saltuaria group (a) redrawn from Muster and Berendonk (2006), the ground beetle Nebria rufescens (b) modified from Holdhaus and Lindroth (1939), and the butterfly Erebia pandrose (c) modified from Varga (1971). The white dots indicate the sampling locations. Abbreviations: A Alps; B Balkans; G Giant Mountains; P Pyrenees; S Scandinavia. In Fig. 3d the geographic extension of major mitochondrial clades is shown, together with the Kimura 2 parameter distance (in %) between clades and within the Northern clade. Solid line Pardosa saltuaria group, chain line N. rufescens, dotted line Erebia pandrose

In order to test the phylogeographic hypothesis derived from the wolf spider data, preliminary results from two further arctic-alpine arthropod taxa are presented here. The ground beetle Nebria rufescens (Fig. 3b) occupies an even larger arctic area than the Pardosa spiders and includes North America (Holdhaus 1954). The area of the butterfly Erebia pandrose, on the other hand, is mainly restricted to Europe (Fig. 3c), with a few populations in the mountains of Central Asia (Cupedo 2007). Within Europe, the distributions of these species are also different (Fig. 3a-c), with N. rufescens resembling more the boreomontane distribution type of de Lattin

(1967), as indicated by occurrences on the British Isles and in some rather low mountain ranges. In marked contrast, E. pandrose shows the typical arctic-alpine disjunction sensu de Lattin (1967), and even in Scandinavia the species' range is limited to the mountains and the far North, while it is restricted to the highest elevations in the Southern European mountains.

In a pilot study, Muster et al. (2009) sequenced the complete mitochondrial ND1 gene from five individuals of each of four (in E. pandrose) and five populations (in N. rufescens) as shown in Fig. 3. To allow the direct comparison of the level of genetic variation, the data set of the wolf spiders was reduced to the first five sequences of each of five similar areas. Both striking similarities and idiosyncratic differences in the resulting genetic structures were found. First, the overall level of sequence divergence was highly different (K2P-distance: Pardosa 2.4%, Nebria 1.3%, Erebia 0.2%) despite the fact that identical genes in co-distributed species were investigated. Analysis of molecular variance (Excoffier et al. 1992, 2005) clearly showed that the populations in the Southern mountains contributed dispro-portionally to the overall genetic differentiation of these taxa.

Despite the different life histories and migration abilities in these three taxa, great similarities in the genetic structures existed (Fig. 4). Most importantly, the haplotypes from the Balkans formed a distinct and highly supported clade in all three study systems. Another common feature was the low divergence among Scandinavia, the Alps, and several rather low Central European mountain ranges (Hercynian Mountains), which always clustered in a "Northern clade" with little geographic structure (except for N. rufescens, where each area has its own unique mutations).

Fig. 4 Neighbour-joining trees showing the phylogenetic relationships of 25 individuals of the Pardosa saltuaria group (a), 25 individuals of Nebria rufescens (b), and 20 individuals of Erebia pandrose (c). All three trees are shown on the same scale, highlighting the different degree of overall sequence divergence. Bootstrap values > 50% from 1,000 replicates are shown at the nodes. Letter abbreviations are the same as in Fig. 3

Fig. 4 Neighbour-joining trees showing the phylogenetic relationships of 25 individuals of the Pardosa saltuaria group (a), 25 individuals of Nebria rufescens (b), and 20 individuals of Erebia pandrose (c). All three trees are shown on the same scale, highlighting the different degree of overall sequence divergence. Bootstrap values > 50% from 1,000 replicates are shown at the nodes. Letter abbreviations are the same as in Fig. 3

The position of the Pyrenees differed among the investigated taxa. While the Pyrenean haplotypes formed a clade as distinct from the Northern populations as the Balkan clade in Pardosa and Erebia, the Pyrenees clearly fell within the Northern clade in Nebria.

It is most likely that the populations from Scandinavia, the Alps, and the Hercynian Mountains are descendants of large populations that have dwelled in the periglacial tundra belt during the last glaciations, or at least that substantial levels of late glacial gene flow among mountain ranges occurred. Similarly, allozyme data from the diving beetle Hydroporus glabriusculus suggest a common late glacial origin of the British and the Scandinavian populations from source populations South of the Northern European ice sheet (Bilton 1994). In the Nebria beetles and in the arctic-alpine burnet moth Zygaena exulans (Schmitt and Hewitt 2004), such late glacial connections obviously existed also between the Alps and the Pyrenees. Thus, the traditional concept of enormous continuous distributions in the periglacial tundra belt during the glaciations (Holdhaus 1954; de Lattin 1967) could be partly corroborated. On the other hand, the Balkans and in some taxa also the Pyrenees have been isolated, and populations there consequently evolved independently for fairly long periods of time, probably since several glacial cycles. They thus represent old relic populations of high conservation importance. Altogether, the populations from disjunct parts of the distribution areas of arctic-alpine disjunct arthropods can be regarded as relics, which originated at very different time horizons.

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