BackgroundThe genomes of higher plants are, on the majority, polyploid, and hybridisation is more frequent in plants than in animals. Both polyploidisation and hybridisation contribute to increased variability within species, and may transfer adaptations between species in a changing environment. Studying these aspects of evolution within a diversified species complex could help to clarify overall spatial and temporal patterns of plant speciation. The Arabidopsis lyrata complex, which is closely related to the model plant Arabidopsis thaliana, is a perennial, outcrossing, herbaceous species complex with a circumpolar distribution in the Northern Hemisphere as well as a disjunct Central European distribution in relictual habitats. This species complex comprises three species and four subspecies, mainly diploids but also several tetraploids, including one natural hybrid. The complex is ecologically, but not fully geographically, separated from members of the closely related species complex of Arabidopsis halleri, and the evolutionary histories of both species compexes have largely been influenced by Pleistocene climate oscillations.ResultsUsing DNA sequence data from the nuclear encoded cytosolic phosphoglucoisomerase and Internal Transcribed Spacers 1 and 2 of the ribosomal DNA, as well as the trnL/F region from the chloroplast genome, we unravelled the phylogeography of the various taxonomic units of the A. lyrata complex. We demonstrate the existence of two major gene pools in Central Europe and Northern America. These two major gene pools are constructed from different taxonomic units. We also confirmed that A. kamchatica is the allotetraploid hybrid between A. lyrata and A. halleri, occupying the amphi-Beringian area in Eastern Asia and Northern America. This species closes the large distribution gap of the various other A. lyrata segregates. Furthermore, we revealed a threefold independent allopolyploid origin of this hybrid species in Japan, China, and Kamchatka.ConclusionsUnglaciated parts of the Eastern Austrian Alps and arctic Eurasia, including Beringia, served as major glacial refugia of the Eurasian A. lyrata lineage, whereas A. halleri and its various subspecies probably survived in refuges in Central Europe and Eastern Asia with a large distribution gap in between. The North American A. lyrata lineage probably survived the glaciation in the southeast of North America. The dramatic climatic changes during glaciation and deglaciation cycles promoted not only secondary contact and formation of the allopolyploid hybrid A. kamchatica, but also provided the environment that allowed this species to fill a large geographic gap separating the two genetically different A. lyrata lineages from Eurasia and North America. With our example focusing on the evolutionary history of the A. lyrata species complex, we add substantial information to a broad evolutionary framework for future investigations within this emerging model system in molecular and evolutionary biology.

BackgroundEffects of polyploidisation on gene flow between natural populations are little known. Central European diploid and tetraploid populations of Arabidopsis arenosa and A. lyrata are here used to study interspecific and interploidal gene flow, using a combination of nuclear and plastid markers.ResultsPloidal levels were confirmed by flow cytometry. Network analyses clearly separated diploids according to species. Tetraploids and diploids were highly intermingled within species, and some tetraploids intermingled with the other species, as well. Isolation with migration analyses suggested interspecific introgression from tetraploid A. arenosa to tetraploid A. lyrata and vice versa, and some interploidal gene flow, which was unidirectional from diploid to tetraploid in A. arenosa and bidirectional in A. lyrata.ConclusionsInterspecific genetic isolation at diploid level combined with introgression at tetraploid level indicates that polyploidy may buffer against negative consequences of interspecific hybridisation. The role of introgression in polyploid systems may, however, differ between plant species, and even within the small genus Arabidopsis, we find very different evolutionary fates when it comes to introgression.

Aim The oceanic Saxifraga rivularis L. presents one of the most extreme disjunctions known in the arctic flora: it has a small amphi-Beringian range and a larger amphi-Atlantic one. It was recently suggested to have had a single allopolyploid origin in Beringia at least one glacial cycle ago, followed by gradual expansion in a more humid period and differentiation into two allopatric subspecies (the Atlantic ssp. rivularis and the Beringian ssp. arctolitoralis). Here we explore the history of its extreme disjunction.Location The amphi-Beringian and northern amphi-Atlantic regions.Methods We obtained amplified fragment length polymorphisms (AFLPs) and chloroplast DNA sequences from 36 populations (287 individuals) and 13 populations (15 individuals), respectively. The data were analysed using principal coordinates analyses, Bayesian clustering methods, and analyses of molecular variance.Results Two distinctly divergent AFLP groups were observed, corresponding to the two described subspecies, but, surprisingly, four of the West Atlantic populations belonged to the supposedly Beringian endemic ssp. arctolitoralis. This was confirmed by re-examination of their morphological characteristics. The overall AFLP diversity in the species was low (26.4% polymorphic markers), and there was no variation in the five investigated chloroplast DNA (cpDNA) regions. There was little geographic structuring of the AFLP diversity within each subspecies, even across the extreme disjunction in ssp. arctolitoralis, across the Bering Sea, and across the Atlantic Ocean, except that most plants from the arctic Svalbard archipelago formed a separate genetic group with relatively high diversity.Main conclusions The extreme disjunction in S. rivularis has evidently formed at least twice. The first expansion from Beringia was followed by allopatric differentiation into one Beringian and one Atlantic subspecies, which are distinctly divergent at AFLP loci but still harbour identical cpDNA haplotypes, suggesting that the expansion was quite recent but before the last glaciation. The next expansion from Beringia probably occurred by means of several long-distance dispersals in the current interglacial, resulting in the colonization of the western Atlantic region by ssp. arctolitoralis. The poor geographic structuring within each subspecies suggests frequent long-distance dispersals from two main Weichselian refugia, one Beringian and one western-central European, but it is possible that the genetic group in Svalbard originates from an additional refugium.

Examples of recurrent homoploid hybrid speciation are few. One often-cited example is Argyranthemum sundingii. This example includes two described species, A. lemsii and A. sundingii, resulting from reciprocal hybridization between A. broussonetii and A. frutescens on Tenerife. The four species and artificial F1 and F2 hybrids have previously been investigated morphologically and cytologically. Here, we examine population differentiation based on amplified fragment length polymorphism to get a better understanding of the genetic relationships among the species and the extent of hybridization. We aim to investigate if there is molecular support for treating the hybrid species as one taxon. Seven parental and four hybrid species populations (149 individuals) were analysed and we scored 85 polymorphic markers. A few (2-5) were private to each species but variably present and mostly rare. Our principal coordinate, STRUCTURE and BAPS analyses and AMOVA resulted in a clear separation of the parental species. The hybrid species were genetically less divergent but not identical. Our data indicate that hybridization and introgression are common in all these species on Tenerife and support the hypothesis that homoploid hybrid speciation has occurred repeatedly. Intrinsic post-zygotic barriers are notoriously weak in Argyranthemum and reproductive isolation and speciation result primarily from strong ecological selection.

When my project is about to finish, it's time for acknowledgements: x Annen. ANNEANNEANNE! Fineste sjefstøsen! My main supervisor Anne K. Brysting. Best supervisor in the world, I'm sure. You are my colleague, my friend, my third mum (I have a modern family), and my hero. As well as scientifically strong, you show a care for and a faith in your students that is really remarkable. Your door is always open, even literally, for all kinds of problems, personal as well as scientifical. If I didn't have you, I would have quitted a long time ago. But you were there, and you kept your faith in me when I didn't. Thanks for all your chatter, your support, and your hugs! I wonder if you'll ever be able to say Vålenga kjaerke! x Barbs. My wonderful co-supervisor Barbara Mable at the Glasgow Uni. Your hospitality when welcoming me to Glasgow meant a lot. And your patience with my ignorance on S-alleles and inheritage and crossings. From now on, my focus will be on our joint project! x Hilde, agent Empetrum, thanks for always being there. Thanks for sharing my tears and my laughter. Thanks to you, Julie, Ragnhild, Flua, Helge, and Einar for quiz nights every Tuesday. And thanks to you, Vir, and Inger for wine colloquia and a nice master period. Snilleskrede: thanks for reading! x Torbaeibi, my wise and nice colleague Tor Carlsen, thanks for all the nice scientific discussions and all the coffea! x Thanks to Svend-Ole Nielsen (my Danish friend), Elisabeth Hubbuck, and Cecilia Leslie for professional help with a messy brain. x Thanks to my family, particularly my father for being supportive, taking me on boat trips, trips to our cottage, and for using me as a source of knowledge in molecular Literature cited

Background: Phylogeography describes the distribution of genes in space and time. Here we analyse the Pleistocene evolutionary history of the Arabidopsis lyrata species complex in arctic-boreal northern America and eastern Russia. Aims: We seek evidence for glacial survival on both sides of the Atlantic, with special emphasis on northern America, and ask how genetic variation within North American populations relates to the putative centre of genetic variation in central Europe. Finally, we comment on how genetic variation observed in North America corresponds to taxonomic units described for the area. Methods: We sequenced DNA markers from the nuclear genome (ribosomal internal transcribed spacers 1 and 2) and chloroplast genome (trnL intron and intergenic spacer trnL-F) from 68 accessions outside Europe. Phylogenetic analysis (tree reconstruction and network building algorithms) were used to infer relationships among DNA haplotypes. Results: We detected reduced genetic variation in northern America compared to Europe, although new DNA types were found. Strong genetic differentiation was found among major regions in northern America. Correlation of genetic data with taxonomy was weak, but for some taxa geographic distribution correlates with distribution of its genetic variation. Conclusions: The A. lyrata complex in North America is separated into two allopatric groups: one is distributed in the Pacific areas from southern Alaska to British Colombia, and the other is found in northern Alaska, central and east Canada, and Greenland. The results suggest glacial survival in and subsequent migration from two different refugia with allele fixation as a result of genetic drift.

According to the Norwegian Diversity Act, practitioners of restoration in Norway are instructed to use seed mixtures of local provenance. However, there are no guidelines for how local seed should be selected. In this study, we use genetic variation in a set of alpine species (Agrostis mertensii, Avenella flexuosa, Carex bigelowii, Festuca ovina, Poa alpina and Scorzoneroides autumnalis) to define seed transfer zones to reduce confusion about the definition of ‘local seeds’. The species selected for the study are common in all parts of Norway and suitable for commercial seed production. The sampling covered the entire alpine region (7–20 populations per species, 3–15 individuals per population). We characterised genetic diversity using amplified fragment length polymorphisms. We identified different spatial genetic diversity structures in the species, most likely related to differences in reproductive strategies, phylogeographic factors and geographic distribution. Based on results from all species, we suggest four general seed transfer zones for alpine Norway. This is likely more conservative than needed for all species, given that no species show more than two genetic groups. Even so, the approach is practical as four seed mixtures will serve the need for restoration of vegetation in alpine regions in Norway.

The newly developed microsatellite markers provide a useful tool for further genetic studies of D. octopetala and its close relatives, addressing population structure as well as phylogeographic patterns. The results of this study support the hypothesis of decreasing genetic diversity with increasing latitude, which may have implications for future adaptability to climate change.