Interspecific and interploidal gene flow in Central European Arabidopsis (Brassicaceae).

BACKGROUND: Effects 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. RESULTS: Ploidal 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. CONCLUSIONS: Interspecific 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.

The evolutionary history of the Arabidopsis lyrata complex: a hybrid in the amphi-Beringian area closes a large distribution gap and builds up a genetic barrier.

BACKGROUND: The 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. RESULTS: Using 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. CONCLUSIONS: Unglaciated 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.

Microsatellite markers show decreasing diversity but unchanged level of clonality in Dryas octopetala (Rosaceae) with increasing latitude.

PREMISE OF THE STUDY: Average arctic temperatures have increased at almost twice the global average in the past 100 years. Most studies on biodiversity along latitudinal gradients have focused on species richness or genetic diversity at lower latitudes, and only a few studies have inferred genetic diversity within a species along a latitudinal gradient at higher latitudes, even though these areas might be most affected by recent climate changes. Here, intraspecific genetic diversity of the arctic-alpine Dryas octopetala (Rosaceae) is studied along a latitudinal gradient to test the hypotheses that genetic diversity decreases and vegetative clonal growth increases with latitude. • METHODS: Ten microsatellite markers have been developed for D. octopetala and analyzed with population genetic methods in five populations along a latitudinal transect spanning from 59.0°N to 79.9°N. • KEY RESULTS: The nine microsatellites that were used in the final analyses resulted in a resolution high enough to distinguish between ramets while providing useful information at a larger geographical scale. Three genetic clusters were indicated, a southern Norway group, a northern Norway group, and a Svalbard group, with corresponding decreasing genetic diversity. No trend was found with regard to clonality along the gradient. • CONCLUSIONS: 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.