Genetic enrichment of the arctic clonal plant Saxifraga cernua at its southern periphery via the alpine sexual Saxifraga sibirica

The Role of Hybridisation in the Making of the Species-Rich Arctic-Alpine Genus Saxifraga (Saxifragaceae)

Evolutionary processes fuelling rapid species diversification are not yet fully understood, although their major contribution to overall patterns of plant biodiversity is well established. Hybridisation is among the least understood of these processes, despite its multifaceted role in speciation processes being widely accepted. Species of the large arctic-alpine genus Saxifraga are notorious for their ability to hybridise; however, the overall role of hybridisation and polyploidisation for the diversification of this genus remains unknown. Here, we provide a comprehensive genus-wide review of hybridisation accounts and ploidy levels. We find that the sections of Saxifraga vary greatly in their propensity to hybridise. The majority of natural hybridisation accounts are from recent localised events (n = 71). Hybridisation hotspots were located in the Pyrenees and the European Alps, thus contrasting with the overall distribution of species richness in the genus. Hybrids or hybrid populations are often short-lived in Saxifraga due to a multitude of reproductive barriers, most commonly low F1 hybrid fertility. However, these barriers are not always fully effective, allowing for backcrossing and the formation of hybrid swarms. In addition, we find that the incidence of polyploidy varies widely across different sections of Saxifraga, with species-rich sections Porphyrion and Saxifraga showing divergent polyploidy proportions. Overall, we show that hybridisation and polyploidisation played differential roles in the diversification of this large genus. Nevertheless, a significant proportion of species are yet to be scrutinised, particularly among the Asian Saxifraga species, illustrating the need for systematic further study to fully unravel the role of hybridisation during the evolution of Saxifraga.

Ecotypic differentiation, hybridization and clonality facilitate the persistence of a cold-adapted sedge in European bogs

Recent research has shown that many cold-adapted species survived the last glacial maximum (LGM) in northern refugia. Whether this evolutionary history has had consequences for their genetic diversity and adaptive potential remains unknown. We sampled 14 populations of Carex limosa, a sedge specialized to bog ecosystems, along a latitudinal gradient from its Scandinavian core to the southern lowland range-margin in Germany. Using microsatellite and experimental common-garden data, we evaluated the impacts of global climate change along this gradient and assessed the conservation status of the southern marginal populations. Microsatellite data revealed two highly distinct genetic groups and hybrid individuals. In our common-garden experiment, the two groups showed divergent responses to increased nitrogen/phosphorus (N/P) availability, suggesting ecotypic differentiation. Each group formed genetically uniform populations at both northern and southern sampling areas. Mixed populations occurred throughout our sampling area, an area that was entirely glaciated during the LGM. The fragmented distribution implies allopatric divergence at geographically separated refugia that putatively differed in N/P availability. Molecular data and an observed low hybrid fecundity indicate the importance of clonal reproduction for hybrid populations. At the southern range-margin, however, all populations showed effects of clonality, lowered fecundity and low competitiveness, suggesting abiotic and biotic constraints to population persistence.

Ex situ conservation storage potential of Saxifraga cernua (Saxifragaceae) bulbils from alpine species

In high latitude and alpine environments many plants show an increase in viviparous reproduction in response to harsh environmental conditions. Low or no seed set means that ex situ conservation in the form of seed banking is not a conservation option for such species. We investigated the potential for bulbils to be stored ex situ in seed banks using traditional storage methods (drying and freezing at −20 °C) and cryopreservation (drying and freezing at −180 °C) as a means of ensuring the long-term conservation of such species. In addition, the impact of drying bulbils to 15% eRH or maintaining initial humidity (60% eRH) was investigated. The study was based on bulbils of the drooping or nodding saxifrage (Saxifraga cernua) collected in Bellalui (commune d’Icogne, Switzerland) at an altitude of 2200 m.a.s.l. Our findings suggest that conservation under traditional seed banking methods or by cryopreservation is a viable option for species producing small (<2 mm) bulbils. This provides new hope for conserving high altitude or latitude plants producing bulbils.

Population genetics of purple saxifrage (Saxifraga oppositifolia) in the high Arctic archipelago of Svalbard

We investigated patterns of genetic variability in Saxifraga oppositifolia in the isolated Arctic Svalbard archipelago. The genetic analysis included genotyping using nine polymorphic microsatellite markers and sequencing of the nuclear internal transcribed spacer region. Among populations, mean allele numbers per microsatellite locus ranged from 2.0 to 2.6, and 9 % of alleles were unique. Observed (H O) and expected (H E) heterozygosities averaged 0.522 and 0.445, respectively. Typically negative but non-significant F IS values (mean -0.173) were found in S. oppositifolia populations. F ST values were relatively low (mean 0.123). The Bayesian structure analysis provided additional information on population genetic structures. Seven out of 11 studied populations, including populations located both near each other and far apart (distances 5-210 km), showed relatively homogeneous clustering patterns, while one population located on a slope in the main settlement of Longyearbyen possessed a unique genetic structure. The Mantel test proved that there is no significant correlation between genetic and geographical distances. Different growth habits (compact, trailing and intermediate) did not possess distinct genetic compositions based on microsatellite variation. Internal transcribed spacer sequencing revealed 12 polymorphic sites. Among 24 sequenced Svalbard samples, eight haplotypes were detected, none shared by the mainland samples. Population genetic structures of S. oppositifolia in Svalbard show that both genetic variation and differentiation levels are modest, outcrossing is the main mating system, and dispersal and gene flow are important, probably attributable to strong winds and human and animal vectors.

Polyploidy in Indian paintbrush (Castilleja; Orobanchaceae) species shapes but does not prevent gene flow across species boundaries

PREMISE OF STUDY

A difference in chromosome numbers (ploidy variation) between species is usually considered a major barrier to gene flow. Therefore, it is surprising that little is known about whether ploidy variation, both within and among species, influences spatial patterns of interspecific hybridization. The role that polyploidy plays in structuring gene flow patterns between three co-occurring Indian paintbrush (Castilleja) species is investigated. •

METHODS

Reciprocal hand pollinations were performed in populations where the three species co-occur with and without variable plants (previous data tested the ancestral "hybrid" history of these variable plants). I measured fruit set, seed production, seed germination, and the DNA content of parent plants and 26 synthesized F(1) hybrids. Data were combined with pollinator fidelity data to estimate the contribution of individual barriers to reproductive isolation. •

KEY RESULTS

Interspecific gene flow could occur in all directions, but barriers were weaker for conspecific vs. heterospecific crosses. Species were nearly fixed for different ploidy levels, but some deviations occurred, primarily in populations with variable plants. Interspecific gene flow could occur across ploidy levels, but it was more likely when species had the same number of chromosomes or when resulting F(1) hybrids had even numbers of chromosomes. Postzygotic reproductive barriers were generally weaker than pollinator fidelity. •

CONCLUSIONS

Polyploidy likely plays a large role in shaping contemporary and historical patterns of gene flow among these species. This study suggests that differences in chromosome numbers among closely related, compatible species might help structure spatial patterns of hybridization.

The phylogeographical history of the Iberian steppe plant Ferula loscosii (Apiaceae): a test of the abundant-centre hypothesis

The geology and climate of the western Mediterranean area were strongly modified during the Late Tertiary and the Quaternary. These geological and climatic events are thought to have induced changes in the population histories of plants in the Iberian Peninsula. However, fine-scale genetic spatial architecture across western Mediterranean steppe plant refugia has rarely been investigated. A population genetic analysis of amplified fragment length polymorphism variation was conducted on present-day, relict populations of Ferula loscosii (Apiaceae). This species exhibits high individual/population numbers in the middle Ebro river valley and, according to the hypothesis of an abundant-centre distribution, these northern populations might represent a long-standing/ancestral distribution centre. However, our results suggest that the decimated southern and central Iberian populations are more variable and structured than the northeastern ones, representing the likely vestiges of an ancestral distribution centre of the species. Phylogeographical analysis suggests that F. loscosii likely originated in southern Spain and then migrated towards the central and northeastern ranges, further supporting a Late Miocene southern-bound Mediterranean migratory way for its oriental steppe ancestors. In addition, different glacial-induced conditions affected the southern and northern steppe Iberian refugia during the Quaternary. The contrasting genetic homogeneity of the Ebro valley range populations compared to the southern Iberian ones possibly reflects more severe bottlenecks and subsequent genetic drift experienced by populations of the northern Iberia refugium during the Pleistocene, followed by successful postglacial expansion from only a few founder plants.

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.

Unexpectedly high genetic variation in large unisexual clumps of the subdioecious plant Honckenya peploides (Caryophyllaceae)

Honckenya peploides is a subdioecious dune plant that reproduces both sexually and by clonal growth. In northwest Spain this species was found to exhibit an extreme spatial segregation of the sexes, and our objective was to investigate genetic variation in unisexual clumps. Genetic variation was studied in six unisexual clumps of H. peploides, three of them exclusively composed of males and three exclusively female. In total, 193 samples were analysed using isozyme analysis and 80 samples were analysed using two AFLP primer combinations. Both techniques revealed considerably high genetic diversity (average proportion of distinguishable genotypes: 0.22 for isozymes and 0.36 for AFLP; average Simpson's D: 0.65 for isozymes and 0.68 for AFLP). Our results show that, in spite of clonal growth, each unisexual clump consists of different genotypes. Genetic diversity within clumps is similar for both sexual morphs. Reasons for unisexuality of the clumps are discussed.

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.