Parallel adaptation to lower altitudes is associated with enhanced plasticity in Heliosperma pusillum (Caryophyllaceae).

High levels of phenotypic plasticity are thought to be inherently costly in stable or extreme environments, but enhanced plasticity may evolve as a response to new environments and foster novel phenotypes. Heliosperma pusillum forms glabrous alpine and pubescent montane ecotypes that diverged recurrently and polytopically (parallel evolution) and can serve as evolutionary replicates. The specific alpine and montane localities are characterized by distinct temperature conditions, available moisture, and light. Noteworthy, the ecotypes show a home-site fitness advantage in reciprocal transplantations. To disentangle the relative contribution of constitutive versus plastic gene expression to altitudinal divergence, we analyze the transcriptomic profiles of two parallely evolved ecotype pairs, grown in reciprocal transplantations at native altitudinal sites. In this incipient stage of divergence, only a minor proportion of genes appear constitutively differentially expressed between the ecotypes in both pairs, regardless of the growing environment. Both derived, montane populations bear comparatively higher plasticity of gene expression than the alpine populations. Genes that change expression plastically or constitutively underlie similar ecologically relevant pathways, related to response to drought and trichome formation. Other relevant processes, such as photosynthesis, rely mainly on plastic changes. The enhanced plasticity consistently observed in the montane ecotype likely evolved as a response to the newly colonized, drier, and warmer niche. We report a striking parallelism of directional changes in gene expression plasticity. Thus, plasticity appears to be a key mechanism shaping the initial stages of phenotypic evolution, likely fostering adaptation to novel environments.

Multiple auto- and allopolyploidisations marked the Pleistocene history of the widespread Eurasian steppe plant Astragalus onobrychis (Fabaceae).

The Eurasian steppes occupy a significant portion of the worldwide land surface and their biota have been affected by specific past range dynamics driven by ice ages-related climatic fluctuations. The dynamic alterations in conditions during the Pleistocene often triggered reticulate evolution and whole genome duplication events. Employing genomic, genetic and cytogenetic tools as well as morphometry we investigate the intricate evolution of Astragalus onobrychis, a widespread Eurasian steppe plant with diploid, tetraploid and octoploid cytotypes. To analyse the heteroploid RADseq dataset we employ both genotype-based and genotype-free methods that result in highly consistent results, and complement our inference with information from the plastid ycf1 region. We uncover a complex and reticulate evolutionary history, including at least one auto-tetraploidization event and two allo-octoploidization events; one of them involved also genetic contributions from other species, most likely A. goktschaicus. The present genetic structure points to the existence of four main clades within A. onobrychis, which only partly correspond to different ploidies. Time-calibrated diffusion models suggest that diversification within A. onobrychis was associated with ice age-related climatic fluctuations during the last million years. We finally argue for the usefulness of uniparentally inherited plastid markers, even in the genomic era, especially when investigating heteroploid systems.

Static and dynamic bending has minor effects on xylem hydraulics of conifer branches (Picea abies, Pinus sylvestris).

The xylem hydraulic efficiency and safety is usually measured on mechanically unstressed samples, although trees may be exposed to combined hydraulic and mechanical stress in the field. We analysed changes in hydraulic conductivity and vulnerability to drought-induced embolism during static bending of Picea abies and Pinus sylvestris branches as well as the effect of dynamic bending on the vulnerability. We hypothesized this mechanical stress to substantially impair xylem hydraulics. Intense static bending caused an only small decrease in hydraulic conductance (-19.5 ± 2.4% in P. abies) but no shift in vulnerability thresholds. Dynamic bending caused a 0.4 and 0.8 MPa decrease of the water potential at 50 and 88% loss of conductivity in P. sylvestris, but did not affect vulnerability thresholds in P. abies. With respect to applied extreme bending radii, effects on plant hydraulics were surprisingly small and are thus probably of minor eco-physiological importance. More importantly, results indicate that available xylem hydraulic analyses (of conifers) sufficiently reflect plant hydraulics under field conditions.

Parallel Differentiation and Plastic Adjustment of Leaf Anatomy in Alpine Arabidopsis arenosa Ecotypes.

Functional and structural adjustments of plants in response to environmental factors, including those occurring in alpine habitats, can result in transient acclimation, plastic phenotypic adjustments and/or heritable adaptation. To unravel repeatedly selected traits with potential adaptive advantage, we studied parallel (ecotypic) and non-parallel (regional) differentiation in leaf traits in alpine and foothill ecotypes of Arabidopsis arenosa. Leaves of plants from eight alpine and eight foothill populations, representing three independent alpine colonization events in different mountain ranges, were investigated by microscopy techniques after reciprocal transplantation. Most traits clearly differed between the foothill and the alpine ecotype, with plastic adjustments to the local environment. In alpine populations, leaves were thicker, with altered proportions of palisade and spongy parenchyma, and had fewer trichomes, and chloroplasts contained large starch grains with less stacked grana thylakoids compared to foothill populations. Geographical origin had no impact on most traits except for trichome and stomatal density on abaxial leaf surfaces. The strong parallel, heritable ecotypic differentiation in various leaf traits and the absence of regional effects suggests that most of the observed leaf traits are adaptive. These trait shifts may reflect general trends in the adaptation of leaf anatomy associated with the colonization of alpine habitats.

Protective Role of Ice Barriers: How Reproductive Organs of Early Flowering and Mountain Plants Escape Frost Injuries.

In the temperate zone of Europe, plants flowering in early spring or at high elevation risk that their reproductive organs are harmed by episodic frosts. Focusing on flowers of two mountain and three early-flowering colline to montane distributed species, vulnerability to ice formation and ice management strategies using infrared video thermography were investigated. Three species had ice susceptible flowers and structural ice barriers, between the vegetative and reproductive organs, that prevent ice entrance from the frozen stems. Structural ice barriers as found in Anemona nemorosa and Muscari sp. have not yet been described for herbaceous species that of Jasminum nudiflorum corroborates findings for woody species. Flowers of Galanthus nivalis and Scilla forbesii were ice tolerant. For all herbs, it became clear that the soil acts as a thermal insulator for frost susceptible below ground organs and as a thermal barrier against the spread of ice between individual flowers and leaves. Both ice barrier types presumably promote that the reproductive organs can remain supercooled, and can at least for a certain time-period escape from effects of ice formation. Both effects of ice barriers appear significant in the habitat of the tested species, where episodic freezing events potentially curtail the reproductive success.

Leaf anatomy of two reciprocally non-monophyletic mountain plants (Heliosperma spp.): does heritable adaptation to divergent growing sites accompany the onset of speciation?

Evolution is driven by natural selection, favouring individuals adapted in phenotypic traits to the environmental conditions at their growing site. To shed light on ecological and (epi-) genetically based differentiation between Heliosperma pusillum and Heliosperma veselskyi, two reciprocally non-monophyletic, but morphologically and ecologically divergent species from the south-eastern Alps, we studied various leaf anatomical traits and investigated chloroplast ultrastructure in leaves of the two species grown either in their natural habitat or in a common garden. The alpine H. pusillum occurs in open, wet rock habitats, whereas its close relative H. veselskyi is restricted to dry, shady habitats below overhanging rocks in the montane belt. H. pusillum exhibited higher thickness of leaves and palisade layers as adjustments and/or adaptations to higher irradiance and a higher stomatal area index reflecting better water availability. Traits were adjusted plastically, but differed between species grown in a common garden, suggesting that the differentiation between the two species is not solely based on phenotypic plasticity but also has a genetic basis. Our study thus supports the hypothesis that differentiation between the highly interfertile species is likely driven by natural selection.