RAD-seq data point to a northern origin of the arctic-alpine genus Cassiope (Ericaceae)

Evolutionary history of the Arctic flora

The Arctic tundra is a relatively young and new type of biome and is especially sensitive to the impacts of global warming. However, little is known about how the Arctic flora was shaped over time. Here we investigate the origin and evolutionary dynamics of the Arctic flora by sampling 32 angiosperm clades that together encompass 3626 species. We show that dispersal into the Arctic and in situ diversification within the Arctic have similar trends through time, initiating at approximately 10-9 Ma, increasing sharply around 2.6 Ma, and peaking around 1.0-0.7 Ma. Additionally, we discover the existence of a long-term dispersal corridor between the Arctic and western North America. Our results suggest that the initiation and diversification of the Arctic flora might have been jointly driven by progressive landscape and climate changes and sea-level fluctuations since the early Late Miocene. These findings have important conservation implications given rapidly changing climate conditions in the Arctic.

The evolutionary history of rice azaleas (Rhododendron tschonoskii alliance) involved niche evolution to a montane environment

PREMISE

The formation of isolated montane geography on islands promotes evolution, speciation, and then radiation if there are ecological changes. Thus, investigating evolutionary histories of montane species and associated ecological changes may help efforts to understand how endemism formed in islands' montane floras. To explore this process, we investigated the evolutionary history of the Rhododendron tschonoskii alliance, which grows in montane environments of the Japanese archipelago and the Korean Peninsula.

METHODS

We studied the five species in the R. tschonoskii alliance and 30 outgroup species, using genome-wide single-nucleotide polymorphisms and cpDNA sequences, in association with environmental analyses.

RESULTS

The monophyletic R. tschonoskii alliance diverged since the late Miocene. Species in the alliance currently inhabit a cold climatic niche that is largely different from that of the outgroup species. We observed clear genetic and niche differentiations between the taxa of the alliance.

CONCLUSIONS

The association of the alliance's evolution with the formation of cooler climates on mountains indicates that it was driven by global cooling since the mid-Miocene and by rapid uplift of mountains since the Pliocene. The combination of geographic and climatic isolation promoted high genetic differentiation between taxa, which has been maintained by climatic oscillations since the Quaternary.

Alpine speciation and morphological innovations: revelations from a species-rich genus in the northern hemisphere

A large number of studies have attempted to determine the mechanisms driving plant diversity and distribution on a global scale, but the diverse and endemic alpine herbs found in harsh environments, showing adaptive evolution, require more studies. Here, we selected 466 species from the genus Saussurea, one of the northern hemisphere's highest-altitude plant genera with high species richness and striking morphological traits, to explore the mechanisms driving speciation and adaptative evolution. We conducted phylogenetic signals analysis and ancestral character estimation to explore the phylogenetic significance of ecological factors. Moreover, we used spatial simultaneous autoregressive (SAR) error models, modified t-tests and partial regression models to quantify the relative effects of ecological factors and morphological diversity upon diversity and endemism of Saussurea. Phylogenetic analyses reveal that geological influences and climate stability exhibit significant phylogenetic signals and that Saussurea originated at a relatively high elevation. Regression models indicate that geological influences and climatic stability significantly affect the diversity and endemism patterns of Saussurea and its morphological innovations. Moreover, morphological innovations in an area show significant contributions to the local diversity and endemism of Saussurea. We conclude that geological influences (mean altitude and topographic heterogeneity), glacial-interglacial climate stability and phylogenetic conservatism have together promoted the speciation and adaptive evolution of the genus Saussurea. In addition, adaptively morphological innovations of alpine species also promote diversification in local regions. Our findings improve the understanding of the distribution pattern of diversity/endemism and adaptive evolution of alpine specie in the whole northern hemisphere.

Relationships between Tertiary relict and circumboreal woodland floras: a case study in Chimaphila (Ericaceae)

BACKGROUND AND AIMS

Tertiary relict and Arctic/circumboreal distributions are two major patterns of Northern Hemisphere intercontinental disjunctions with very different histories. Each has been well researched, but members of one biome have generally not been incorporated in the biogeographical analyses of the other, and links or transitions between these two biomes have rarely been addressed.

METHODS

Phylogenies of Chimaphila were generated based on cpDNA and nuclear ITS, using Bayesian and maximum likelihood methods. A time-calibrated phylogeny was generated using BEAST. Ancestral area reconstruction was inferred using both statistical dispersal-vicariance analysis and a dispersal-extinction-cladogenesis model.

KEY RESULTS

The Chimaphila crown group was estimated to have originated in the early Miocene. The lineages of C. umbellata diverged early, but its present circumboreal distribution was not achieved until around the middle Pliocene or later. Sister to this is a clade of four species with Tertiary relict distribution. Among these, two expansions occurred from North America to Asia, probably via the Bering Land Bridge, generating its current disjunctions.

CONCLUSIONS

Our data concur with a few other studies, indicating that the circumboreal woodland biome has an older origin than most true Arctic-alpine taxa, having gradually recruited taxa since the early Oligocene. For the origin of Asia-North America disjunctions in Chimaphila, an 'out-of-America' migration was supported. It is not clear in which direction Pyroloideae lineages moved between Tertiary relict disjunctions and Arctic/circumboreal distributions; each biome might have recruited species from the other.

RAD-Seq Data Point to a Distinct Split in Liriodendron (Magnoliaceae) and Obvious East–West Genetic Divergence in L. chinense

Liriodendron is a Tertiary period relic tree genus with a typical East Asian and North American disjunction distribution pattern. As an angiosperm base group of trees, Liriodendron provides a valuable resource for the study of evolution processes. Here, we reconstruct the phylogeny and population genetic structure of Liriodendron based on the restriction site-associated DNA sequencing (RAD-Seq) of a wide collection of individuals from 16 populations. Our results reveal a clear phylogenetic break between L. chinense and L. tulipifera and obvious genetic divergence between the eastern and western populations of L. chinense, which are consistent with the patterns of geographical distributions. The phylogeographic history and long-term geographical isolation of the genus may be responsible for this pattern. Furthermore, a closer relationship was found between L. tulipifera and the eastern populations of L. chinense, indicating the ancient phylogeny of L. chinense in this area. The results of this study will aid in the development of scientific strategies for the conservation and utilization of the Liriodendron germplasm.

Is the East Asian flora ancient or not?

The East Asian flora (EAF) is a key biodiversity hotspot for understanding the origin and evolution of Northern Hemisphere floras, but there is an ongoing debate on whether it is a museum or a cradle for seed plants. Within the EAF, two main floras, the Sino-Himalayan Flora (mainly the Rhododendron Flora) and the Sino-Japanese Flora (mainly the Metasequoia Flora), have been recognized. Previous studies suggested that the EAF is ancient and the Metasequoia Flora is older than the Rhododendron Flora. To test this hypothesis, we synthesized molecular as well as fossil data on seed plants, focusing on the biogeographical origins and historical evolution of the EAF. We compared the ages of its two constituent floras, and examined the impact of the Asian monsoon and other environmental changes on the development of EAF through meta-analysis. Our results suggest that the EAF might be relatively young, with most of its clades originating since the Miocene. The Rhododendron Flora and the Metasequoia Flora are probably of a similar age. The formation and development of the Asian monsoon might have been the main factors that have driven the evolution of EAF. In the Rhododendron Flora, the north-south mountain chains increased the concentration of species and reduced extinction, and the barriers between the east and west have resulted in species differentiation, which triggered it to become a diversity center. The EAF appears to have multiple biogeographical origins, having closely affiliated not only with other floras in the Northern Hemisphere, but also with Gondwanan floras.

Unveiling the Identity of Wenwan Walnuts and Phylogenetic Relationships of Asian Juglans Species Using Restriction Site-Associated DNA-Sequencing

Juglans species have considerable ecological and economic value worldwide. In China, Wenwan walnuts have been collected by aristocrats and noblemen for more than 2000 years. As a diversity center of Asian Juglans, five species are widely distributed in China. The most famous of these is Mahetao (J. hopeiensis), which is an uncharacterized species that is mostly cultivated. Wild J. hopeiensis individuals are very rare and are endemic to Hebei Province. Because of the minimal variations in previously used molecular markers and the heterogeneity between chloroplast and nuclear genomes, determining the phylogenetic relationships among the Juglans species has been challenging, and has hindered subsequent evolutionary inferences. In this study, we collected enough materials for both cultivated and wild Mahetao to construct well-resolved phylogenetic trees for Asian Juglans species. We used a high-throughput genome-wide restriction site-associated DNA sequencing method. Consequently, the identity of J. hopeiensis has been clearly resolved. Our results indicate that J. hopeiensis is a hybrid of J. regia and J. mandshurica. However, J. hopeiensis, J. regia and J. sigillata should be considered as a single species from section Juglans. Additionally, J. ailantifolia, J. cathayensis, and J. mandshurica likely represent one species from section Cardiocaryon according to morphological and molecular studies. These results are supported by population structure analysis and morphological comparison. We propose that J. hopeiensis trees growing in the wild should be conserved because of the economic value of their nuts. These trees may be of particular importance to impoverished communities. Furthermore, they may serve as a valuable genetic resource relevant for enhancing the production of edible walnuts. The 2b-RAD method is a viable option for future phylogenetic studies of Juglans species as well as other plant species.