Woolly, cushion, rosette: drivers of diversity in the alpine Saussurea genus

What are the mechanisms driving speciation and adaptative evolution in Saussurea, a highly diverse genus of alpine herbaceous plants?

Plants in alpine ecosystems are exposed to extreme environmental stresses, including low temperature, poor soil quality, strong winds and UV radiation. Despite this, the biodiversity of mountain ecosystems is extremely rich. A large number of studies have tried 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 research.

In their new study published in AoBP, Zhang et al. focus on the diversity, endemism and adaptation of the Saussurea genus, one of the northern hemisphere’s highest-altitude plant genera. Theuppermost altitudinal limit of Saussurea (ca. 6300 m) is the highest altitude of seed plants on record. Saussurea is one of the largest genera in the Asteraceae family, containing 460–490 herbaceous species. They have evolved a high diversity of specialized morphological traits adapting them to the wide range of environmental stresses experienced in mountain regions. Within the genus, there are species with woolly leaves, species exhibiting cushion and rosette forms, and even species with “greenhouse” bracts that help to maintain warmer inflorescence temperatures. This diversity makes Saussurea an excellent model to study speciation, diversification, and distribution of mountain species.

Saussurea medusa in the Baima Snow Mountain of the Qinghai-Tibet Plateau. Image credit: Y. Zhang.

Zhang et al. selected 466 species from the Saussurea genus to explore the mechanisms driving speciation and adaptative evolution in these extreme ecosystems. Phylogenetic analyses revealed that geological influences and climate stability exhibit significant phylogenetic signals and highlight that Saussurea originated at a relatively high elevation. Regression models indicated that geological influences and climatic stability significantly affect the diversity and endemism patterns of Saussurea and its morphological innovations. In addition, morphological innovations in a specific area show significant contributions to the local diversity and endemism of Saussurea species.

The study reveals the important effects of geological, glacial and phylogenetic factors on the diversity patterns of alpine taxa. The findings improve the understanding of the distribution pattern of diversity/endemism and adaptive evolution of alpine species in the northern hemisphere. The authors hope that future work will build upon their findings by sampling more of the Saussurea genus and using genomic approaches to reveal the specific adaptative mechanisms for special morphological traits in alpine plants.

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