Island-like mountain radiations in Asia: The case study of the genera Saussurea and Jurinea

Abstract

Evolutionary radiations represent events in which many species or lineages evolved from a common ancestor in a short period of time. Many plant radiations have been triggered by island-like ecological opportunities following mountain uplift; the mountain ranges with the steepest and widest environmental gradients, such as the Andes, are home of exceptional regional species pools (and also exceptional endemism rates), mainly derived from evolutionary radiations. The Himalayan-Qinghai-Tibetan Plateau (HQTP) and the adjacent Hengduan Mountains (HM) are considered one of the main biodiversity hotspots of the world thanks to its richness in species and endemics. Both regions show extreme altitudinal ranges compressed in short distances as a consequence of the collision of the Indian and Eurasian plates, and thus constitute ideal evolutionary scenarios to study diversification processes in mountain regions. We have identified two possible cases of alpine radiations in the Saussurea-Jurinea complex (Compositae-Cardueae), involving some 550 species in total. Saussurea shows an amazing number of species (more than 300) in the HQTP and Hengduan mountains, although a considerable number of species are also found on the west side of the mountains of Middle Asia (Tian Shan and Pamir-Alay). Jurinea, in contrast, has the highest number of species (150 sp.) in the Tian Shan and Pamir-Alay. Our general objectives are the following: a) To carry out an extensive sampling of Saussurea and Jurinea, especially centred in the two main radiation areas in the HQTP and Tian Shan mountains; b) To generate well-resolved phylogenies of both genera using a multi-loci approach through next-generation-sequencing (NGS) analyzed by Bayesian inference and parsimony, and explore coalescent-based species tree estimation with our NGS data set; c) On the basis of the new phylogenies, conduct phylogenetic comparative analyses and multi-model biogeographical inference to address the following questions: Do the alpine species of both genera in Asian mountains constitute clades with clearly higher rates of diversification than their lowland relatives? How many independent radiations took place in the complex? If several, did they occur at the same period, and are they comparable in terms of speciation rates? Which factors (intrinsic or extrinsic) shape species radiations, i.e., do the radiations follow a geographical model, an adaptive model, or a mixed model? Our results will be a major contribution to the study of alpine radiations especially in the HQTP, and will open a methodological pathway for the analysis of very large radiations in other genera.