Format: ORAL

Carolina M. Siniscalchi1, Joseph L.M. Charbonneau2, Michael B. Belitz3, Aliasghar A. Maassoumi4, Tajinder Singh5, Heather R. Kates3, Douglas E. Soltis3, Pamela S. Soltis3, Robert P. Guralnick3, Ryan A. Folk5

1 General Libraries, Mississippi State University, Mississippi State, USA 2 Department of Ecology and Evolutionary Biology, University of Michigan, Ann Arbor, USA 3 Florida Museum, University of Florida, Gainesville, USA 4 Research Institute of Forests and Rangelands, Tehran, Iran 5 Department of Biological Sciences, Mississippi State University, Mississippi State, USA

Astragalus (Fabaceae) represents a remarkable successful radiation of highly similar species in temperate, cold arid regions of Earth. The factors behind its extraordinary diversity have attracted attention from systematists and biogeographers, especially on what sets it apart from less species-rich relatives. Yet its status as the iconic plant megagenus has both highlighted and hindered study of this fascinating diversity. Here, using phylogenomics and an extensive sampling, we ask if Astragalus diversification is uniform or characterized by radiation bursts. We additionally test whether the species diversity of Astragalus is attributable specifically to its predilection for cold and arid habitats, types of soils, or if it is influenced by chromosome evolution. Finally, we test whether Astragalus originated in central Asia as proposed and whether niche evolutionary shifts were subsequently associated with the colonization of other continents. Our results show the importance of heterogeneity in the diversification of Astragalus, with upshifts associated with the earliest divergences but not strongly tied to any of the tested abiotic factor or biogeographic regionalization. Chromosome number was the only factor potentially correlated with diversification. In contrast to diversification shifts, biogeographic shifts have a strong association with the abiotic environment and highlight the importance of central Asia as a biogeographic gateway. Our investigation shows the importance of phylogenetic and evolutionary studies of logistically challenging mega-radiations. Our findings reject any simple key innovation behind high diversity and underline the often nuanced, multifactorial processes leading to species-rich clades.