MIOCENE ORIGIN OF THE CHARACTERISTIC BROAD-LEAVED EVERGREEN SHRUB AMMOPIPTANTHUS (LEGUMINOSAE) IN THE DESERT FLORA OF EASTERN CENTRAL ASIA

Ammopiptanthus is the only broad-leaved evergreen angiosperm genus in the deserts of eastern central Asia. Evolutionary history of Ammopiptanthus was examined by means of phylogenetic and molecular dating analyses with a more extensive sampling than ever to shed light on the origin of the genus and the eastern central Asian flora. MatK and nrITS sequences were analyzed using parsimony and Bayesian methods. Divergence time of Ammopiptanthus was estimated using Bayesian dating analyses with relaxed clock model. Both the nrITS-based phylogeny and the matK-based phylogeny gave similar results that Ammopiptanthus is monophyletic and sister to a clade including Sophora and Ammodendron. Low levels of molecular variation were detected in both the plastid and the nuclear DNA regions within Ammopiptanthus. The two species of the genus cannot be distinguished by the analyses. The stem age of Ammopiptanthus was estimated to be 19.6 million years (Myr; 95% highest posterior density [HPD]: 11.2-28.2 Myr) by the matK data and 21.8 Myr (95% HPD: 13.8-30.0 Myr) by nrITS analysis. The estimated divergence time favors the hypothesis that Ammopiptanthus is a Tertiary relic genus descended from the broad-leaved evergreen Tethyan flora. With increasingly intensified aridification of the region, Ammopiptanthus, together with other characteristic genera in the modern desert flora of eastern central Asia, was able to successfully establish themselves because of high ecological capacities in adaptation to local environments and have persisted for a long time.