Molecular phylogeny and divergence times of Astragalus section Hymenostegis: An analysis of a rapidly diversifying species group in Fabaceae

Abstract

The taxa of Astragalus section Hymenostegis are an important element of mountainous and steppe habitats in Southwest Asia. A phylogenetic hypothesis of sect. Hymenostegis has been obtained from nuclear ribosomal DNA internal transcribed spacer (ITS) and plastid ycf1 sequences of up to 303 individuals from 106 species, including all 89 taxa currently assigned to sect. Hymenostegis, 14 species of other Astragalus sections, and two species of Oxytropis and one Biserrula designated as outgroups. Bayesian phylogenetic inference and parsimony analyses reveal that three species from two other closely related sections group within sect. Hymenostegis, making the section paraphyletic. DNA sequence diversity is generally very low among Hymenostegis taxa, which is consistent with recent diversification of the section. We estimate that diversification in sect. Hymenostegis occurred in the middle to late Pleistocene, with many species arising only during the last one million years, when environmental conditions in the mountain regions of Southwest and Central Asia cycled repeatedly between dry and more humid conditions.

Figure 1

Map of the distribution area of the sect. Hymenostegis species. Brown shading indicates differences in species numbers per area from 1–2 (light brown) species to 25–30 species (dark brown).

Figure 2

Bayesian phylogenetic tree based on nrDNA ITS sequences and including one individual per sect. Hymenostegis species. Clade support values (BI pp/MP bs) are given at the branches of the tree. Asterisks at branches indicate pp ≥ 0.98 and bs ≥ 80%, respectively. Blue numbers provide mean node ages in million years estimated through a Beast analysis. Asterisks behind species names indicate new species names currently in the process of valid publication, while species names given in bold face mark species formerly not included within sect. Hymenostegis. Colors of species names refer to distribution areas given in Fig. 3.

Figure 3

Phylogenetic tree obtained by Bayesian phylogenetic inference of the combined nuclear rDNA ITS and chloroplast ycf1 datasets of 64 Astragalus and one outgroup accession. Clade support values (BI pp/MP bs) are given at the branches of the tree. Asterisks at branches indicate pp ≥ 0.98 and bs ≥ 80%, respectively. Asterisks behind species names indicate new species names currently in the process of valid publication, while species names given in bold face mark species formerly not included within sect. Hymenostegis. Regions of origin of the species are color-coded and the areas are provided in the inserted map.