Molecular phylogenetics of annual Astragalus (Fabaceae) and its systematic implications

Annual Astragalus spp. are important elements in the floras of Mediterranean areas of Africa, Europe and the deserts of South-West Asia. By reconstructing phylogenetic trees including c. 75% of Old World annual Astragalus spp. and representatives of allied perennial taxa, we sought novel evolutionary insights regarding the phylogenetic position and origins of these species. Our analyses of nuclear ribosomal DNA internal transcribed spacer and plastid trnK/matK sequences included 171 taxa of Astragalus plus nine outgroup taxa to recover the main clades in the genus. Character mapping for five selected morphological characters was conducted on the trees obtained, with a focus on the annual vs. perennial habit. Annual Astragalus spp. have originated in eight clades independently. Although the annual habit is the derived state in most clades recovered, at least two reversions to perenniality are found: one among the early branching lineages and another in the New World Astragalus clade. Our results suggest that the annual strategy has been an important force of selection leading to speciation in several clades of Astragalus and in different geographical areas. Geological events that have favoured the establishment of the annual habit in xeric habitats have probably also favoured the radiation of cushion-forming plants in the adjacent mountainous regions.