Evidence on the Monophyly of Astragalus (Fabaceae) and its Major Subgroups Based on Nuclear Ribosomal DNA ITS and Chloroplast DNA trnL Intron Data

Abstract

Phylogenetic relationships among 115 species representing the legume genus Astragalus and 12 related genera were inferred from an analysis of nucleotide sequence variation in the internal transcribed spacers and 5.8S gene of nuclear ribosomal DNA. For a subset of these taxa, the ITS data were supplemented by sequences from the chloroplast trnL intron. Phylogenies derived from maximum parsimony and neighbor- joining analyses of sequence and insertion/ deletion characters all suggest that the vast majority of Astragalus is monophyletic (with the exception of outlier species). All New World Astragalus species with aneuploid chromosome numbers (n = 11-15) form a monophyletic group (Neo-Astragalus), which now includes the Mediterranean aneuploid Astragalus echinatus. Other Old World aneuploid species are not closely related to Neo-Astragalus, but rather are found among Old World euploid (n = 8, 16) groups. Similarly, the relatively few North American species with euploid numbers are not the closest relatives to Neo-Astragalus but are dispersed among divergent Old World groups that include both aneuploid and euploid species. The histor- ically allied genus Oxytropis is not nested within Astragalus, but forms a separate clade within the larger Astragalean clade. The proposed segregate genera Astracantha (Eurasian) and Orophaca (North American) are clearly nested within Astragalus s. str. South American species of Astragalus are nested within Neo- Astragalus and comprise at least two independently derived clades (along with their close North American relatives), as previously suggested by morphology. Parsimony reconstructions of characters that have been used in the traditional subgeneric taxonomy of the genus were examined and show high levels of homoplasy. Preliminary estimates of the absolute rate of species diversification in Astragalus suggest it may be higher than in some other, often cited, continental or insular adaptive radiations in angiosperms.