Astragalus (Fabaceae): A molecular phylogenetic perspective

Nucleotide sequences of the plastid matK gene and nuclear rDNA internal transcribed spacer region were sampled from Astragalus L. (Fabaceae), and its closest relatives within tribe Galegeae, to infer phylogenetic relationships and estimate ages of diversification. Consistent with previous studies that emphasized sampling for nrDNA ITS primarily within either New World or Old World species groups, Astragalus, with the exception of a few morphologically distinct species, is strongly supported as monophyletic based on maximum parsimony and Bayesian analyses of matK sequences as well as a combined sequence dataset. The matK data provides better resolution and stronger clade support for relationships among Astragalus and traditionally related genera than nrDNA ITS. Astragalus sensu stricto plus the genus Oxytropis are strongly supported as sister to a clade composed of strictly Old World (African, Australasian) genera such as Colutea, Sutherlandia, Lessertia, Swainsona, and Carmichaelia, plus several morphologically distinct segregates of Eurasian Astragalus. Ages of these clades and rates of nucleotide substitution estimated from a fossil-constrained, rate-smoothed, Bayesian analysis of matK sequences sampled from Hologalegina indicate Astragalus diverged from its sister group, Oxytropis, 12-16 Ma, with divergence of Neo-Astragalus beginning ca. 4.4 Ma. Estimates of absolute rates of nucleotide substitution for Astragalus and sister groups, which range from 8.9 to 10.2x10(-10) substitutions per site per year, are not unusual when compared to those estimated for other, mainly temperate groups of papilionoid legumes. The results of previously published work and other recent developments on the phylogenetic relationships and diversification of Astragalus are reviewed.