Résumé
We have estimated the potential phylogenetic utility of the ribosomal external transcribed spacer (ETS) from the nuclear ribosomal region. The ETS was sequenced from 13 annual Medicago (Fabaceae) species upstream a highly conserved motive which was found among many different organisms. In the genus Medicago, the ETS was found to evolve 1.5 times faster than the internal transcribed spacer and to be 1.5 times more informative. Reduced ribosomal maturation process constraints on ETS are proposed to explain the different evolutionary rates between the two spacers. Maximal phylogenetic resolution and support was obtained when the two spacers were analyzed together. No incongruence between the two spacers was found and ETS appears to be a valuable source of information for solidifying ITS plant phylogeny. The phylogeny obtained in Medicago suggests that none of the three subsections included in the study is monophyletic.