RIBOSOMAL EXTERNAL AND INTERNAL TRANSCRIBED SPACERS - COMBINED USE INTHE PHYLOGENETIC ANALYSIS OF MEDICAGO (LEGUMINOSAE)

We have estimated the potential phylogenetic utility of the ribosomal external transcribed spacer (ETS) from the nuclear ribosomal region. T he ETS was sequenced from 13 annual Medicago (Fabaceae) species upstre am a highly conserved motive which was found among many different orga nisms. In the genus Medicago, the ETS was found to evolve 1.5 times fa ster than the internal transcribed spacer and to be 1.5 times more inf ormative. Reduced ribosomal maturation process constraints on ETS are proposed to explain the different evolutionary rates between the two s pacers. Maximal phylogenetic resolution and support was obtained when the two spacers were analyzed together. No incongruence between the tw o spacers was found and ETS appears to be a valuable source of informa tion for solidifying ITS plant phylogeny. The phylogeny obtained in Me dicago suggests that none of the three subsections included in the stu dy is monophyletic.