The internal transcribed spacer region of the nrDNA (ITS) is widely used to reconstruct phylogenetic relationships at low taxonomic levels. In some groups, however, sequence variation in the ITS region is not sufficient to provide resolution of interspecific relationships. For example, studies in Lessingia (Astereae) using ITS sequence data provided little resolution of relationships among taxa due to low levels of sequence variation. We are investigating the phylogenetic utility of the external transcribed spacer (ETS) in Astereae, specifically in Lessingia and related genera. The ETS is flanked by the 18S and the non-transcribed spacer (NTS) of nrDNA. Using universal primers (therefore widely applicable in plants) and long-distance PCR we amplified a large DNA segment (more than 5 kb) encompassing a portion of the 26S, the NTS, the ETS, and a portion of the 18S from various members of Astereae. After sequencing the 3' end of this large fragment we designed a primer that anneals within the ETS. This primer, used in combination with a primer in the 18S, generated a PCR fragment approximately 500 bp in size representing the 3' end of the ETS. To date, we have successfully amplified this region in several members of the Astereae (Aster, Ericameria, Erigeron, Felicia, Grangea, Grindelia, Haplopappus, Hazardia, Isocoma, Lessingia, Rigiopappus, Solidago, Xanthisma, and, Xylorhiza) and in two members of the Anthemideae (Achillea and Artemisia). Preliminary phylogenetic analyses using partial ETS sequences produce topologies that are congruent with those generated using ITS sequence data. Additional support is provided for some clades that are weakly supported in ITS trees. We are hopeful that additional information from the ETS region in combination with ITS data will provide increased resolution relative to the use of ITS sequences alone.

Key words: Astereae, Compositae, ETS, external transcribed spacer, Lessingia, phylogeny