In a continuing effort to gain insight into the systematics of Rubus (Rosaceae), we examined the external transcribed spacer (ETS) of nuclear ribosomal DNA (nrDNA) to assess its suitability for phylogenetic analysis. We sampled four Rubus species (R. arcticus, R. idaeus, R. odoratus, and R. trivialis) representing four subgenera and corresponding to distinct clades based on previous analysis of ITS region sequences. We PCR amplified the entire nrDNA intergenic spacer (IGS) using universal primers (18S-IGS and 26S-IGS). The size of IGS in Rubus is approximately 2.5 kb. We sequenced the 3' end of ETS using primer 18S-E. ETS sequences of these species can be readily aligned by eye. Pairwise divergences among the four species noted from the ETS data range from 4.0 to 5.9% with a mean of 4.5%, whereas data from the ITS region for these same species range from 4.4 to 9.3% with a mean of 6.8%. Percent variable/parsimony-informative sites for ETS and ITS 1 + ITS 2 are 10.2/0.7% and 12.8/1.0% respectively. In contrast to published studies of ETS sequences in other plant taxa, variation in ETS is somewhat lower than for ITS in these four Rubus species. However, we have only sequenced the 3' portion of ETS, which is expected to be conserved.

Key words: ETS, nrDNA, phylogeny, Rosaceae, Rubus