The need for biparentally inherited, nuclear DNA markers useful at lower taxonomic levels in plant systematics research has revealed several exciting possibilities. One includes the granule-bound starch synthase I (GBSSI) gene comprising 13 translated exons and 12 introns in most species examined. Rubus (Rosaceae) typifies a group where GBSSI sequences may yield valuable information. Sequences of the nrDNA ITS region and, more recently, of three chloroplast DNA regions initially provided insight into the systematics of Rubus. However, none offers sufficient numbers of phylogenetically informative characters to generate robust hypotheses of relationship. Thus, we sequenced the 5' end of the GBSSI gene for five Rubus species representing three clades. GBSSI in Rubus and Rosa (both Rosoideae) occurs at two paralogous loci; GBSSI-1 and GBSSI-2. The region we sequenced includes seven complete exons, two partial exons, and eight introns. Total number of aligned nucleotides is 2157 using Rosa as an outgroup, with exons and introns ranging in length from 64 to 244 bp and from 64 to 477 bp, respectively. Analysis of exons and introns or introns alone recovers a single most parsimonious tree which is consistent with both ITS and chloroplast phylogenies generated previously. Although pairwise divergences for distantly related species are slightly lower for GBSSI-1 introns than for ITS 1 + ITS 2, they are notably higher for closely related species; divergence between R. cuneifolius and R. hispidus is 0.4% for ITS but 2.1% for GBSSI-1. Percent parsimony-informative characters is also higher in GBSSI-1 (6.0%) than in ITS (2.0%). Given that aligned sequences of GBSSI-1 introns are nearly 2.5-fold longer than ITS 1 + ITS 2 (1216 bp vs. 493 bp, respectively) and show higher levels of divergence, we believe the 5' region of GBSSI-1 holds great promise for phylogeny reconstruction of Rubus and potentially other plant groups, especially at lower taxonomic levels.

Key words: GBSSI, phylogeny, Rosaceae, Rubus, systematics