DISCORDANCE BETWEEN NUCLEAR AND CHLOROPLAST PHYLOGENIES IN THE HEUCHERA GROUP (SAXIFRAGACEAE)

Various factors, including taxon density, sampling error, convergence, and heterogeneity of evolutionary rates, can potentially lead to inco ngruence between phylogenetic trees based on different genomes. Partic ularly at the generic level and below, chloroplast capture resulting f rom hybridization may distort organismal relationships in phylogenetic analyses based on the chloroplast genome, or genes included therein. However, the extent of such discord between chloroplast DNA (cpDNA) tr ees and those trees based on nuclear genes has rarely been assessed. W e therefore used sequences of the internal transcribed spacer regions (ITS-1 and ITS-2) of nuclear ribosomal DNA (rDNA) to reconstruct phylo genetic relationships among members of the Heuchera group of genera (S axifragaceae). The Heuchera group presents an important model for the analysis of chloroplast capture and its impact on phylogenetic reconst ruction because hybridization is well documented within genera (e.g., Heuchera), and intergeneric hybrids involving six of the nine genera h ave been reported. An earlier study provided a well-resolved phylogene tic hypothesis for the Heuchera group based on cpDNA restriction-site variation. However, trees based on ITS sequences are discordant with t he cpDNA-based tree. Evidence from both morphology and nuclear-encoded allozymes is consistent with the ITS trees, rather than the cpDNA tre e, and several points of phylogenetic discord can clearly be attribute d to chloroplast capture. Comparison of;the organellar and ITS trees a lso raises the strong likelihood that ancient events of chloroplast ca pture occurred between lineages during the early diversification of th e Heuchera group. Thus, despite the many advantages and widespread use of cpDNA data in phylogeny reconstruction, comparison of relationship s based on cpDNA and ITS sequences for the Heuchera group underscores the need for caution in the use of organellar variation for retrieving phylogeny at lower taxonomic levels, particularly in groups noted for hybridization.