Following (1) the large scale molecular phylogeny of seed plants based on plastid rbcL gene sequences (published in 1993 by Chase et al., Ann. Missouri Bot. Gard. 80: 528-580) and (2) the 18S nuclear phylogeny of flowering plants (published in 1997 by Soltis et al., Ann. Missouri Bot. Gard. 84: 1-49), we present a phylogenetic analysis of flowering plants based upon a second plastid gene, atpB, analyzed separately and in combination with rbcL sequences for 357 taxa. Despite some discrepancies, the atpB-based phylogenetic trees were highly congruent with those derived from the analysis of rbcL and 18S rDNA, and the combination of atpB and rbcL DNA sequences (comprising ca. 3000 base pairs) produced increased bootstrap support for many major sets of taxa. The angiosperms are divided into two major groups: noneudicots with inaperturate or uniaperturate pollen (monocots plus Laurales, Magnoliales, Piperales, Ceratophyllales, and Amborellaceae-Nymphaeaceae-Illiciaceae) and the eudicots with triaperturate pollen (particularly asterids and rosids). Based on rbcL alone and atpB/rbcL combined, the noneudicots (excluding Ceratophyllum) are monophyletic, whereas they form a grade in the atpB trees. Ceratophyllum is sister to the rest of angiosperms whith rbcL alone and in the combined atpB/rbcL analysis, whereas with atpB alone, Amborellaceae, Nymphaeaceae, and Illiciaceae/Schisandraceae form a grade at the base of the angiosperms. The phylogenetic information at each codon position and the different types of substitutions (observed transitions and transversions in the trees versus pairwise comparisons) were examined; taking into account their respective consistency and retention indices, we demonstrate that third codon positions and transitions are the most useful characters in these phylogenetic reconstructions. This study further demonstrates that phylogenetic analysis of large matrices is feasible.

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To explore the feasibility of parsimony analysis for large data sets, we conducted heuristic parsimony searches and bootstrap analyses on separate and combined DNA data sets for 190 angiosperms and three outgroups. Separate data sets of 18S rDNA (1,855 bp), rbc L (1,428 bp), and atp B (1,450 bp) sequences were combined into a single matrix 4,733 bp in length. Analyses of the combined data set show great improvements in computer run times compared to those of the separate data sets and of the data sets combined in pairs. Six searches of the 18S rDNA rbc L atp B data set were conducted; in all cases TBR branch swapping was completed, generally within a few days. In contrast, TBR branch swapping was not completed for any of the three separate data sets, or for the pairwise combined data sets. These results illustrate that it is possible to conduct a thorough search of tree space with large data sets, given sufficient signal. In this case, and probably most others, sufficient signal for a large number of taxa can only be obtained by combining data sets. The combined data sets also have higher internal support for clades than the separate data sets, and more clades receive bootstrap support of 50% in the combined analysis than in analyses of the separate data sets. These data suggest that one solution to the computational and analytical dilemmas posed by large data sets is the addition of nucleotides, as well as taxa.

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