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 on 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 approximately 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 in the atpB trees they form a grade. Ceratophyllum is sister to the rest of angiosperms with 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 vs. 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.

Sequence analyses of the plastid genes atpB and rbcL support an expanded order Malvales. Within this alliance, core Malvales are clearly supported and comprise most genera that have previously been included in Sterculiaceae, Tiliaceae, Bombacaccae, and Malvaceae. Additional well supported malvalean alliances include the bixalean clade (Bixaceae, Diegodendraceae, and Cochlospermaceae), the cistalean clade (Cistaccac, Dipterocarpaceae, and Sarcolaenaceae) and Thymelaeaceae (including Gonystyloideae and Aquilarioideae). Our results indicate sister-group relationships between ( I ) Neuradaceae and the cistalean clade;

The breeding of new, high-quality citrus cultivars depends on dependable information about the relationships of taxa within the tribe Citreae; therefore, it is important to have a well-supported phylogeny of the relationships between species not only to advance breeding strategies, but also to advance conservation strategies for the wild taxa. The recent history of the systematics of Citrus (Rutaceae: Aurantioideae) and its allies, in the context of Rutaceae taxonomy as a whole, is reviewed. The most recent classification is tested using nine cpDNA sequence regions in representatives of all genera of the subfam. Aurantioideae (save Limnocitrus) and numerous species and hybrids referred to Citrus s.l. Aurantioideae are confirmed as monophyletic. Within Aurantioideae, tribe Clauseneae are not monophyletic unless Murraya s.s. and Merrillia are removed to Aurantieae. Within tribe Aurantieae, the three traditionally recognized subtribes are not monophyletic. Triphasiinae is not monophyletic unless Oxanthera is returned to Citrus (Citrinae). Balsamocitrinae is polyphyletic. Feroniella, traditionally considered allied closely to Limonia (=Feronia), is shown to be nested in Citrus. The proposed congenericity of Severinia and Atalantia is confirmed. The most recent circumscription of Citrus is strongly supported by this analysis, with hybrids appearing with their putative maternal parents. The genus was resolved into two clades, one comprising wild species from New Guinea, Australia, and New Caledonia (formerly Clymenia, Eremocitrus, Microcitrus, Oxanthera), but surprisingly also Citrus medica, traditionally believed to be native in India. The second clade is largely from the Asian mainland (including species formerly referred to Fortunella and Poncirus).

Sequence data for plastid rbcL and atpB from members of Anacardiaceae, Burseraceae, Cneoraceae, Meliaceae, Ptaeroxylaceae, Rutaceae, and Simaroubaceae were analyzed cladistically to evaluate the familial and subfamilial circumscriptions of Rutaceae. Taxa representing all subfamilies and tribes were sampled. The analysis shows that Rutaceae are paraphyletic, with Spathelia and Dictyoloma (Rutaceae), Harrisonia (Simaroubaceae), Cneorum (Cneoraceae), and Ptaeroxylon (Ptaeroxylaceae) forming a clade sister to all other Rutaceae. Circumscription of Rutaceae to include all of these taxa is recommended. This analysis indicates that Simaroubaceae and Meliaceae are the outgroups closest to Rutaceae. Correlation of the molecular phylogenies with biochemical data indicates that chemotaxonomic information is more reliable than fruit type as an indicator of familial and subfamilial circumscriptions. The subfamilial classification needs revision; none of the subfamilies of more than one genus is monophyletic.

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