Molecular Phylogenetic Relationships Among Angiosperms: An Overview Based on rbcL and 18S rDNA Sequences

Abstract

During only the past 4 years, two large data sets of DNA sequences have greatly clarified the broad picture of angiosperm relationships and evolution. By far the more extensive of these two data sets is that based on the chloroplast gene rbcL, with sequences representing 499 species of seed plants [1]. More recently, a smaller data set representing 223 angiosperms has been compiled for the nuclear 18S ribosomal RNA gene [2]. The general structural features, rate of evolution, and phylogenetic utility of both genes have been previously reviewed [3,4] and are not discussed here. For the first time, species representing the diversity of angiosperms have been sequenced for both a chloroplast and a nuclear gene. Visual inspection of the rbcL and 18S rDNA topologies suggests a high degree of overall concordance. Particularly noteworthy is the support provided by the 18S rDNA topology for those nontraditional relationships suggested by the rbcL trees. For example, 18S rDNA sequence analysis confirms the rbcL-based inferences that Droseraceae and Nepenthaceae are closely related to the Caryophyllidae and that a number of members of Rosidae and Dilleniidae have as their closest relatives members of Asteridae.