A COMPARISON OF ANGIOSPERM PHYLOGENIES FROM NUCLEAR 18S RDNA AND RBCLSEQUENCES

To investigate the phylogenetic utility of entire, nuclear-encoded sma ll subunit (18S) ribosomal DNA sequences, we compared the rate of evol ution and phylogenetic resolution of entire 18S sequences with those f or the chloroplast gene rbcL using a suite of 59 angiosperms and 3 gym nosperms (Gnetum, Ephedra, and Zamia) as outgroups. For rbcL, 482 (33. 6%) of the 1431 base positions were phylogenetically informative, wher eas for 18S rDNA 341 (18.4%) of the 1853 positions were informative. P airwise comparisons within the angiosperms show that rbcL is generally about three times more variable than 18S rDNA. However, because the 1 8S region is approximately 400 base pairs longer than rbcL, the ratio of the number of phylogenetically informative sites per molecule is on ly about 1.4 times greater for rbcL compared to 18S rDNA. Not only are sites more variable in rbcL than in 18S rDNA, but this variability is more evenly distributed over the length of rbcL. In contrast, 18S rDN A shows highly variable regions interspersed with regions of extreme c onservation. Minimum-length Fitch trees were constructed for each matr ix, and the results were compared to a tree derived from a previous gl obal analysis of rbcL sequences based on 499 seed plants. Parsimony an alyses showed that several clades are strongly supported by both data sets, such as Gnetales, monocots, paleoherbs, Santalales, and various clades within Rosidae s.l. and Asteridae s.l. Some clades (e.g., Santa lales) have higher base substitution rates for 18S rDNA, permitting th e assessment of inter- and intrafamilial relationships. This comparati ve study indicates that 18S rDNA sequences contain sufficient informat ion to conduct phylogenetic studies at higher taxonomic levels (family and above) within angiosperms, rDNA sequences are best applied to suc h deep divergences, but the amount of variation differs significantly among taxonomic groups.