ANGIOSPERM PHYLOGENY INFERRED FROM 18S RIBOSOMAL DNA-SEQUENCES

Parsimony analyses were conducted for 223 species representing all maj or groups of angiosperms using entire 18S ribosomal DNA (rDNA) sequenc es. Although no search swapped to completion, the topologies recovered are highly concordant with those retrieved via broad analyses based o n the chloroplast gene rbcL. The general congruence of 18S rDNA and rb cL topologies further clarifies the broad picture of angiosperm phylog eny. In all analyses, the first-branching angiosperms are Amborellacea e, Austrobaileyaceae, Illiciaceae, and Schisandraceae, all woody magno liids. These taxa are always followed by the paleoherb family Nymphaea ceae. This same general order of early-branching taxa is preserved wit h several suites of outgroups. In most searches, the remaining early-b ranching taxa represent Piperales and other orders of subclass Magnoli idae (sensu Cronquist). With the exception of Acorus, the monocots are supported as monophyletic and typically have as their sister Ceratoph yllum. In most analyses, taxa with uniaperturate pollen form a grade a t the base of the angiosperms; a large eudicot clade is composed prima rily of taxa having triaperturate pollen. Two large subclades are pres ent within the eudicots, one consisting largely of Rosidae and a secon d corresponding closely to Asteridae sensu late. Subclasses Dilleniida e and Hamamelidae are highly polyphyletic. These data sets of 185 rDNA sequences also permit an analysis of the patterns of molecular evolut ion of this gene. Problems deriving from both the prevalence of indels and uncertain alignment of 18S rDNA sequences have been overstated in previous studies. With the exception of a few well-defined regions, i nsertions and deletions are relatively uncommon in 18S rDNA; sequences are therefore easily aligned by eye across the angiosperms. Indeed, s everal indels in highly conserved regions appear to be phylogeneticall y informative. Initial analyses suggest that both stem and loop bases are important sources of phylogenetic information, although stem posit ions are prone to compensatory substitutions. Of the stem changes anal yzed, only 27% destroy a base-pairing couplet; 73% maintain or restore base pairing.