Noncoding sequences from the slowly evolving chloroplast inverted repeat in addition to rbcL data do not support gnetalean affinities of angiosperms

Abstract

We developed PCR primers against highly conserved regions of the rRNA operon located within the inverted repeat of the chloroplast genome and used these to amplify the region spanning from the 3' terminus of the 23S rRNA gene to the 5' terminus of the 5S rRNA gene. The sequence of this roughly 500-bp region, which includes the 4.5S rRNA gene and two chloroplast intergenic transcribed spacer regions (cpITS2 and cpITS3), was determined from 20 angiosperms, 7 gymnosperms, and 16 ferns (21,700 bp). Sequences for the large subunit of ribulose bisphosphate carboxylase/oxygenase (rbcL) from the same or confamilial genera were analyzed in both separate and combined data sets. Due to the low substitution rate in the inverted repeat region, noncoding sequences in the cpITS region are not saturated with substitutions, in contrast to synonymous sites in rbcL, which are shown to evolve roughly six times faster than noncoding cpITS sequences. Several length polymorphisms with very clear phylogenetic distributions were detected in the data set. Results of phylogenetic analyses provide very strong bootstrap support for monophyly of both spermatophytes and angiosperms. No support for a sister group relationship between Gnetales and angiosperms in either cpITS or rbcL data was found. Rather, weak bootstrap support for monophyly of gymnosperms studied and for a basal position for the aquatic angiosperm Nymphaea among angiosperms studied was observed. Noncoding sequences from the inverted repeat region of chloroplast DNA appear suitable for study of land plant evolution.