Evolution of a RNA polymerase gene family in Silene (Caryophyllaceae) -: Incomplete concerted evolution and topological congruence among paralogues

Four low-copy nuclear DNA intron regions from the second largest subunits of the RNA polymerase gene family (RPA2, RPB2, RPD2a, and RPD2b), the internal transcribed spacers (ITSs) from the nuclear ribosomal regions, and the rps16 intron from the chloroplast were sequenced and used in a phylogenetic analysis of 29 species from the tribe Sileneae (Caryophyllaceae). We used a low stringency nested polymerase chain reaction (PCR) approach to overcome the difficulties of constructing specific primers for amplification of the low copy nuclear DNA regions. Maximum parsimony analyses resulted in largely congruent phylogenetic trees for all regions. We tested overall model congruence in a likelihood context using the software PLATO and found that ITSs, RPA2, and RPB2 deviated from the maximum likelihood model for the combined data. The topology parameter was then isolated and topological congruence assessed by nonparametric bootstrapping. No strong topological incongruence was found. The analysis of the combined data sets resolves previously poorly known major relationships within Sileneae. Two paralogues of RPD2 were found, and several independent losses and incomplete concerted evolution were inferred. The among-site rate variation was significantly lower in the RNA polymerase introns than in the rps16 intron and ITSs, a property that is attractive in phylogenetic analyses.