Phylogeny of the eudicot order Malpighiales: analysis of a recalcitrant clade with sequences of the petD group II intron

Malpighiales are one of the most diverse orders of angiosperms. Molecular phylogenetic studies based on combined sequences of coding genes allowed to identify major lineages but hitherto were unable to resolve relationships among most families. Spacers and introns of the chloroplast genome have recently been shown to provide strong signal for inferring relationships among major angiosperm lineages and within difficult clades. In this study, we employed sequence data of the petD group II intron and the petB-petD spacer for a set of 64 Malpighiales taxa, representing all major lineages. Celastrales and Oxalidales served as outgroups. Sequence alignment was straightforward due to frequent microstructural changes with easily recognizable motifs (e. g., simple sequence repeats), and well defined mutational hotspots. The secondary structure of the complete petD intron was calculated for Idesia polycarpa as an example. Domains I and IV are the most length variable parts of the intron. They contain terminal A/T-rich stem-loop elements that are suggested to elongate independently in different lineages with a slippage mechanism earlier reported from the P8 stem-loop of the trnL intron. Parsimony and Bayesian analyses of the petD dataset yielded trees largely congruent with results from earlier multigene studies but statistical support of nodes was generally higher. For the first time a deep node of the Malpighiales backbone, a clade comprising Achariaceae, Violaceae, Malesherbiaceae, Turneraceae, Passifloraceae, and a Lacistemataceae-Salicaceae lineage received significant statistical support (83% JK, 1.00 PP) from plastid DNA sequences.