Using progesterone 5β-reductase, a gene encoding a key enzyme in the cardenolide biosynthesis, to infer the phylogeny of the genus Digitalis

The progesterone 5β-reductase (5β-POR), a key enzyme in the cardenolide biosynthesis, was sequenced for 21 species of Digitalis and Isoplexis to infer phylogenetic and biogeographic relationships. This new secondary metabolism molecular marker was compared to the previously applied nuclear ITS and plastid trnL-F sequences. The results from separate analyses show high congruence within the genus Digitalis and support the conclusion that all species of Isoplexis have a common origin and are embedded in Digitalis. The genus Isoplexis therefore should be reduced to sectional rank within the genus Digitalis. The sequence analyses give further evidence that additional sequence data increase support for relationships. It demonstrates that poorly supported relationships in smaller data sets may lead to erroneous conclusions about the evolution of the investigated taxa.