Phylogeny of Weigela and Diervilla (Caprifoliaceae) based on nuclear rDNA ITS sequences:: Biogeographic and taxonomic implications

Phylogenetic relationships among 16 taxa representing 11 species of Weigela and three species of Diervilla were inferred from nucleotide sequence variation in the internal transcribed spacer (TTS) regions of 18-26 S nuclear ribosomal DNA. Phylogenetic trees were obtained using parsimony, distance, and maximum likelihood methods. The TTS phylogenies did not support the monophyly of Weigela, which comprises three major clades. The first clade includes a single species, W. middendorffiana, which is more closely related with American genus, Diervilla. The second clade consisted of a single species, W. maximowiczii, but its relationship to other Weigela species is equivocal. The third one is a strongly supported core clade which contains the remaining Weigela species. Within the core group, W. hortensis exhibits the most distinct ITS sequence type. The hybrid origin of the species from the putative parents, W. florida and W. subsessilis, is not supported. The ITS sequence evidence favors the taxonomic hypothesis proposed by Nakai who recognized four distinct evolutionary lines (Diervilla, Macrodiervilla, Weigelastrum and Weigela). However, recognition of the two sections, Calysphyrum and Weigela was not supported in the ITS phylogenies. Substantial morphological differences between Diervilla and Weigela middendorffiana do not support the morphological stasis concept which has been considered as a general evolutionary mode among the Asian and North American disjunct taxa.