Genetic Relationships among Cultivated Species of Rhododendron L. Section Pentanthera G.Don Based on DNA Sequence Variation of the Internal Transcribed Spacer (ITS) Region

Rhododendron section Pentanthera has traditionally been viewed as a group of closely related species due to the relative lack of distinctive morphological characteristics separating species and numerous reports in the literature of artificial and natural fertile hybrids that produce fertile progeny. Significant variation within species has created difficulties in efforts to assemble these taxa into well-defined groups. Genetic relationships among cultivated specimens of the 15 currently recognized species in Rhododendron section Pentanthera were derived from sequence comparisons of the internal transcribed spacer (ITS) region. Sequences of the entire ITS region including ITS1, ITS2, and the 5.8S subunit were generated by direct sequencing of polymerase chain reaction (PCR) amplified fragments. Rhododendron vaseyi A.Gray, Rhododendron section Rhodora (L.) G.Don was used as an outgroup. Aligned sequences of the 16 taxa resulted in 690 characters. A distance matrix of sequence divergence was calculated using Kimura's two parameter model. A bootstrap analysis was performed and a phenogram was constructed using MEGA. A phenetic assay rather than a phylogenetic analysis was performed because the ITS region contained only eight (1.16%) phylogenetically informative sites. The entire ITS region contained 41 variable sites (5.94%). Five species had identical ITS sequences. Seven additional species differed only by one or two base pair substitutions and/or the addition or deletion of one or two base pairs. R. luteum Sweet, R. occidentale A. Gray, R. molle (Blume) G.Don, and the outgroup, R. vaseyi had noticeable variation (base substitutions, additions, and deletions) compared to the other species. Divergence values among the taxa were extremely low, ranging from 0.00% to 3.51%. This molecular data, and previous hybridization studies, do not support the accepted taxonomic treatment of the section.