Contribution of the Rhododendron ripense Makino chloroplast genome to the development of evergreen azalea cultivars

Evergreen azalea is one of the most important ornamental shrubs and pot plants in temperate zones worldwide. In Japan, hundreds of azalea cultivars have been bred based on the genetic diversity of wild species and various accumulated mutants since the middle of the 17th century. Japanese cultivar groups such as Edo-kirishima, Kurume-tsutsuji, Ryūkyū-tsutsuji, Hirado-tsutsuji, and Satsuki have been developed by selection and crossing, and many cultivars have been exported to Western countries and utilized as breeding materials for pot and garden azalea. Rhododendron ripense Makino, which grows on riverside rocks and is endemic to Japan, is one of the best ornamental species because of its high adaptability to environmental conditions. We have focused on the genetic contribution of this wild species to evergreen azalea cultivars, and developed a PCR-RFLP identification marker of R. ripense cpDNA based on a species-specific sequence of the trn L-F region. The R. ripense cpDNA specific marker has been in Japanese large-flowered groups, all Ryūkyū and Ōkirishima cultivars, and half of all Hirado cultivars have the R. ripense cpDNA type. Most Japanese small flower cultivars, such as Edo-kirishima, Kurume and Satsuki have non-R. ripense type cpDNA. Italian large-flowered cultivars also tend to be the R. ripense cpDNA type. Furthermore, all pot azalea cultivars of the Indian and Simsii groups possess R. ripense type cpDNA. These results clarified the cytoplasmic contribution of R. ripense not only to Japanese large flower cultivars, but also to Western azalea cultivars. Although R. simsii has been considered to be the main ancestral species of pot azalea, R. ripense should be recognized as the cytoplasmic parent of these cultivars. The ornamental value and adaptive environmental trait originating from R. ripense should be reviewed to elucidate the development history of evergreen azalea cultivars.