Chloroplast DNA polymorphisms and morphological variation in Japanese wild azaleas, the origin of evergreen azalea cultivars

In Japan, many azalea cultivars have been selected from the endogenous species and natural hybrids of evergreen azalea since the Edo era (1603 similar to 1867). Mt. Kirishima, the volcanic mountain mass in south Japan, is one of the gene centers of evergreen azaleas. Rhododendron kiusianum Makino inhabits above 1000m in this mountain mass, while R. kaempferi Planch. is distributed below 600m. Presumable natural hybrid populations are found in the intermediate region (1000 similar to 600m) of these two distributions. Two species were clearly distinguished by morphological investigation. R. kiusianum had small pink-purple flowers and small elliptical leaves, whereas R. kaempferi had bigger red-orange flowers with deep colored blotches and large oblong leaves. Interspecific hybrids showed varied phenotype within the range of two species, especially flower colors and leaf shapes, and a gradual dine of these morphological characters along the altitude was seen. PCR-RFLP analysis of chloroplast DNA (cpDNA) could detect the specific bands for two species in 16S rDNA region when digested with HhaI restriction enzyme. Populations of interspecific hybrids were composed of individuals which had a banding pattern of either R. kiusianum or R. kaempferi. This indicates R. kiusianum and R. kaempferi are clearly distinct species and natural hybrid populations consist of individuals which have one of two cpDNA. Most of cultivars in Edo-Kirishima and Kurume azalea possessed a banding pattern of R. kaempferi.