Azaleodendrons: Investigating Parentage, Fertility, and Effects of Polyploidy Among Hybrids of Deciduous Azaleas and Evergreen Rhododendrons

No Thumbnail Available



Journal Title

Series/Report No.

Journal ISSN

Volume Title



Studies were conducted in order to develop a fertile form of the azaleodendron cultivar Rhododendron L. 'Fragrant Affinity' and determine parentage of this and two other azaleodendron cultivars. Wide hybridization can potentially lead to recombination of diverse traits and creation of unique phenotypes, but these hybrids are often sterile as is the case with the inter-subgeneric hybrid R. 'Fragrant Affinity'. Sterility in wide hybrids can either be genic or chromosomal; the latter may be overcome by induction of polyploidy which can restore chromosome homology and fertility. Initial cytological studies of 'Fragrant Affinity' showed bridges between bivalents in metaphase I and laggard chromosomes in anaphase I. In this study we successfully developed an allotetraploid form of R. 'Fragrant Affinity' using oryzalin (4-(dipropylamino)-3, 5-dinitro-benzenesulfonamide) as a mitotic inhibitor and chromosomal doubling agent. Genome sizes (2C) were determined using flow cytometry and found to be approximately 1.6 and 3.2 pg for the diploid and allotetraploid, respectively. Pollen viability, determined by staining and germination tests, was 4% and 0%, respectively for the diploid and 68% and 45%, respectively for the allotetraploid. No seeds were produced when the diploid R. 'Fragrant Affinity' was used as a maternal parent when crossed with pollen from viable diploid and tetraploid parents. The allotetraploid produced viable seeds and seedlings when crossed with pollen from either diploid or tetraploid parents, including self pollination, demonstrating restored male and female fertility. Additional crosses were successfully completed using the allotetraploid as part of an ongoing breeding program to develop new fragrant, cold hardy, evergreen rhododendron. Morphological analysis has historically been used to determine parentage of unknown hybrids. This can be difficult when the purported parental taxa have very similar morphology as in the case of the three azaleodendron cultivars R. 'Fragrans', 'Fragrans Affinity', and 'Fragrant Affinity'. The three cultivars are purported hybrids of R. catawbiense Michx. or R. ponticum L. and R. viscosum (L.) Torr., the three cultivars are very similar morphologically as are the purported parents. In this study, morphological and AFLP analyses were conducted to determine if these are unique cultivars or clones and to elucidate the parents. The three cultivars, potential parents from subgenera Hymenanthes (Blume) K.Koch (evergreen rhododendrons) and Pentanthera (G.Don) Pojarkova (deciduous azaleas), and related taxa from each subgenus were evaluated using 31 primer combinations. Morphological comparison indicated that R. ponticum is likely the evergreen rhododendron parent of R. 'Fragrant Affinity' based on intermediate petiole and leaf blade lengths and floral characteristics. Genetic similarity calculated using Jaccard's coefficient of similarity was highest between the hybrids and R. ponticum among the evergreen rhododendrons and R. viscosum among the deciduous azaleas, respectively. Genetic similarities among the three azaleodendrons indicated that they are unique cultivars and not synonyms, but likely share the same parental species. A dendrogram generated using the genetic similarity matrix grouped taxa into generally accepted taxonomic groups and nested R. ponticum with the hybrids suggesting it is the evergreen rhododendron parent. Additionally, a plot of the first three principle components also showed R. ponticum to be grouped more closely with the hybrids. Furthermore, there were 18 fragments unique to R. ponticum and the hybrids, however no unique bands were found for the purported deciduous azalea parent, suggesting the original parent may have been a hybrid.



polyploidy, plant breeding, cytology, azaleodendron, AFLP, genome size, flow cytometry, fertility





Horticultural Science