Rhododendron L. is an extremely diverse genus with eight subgenera and more than 1000 species (Chamberlain et al., 1996). This diversity, combined with broad crossability, has led to the development of more than 28,000 cultivars registered with the Royal Horticulture Society (RHS), including 14,298 rhododendrons, 12,989 azaleas, and 108 azaleodendrons (intersubgeneric hybrids between azaleas and rhododendrons) (Leslie, 2002). Thus, this genus is among the most extensively developed and widely grown in the world. Many superior cultivars continue to be used in breeding programs, but are often of unknown parentage.
Three azaleodendron cultivars exist in the trade with similar names: ‘Fragrans’, ‘Fragrans Affinity’, and ‘Fragrant Affinity’. ‘Fragrans’ is a purported hybrid of R. catawbiense Michx. and R. viscosum L. (Torr.) that was introduced by Paxton, of Chandler & Sons Nursery, London, in 1843. It is described as, “A sweet-scented azaleodendron, fast-growing and compact. Trusses of small flowers, pale mauve with centers lighter to white” (Salley and Greer, 1986, p. 110). ‘Fragrans Affinity’ was found in a group of deciduous azalea seedlings at Greer Gardens, Eugene, Ore., in the 1950s. Harold Greer (pers. comm., May 2004) speculated that it may be a hybrid of R. ponticum L. and R. viscosum or R. catawbiense and R. viscosum. The plant was named ‘Fragrans Affinity’ because of its similarity to ‘Fragrans’. The history of ‘Fragrant Affinity’ is poorly documented. A plant was received in 2000 from the late Dr. A. Kehr, who indicated it was a hybrid between R. viscosum and R. catawbiense. The name ‘Fragrant Affinity’ is not registered with the RHS or the American Rhododendron Society, and the ultimate origin of this material is unknown. Although ‘Fragrant Affinity’ is sterile, an allotetraploid form of ‘Fragrant Affinity’, named ‘Fragrant Affinity Tetra’, was developed that exhibited restored fertility (Contreras et al., 2007). All three are semievergreen and closely resemble the description provided by Salley and Greer (1986) of ‘Fragrans’ (pers. obs.).
The suspected parents of these azaleodendrons are distinct taxonomically and are classified in separate subgenera. Rhododendron catawbiense and R. ponticum are in subgenus Hymenanthes (Blume) K.Koch, section Ponticum G.Don, subsection Pontica Sleumer. This subsection contains evergreen species from North America, Europe, and Asia, including R. hyperythrum L. and R. maximum L. Rhododendron viscosum is in subgenus Pentanthera (G.Don) Pojarkova, section Pentanthera G.Don. This section contains other fragrant, deciduous species from North America including R. arborescens (Pursh) Torrey, R. atlanticum (Ashe) Rehd., R. canescens (Michx.) Sweet, and R. periclymenoides (Michx.) Shinners (Chamberlain et al., 1996).
Parentage of these azaleodendrons is difficult to determine because these hybrids were not the result of controlled pollinations. Furthermore, use of traditional morphological comparison to clarify parentage is complicated because of the number of species that could potentially be involved and the similarity of traits among species within each subgenus. Molecular techniques have been used widely to assess genetic relationships among plants. The amplified fragment length polymorphism (AFLP) technique (Vos et al., 1995; Zabeau and Vos, 1993), in particular, has been used by many scientists to distinguish between species as well as cultivars of the same species (DeHaan et al., 2003; Mellish et al., 2002; Paul et al., 1997; Perera et al., 1998; Zhang et al., 2000). Because of the inclusion of a restriction endonuclease digestion step, AFLPs have the advantage of being highly reproducible in comparison with random amplified polymorphic DNA markers. Milla et al. (2005) reported that reproducibility of markers used in a study of Arachis L. germplasm ranged from 96% to 100%. Also, AFLP analysis has a very high resolution, requires no prior knowledge of the genomes being studied (Vos et al., 1995), and has the capacity to screen simultaneously for many DNA regions distributed throughout the genome, thus producing hundreds of genetic markers (Mueller and Wolfenbarger, 1999).
Amplified fragment length polymorphism markers have been used to confirm hybridity in a number of plants. Teo et al. (2002) confirmed the hybrid status of Mangifera odorata Griff. Kiew et al. (2003) assessed the hybrid status of four genera of Malesian plants. Beismann et al. (1997) differentiated between Salix alba L., S. fragilis L., and their hybrid S. ×rubens Schrank when morphological analysis was determined to be inconclusive.
The objectives of this study were 1) to elucidate the progenitor species of ‘Fragrans’, ‘Fragrans Affinity’, and ‘Fragrant Affinity’; and 2) to determine whether these cultivars are all distinct clones or possibly synonyms.
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