Deutzia species are a valuable group of temperate landscape plants grown primarily for their profusions of showy white to pink flowers produced in late spring. Several species have been widely cultivated in Europe since the first Deutzia were imported from Japan in the early 18th century. Deutzia gained added global popularity in the early 19th century due to the introduction of additional Asian species to cultivation (Styer and Stern, 1979). With access to this diverse germplasm, many additional Deutzia hybrids and cultivars were developed through the breeding and selection efforts of Victor Lemoine, his family, and the Lemoine Nursery staff in the 19th and early–mid 20th centuries in Nancy, France (Wyman, 1971). The development of new hybrids and cultivars has continued since then.
The taxonomic history of Deutzia has seen it placed within both the Saxifragaceae and the Philadelphaceae families, although it is currently accepted as a member of Philadelpheae within Hydrangeaceae (Soltis et al., 1995; Stevens, 2001). Deutzia is most closely allied with Kirengoshoma, a small genus of rhizomatous perennials that share several morphological traits with Deutzia (Hufford et al., 2001). Deutzia represents a disjunct genus with species occurring in eastern Asia and Central America. The genus has traditionally been divided into three sections based on morphological differences, with the Asian sections Deutzia and Mesodeutzia differing in the aestivation of the petals (valvate/induplicate in Deutzia and imbricate in Mesodeutzia). The central American Neodeutzia, rarely treated as a separate genus, differs from its Asian relatives in the number of stamens, with 12 to 15 in Neodeutzia compared with 10 in Deutzia and Mesodeutzia (Hwang, 1993; Styer and Stern, 1979; Zaikonnikova, 1975). Kim et al. (2015) constructed a phylogeny of the genus and suggested that polyphyletic sections Deutzia and Mesodeutzia should be merged into a single monophyletic section (Deutzia/Mesodeutzia), thus reducing the number of sections to two.
Polyploidy has had an important role in the evolution and divergence of angiosperms (Soltis et al., 2015; Wendel, 2015). Repeated cycles of whole genome duplication (sometimes coupled with hybridization) can lead to reproductive isolation, genomic rearrangements, enzymatic multiplicity, epigenetic changes, and novel phenotypes that contribute to biodiversity and speciation (Adams and Wendel, 2005; Chen and Ni, 2006; Chen and Yu, 2013; Hegarty and Hiscock, 2008; Laport and Ng, 2017; Madlung, 2013). For plant breeders, knowledge of ploidy is particularly important because it influences reproductive compatibility, fertility of hybrids, and gene expression (Ranney, 2006). There have been numerous cytological studies of Deutzia, beginning with Sax (1931) at the Arnold Arboretum (Boston, MA). Compared with other genera within Philadelpheae, Deutzia exhibits extreme variability in ploidy, with 2n = 2x = 26 to 2n = 14x = 182 (Table 1), possibly with ploidy variations within species. The other speciose genus in this tribe, Philadelphus, has a similar number of species (≈65) as Deutzia (Dirr, 2009). However, unlike Deutzia, polyploidy is not found in Philadelphus. The greater ploidy variation in Deutzia may contribute to its higher degree of morphological variation when compared with other genera in this tribe (Sax, 1931). Both Deutzia and Philadelphus, as well as Fendlerella, also of Philadelpheae, have a base chromosome number of x = 13 (Sax, 1931; Ward, 1984).
Previous cytological reports for Deutzia species.
Genome size (DNA content) can reflect biodiversity, genome evolution, and taxonomic relationships (Laport and Ng, 2017; Ranney et al., 2018; Rounsaville and Ranney, 2010; Soltis et al., 2015). Genome size data can also be used to estimate ploidy in closely related taxa when properly calibrated with known cytological standards (Jones et al., 2007; Lattier et al., 2014; Parris et al., 2010; Rounsaville and Ranney, 2010; Shearer and Ranney, 2013). A previous report of the genome size of Deutzia was the first report of genome size of Hydrangeaceae. Using Feulgen densitometry, Hanson et al. (2001) determined that the 1Cx genome size of D. prunifolia is 1.9 pg. The more recent development of flow cytometry has provided a more accurate and efficient method of determining genome size (Doležel et al., 2007). We are not aware of other reports of genome size of Deutzia using flow cytometry.
Despite the extensive use and wide cultivation of Deutzia, correct identification of the species is challenging and problematic, and differentiation between species is often subtle (Dirr, 2009). With more than 60 species of Deutzia, it is possible that many species and cultivars have been traded under incorrect names for years. However, an excellent key for Deutzia that provides clear distinctions among taxa was developed by Zaikonnikova (1975).
The broad genetic diversity and array of desirable commercial traits of Deutzia provide a valuable base for further breeding and development of future cultivars. However, confusion regarding the proper identity and lack of information regarding cytogenetics of particular accessions and cultivars constrain the development of informed breeding strategies. The objectives of this study were to validate the identification and determine genome sizes and estimated ploidy of an extensive collection of Deutzia species, hybrids, and cultivars.
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