Dogwoods include more than 50 species of shrubs, small trees, and a few herbaceous perennials with distribution that ranges across the northern hemisphere and rarely into the southern hemisphere (Eyde, 1988; Fan and Xiang, 2001; Reed, 2004; Xiang et al., 2006). Many of these species are valuable landscape plants and are frequently cultivated for their four-season attributes including attractive flowers, fruit, foliage, bark, and form (Cappiello and Shadow, 2005). According to the Census of Horticultural Specialties (USDA-NASS, 2010), 2009 sales of dogwoods totaled more than $30.9 million in the United States.
Considerable research has focused on determining the systematic relationships among dogwoods (Fan and Xiang, 2001, 2003; Xiang et al., 1993, 1996, 1998, 2002, 2006). A species-level phylogeny completed by Xiang et al. (2006), supported major clades within the genus including the big-bracted dogwoods (BB) comprising subgenus Cynoxylon and Syncarpea, the dwarf dogwoods (DW) comprising subgenus Arctocrania, and the cornelian cherries (CC) comprising subgenus Cornus.
Polyploidy, also referred to as whole genome duplication, has played a significant role in the evolution and diversification of angiosperms (Soltis and Burleigh, 2009; Soltis et al., 2004, 2009). Polyploidization may lead to reproductive isolation, rapid genomic rearrangements, novel patterns of gene expression, and adaptations, which can ultimately lead to divergence and speciation (Adams and Wendel, 2005; Comai, 2005; Hegarty and Hiscock, 2008; Soltis and Burleigh, 2009). Knowledge of ploidy levels is important for plant breeders because it can influence fertility, crossability, segregation, and gene expression (Chen and Ni, 2006; Soltis et al., 2004). Cytological studies on dogwoods have found the base chromosome number for BB to be x = 11, for CC to be x = nine or 10, and for DW to be x = 11 with both diploids and tetraploids found in the DW (Dermen, 1932; Goldblatt, 1978; Xiang and Eyde, 1995; Xiang et al., 2006). Chromosome counts for C. canadensis (DW) vary and include diploid (2n = 2x = 22) (Murrell, 1994; Packer, 1964; Zhang et al., 2008) and tetraploid (2n = 4x = 44) (Dermen, 1932; Löve and Löve, 1982) assessments.
Genome size data reflect fundamental biodiversity characters and can be reflective of genome evolution and taxonomic relationships (Greilhuber, 1998; Rounsaville and Ranney, 2010; Zonneveld and Duncan, 2010; Zonneveld et al., 2005). Genome size data can also be used to determine ploidy levels among closely allied species when properly calibrated (Jones et al., 2007; Palmer et al., 2009; Parris et al., 2010; Rounsaville and Ranney, 2010). In some cases, when parents vary considerably in genome sizes, hybrids can be verified based on intermediate values (Galbraith et al., 2005; Keller et al., 1996; Parris et al., 2010).
Despite the considerable breeding, selection, and widespread cultivation of dogwoods, sampling for genome size and ploidy level of species, cultivars, and hybrids has been limited and little is known concerning ploidy level and genome size of specific cultivars and hybrids. The objectives of this study were to determine relative genome sizes and ploidy levels of diverse taxa within Cornus, specifically for the BB, CC, and DW clades.
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