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Genome size variation can be used to investigate biodiversity, genome evolution, and taxonomic relationships among related taxa. Plant breeders use genome size variation to identify parents useful for breeding sterile or improved ornamentals. Lilacs (Syringa) are deciduous trees and shrubs valued for their fragrant spring and summer flowers. The genus is divided into six series: Syringa (Vulgares), Pinnatifoliae, Ligustrae, Ligustrina, Pubescentes, and Villosae. Reports conflict on genome evolution, base chromosome number, and polyploidy in lilac. The purpose of this study was to investigate genome size and ploidy variation across a diverse collection. Flow cytometry was used to estimate monoploid (1Cx) and holoploid (2C) genome sizes in series, species, cultivars, and seedlings from parents with three ploidy combinations: 2x x 2x, 2x x 3x, and 3x x 2x. Pollen diameter was measured to investigate the frequency of unreduced gametes in diploid and triploid Syringa vulgaris cultivars. Three triploids of S. vulgaris were observed: ‘Aucubaefolia’, ‘Agincourt Beauty’, and ‘President Gr evy’. Across taxa, significant variations in 1Cx genome size were discovered. The smallest and largest values were found in the interspecific hybrids S. ·laciniata (1.32 ± 0.04 pg) and S. ·hyacinthiflora ‘Old Glory’ (1.78 ± 0.05), both of which are in series Syringa. Series Syringa (1.68 ± 0.02 pg) had a significantly larger 1Cx genome size than the other series. No significant differences were found within series Pubescentes (1.47 ± 0.01 pg), Villosae (1.55 ± 0.02 pg), Ligustrina (1.49 ± 0.05 pg), and Pinnatifoliae (1.52 ± 0.02 pg). For S. vulgaris crosses, no significant variation in 2C genome size was discovered in 2x x 2x crosses. Interploid crosses between ‘Blue Skies’ (2x) and ‘President Gr evy’ (3x) produced an aneuploid population with variable 2C genome sizes ranging from 3.41 ± 0.03 to 4.35 ± 0.03 pg. Only one viable seedling was recovered from a cross combination between ‘President Gr evy’ (3x) and ‘Sensation’ (2x). This seedling had a larger 2C genome size (5.65 ± 0.02 pg) than either parent and the largest 2C genome size currently reported in lilac. ‘Sensation’ produced 8.5% unreduced pollen, which we inferred was responsible for the increased genome size. No unreduced pollen was discovered in the other diploids examined. Increased ploidy may provide a mechanism for recovering progeny from incompatible taxa in lilac breeding. Genome size variation can be used to investigate biodiversity, taxonomic relationships, and genome evolution among related taxa (Greilhuber, 1998; Rounsaville and Ranney, 2010; Shearer and Ranney, 2013; Zonneveld and Duncan, 2010; Zonneveld et al., 2005). Studies on genome evolution focus on large, structural changes in sequence or fluctuations in genome size resulting from natural phenomena including polyploidy, chromosome fission/fusion, and interploid hybridization (Soltis and Soltis, 2012). Genome size variation can also be used by plant breeders to identify parents for wide hybrids among parent taxa. Interspecific hybrids have been shown to have genome sizes intermediate between their parents in other woody ornamentals such as Cornus (Shearer and Ranney, 2013) and Magnolia (Parris et al., 2010). When combining genome sizes with their corresponding chromosome counts, genome size data can be used to discover ploidy variation among related taxa (Contreras et al., 2013; Lattier, 2016; Parris et al., 2010; Shearer and Ranney, 2013). Polyploidy, or whole genome duplication, is a driving force in evolution and occurs naturally through somatic mutations in meristematic cells and through unreduced gametes (Harlan and deWet, 1975; Ranney, 2006). There are two broad categories of polyploidy; autopolyploidy is the duplication of a single genome, whereas allopolyploidy is the combination of two or more different genomes and an associated duplication event (Chen and Ni, 2006). The identification and induction of polyploidy can be valuable tools for plant breeding. Irregular meiosis in gametes can result in sterility, whereas ‘‘gigas’’ effects of somatic cells can result in thicker, glossier cuticles, enlarged flowers, or enlarged fruit (Ranney, 2006). In addition, polyploids have been used to overcome interploid hybridization barriers (Ranney, 2006) and to restore fertility in wide hybrids of ornamentals such as Rhododendron ‘Fragrant Affinity’ and ·Chitalpa tashkentensis (Contreras et al., 2007; Olsen et al., 2006). Syringa is a genus of deciduous, woody trees, and shrubs grown primarily for their heavy spring and summer blooms of fragrant flowers. Syringa comprised 21–28 species that are part of the monophyletic subfamily Oleoideae in family Oleaceae and are closely allied with Ligustrum (Li et al., 2002;Wallander and Albert, 2000). Recent taxonomic studies divide the genus into six series: Syringa, Pinnatifoliae, Ligustrae, Ligustrina, Pubescentes, and Villosae (Li et al., 2012). Most species are native to eastern Asia while two species, S. vulgaris and S. josikaea, are native to southeastern Europe (Kim and Jansen, 1998). Most cultivar development over centuries of breeding has focused on improvements of common lilac (S. vulgaris) within series Syringa. Lilacs have proven to be important ornamental crops, yet little is known about how nuclear genome varies among series, species, hybrids, and cultivars. A survey of genome Received for publication 14 June 2017. Accepted for publication 13 July 2017. This research was funded in part by the Oregon Department of Agriculture. We acknowledge the assistance of Mara Friddle, Kim Shearer, and Aleen Haddad in this research. Graduate Research Assistant. Corresponding author. E-mail: ryan.contreras@oregonstate.edu. J. AMER. SOC. HORT. SCI. 142(5):355–366. 2017. 355 size (C-value) and ploidy level within Syringa would contribute to the call for a global census of angiosperm C-values (Galbraith et al., 2011). Although genome sequencing is a powerful tool for studying gene function, C-values calculated from sequencing data tend to underestimate true genome size (relative to flow cytometry) because of misassembly and the inability to sequence through repetitive regions of the genome (Bennett and Leitch, 2011). Flow cytometry measurements of genome size have proven useful for the identification of species, hybrids, polyploids, and polyploid series (Galbraith et al., 2011). In genera such as lilac with a long history of breeding and cultivation, variation in genome size and chromosome number can occur from interspecific hybridization, unreduced gametes, and the induction of autopolyploids. Interspecific hybridization has been a valuable tool for producing many new cultivars of lilac (Table 1). Two reports on genome size estimates in lilac focused on two European species, S. vulgaris and S. josikaea. Siljak-Yakovlev et al. (2010) reported S. vulgaris to have a 2C genome size of 2.4 pg based on propidium iodide flow cytometry. Olszewska and Osiecka (1984) reported S. josikaea to have a 2C genome size of 2.6 pg based on Feulgen cytophotometry. Despite the paucity of genome size estimates in lilac, much effort has been dedicated to studying chromosome number variation in lilac and the Oleaceae. Phylogenetic analysis has determined the ancestral state of the Oleaceae to be diploid (Taylor, 1945). Cyto-taxonomy divides the Oleaceae into two groups according to basic chromosome number with the first group consisting ofMendora (x = 11), Jasminum (x = 13), Fontanesia (x = 13), Forsythia (x = 14), and Abeliophyllum (x = 14). The second group (originally designated as subfamily Oleoideae) consists of genera with x= 23, includingOlea, Syringa,Ligustrum,Fraxinus,Osmanthus, Forestiera, Phillyrea, Osmarea, and Chionanthus (Taylor, 1945). Lilacs are primarily diploids with basic chromosome numbers reported at x = 22, 23, or 24 (Darlington and Wylie, 1956). Sax (1930) reported the ‘‘fundamental’’ chromosome number in lilac to be x = 12 and hypothesized that ancestral polyploidization of an x = 11 or x = 12 cytotype was responsible for the variation in chromosome numbers, with the x = 23 cytotype resulting from the loss of a pair of chromosomes. By contrast, Taylor (1945) reported that most wild-type lilac specimens are x = 23 cytotypes, not x = 22 or x = 24. The prevalence and stability of the x = 23 cytotype throughout the Oleaceae, illustrated by Taylor (1945) and Stebbins (1940), indicates that the x = 23 cytotype likely predates other cytotypes in Syringa and originated as the result of allopolyploidy between ancestral Oleaceae taxa of two cytotypes, x = 11 and x = 12. Therefore, the variation in chromosome number observed in common lilac is likely the result of aneuploidy over centuries of plant collection and wide hybridization. Aside from theories of ancestral allopolyploidy, no reports exist to confirm polyploidy in wild or cultivated lilac populations. In addition, no reports of natural polyploidy exist for the closely related genus Ligustrum. However, natural polyploidy has been discovered in other related genera in Oleaceae. Taylor (1945) reported tetraploids in Mendora, tetraploids and triploids in Jasminum, tetraploids and hexaploids in Fraxinus, and hexaploids in Osmanthus. In white ash (Fraxinus), the tetraploid F. smallii and hexaploids, such as F. biltmoreana and F. profunda, are hypothesized to have allopolyploid origins (Miller, 1955; Nesom, 2010; Santamour, 1962). Early efforts producing artificial polyploids in lilac were reported to be successful. In the middle of the 20th century, Karl Sax produced colchicine-induced autopolyploids of S. vulgaris at the Arnold Arboretum (Fiala and Vrugtman, 2008). Fiala reportedly produced tetraploid forms of S. julianae, S. komarowii, S. ·prestoniae, S. wolfii, S. yunnanensis, S. vulgaris, S. oblata, and S. ·hyacinthiflora (Fiala and Vrugtman, 2008). Despite these previous reports of induced polyploidy, no cytological evidence exist

Ploidy and Genome Size in Lilac Species, Cultivars, and Interploid Hybrids