HORTSCIENCE 52(4):547–553. 2017. doi: 10.21273/HORTSCI11432-16 Johnson, 1979; Oginuma et al., 1990). Poly- ploidy is rare, and it is known from only two species. U. americana is known to include Genome Size Variation in Elms (Ulmus tetraploids with 2n = 56 (Karrfalt and Karnosky, 1975) as well as diploids (Whittemore and spp.) and Related Genera Olsen, 2011), and Hemiptelea davidii has two 1 reported chromosome numbers, 2n =56(Fu Alan T. Whittemore et al., 1998) and 2n = 84 (Oginuma et al., 1990), U.S. National Arboretum, 3501 New York Avenue NE, Washington, DC presumably tetraploid and hexaploid numbers 20002-1958 based on x = 14. Elms in North America and Europe have Zheng-Lian Xia suffered high mortality from two diseases: U.S. National Arboretum, Room 124, Building 010A, BARC-West, 10300 DED, caused by several species of fungi Baltimore Avenue, Beltsville, MD 20705 native to East Asia (Brasier, 1991, 2001), and elm yellows (elm phloem necrosis), Additional index words. DNA content, elm, flow cytometry, Hemiptelea, Planera, Ulmus, caused by a phytoplasma (Mittempergher, Zelkova 2000; Sinclair, 2000). Because of the horti- cultural importance of elms, there has been Abstract. Elms (Ulmus spp.) are iconic street and landscape trees, but their use is much interest in selection and breeding for currently limited by susceptibility to disease, especially Dutch elm disease (DED). disease tolerance in the genus (Dunn, 2000). Improved access to disease-resistant germplasm will be of great benefit for ongoing To date, elm breeding has mostly in- breeding and selection programs, but these programs have been limited historically by volved species native to Europe and North uncertain relationships among Ulmus species, especially the North American species America, although the fungi that cause DED and their putative Old World relatives. Estimates of genome size from 28 species are native to eastern Asia; the North Amer- representing both subgenera of Ulmus (subg. Ulmus and subg. Oreoptelea)andsix ican and European elm species have low species in the related small genera Zelkova, Hemiptelea,andPlanera were estimated levels of resistance. Although Ulmus is most using flow cytometry. Genome-size estimates were calibrated using seven elms with diverse in East Asia, with 21 species native known chromosome counts. Results strongly supported the subgeneric classification of to China alone (Fu et al., 2004), until re- Wiegrefe et al. Monoploid genome size was found to be quite constant within the cently, relatively little germplasm from this subgenera of Ulmus they recognized and within the small genera, and polyploidy is area has been available to Western breeders. uncommon in these plants. However, there are consistent differences in genome size The work of Fu et al. (2004), which reduces between the subgenera of Ulmus and between them and the smaller genera, and these several of the names used in previous liter- differences can be used to place species in their proper taxon, knowledge which can be ature, including U. japonica (Rehder) Sarg. useful in identifying disease-resistant germplasm that may be compatible with Ulmus 1907 not Sieb. 1830, U. propinqua Koidz., americana and other North American taxa. Two Asian species that have sometimes and U. wilsoniana C.K. Schneid, to syno- been considered to be related to North American species now placed in subg. Oreoptelea nyms of U. davidiana var. japonica (Rehder) were tested. The Himalayan Ulmus villosa has a much smaller genome than either of the Nakai, indicates that the level of diversity is subgenera, indicating that its relationship with other elms is rather remote. It may be even lower than what was previously thought; a source of novel genes in Ulmus, but our results indicate it is not close to U. americana thus, Warren (2000) lists four Asian elm or other New World species. In contrast, results from the rare Chinese species Ulmus species (U. japonica, U. parvifolia, U. pumila, elongata support its placement in subg. Oreoptelea. It is the only close relative of the and U. wilsoniana) that have contributed to North American elms that is native to Asia, where DED is believed to have originated, commercial cultivars now available in the and its response to DED infection should be evaluated. west, but using the taxonomy of Fu et al. (2004), this list includes only three valid The genus Ulmus L. (the elms) holds the Northern Hemisphere, Zelkova Spach, species, U. davidiana var. japonica, U. a preeminent place in North American and Hemiptelea Planch., and Planera J. F. Gmel. parvifolia,andU. pumila, which are now European horticulture. Ulmus spp. have (Wiegrefe et al., 1998). The genus Zelkova recognized. served as iconic street and landscape trees Spach, with five or six species disjunct across In the 1980s and 1990s, much new elm in both of these continents (Campanella, Eurasia (Denk and Grimm, 2005), has also germplasm was introduced to North America 2003; Dunn, 2000). Elms have also served become important in American horticulture. from China through the efforts of the late many other purposes in other Northern Hemi- The other two genera have one species each. George Ware (Ware, 1995). Chromosome sphere cultures (Heybroek, 2015). The genus Hemiptelea davidii (Hance) Planch., native counts for many of these introductions were consists of 20–40 species, widespread in the to northeastern China and Korea, is used as published, but the plants were juvenile and north temperate zone and extending south a small tree or clipped into a thorn hedge in not flowering at the time they were studied into tropical mountains in both hemispheres China, but it is not used in American horti- (Santamour and Ware, 1997). Most of these (Fu et al., 2004; Sherman-Broyles, 1997). culture. Genotypes from Inner Mongolia, trees are now producing fruit and showing The closest relatives of Ulmus are three China, are noted for their red fall color adult bark characteristics, both of which are small genera native to temperate regions of (Deligen, 2006), but have not yet been in- important for identification. In addition, tax- troduced to the West. Planera aquatica J. F. onomic treatments of the Chinese species Gmel., native to seasonally flooded river- have been published (Fu et al., 1998, 2004), bottoms in the southeastern United States allowing more accurate identifications of the Received for publication 1 Nov. 2016. Accepted (Godfrey, 1988), has been little used outside Asian species. It has thus been possible to for publication 21 Feb. 2017. its native range, but it grows as a small tree in correct some misidentifications in Ware’s We thank Michael S. Dosmann and Kathryn gardens and may be valuable for its tolerance Asian germplasm. Richardson (Arnold Arboretum, Jamaica Plain, of heat, flooding, and poorly drained soils. Unfortunately, the relevance of this ma- MA) and Matt Lobdell, Kris Bachtell, and Marlene Studies of chromosome number and terial to research and breeding on U. ameri- Hahn (Morton Arboretum, Lisle, IL) for providing plant material for analysis from their living collec- structure have found very limited genomic cana is uncertain. The difficulty of crossing tions. Susan Bentz and Kevin Conrad provided divergence in the group. All members of tetraploid U. americana with other Ulmus assistance in various ways. these four genera that have been studied spp. has often been attributed to ploidy 1Corresponding author. E-mail: Alan.Whittemore@ have chromosome numbers based on x =14, differences, but Ager and Guries (1982) and ars.usda.gov. with no aneuploid variation (Goldblatt and Bob et al. (1986) demonstrate that crossing HORTSCIENCE VOL. 52(4) APRIL 2017 547 barriers between U. americana and several Ulmus), whereas Fu et al. (1979, 1998) a stain. Carrying out flow cytometry using PI diploid elm species are not ploidy-related. placed the Chinese species U. elongata in and calibrating the work using additional Studies of interspecific hybridization in this group, based on the characteristics of its trees with known chromosome numbers will Ulmus have shown that different combina- inflorescence and fruit. If these placements give us a firmer understanding of variation tions of parents show different levels of are correct, U. villosa and U. elongata would in genome size. In addition, a broad survey compatibility (Hans, 1981; Townsend, 1975), fall within Ulmus subg. Oreoptelea as de- of nuclear DNA content in Ulmus and re- but the planning of controlled breeding pro- fined by Wiegrefe et al. (1994), and these lated genera using flow cytometry could grams was limited in the past because tradi- poorly known species would be the only provide more information on the distribu- tional classifications of Ulmus did not seem close relatives of U. americana native to tion of natural polyploids, and reveal differ- to reflect relationships adequately (Hans, Asia, where DED is believed to have origi- ences in genome size between the genera 1981). nated. In this case, it would be worth in- and subgenera. Knowledge of genome-size The infrageneric classification of Ulmus vestigating them as possible sources of variation, in turn, can help to place species has now been placed on a more solid footing resistance genes that would be more compat- whose relationships are uncertain. In view by the work of Wiegrefe et al. (1994). All ible with the genetic background of U. of the importance of Ulmus and Zelkova in elms available to these authors were placed in americana and other species of subg. Oreop- American horticulture, and especially the two well-marked subgenera, Ulmus subg. telea than the species of subg. Ulmus, the need to find disease-resistant germplasm, Ulmus and Ulmus subg. Oreoptelea (Spach) only DED-resistant species that have been abroadsurveyofUlmus and related genera Planch. This has presented a problem for the studied to date. Santamour (1979) presented using flow cytometry was conducted, em- American elm breeders.