The genus Ulmus L. (the elms) holds a preeminent place in North American and European horticulture. Ulmus spp. have served as iconic street and landscape trees in both of these continents (Campanella, 2003; Dunn, 2000). Elms have also served many other purposes in other Northern Hemisphere cultures (Heybroek, 2015). The genus consists of 20–40 species, widespread in the north temperate zone and extending south into tropical mountains in both hemispheres (Fu et al., 2004; Sherman-Broyles, 1997).
The closest relatives of Ulmus are three small genera native to temperate regions of the Northern Hemisphere, Zelkova Spach, Hemiptelea Planch., and Planera J. F. Gmel. (Wiegrefe et al., 1998). The genus Zelkova Spach, with five or six species disjunct across Eurasia (Denk and Grimm, 2005), has also become important in American horticulture. The other two genera have one species each. Hemiptelea davidii (Hance) Planch., native to northeastern China and Korea, is used as a small tree or clipped into a thorn hedge in China, but it is not used in American horticulture. Genotypes from Inner Mongolia, China, are noted for their red fall color (Deligen, 2006), but have not yet been introduced to the West. Planera aquatica J. F. Gmel., native to seasonally flooded riverbottoms in the southeastern United States (Godfrey, 1988), has been little used outside its native range, but it grows as a small tree in gardens and may be valuable for its tolerance of heat, flooding, and poorly drained soils.
Studies of chromosome number and structure have found very limited genomic divergence in the group. All members of these four genera that have been studied have chromosome numbers based on x = 14, with no aneuploid variation (Goldblatt and Johnson, 1979; Oginuma et al., 1990). Polyploidy is rare, and it is known from only two species. U. americana is known to include tetraploids with 2n = 56 (Karrfalt and Karnosky, 1975) as well as diploids (Whittemore and Olsen, 2011), and Hemiptelea davidii has two reported chromosome numbers, 2n = 56 (Fu et al., 1998) and 2n = 84 (Oginuma et al., 1990), presumably tetraploid and hexaploid numbers based on x = 14.
Elms in North America and Europe have suffered high mortality from two diseases: DED, caused by several species of fungi native to East Asia (Brasier, 1991, 2001), and elm yellows (elm phloem necrosis), caused by a phytoplasma (Mittempergher, 2000; Sinclair, 2000). Because of the horticultural importance of elms, there has been much interest in selection and breeding for disease tolerance in the genus (Dunn, 2000).
To date, elm breeding has mostly involved species native to Europe and North America, although the fungi that cause DED are native to eastern Asia; the North American and European elm species have low levels of resistance. Although Ulmus is most diverse in East Asia, with 21 species native to China alone (Fu et al., 2004), until recently, relatively little germplasm from this area has been available to Western breeders. The work of Fu et al. (2004), which reduces several of the names used in previous literature, including U. japonica (Rehder) Sarg. 1907 not Sieb. 1830, U. propinqua Koidz., and U. wilsoniana C.K. Schneid, to synonyms of U. davidiana var. japonica (Rehder) Nakai, indicates that the level of diversity is even lower than what was previously thought; thus, Warren (2000) lists four Asian elm species (U. japonica, U. parvifolia, U. pumila, and U. wilsoniana) that have contributed to commercial cultivars now available in the west, but using the taxonomy of Fu et al. (2004), this list includes only three valid species, U. davidiana var. japonica, U. parvifolia, and U. pumila, which are now recognized.
In the 1980s and 1990s, much new elm germplasm was introduced to North America from China through the efforts of the late George Ware (Ware, 1995). Chromosome counts for many of these introductions were published, but the plants were juvenile and not flowering at the time they were studied (Santamour and Ware, 1997). Most of these trees are now producing fruit and showing adult bark characteristics, both of which are important for identification. In addition, taxonomic treatments of the Chinese species have been published (Fu et al., 1998, 2004), allowing more accurate identifications of the Asian species. It has thus been possible to correct some misidentifications in Ware’s Asian germplasm.
Unfortunately, the relevance of this material to research and breeding on U. americana is uncertain. The difficulty of crossing tetraploid U. americana with other Ulmus spp. has often been attributed to ploidy differences, but Ager and Guries (1982) and Bob et al. (1986) demonstrate that crossing barriers between U. americana and several diploid elm species are not ploidy-related. Studies of interspecific hybridization in Ulmus have shown that different combinations of parents show different levels of compatibility (Hans, 1981; Townsend, 1975), but the planning of controlled breeding programs was limited in the past because traditional classifications of Ulmus did not seem to reflect relationships adequately (Hans, 1981).
The infrageneric classification of Ulmus has now been placed on a more solid footing by the work of Wiegrefe et al. (1994). All elms available to these authors were placed in two well-marked subgenera, Ulmus subg. Ulmus and Ulmus subg. Oreoptelea (Spach) Planch. This has presented a problem for the American elm breeders. U. americana belongs to subg. Oreoptelea, which is predominately North American (only a single Old World species, the European U. laevis, was placed here by Wiegrefe et al.), whereas all of the elm species studied by Wiegrefe et al. that are native to eastern Asia (the area where the fungi causing DED are native) belong to subg. Ulmus. Past attempts to breed resistance to DED from other elm species into U. americana have not met with success, but they involved crossing U. americana (of subg. Oreoptelea) with species of subg. Ulmus (especially U. pumila). Quite a few hybrid combinations have been reported to yield at least occasional fruit (Townsend, 1975), but the only hybrid cultivars that have been released are hybrids among the European and Asian species of subg. Ulmus (Warren, 2000), indicating limited genetic compatibility between the subgenera.
However, two Asian species that were not available to Wiegrefe et al. have been placed by some authors in sect. Chaetoptelea (Liebm.) C.K. Schneid., a group of species that falls within Ulmus subg. Oreoptelea as defined by Wiegrefe et al. (1994). If true, then sources of resistance genes that are more compatible with the North American elms may be available in the genus. The Himalayan species U. villosa was placed in this group by Grudzinskaya (1974) and Richens (1983), based on the characteristics of its fruit (its inflorescence resembles that of subg. Ulmus), whereas Fu et al. (1979, 1998) placed the Chinese species U. elongata in this group, based on the characteristics of its inflorescence and fruit. If these placements are correct, U. villosa and U. elongata would fall within Ulmus subg. Oreoptelea as defined by Wiegrefe et al. (1994), and these poorly known species would be the only close relatives of U. americana native to Asia, where DED is believed to have originated. In this case, it would be worth investigating them as possible sources of resistance genes that would be more compatible with the genetic background of U. americana and other species of subg. Oreoptelea than the species of subg. Ulmus, the only DED-resistant species that have been studied to date. Santamour (1979) presented evidence that U. villosa shows resistance to DED, but DED resistance has never been studied in U. elongata (Smalley and Guries, 2000). Finding other characteristics that distinguish the subgenera could confirm or refute the placement of these two species in subg. Oreoptelea, which in turn could provide direction for future study of how elm species respond to these diseases and for future elm breeding.
Flow cytometry provides a measure of nuclear DNA content and a method for examining genomic diversification that is much faster and easier than chromosome counts. Flow cytometry can be carried out on most tissues of the plant, and it provides a different view of diversity in a group of plants than chromosome number. A recent flow cytometry survey of U. americana (Whittemore and Olsen, 2011) revealed unexpected variation in the species. It is desirable to extend this work for several reasons. First, the study of Whittemore and Olsen quantified the DNA by staining with 4′,6-diamidino-2-phenylindole (DAPI) and calibrated the measurements with a single chromosome count. The staining reaction of DAPI is specific to AT bps, so it gives an accurate estimate of total DNA only if the percent AT in the genome is similar in the study organism and the internal standard (Dolezel et al., 2007a). This is not a problem if DNA is visualized using an intercalating dye such as propidium iodide (PI) rather than a stain. Carrying out flow cytometry using PI and calibrating the work using additional trees with known chromosome numbers will give us a firmer understanding of variation in genome size. In addition, a broad survey of nuclear DNA content in Ulmus and related genera using flow cytometry could provide more information on the distribution of natural polyploids, and reveal differences in genome size between the genera and subgenera. Knowledge of genome-size variation, in turn, can help to place species whose relationships are uncertain. In view of the importance of Ulmus and Zelkova in American horticulture, and especially the need to find disease-resistant germplasm, a broad survey of Ulmus and related genera using flow cytometry was conducted, emphasizing species of uncertain relationship, and germplasm that has only been recently introduced to American horticulture and not well characterized.
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