The american elm, Ulmus americana (Ulmaceae), is a tall, graceful tree that is native to the eastern United States and adjacent Canada (Bey, 1990). The species has frequently been planted as an ornamental and shade tree throughout temperate parts of the world, but use of the tree as an ornamental has been heavily limited by dutch elm disease, caused by two exotic wilt fungi, Ophiostoma ulmi (Buisman) C Nannf. and O. novo-ulmi Brasier (Brasier, 1991, 2001). Recent screening and breeding work has produced several cultivars that tolerate the disease (Townsend et al., 2005), and work is underway to breed this resistance into a wider variety of genetic backgrounds, with the aim of restoring american elm as an important and useful in both urban and forested landscapes (Pinchot et al., 2017).
Most elm species are diploid, with 2n = 28 (Santamour and Ware, 1997). Polyploids are known in only one elm species, U. americana, with diploid and tetraploid populations known in the wild (Whittemore and Olsen, 2011; Whittemore and Xia, 2017) and two triploids known in cultivation (Santamour and Bentz, 1995; Sherald et al., 1994). Understanding the complex genetics of american elm is important if breeding and selection work is to be carried out efficiently.
The cultivated triploid NPS 3-487 (released commercially as U. americana ‘Jefferson’) was studied by Sherald et al. (1994), who found that 17% of the fruit on the triploid contained viable seed. This figure is low compared with tetraploid U. americana they studied, in which 89% of the fruit contained viable seed, but it is an unusually high figure for a sexually reproducing triploid. This tree is particularly interesting from a horticultural standpoint because it is known to show very high levels of tolerance to dutch elm disease (Townsend et al., 2005). In the course of surveying wild U. americana, a natural pentaploid was found in western Nebraska (discussed subsequently), and this also showed substantial seed set.
Spontaneous triploids and other odd polyploids are known in many groups of plants, but they are normally sexually sterile or nearly so (Grant, 1981). However, some triploids reproduce via apomixis (Whitton et al., 2008) or by unusual chromosomal systems that allow permanent propagation of odd ploidies (Grant, 1981). In cases where triploids set seed sexually, they can serve as a genetic bridge, allowing gene exchange between diploids and tetraploids, both in natural populations and in artificial plant breeding. Transfer of alleles between diploids and tetraploids has played an important evolutionary role in some taxa (Kim et al., 2008). The effectiveness of triploids as a bridge between diploids and tetraploids depends on the fertility of the triploid and the chromosomal complement of the backcrosses (Burton and Husband, 2000; Ramsey and Schemske, 1998). The meager literature on the chromosomal complements of backcrosses between triploids and plants with even ploidy numbers is reviewed by Ramsey and Schemske (1998). Although meiosis in triploids produces a preponderance of aneuploid pollen with ≈1.5 sets of chromosomes, many triploids produce progeny that are euploid or close to euploid. The progeny of triploids fertilized with pollen from tetraploids were predominantly tetraploid [sometimes with one extra or one missing chromosome (i.e., 2n = 4x ± 1)] in the majority of cases, but this varied greatly among taxa, covering the whole range from 100% tetraploid (in Aquilegia chrysantha A. Gray × flabellata Siebold & Zucc.) to 75% aneuploid (in Triticum durum Desf. × Aegilops longissima Schweinf. & Muschl.) (Ramsey and Schemske, 1998).
The progeny of the triploid american elm NPS 3-487 that were described by Sherald et al. (1994) no longer exist, but the original parent tree is alive on the National Mall in Washington, DC, and still producing heavy crops of seed. To test whether these seeds are the result of sexual reproduction and to estimate the chromosomal complement of the seeds, nuclear DNA content was estimated by flow cytometry on a large sample of its seeds. Representative U. americana from surrounding plantings were also analyzed to estimate the ploidy level of the pollen available for fertilizing the triploid. A small sample of seeds was obtained from a wild pentaploid elm from Nebraska, and two tetraploid trees close to it were also analyzed.
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