A Dutch elm disease resistant triploid elm

Abstract

A naturally occurring triploid elm hybrid was found in the American elm, Ulmusamericana L., planting on the National Mall in Washington, D.C. Chromosome examinations of mitosis in root tips and meiosis in pollen mother cells showed a chromosome complement of 2n = 3x = 42. The chromosome alignment at meiotic metaphase I was predominantly 14 bivalents and 14 univalents, indicating that one parent was an American elm, which contributed the bivalents through autosyndetic pairing. When DNA underwent restriction digestion with HindIII and probed with a 9.5-kilobase cloned ribosomal DNA fragment from pea, Pisumsativum L., the triploid hybrid showed a band not found in American elms. Seed from the open-pollinated parent tree had low viability and seedlings were highly variable in height, leaf size, and shape. The small leaves of some seedlings suggest that the other parent was a species with leaves smaller than American elm. The hybrid was found to be resistant to Ophiostomaulmi (Buis.) Nannf. When twig crotches were inoculated, only 14% developed wilt compared with 63% in the American elm. None of the 22 hybrid trees inoculated developed systemic wilt compared with 8 of the 18 American elms inoculated. The hybrid, which is easily propagated by softwood cuttings, has rapid growth, good crown structure, and many characteristics of the American elm.

Ulmus americana (Ulmaceae) is a polyploid complex

“…Thirty‐six of these species are diploid (2 n = 2 x = 28), whereas U. americana has generally been reported to be tetraploid (2 n = 4 x = 56; Santamour, 1993; Santamour and Ware, 1997). Triploid counts have been reported twice for U. americana , both times from cultivated trees growing on the National Mall in Washington, D.C. (Sherald et al, 1994; Santamour and Bentz, 1995). Sherald et al (1994) provided a detailed analysis of one of these trees, NPS 3‐487 (since released as a commercial cultivar, U. americana ‘Jefferson’).…”

“…Triploid counts have been reported twice for U. americana , both times from cultivated trees growing on the National Mall in Washington, D.C. (Sherald et al, 1994; Santamour and Bentz, 1995). Sherald et al (1994) provided a detailed analysis of one of these trees, NPS 3‐487 (since released as a commercial cultivar, U. americana ‘Jefferson’). They found that meiosis in this tree is characterized by formation of 14 bivalents and 14 univalents, and they showed that Hind III‐digested ribosomal RNA of the triploid showed two bands, one shared with typical tetraploid U. americana and one unique band.…”

Defence responses in the xylem of Ulmus americana cultivars after inoculation with Ophiostoma novo‐ulmi

“…While most American elms are tetraploid, diploid and triploid specimens have been identified (Whittemore & Olsen, ). In a study examining the triploid American elm “Jefferson,” Sherald, Santamour, Hajela, Hajela and Sticklen () found “Jefferson” had higher levels of resistance to DED compared with wild‐type American elms. Ploidy levels have been shown to have an impact on xylem characteristics in other plant species (Hao, Lucero, Sanderson, Zacharias & Holbrook, ; Maherali, Walden & Husband, ), which could help explain the differences in resistance.…”

“…While most American elms are tetraploid, diploid and triploid specimens have been identified (Whittemore & Olsen, 2011). In a study examining the triploid American elm "Jefferson," Sherald, Santamour, Hajela, Hajela and Sticklen (1994)…”

Proceedings of the American elm restoration workshop 2016

“…Tolerance to DED tested by twig-crotch inoculations (Sherald et al 1994) and stem inoculations (Townsend et al 2005). …”