Seeds from two odd-polyploid Ulmus americana L., both open-pollinated from surrounding tetraploid trees, were analyzed for nuclear genome size by using flow cytometry. Seeds from the triploid were predominantly aneuploid, with DNA content intermediate between triploid and tetraploid levels, but substantial numbers of tetraploid and pentaploid seeds were observed. No seeds of even ploidy were found in progeny of the pentaploid; seeds of the pentaploid were pentaploid, aneuploid with DNA content intermediate between tetraploid and pentaploid levels, or aneuploid with DNA content intermediate between pentaploid and hexaploid levels. Seeds from both trees often gave two peaks via flow cytometry, indicating the presence of two genetically distinct embryos in the same seed. The frequency of polyembryony in the sample is much higher than the frequency of seeds that yield multiple seedlings, suggesting that the formation of two genetically distinct embryos, followed by abortion of one embryo, is common in elms.
Elms (Ulmus spp.) are iconic street and landscape trees, but their use is currently limited by susceptibility to disease, especially Dutch elm disease (DED). Improved access to disease-resistant germplasm will be of great benefit for ongoing breeding and selection programs, but these programs have been limited historically by uncertain relationships among Ulmus species, especially the North American species and their putative Old World relatives. Estimates of genome size from 28 species representing both subgenera of Ulmus (subg. Ulmus and subg. Oreoptelea) and six species in the related small genera Zelkova, Hemiptelea, and Planera were estimated using flow cytometry. Genome-size estimates were calibrated using seven elms with known chromosome counts. Results strongly supported the subgeneric classification of Wiegrefe et al. Monoploid genome size was found to be quite constant within the subgenera of Ulmus they recognized and within the small genera, and polyploidy is uncommon in these plants. However, there are consistent differences in genome size between the subgenera of Ulmus and between them and the smaller genera, and these differences can be used to place species in their proper taxon, knowledge which can be useful in identifying disease-resistant germplasm that may be compatible with Ulmus americana and other North American taxa. Two Asian species that have sometimes been considered to be related to North American species now placed in subg. Oreoptelea were tested. The Himalayan Ulmus villosa has a much smaller genome than either of the subgenera, indicating that its relationship with other elms is rather remote. It may be a source of novel genes in Ulmus, but our results indicate it is not close to U. americana or other New World species. In contrast, results from the rare Chinese species Ulmus elongata support its placement in subg. Oreoptelea. It is the only close relative of the North American elms that is native to Asia, where DED is believed to have originated, and its response to DED infection should be evaluated.