Ploidy of seeds from odd-polyploid American elms (Ulmus americana)

Authors: Whittemore, Alan; Xia, Zheng-Lian

Submitted to: Journal of the American Society for Horticultural Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/31/2020
Publication Date: 4/4/2020
Citation: Whittemore, A.T., Xia, Z. 2020. Ploidy of seeds from odd-polyploid American elms (Ulmus americana). Journal of the American Society for Horticultural Science. 145:186-192. https://doi.org/10.21273/JASHS04828-19.
DOI: https://doi.org/10.21273/JASHS04828-19

Interpretive Summary: Plant breeding often seeks to transfer desirable genes between related species. It is difficult to cross such species successfully if the two species have different chromosome numbers because the hybrid will have an unbalanced chromosome set. In order to facilitate breeding of American elm trees with different chromosome numbers, ARS scientists at the U.S. National Arboretum examined the offspring of a triploid American elm to test its fertility and determine chromosome inheritance. This triploid showed a surprisingly high level of fertility, and the results suggest that the extra (unpaired) chromosomes are distributed randomly among the gametes. They also found evidence of multiple embryos in some seeds, resulting from independent fertilizations. These results will help plant breeders develop strategies for transferring desirable genes between diploid and tetraploid American elm, to allow desirable characters from the two populations to be combined in one cultivar.

Technical Abstract: Seeds from two odd-polyploid Ulmus americana, both open-pollinated from surrounding tetraploid trees, were analyzed for nuclear genome size using flow cytometry. Seeds from the triploid were predominantly aneuploid, with DNA content intermediate between triploid and tetraploid levels, but significant 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 with 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.