|CAMPBELL, JULIAN - Collaborator
|CARLSON, CRAIG - Cornell University
|ATHA, DAVID - New York Botanical Garden
Submitted to: International Journal of Plant Sciences
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/11/2017
Publication Date: 5/30/2018
Citation: Whittemore, A.T., Campbell, J.J., Xia, Z., Carlson, C.H., Atha, D., Olsen, R.T. 2018. Ploidy variation in Fraxinus L. (Oleaceae) of eastern North America. International Journal of Plant Sciences. 179(5):377-389. Available: https://www.journals.uchicago.edu/doi/10.1086/696688.
Interpretive Summary: The North American ashes (the genus Fraxinus) are one of the most important tree crops for the $4.7-billion per year nursery industry. However, this native tree is being lost rapidly due to the spread of Emerald Ash Borer, an insect pest recently introduced from Asia. Efforts to conserve ash tree genetic resources have been hampered by uncertain taxonomic classification, with different authors recognizing as few as five or as many as twelve species in eastern North America. Since speciation in Fraxinus often involves polyploidy (i.e. acquiring extra chromosome sets), we carried out a broad survey of genome size in native ash by sampling 303 wild ash collections, representing all twelve putative species across much of the eastern United States. Our survey validated some controversial native ash species, and suggests that further work is needed in some groups before a stable, accurate classification can be produced. This work has allowed us to identify ash populations that have not been used in horticulture, but which show economic promise and need to be conserved while strategies for controlling the borer are identified. These results will be used by taxonomists, geneticists, conservation organizations, and tree breeders working on ash.
Technical Abstract: Premise of the Research Ash (Fraxinus spp.), once a dominant tree genus in eastern North America, is now endangered by the rapid spread of Emerald Ash Borer. Conservation efforts have been hampered by serious disagreements about the taxonomy of North American Fraxinus. Polyploidy has been important in speciation in the genus, so ploidy level can help to establish natural species boundaries in the group. Methodology Flow cytometry, calibrated with known chromosome counts, was used to assess genome size and infer ploidy for 303 accessions of Fraxinus spp., representing all twelve putative species recognized in the United States east of the hundredth meridian. Pivotal Results Monoploid genome size varied from 1Cx=0.725 for F. quadrangulata (sect. Dipetalae) to 1Cx=0.925 pg (sect. Melioides), and polyploidy is found in several species of sect. Melioides. Taxonomic separation of polyploid F. biltmoreana from the diploid F. americana and recognition of F. profunda as a valid species are both strongly supported, but results do not support separation of glabrous plants from F. biltmoreana as F. smallii. Both diploid and tetraploid populations were found in F. caroliniana. Fraxinus profunda proved to be octaploid, not hexaploid as previously reported. Conclusions, further work is needed on F. biltmoreana, F. caroliniana, and F. pauciflora, where taxonomic concepts are still not well supported by the genetic data. Results point out the importance of establishing an accurate taxonomy when undertaking conservation work. Conservation efforts in eastern North American Fraxinus will not conserve the genetic diversity if only a few broadly-defined species are recognized, but well-founded segregate species must be taken into account.