Ploidy Variation inFraxinusL. (Oleaceae) of Eastern North America: Genome Size Diversity and Taxonomy in a Suddenly Endangered Genus

Premise of research. Ash (Fraxinus spp.), once dominant forest trees in eastern North America, are now endangered by the rapid spread of the emerald ash borer. Fraxinus is the target of conservation programs, but sampling genetic resources for conservation requires a knowledge of the taxonomy in order to ensure that genetically distinct groups are being conserved, and this has been hampered by serious disagreements about the taxonomy of North American Fraxinus. Polyploidy has been important in speciation in the genus, so ploidy level determination 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 12 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 was found in several species of sect. Melioides. Taxonomic separation of polyploid F. biltmoreana from the diploid F. americana and the recognition of F. profunda as a valid species are both strongly supported, but the 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, wherein taxonomic concepts are still not well supported by the genetic data. Results indicate the importance of establishing an accurate taxonomy when undertaking conservation work. Conservation efforts in eastern North American Fraxinus are less likely to conserve the full scope of genetic diversity within the genus if only a few broadly defined species are recognized. It is critical that well-founded segregate species be taken into account.