Ploidy Variation in Fraxinus L. (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...