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Dogwoods (Cornus spp.) are valuable nursery crops grown as landscape plants throughout much of the world. Although there has been considerable work on breeding and selecting dogwoods, there is little information available on genome sizes (DNA content) and ploidy levels within the genus, particularly for specific clones and cultivars. Our objective was to conduct a survey of relative genome sizes and ploidy levels of dogwood taxa representative of the big-bracted, cornelian cherry, and dwarf dogwood clades. Flow cytometry was used to determine relative genome sizes and ploidy levels of 94 accessions of various species, hybrids, and cultivars. Traditional cytology was performed on root cells of representative taxa to calibrate genome sizes with ploidy level. All dogwood accessions tested were diploid with the exception of C. canadensis that was tetraploid and the hybrid cultivar C. KN30-8 that was triploid. Relative genome sizes varied by clade, subgenus, and species with 1Cx values ranging from 1.07 pg for C. canadensis to 5.08 pg for C. eydeana. Relative genome sizes were also valuable for confirming hybridity of interspecific crosses in cases in which parents varied substantially in relative genome size and hybrids were intermediate. A broad range of interspecific hybrids was documented including C. capitata × florida, C. capitata × kousa, C. elliptica × florida, C. hongkongensis × florida, C. kousa × elliptica, and C. kousa × florida. These results provide further insights into the cytogenetics, reproductive biology, crossability, and relative genome sizes of dogwoods.

Styrax L. is a genus of ≈130 species, many with horticultural traits of interest, though sparsely represented in cultivation. Previous studies reporting chromosome counts have established a base chromosome number of x = 8 and provided evidence of diploid, tetraploid, pentaploid, and hexaploid levels within the genus, including the more commonly cultivated Styrax japonicus Siebold & Zucc. being reported as pentaploid. Little has been reported relative to genome size within the Styracaceae. We conducted flow cytometry using fluorochromes propidium iodide (PI) and 4′,6-diamidino-2-phenylindole (DAPI) to estimate relative 2C genome size and ploidy of 47 accessions representing 15 species of Styrax and base composition of a subset of 9 accessions representing 6 species. Cytologic analysis of chromosomes was completed on Styrax confusus Hemsl. for ploidy calibration to genome size. Diploids, tetraploids, pentaploids, hexaploids, and octoploids were identified. Relative 2C genome size ranged from 1.38 to 6.61 pg. This survey confirms published ploidy for Styrax americanus Lam., Styrax obassia Siebold & Zucc., Styrax redivivus (Torr.) L.C.Wheeler (diploid), and S. japonicus (pentaploid); suggests alternate ploidy levels and/or ploidy series for Styrax grandifolius Aiton (diploid) and Styrax hookeri C.B.Clarke (octoploid); and establishes ploidy levels for Styrax glabrescens Benth., Styrax odoratissimus Champ. ex Benth., Styrax officinalis L., Styrax platanifolius Engelm. ex Torr., Styrax ramirezii Greenm., Styrax wilsonii Rehder (diploid), Styrax confusus, Styrax hemsleyanus Diels (tetraploid), and Styrax fortunei Hance (diploid and hexaploid).