Germplasm characterization for Phlox, Rudbeckia and Coreopsis: determination of genome size by flow cytometry

The Ornamental Plant Germplasm Center acquires, characterizes and enhances herbaceous ornamental germplasm in the public domain both for conservation and for use by the greenhouse and nursery industry. Recent efforts in developing comprehensive germplasm collections of North American native species of Phlox, Rudbeckia and Coreopsis have resulted in the acquisition of over 580 accessions from approximately 60 species that are being characterized for ornamental potential based on habit, flower & foliage attributes, and cut-flower longevity; and also for ploidy (relative DNA content). A survey of the 2C value from flow-cytometric analyses in these accessions was undertaken to guide potential interspecific hybridization studies for germplasm enhancement. In the 29 species of Phlox that were surveyed, the holoploid DNA content ranged from 7.8 pg (P. subulata) to 26.9 pg (P. villosissima). For the 20 species of Rudbeckia, the variation in holoploid DNA content extended from 7.3 pg (R. amplexicaulis) to 23.3 pg (R. scabrifolia); whereas for the 18 species of Coreopsis surveyed, the range was 4.6 pg (C. bigelovii) to 17.5 pg (C. delphinifolia). Whereas each of the genera displayed a similar fold-variation in genome size, the patterns of variation were slightly different. Each genus also had species that included diploid and polyploid individuals; in some cases the polyploids were cultivars whereas in others they were accessions of wild origin. Variation in DNA content consistent with populations that are diploid, tetraploid and hexaploid was identified for accessions of Phlox subulata. Similar variation in DNA content, equivalent to diploid and tetraploid populations, were also found in Phlox pilosa, P. nivalis, P. glaberrima and P. pulchra as well as in Rudbeckia fulgida, R. hirta, R. triloba and R. laciniata. Tetraploid individuals were also identified in Coreopsis grandiflora, C. rosea and C. verticillata. Genome size and ploidy estimations of germplasm for these important ornamentals provide useful information that may facilitate improvement of the crops.