Genome size and ploidy levels of creeping phlox and related germplasm of mat-forming taxa from eastern and western North America

The eastern North American creeping phlox (Phlox subulata L.) is a widely cultivated flowering groundcover with a history of breeding and selection. Little is known about genome size variation and ploidy of P. subulata and related taxa. Mean holoploid (2C) and monoploid (1Cx) genome sizes and ploidy were analyzed with flow cytometry for a germplasm collection (n = 53) of 11 morphologically similar creeping phlox taxa from natural plant populations, cultivars and hybrids obtained from nursery sources, and the related Microsteris gracilis. Holoploid genome sizes of accessions from natural populations were more variable than cultivated taxa and ranged from 7.47 to 22.86 pg and corresponded to diploid (2n = 2x = 14) tetraploid (2n = 4x = 28), and hexaploid (2n = 6x = 42) levels, but most accessions were diploid and genome size ranged from 7.60 to 8.47 pg. Two tetraploid accessions were discovered, but hexaploids were limited to one population of P. subulata. Most accessions consisted of a single cytotype, but intrapopulation differences in holoploid genome size were found among P. subulata and P. nivalis. The monoploid genome size of M. gracilis differed significantly from all Phlox, supporting separation of the genera. All cultivar accessions were diploid with genome sizes similar to wild diploid P. subulata, however Phlox × procumbens had a mean genome size (8.73 pg) intermediate to parental taxa, P. stolonifera × P. subulata, supporting hybrid origin. Knowledge of cytotype variation in Phlox germplasm will be useful for plant breeders, systematists, and conservationists.