The genus Phlox is a staple of gardens worldwide that includes species admired for beauty and versatility in gardens, constructed landscapes, containers, and as cut flowers. Extensive breeding and selection has occurred in three primary species: Phlox drummondii, P. paniculata, and P. subulata, but the genus includes other species with ornamental value. Phlox L. (Polemoniaceae) includes approximately 65 species primarily endemic to North America; 20-23 species occur in the eastern U.S. and 40-45 in the west. The eastern species are a polymorphic group organized into 6 subsections that include the three main cultivated species and up to 20 related, rarely cultivated species. Thus, the species diversity of Phlox has barely been applied for ornamental use. The widespread availability of diverse germplasm can contribute not only to new cultivated forms but also to a greater understanding of species diversity and relationships. Such interest has made Phlox a priority genus for conservation at the Ornamental Plant Germplasm Center. This work describes the development, partial characterization, and manipulation of Phlox germplasm.
Phlox germplasm collection development began in 2010 with an effort to collect all eastern species from natural populations throughout their native ranges; 187 accessions were collected from wild populations of 22 eastern species during a series of expeditions. Another 166 accessions were of cultivated origin; these were used for comparison to wild-collected material. The 353 accessions represent the most comprehensive germplasm collection of Phlox to date.
This germplasm was first characterized by estimation of genome size using flow cytometry and ploidy estimates by chromosome counts. Genome size was surveyed in 287 accessions; 165 accessions were of wild origin and the rest were cultivars. Most accessions were diploid, but genome size was variable; both tetraploid and hexaploid populations were found in a species where it had not been previously reported. The 122 cultivated selections were also primarily diploid, but anueploids were discovered suggesting previous polyploid breeding. These data have important implications in the evolution, adaptation, conservation, and commercial breeding of Phlox.
Molecular characterization included a limited application of microsatellite (SSR) marker analysis of 8 populations, 5 diploid and 3 tetraploid, and 61 individuals from the P. pilosa complex. This species complex had high genetic diversity and moderate population structuring, typical of outcrossing species with gametophytic self - incompatibility. The data indicated that P. pilosa is a rich source of genetic diversity for germplasm collections and enhancement.
Three interspecific hybridization experiments were performed to identify gene pools for Phlox germplasm enhancement; in two studies there was at least one recurring parent and the other was a partial diallel. Interspecific hybridization was most successful among closely related species, and success decreased with increasing phylogenetic distance; however, even crosses between closely related species failed to produce seeds, emphasizing that selection of parental taxa is critical. Success at recreating previously described interspecific hybrids was limited, but at least one was resynthesized. Unlike previous failed attempts at interploid hybridization, some interploid crosses were successful and resulted in morphologically intermediate, aneuploid taxa with intermediate genome sizes to the parents.