Determining Ploidy Level in Guayule Breeding Populations.

The domestication and cultivation of guayule (Parthenium argentatum Gray) as a perennial natural rubber crop has been intermittent in the southwestern United States. Continued genetic improvement through modern plant breeding is vitally needed to realize yield potential and suitability for successful commercialization. Natural differences in ploidy levels among individuals are one of several factors that have slowed the rate of genetic gain in guayule breeding programs, thus knowledge of ploidy levels in guayule germplasm would help accelerate the development of stable, high yielding cultivars. The objectives of this study were to 1) adapt flow cytometry for ploidy level analysis in guayule and 2) use flow cytometry to evaluate the ploidy levels in three breeding populations. The first population consisted of 34 guayule accessions available from the National Plant Germplasm System, the second population consisted of single plants from a genetically modified population, and the third population consisted of single plants from an open pollinated population derived from diploid guayule lines. A natural polyploid series that ranged from diploids (2n=2x=36) to pentaploids (2n=5x=90) was detected, with 4x as the predominant ploidy level in all populations. In addition, accessions in the first population with multiple ploidy levels (i.e., mixed ploidy) were observed, which suggests that the ploidy level of a guayule plant should not be taken for granted. Notably, linkage of ploidy level data in this study to that of pedigrees uncovered complex ploidy level variation in guayule breeding programs, which was found to be perfectly concordant with existing ploidy level data supported by chromosome counting. Importantly, this work serves as the basis for future breeding efforts as well as QTL analysis and association mapping studies in guayule.