Complex Ploidy Level Variation in Guayule Breeding Programs

Abstract

Guayule ( Parthenium argentatum A. Gray) is a potential source of natural rubber, but attempts to domesticate and cultivate this perennial crop for large‐scale production in the southwestern United States have been intermittent over the past century. Genetic improvement through modern plant breeding is needed to increase its yield potential and suitability for commercialization. Natural variation for ploidy level is extensive among individuals in wild guayule populations, but less is known about the extent of ploidy level variation in guayule breeding germplasm. Because ploidy variation is among the factors that slow the rate of genetic gain in guayule breeding programs, determining the ploidy level of publicly available guayule accessions would help to accelerate the development of stable, high yielding cultivars. To that end, we adapted flow cytometry to examine the ploidy of 34 guayule accessions available from the National Plant Germplasm System. The data revealed a natural polyploid series ranging from diploid (2 n = 2 x = 36) to pentaploid (2 n = 5 x = 90), with 4 x being the predominant ploidy. Interestingly, not all plants sampled from an accession had the same ploidy level (mixed ploidy). Notably, the integration of ploidy and pedigree data uncovered complex ploidy variation in guayule breeding programs. The frequency and range of ploidy variation observed in this germplasm will help to direct future breeding efforts as well as linkage analysis and genome‐wide association studies.