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 (2n = 2x = 36) to pentaploid (2n = 5x = 90), with 4x 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.

Genomic diversity and phylogenetic relationships in the genus Parthenium (Asteraceae)

“…Contemporary nursery material of wild quinine, a Parthenium species distantly related to guayule (Rollins, 1950;Kumar et al, 2009), was selected as the outgroup for phylogenetic analysis. A subset of these accessions has been previously analyzed for ploidy, chromosome number, and nuclear genome size (Gore et al, 2011;Sanchez et al, 2014) and this study expands those analyses to a further five accessions, providing ploidy, chromosome number, and nuclear genome size information for the accessions that were genetically characterized with SNP markers (Table 1). From the 62 plant samples analyzed via GBS, we obtained 48 million reads that passed quality control.…”

“…Following Gore et al (2011), we selected representative plants within a ploidy level of the five accessions and conducted mitotic chromosome counts in root tip cells as described in Sanchez et al (2014). Counts of metaphase chromosomes from images were independently confirmed by two people (authors Paul L. Sanchez and Bernd Friebe).…”

“…Gray), a native plant of the desert regions of northern Mexico and southwestern United States, has been intermittently improved and cultivated as a source of natural rubber over the past century (reviewed in Hammond and Polhamus, 1965; Thompson and Ray, 1989). However, development of high rubber yielding cultivars has met with only partial success, in large part due to a combination of limited genetic diversity in the source of germplasm (Thompson and Ray, 1989), extensive polyploidy variation within and among the available guayule accessions (Gore et al, 2011), and a complex mode of reproduction that is characterized by sporophytic self-incompatibility in diploid plants and facultative apomixis (diplospory type) in polyploid plants (reviewed in Ray et al, 2005;Thompson and Ray, 1989). Recently, Gore et al (2011) successfully adapted automated flow cytometry methods to efficiently characterize ploidy level variation in guayule breeding programs.…”

“…However, development of high rubber yielding cultivars has met with only partial success, in large part due to a combination of limited genetic diversity in the source of germplasm (Thompson and Ray, 1989), extensive polyploidy variation within and among the available guayule accessions (Gore et al, 2011), and a complex mode of reproduction that is characterized by sporophytic self-incompatibility in diploid plants and facultative apomixis (diplospory type) in polyploid plants (reviewed in Ray et al, 2005;Thompson and Ray, 1989). Recently, Gore et al (2011) successfully adapted automated flow cytometry methods to efficiently characterize ploidy level variation in guayule breeding programs. However, in order to enable modern genomics-assisted breeding approaches for the accelerated improvement of this nascent crop, a thorough characterization of both polyploidy variation and genetic diversity within the guayule germplasm pool is essential.…”

“…Building upon previously published data for 10 guayule, mariola, and Parthenium hybrid accessions (Gore et al, 2011;Sanchez et al, 2014), we used flow cytometry to expand ploidy and nuclear genome size characterization to four interspecific hybrids between guayule and other Parthenium species. Additionally, we estimated the nuclear genome size and ploidy level of a wild quinine accession, a first for this Parthenium species.…”