DNA Content and Ploidy Determination of Bromegrass Germplasm Accessions by Flow Cytometry

Species of the genus Bromus represent ploidy states from diploid to decaploid. Ploidy determination of Bromus germplasm is necessary before it can be effectively used in breeding or genetic studies. The objective of this study was to characterize the ploidy of 322 accessions of four Bromus species [Bromus inermis Leyss, B. riparius Rehm, B. biebersteinii Roem and Schult., and B. inermis ssp. pumpellianus (Scribn) Wagnon] that are in the USDA National Plant Germplasm System (NPGS). Flow cytometry was used to determine DNA content of 10 plants of each accession. Mean DNA contents were correlated to ploidy level with root tip chromosome counts on selected accessions whose DNA content indicated that they represented different ploidy levels. On the basis of DNA content (pg 2C⁻¹ = DNA content of a diploid somatic nucleus) and chromosome counts, mean DNA content and chromosome number was 22.62 pg 2C⁻¹ for octaploid B. biebersteinii (2n = 8x = 56), 26.07 pg 2C⁻¹ for decaploid B. biebersteinii (2n = 10x = 70), 11.74 pg 2C⁻¹ for tetraploid B. inermis (2n = 4x = 28), 22.28 pg 2C⁻¹ for octaploid B. inermis (2n = 8x = 56), 22.72 pg 2C⁻¹ for octaploid B. inermis ssp. pumpellianus (2n = 8x = 56), 26.5 pg 2C⁻¹ for decaploid B. inermis ssp. pumpellianus (2n = 10x = 70), 6.14 pg 2C⁻¹ for diploid B. riparius (2n = 2x = 14), 22.15 pg 2C⁻¹ for octaploid B. riparius (2n = 8x = 56), and 26.64 pg 2C⁻¹ for decaploid B. riparius (2n = 10x = 70). Standard deviations of the mean values were 0.88 pg 2C⁻¹ or less. Most B. inermis and B. inermis ssp. pumpellianus accessions were octaploid (93.75%), while the majority of the B. riparius and B. biebersteinii were decaploid (92.30%). The B. inermis and related species in the USDA NPGS were collected primarily from areas in the former USSR. The NPGS bromegrass germplasm could be enhanced by collections from western and central Europe, the Middle East, and China.