Submitted to: South African Journal of Botany
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: December 27, 2011
Publication Date: January 25, 2012
Citation: Burson, B.L., Actkinson, J., Jessup, R.W., Hussey, M.A. 2012. Ploidy determination of buffel grass accessions in the USDA National Plant Germplasm System collection by flow cytometry. South African Journal of Botany. 79:91-95. Interpretive Summary: Buffelgrass is an important forage and range grass that grows in the warmer and drier areas of the world because it can grow and produce with limited moisture. Over the years, scientists have collected plants of this grass primarily from Africa and Asia and put them in the USDA National Plant Germplasm System (NPGS) for other scientists to use. Plant Breeders from throughout the world, who are working with buffelgrass, regularly obtain seed of these grasses from the NPGS and use the plants in their breeding programs. Buffelgrass plants frequently have a different number of chromosomes and it is important to the breeder that he or she uses plants in his or her breeding program that have the same number of chromosomes. Counting buffelgrass chromosomes with a microscope is difficult because they are small and the process takes a lot of time. Because of this, the chromosome number of only a few plants in the buffelgrass collection in the NPGS is known. A less time consuming approach of determining or estimating the chromosome number of a plant is to measure the DNA content in the nuclei in its cells. This is done with an instrument called a flow cytometer. Using a flow cytometer, we determined the DNA content of 568 different buffelgrass plants that had been deposited in the buffelgrass collection. This information is of value because it will be very helpful to buffelgrass breeders when they are selecting plants to use in their breeding programs. Knowing this information will greatly increase their efficiency which will be a monetary savings.
Technical Abstract: Buffelgrass [Pennisetum ciliare (L.) Link syn. Cenchrus ciliaris L.] is an important forage and range grass in many of the semi-arid tropical and subtropical regions of the world. The species reproduces primarily by apomixis but it is highly diverse because a wide array of different apomictic ecotypes exist in nature. The buffelgrass collection in the USDA National Plant Germplasm System (NPGS) reflects this diversity. The most commonly reported chromosome number for buffelgrass is 2n=4x=36 but other numbers have been reported. Little is known regarding the chromosome number of most buffelgrass accessions in the NPGS. This information is needed because many accessions are used in hybridization and breeding programs. Because buffelgrass chromosomes are small and sometimes difficult to count, the ploidy level of most accessions in the NPGS collection was determined by measuring the DNA content of each accession using flow cytometry and their ploidy levels were predicted based on this information. Five hundred and sixty-eight accessions were analyzed, and based on their DNA content, 308 were tetraploids with 36 chromosomes, 139 were pentaploids with 45 chromosomes, 20 were hexaploids with 54 chromosomes, two were septaploids with 63 chromosomes, and 99 were aneuploids. Eighty-three of the aneuploids were in the 37 to 44 chromosome range and 16 were in the 46 to 53 chromosome range. These findings show the variation in chromosome number within this apomictic taxon and demonstrate the importance of knowing the chromosome number of an accession prior to using it in a breeding program. They also demonstrate how fertilization of unreduced gametes combined with facultative apomictic reproduction has increased the ploidy levels of different ecotypes and created a range of aneuploids within the species.