Estimation of genome size and physical mapping of ribosomal DNA in diploid and tetraploid guineagrass (Panicum maximum Jacq.)

Guineagrass (Panicum maximum Jacq.) is one of the most important tropical forage grasses due to its high productivity and good forage qualities. Although P. maximum is important forage, basic genomic information such as genome size and ribosomal DNA (rDNA) ideogram information have not been reported. In this study, the DNA contents of one diploid strain and three tetraploid cultivars of P. maximum were examined by flow cytometry and rDNA loci were physical mapped by fluorescence in situ hybridization (FISH). The DNA content of the diploid strain was determined to be approximately 500 Mbp, which is of similar size to Oryza sativa and represents a relatively small genome size when compared to other members of the plant kingdom. The tetraploid cultivars were determined to exhibit a genome size of approximately 1000 Mbp. FISH with rDNA probes was applied to diploid and tetraploid strains. The numbers of 5S rDNA loci in the diploid and the tetraploids were two and four, respectively. The number of 45S rDNA loci varied from four to eight among the tetraploids. Constructed quantitative ideograms of diploid strain by image analysis indicated that the difference in the number of 45S rDNA is dependent on the presence or absence of the locus on chromosome 2. The determined DNA content and constructed ideograms in this study are useful for future research efforts on P. maximum in areas focused on cultivar development, gene isolation of apomixis loci and mutation research.