Cytological Pattern Reveals Genome Downsizing in Cynodon dactylon (L.) Pers along the Longitudinal Gradient

Understanding the cytological pattern of genome size and ploidy level of the bermudagrass (Cynodon dactylon) is vital to explore the evolution pattern and breeding of the species. To study the diversification of the cytological pattern of bermudagrass along the longitudinal gradient, the genome size and ploidy level were measured and explored with the relationship to climate factors. The corresponding ploidy level was verified through the mitotic chromosome counts method. Bermudagrass accessions ploidy level included diploids, triploid, tetraploid, pentaploid and hexaploid with a basic chromosome number of x = 9. The major ploidy level was tetraploid (45%) and aneuploidy was commonly discovered in collected regions. Mean genome size of bermudagrasswas was estimated to be 1.31 pg/1Cx along longitudinal gradient. The 1Cx values of diploid were higher than that of triploid and tetraploid, while the tetraploid had minimum basic genome size. In the current study, we observed that genome downsizing exists in tetraploids of Cynodon dactylon. Tetraploids have a wider distribution than other ploidy levels, especially in arid areas, occupying a relatively high proportion. In addition, at the same ploidy level, genome size was remarkably variable in the current study. The coefficient of determination analysis showed that longitude and mean annual rainfall were significantly correlated to genome size rather than ploidy level. This cytological study will be helpful for further genetic mechanisms and molecular characteristics to landscape adaptation of bermudagrass.