Genome Evolution in the Genus Sorghum (Poaceae)

Background and Aims The roles of variation in DNA content in plant evolution and adaptation remain a major biological enigma. Chromosome number and 2C DNA content were determined for 21 of the 25 species of the genus Sorghum and analysed from a phylogenetic perspective. Methods DNA content was determined by flow cytometry. A Sorghum phylogeny was constructed based on combined nuclear ITS and chloroplast ndhF DNA sequences. Key Results Chromosome counts (2n = 10, 20, 30, 40) were, with few exceptions, concordant with published numbers. New chromosome numbers were obtained for S. amplum (2n = 30) and S. leiocladum (2n = 10). 2C DNA content varies 81-fold (127‐1030 pg) among the 21 Sorghum species. 2C DNA content varies 36-fold from 127 pg to 460 pg among the 2n = 10 species and 58-fold (152‐879 pg) among the 2n = 20 species. The x = 5 genome size varies over an 88-fold range from 026 pg to 230 pg. The mean 2C DNA content of perennial species (620pg) issignificantlygreater thanthe mean (292pg) of theannuals. Amongthe 21 speciesstudied, the mean x =5 genome size of annuals (115 pg) and of perennials (129 pg) is not significantly different. Statistical analysis of Australian species showed: (a) mean 2C DNA content of annual (289 pg) and perennial (773 pg) species is significantly different; (b) mean x = 5 genome size of perennials (166 pg) is significantly greater than that of the annuals (109 pg); (c) the mean maximum latitude at which perennial species grow (254 degrees) is significantly greater than the mean maximum latitude (176) at which annual species grow. Conclusions The DNA sequence phylogeny splits Sorghum into two lineages, one comprising the 2n = 10 species with large genomes and their polyploid relatives, and the other with the 2n = 20, 40 species with relatively small genomes. An apparent phylogenetic reduction in genome size has occurred in the 2n = 10 lineage. Genome size evolution in the genus Sorghum apparently did not involve a ‘one way ticket to genomic obesity’ as has been proposed for the grasses. a 2005 Annals of Botany Company