Evolution of genome size in the grasses

The grasses (Poaceae) exhibit marked conservation of gene content and order (synteny and colinearity), a feature that promises the extension of genomic information from one grass species to another using a unified comparative approach. Grasses also show striking differences in the size of their genomes. Since this complicates the treatment of the grass family as a model genetic system, there is need to better understand the patterns and processes that drive genome size evolution. This requires knowledge of phylogenetic relationships, especially deep branching patterns that unify the grass subfamilies. In this study, a phylogeny of 66 grass species with known genome size, most of them diploid, was assembled. The phylogeny described relationships integrated from shared and derived characteristics in molecular and morphological data and the branching order of basal lineages recently inferred from RNA structure and large-scale chromosomal rearrangements. Evolutionary changes in genome size that exclude the effect of polyploidization were traced along the branches of the tree using parsimony methods of character state reconstruction. Most levels of change did not exceed twofold, and few exceeded threefold. The frequency of changes in genome size appears to decrease exponentially with the magnitude of change. The ratio of increases-to-decreases in genome size increased in the order Ehrhartoideae, PACCAD (panicoids, arundinoids, chloridoids, centothecoids, aristidoids, and danthonioids), and Pooideae clades. However, there were clear patterns of increase and decrease in all major clades, and notable genome size changes in the Pooideae and Chloridoideae subfamilies. This shows different tendencies in genome size diversification in these major grass lineages. Depending on the tracing method, the genome of the ancestor of the grass family had 3.0 to 5.2 pg DNA per 2C nucleus. Results extend early proposals that suggest genome size has both increased and decreased along grass lineages, and show that different models of character evolution imparted different frequencies and levels of change along the branches of the trees.