The Bimodal Distribution of Genic GC Content Is Ancestral to Monocot Species

ayu namira;

In grasses such as rice or maize, the distribution of genic GC content is well known to be bimodal. It is mainly driven by GC contentat third codon positions (GC3 for short). This feature is thought to be specific to grasses as closely related species like banana have a unimodal GC3 distribution. GC3 is associated with numerous genomics features and uncovering the origin of this peculiar distribution will help understanding the potential roles and consequences of GC3 variations within and between genomes. Until recently, the origin of the peculiar GC3 distribution in grasses has remained unknown. Thanks to the recent publication of several complete genomes and transcript omes of non grass monocots,we studied more than 1,000 groups of one-to-one or thologous genesinseven grasses and three outgroup species (banana, palm tree, and yam). Using a maximum likelihood-based method, we reconstructed GC3at several ancestral nodes. We found that the bimodal GC3 distribution observed inextant grasses isancestral to bothgrasses and most monocot species, and that other species studied here have lost this peculiar structure. We also found that GC3 in grass line agesis globally evolving very slowly and that the decreasing GC3 gradient observed from 50 to30 along coding sequences is also conserved and ancestraltomonocots. This result strongly challenges the previous views on the specifi city of grass genomes and we discuss its implications for the possible causes of the evolution of GC content in monocots