On the Origin and Evolutionary Consequences of Gene Body DNA Methylation

In plants, CG DNA methylation is prevalent in the transcribed regions of many constitutively expressed genes (gene body methylation; gbM), but the origin and function of gbM remain unknown. Here we report the discovery that Eutrema salsugineum has lost gbM from its genome, the first known instance for an angiosperm. Of all known DNA methyltransferases, only CHROMOMETHYLTRANSFERASE 3 (CMT3) is missing from E. salsugineum. Identification of an additional angiosperm, Conringia planisiliqua, which independently lost CMT3 and gbM supports that CMT3 is required for the establishment of gbM. Detailed analyses of gene expression, the histone variant H2A.Z and various histone modifications in E. salsugineum and in Arabidopsis thaliana epiRILs found no evidence in support of any role for gbM in regulating transcription or affecting the composition and modifications of chromatin over evolutionary time scales.