Beyond transcriptional silencing: Is methylcytosine a widely conserved eukaryotic DNA elimination mechanism?

Methylation of cytosine DNA residues is a well-studied epigenetic modification with important roles in formation of heterochromatic regions of the genome, and also in tissue-specific repression of transcription. However, we recently found that the ciliate Oxytricha uses methylcytosine in a novel DNA elimination pathway important for programmed genome restructuring. Remarkably, mounting evidence suggests that methylcytosine can play a dual role in ciliates, repressing gene expression during some life-stages and directing DNA elimination in others. In this essay, I describe these recent advances in the DNA methylation field and discuss whether this unexpected novel role for methylcytosine in DNA elimination might be more widely conserved in eukaryotic biology, particularly in apoptotic pathways.