What determines whether the viral DNA is methylated or non-methylated at different stages of development?
You have to realize that the genome is full of viruses, and they are generally silent, which means they’re methylated. And organisms have a major interest in silencing viral genes that get into the genome. Otherwise they’d screw up the genome. So this is an evolutionarily conserved mechanism to silence these transposable elements, these viruses. And you can argue that this is most important in the early embryonic lineage, which gives rise to germ cells. We later found out that these early embryonic cells express several methyltransferases. The one we knocked out is the main one, but it only serves to maintain methylation once methylation has already been established by a different enzyme. It propagates the methylation signal from one cell division to the next. It doesn’t establish new methylation on its own. The job of establishing methylation is done by enzymes called de novo methyltransferases. So if you put a virus into an early cell and it’s not methylated, then the maintenance met
