Regarding mechanisms, I was under the impression that DNA methylation was widely accepted as a primary mechanism for epigenetic inheritance. Not so much, or is that one of those artificial scenarios you referenced?
That’s correct but you’re (understandably) confusing “epigenetic inheritance” with “trans-generational epigenetic inheritance”.
The first (i.e. epigenetic inheritance, particularly via DNA methylation) is a ubiquitous biological process by which a state is encoded in the cell and can be passed down to daughter cells upon cell division. Hence the daughter cells inherit the mother cell’s state. At its simplest, it just acts like a “bookmark” in the genome which informs the cell what parts to read, and what parts to skip over.
But things are different when talking about multi-cellular organisms that pass heritable information through the germ line. This is what trans-generational refers to. For the longest time it was assumed that no epigenetic information could be transmitted through the germ line due to a concept known as the “Weismann barrier” (think of it as the librarian: upon return, she removes all the bookmarks from the books). We now know that, under specific circumstances, this barrier can be “violated”, and “leaks”. However, unlike normal (genetic) inheritance, this leak does not allow a structured transmission of information according to our current understanding (that is, a bit of information encoding a phenotype “A” wouldn’t necessarily cause the same phenotype in the offspring; it would simply lead a perturbation). There are more specific exceptions to this rule which, for instance in nematode worms, allow a more structured information transmission (though it’s an open question whether DNA methylation is causally implicated).
Thanks for that clarification. I wish the popular literature on the topic took similar pains to point this out. I can’t help but wonder if this is the root of much of the misguided interest in epigenetics.
> I can’t help but wonder if this is the root of much of the misguided interest in epigenetics.
In a word: yes. Even within the field this causes confusion (researchers of course know the distinction in principle, but the term “inheritance” is suggestive, and has led more than one person down the garden path). To make matters worse, “epigenetics” also can refer to several different things.
The first (i.e. epigenetic inheritance, particularly via DNA methylation) is a ubiquitous biological process by which a state is encoded in the cell and can be passed down to daughter cells upon cell division. Hence the daughter cells inherit the mother cell’s state. At its simplest, it just acts like a “bookmark” in the genome which informs the cell what parts to read, and what parts to skip over.
But things are different when talking about multi-cellular organisms that pass heritable information through the germ line. This is what trans-generational refers to. For the longest time it was assumed that no epigenetic information could be transmitted through the germ line due to a concept known as the “Weismann barrier” (think of it as the librarian: upon return, she removes all the bookmarks from the books). We now know that, under specific circumstances, this barrier can be “violated”, and “leaks”. However, unlike normal (genetic) inheritance, this leak does not allow a structured transmission of information according to our current understanding (that is, a bit of information encoding a phenotype “A” wouldn’t necessarily cause the same phenotype in the offspring; it would simply lead a perturbation). There are more specific exceptions to this rule which, for instance in nematode worms, allow a more structured information transmission (though it’s an open question whether DNA methylation is causally implicated).