a bonobo humanity?

Gametes are gametes because of epigenetic modifications in their pro-nuclei, but they have to lose these modifications, or transform them, when they come together to form zygotes. The male pro-nucleus DNA methylation is stripped away immediately after sperm penetrates egg. The egg pronucleus undergoes the same process, but more gradually. It’s like a wiping away of epigenetic memory, creating totipotency, which becomes a more limited pluripotency as the blastocyst, with its inner cell mass (ICM), forms. 

The ICM cells begin differentiating through the regulation of some key genes. For example, a gene codes for a protein that switches on a set of genes, which code for proteins in a cascading effect. But it’s not quite a matter of switching genes on or off, it’s rather more complex. The process is called gene reprogramming, and it’s of course done effortlessly during every reproductive cycle. Artificial reprogramming of the kind carried out by Yamanaka and others, an essential part of cloning, hasn’t come close to this natural process that goes on in mammals and other species every day.

Clearly, though the epigenetic reprogramming for the female pronucleus is different from that carried out more swiftly in the male. As Carey puts it, ‘the pattern of epigenetic modifications in sperm is one that allows the male pronucleus to be reprogrammed relatively easily.’ Human researchers haven’t been particularly successful in reprogramming an adult nucleus by various methods, such as transferring it to a fertilised egg or treating it with the four genes isolated by Yamanaka. The natural process of gene reprogramming eliminates most of the epigenetic effects accumulated in the parent genes, but as the reprogramming is a different process in the male and female pro-nucleus, this shows that they aren’t functionally equivalent. There is a ‘parent-of-origin effect’. Experiments done on mice to explore this effect found that DNA methylation, an important form of chromatin modification (and the first one discovered), was passed on to offspring by the female parent. That’s to say, DNA from the female was more heavily methylated than that from the male. Carey describes the DNA as ‘bar-coded’ as coming from the male or the female. The common term for this is imprinting, and it’s entirely epigenetic.

Imprinting has been cast by Carey, and no doubt others, as an aspect of the ‘battle of the sexes’. This battle may well be imprinted in the pronuclei of the fertilised egg. Here’s how Carey puts the two opposing positions:

Male: This pregnant female is carrying my genes in the form of this foetus. I may never mate with her again. I want my foetus to get as big as possible so that it has the greatest chance of passing on my genes.

Female: I want this foetus to pass on my genes. But I don’t want it to be at the cost of draining me so much that I never reproduce again. I want more than this one chance to pass on my genes.

So there’s a kind of balance that has developed in we eutherian mammals, in a battle to ensure that neither sex gains the upper hand. Further experiments on mice in recent times have explored how this battle is played out epigenetically. I’ll look at them in the next post in this series.

Reference

The Epigenetics Revolution, by Nessa Carey, 2011