So for most of this week we've been thinking about mammalian X inactivation and how that and how this ensures dosage compensation in mammals. But now I want to think about what we've actually learnt from how dosage compensation happens in worms and in flies. So while in mammals we know we inactivate one of the two X chromosomes and this means that you have only one active X chromosome in a female cell and one active X chromosome in a male cell. If we now consider what happens in worms, in worms you have hermaphrodites that are xx and males that have a single X chromosome. Rather than having inactivation of one of the two X chromosomes in hermaphrodites which would be similar to what happens in mammals in fact you get down regulation of both X chromosomes in hermaphrodites by 50%. So this seems to be quite unrelated to what happens in mammals. In flies by contrast we have female has two X chromosomes and male has one X and one Y just like in mammals. But, now the females don't have any form of X inactivation but, rather it's the male X chromosome which is upregulated two fold. And so, the way that you get accommodation for the fact that you need to have equivalent dose of X linked gene between the sexes is that you up regulate rather than down regulating at all. So for a very long time it seemed that these 3 different processes had evolved distinctly or perhaps unrelated. But in recent years we've actually found out that there seemed to be more similarities than perviously thought. So while we have this up regulation in flies of the male X chromosome but nothing happening in the females its recently become clear although still relatively controversial that in mammals we not only have inactivation of one of the X chromosomes in females but we have up regulation of that active X chromosome which is found the single active X chromosome in females and males just like happens in the flies. So this is relatively confusing to think about. Why is it that we should have this up regulation of the active X chromosome. But I guess whats thought, is that probably got to do with the dose of X linked genes compared with all the autosomes. We have two copies of autosomes and therefore. you get the production of, of protein at two times the rate that you would from a chromosome that only occurs in one copy. Whereas, if you upregulate that single active X chromosome, you should have an equivalent dose overall of the X linked genes compared with the autosomes. To be able to find how this occurred in mammals, it was actually needed to be studied at a genome-wide level. So you can't study a single gene on the X chromosome compared with a single autosomal gene, because the level of expression of those particular genes may not represent what happens in total. They may undergo particular tissue specific silencing for example or different expression levels based on that function of the gene that’s required. However if you study all of the X linked genes, now a thousand of them also compared with all of the autosomal genes by studying the expression of all of these genes you can then begin to see whether or not, what the dose of X linked genes is compared to the dose of autosomal genes. So as I said, this is still relatively controversial, but it appears that maybe we have something related to what happens in flies as well as X inactivation which we've spent a lot of time thinking about. So, then if there are perhaps some similarities in the mechanisms similarities in the consequences, what about those mechanisms? What about the molecular mechanisms behind this upregulation? Well, for the down regulation that happens in worms, first of all, we know that down regulation by 50% of both X chromosomes in hermaphrodite involves SMC proteins. These SMC proteins are structural maintenance of chromosomes proteins. And they are involved in complexes called the condensin complex, or condensin-like complexes, which, when the chromosomes are actually condensed in mitosis, these are the proteins that are involved in condensing that chromatin down. So it kind of makes sense that they would be involved in the formation of a silent or at least some repression that could involved in this worm dosage compensation, but quite interestingly, the Smchd1 protein, which I mentioned as it seems to be involved in the maintenance of X inactivation is also an SMC protein. Its not one of the core SMC proteins but it has an SMC hinge domain as its given in its name. And so there might be some relationship between the down regulation of two X chromosomes by half in the worms. And the complete inactivation of the complete inactivation that occurs in the mammalian single X chromosome that's inactivated. If we now think about the flies, where we see the upregulation of the Y, of the male X chromosome, then we know this involves long noncoding RNAs for up regulation, and these are called RNA on the X or 1, so roX1 and also roX2. We also know it involves histone acetyltransferases, which we know, acetylation is involved in activation, and an RNA-DNA helicase, and this helicase is involved in unwinding chromatin. And finally we know that, which has been found more recently, that the active X chromosome in the male fly is involved, is found in an active nuclear compartment, so this is like the opposite of what's found for X inactivation. So rather then create a silent nuclear compartment. There is an active nuclear compartment that's found, that's enriched in RNA polymerase 2 because it's enriched in those transcription factories that we talked about in week two. So, very recently just in 2013, in mammals it was found that also at least in human cells, you can find a long non-coding RNA which is associated with the active X chromosome, in both females and males. And this long noncoding RNA is called XACT. This is still highly controversial but potentially we have some similar mechanism in mammals that exists for this upregulation of the X the active X chromosome as what happens in flies. Further more, the histone acetyltransferase, which is involved in flies, if you look at the similar gene that occurs in mammals, it also seems to be occurred in mammalian X upregulation, and so indeed although we thought that these mechanisms were originally completely distinct, and then X inactivation and X upregulation in flies were different. Now it would seem that we seem to have this extra, upregulation, as well, and that there are similarities beyond what we first imagined. So why is it, I, want to come back to this idea of why is it that we would undergo both down-regulation and upregulation. So, it's possible that because as i said we need to have not only balancing of the X linked genes' between the sexes but also relative to the autosomes why is it we choose to have a somewhat more complicated mechanism where we both inactivate and upregulate. Rather than perhaps just doing what the flies do and upregulating the single X chromosome is really still not clear. And that's going to be something that's interesting to work out why we've evolved this way because it doesn't seem like its the most parsimonious explanation for the way it should occur. So I'd like to alert you we're now at the end of dosage compensation and we're going to have one final lecture this week which is a little bit more about the fly. But I'd like to alert you to an animation that exists on this website that's given. This animation is, the parental animation from which I've taken all of the snippets of animation I've shown for the first three weeks. this is a 13 minute long animation which goes through X inactivation. And it's really worth looking at for your own revision of what goes on. There's a little bit of additional background at the beginning on just genetics. And at the end, a little bit on perhaps how it might relate to disease. So I think this is well worth your time to watch.
