Is somatic vs. germline distinction definitive?

[adamq1]

I've read that somatic mutations don't affect gernline cells because gametes and so on are set aside very early in embryonic development.

Is this distinction between soma and germline very well supported or is there still wiggle room? For eg, could there be a mechanism such that retroviruses 'copy and paste' mutations from somatic cells into the germline or something like that?

 

[mishrashubham]

It is called the Weismann Barrier and is considered pretty impenetrable in animals (somatic and germline cells do not have a clear distinction in plants due to them being haplodiplontic and hence having gametophytes). However Ted Steele, an Australian immunologist, has proposed a possible mechanism to be breached although I couldn't find any experiments or studies that validate or confirm his results. Someone with better searching capability might help.

 

[adamq1]

Thanks, i will see what i can find from your reference to Ted Steele.

I found some info about the weismann barrier, but as far as i can make out it applies only to the interaction between DNA and RNA, and doesn't really exclude RNA 'carriers' like retroviruses. I think i might have put that badly but just trying to learn! It's a nightmare trying to wade through unfamiliar terminology.

What prompted me to ask the question was http://www.scientificamerican.com/article.cfm?id=ductile-helix-jumping" paper), which appears to indicate that retrotransposon mutations influence brain function. My layman's read of this is that if experience affects RNA and DNA in brain cells through retrotransposition, and if these modifications made it into gamete cells by means of retroviruses, this would be a mechanism of evolution supplemental to natural selection?

I was trying to find out if the idea is plausible or just nonsense. Sorry if this breaks rules about speculation.

 

[mishrashubham]

Thanks, i will see what i can find from your reference to Ted Steele.

I found some info about the weismann barrier, but as far as i can make out it applies only to the interaction between DNA and RNA, and doesn't really exclude RNA 'carriers' like retroviruses. I think i might have put that badly but just trying to learn! It's a nightmare trying to wade through unfamiliar terminology.

Where did you read about it? Wikipedia is a good start. You might be confusing it with the central dogma which talks about DNA and RNA.

...and if these modifications made it into gamete cells by means of retroviruses, this would be a mechanism of evolution supplemental to natural selection?

That is precisely what Steele is trying to imply.

 

[adamq1]

I've been reading on wikipedia and definitely getting myself confused. I found this http://www.epigenie.com/Epigenetics.html" that suggests epigenetics might be a bit over-hyped?

 

[mishrashubham]

I've been reading on wikipedia and definitely getting myself confused. I found this http://www.epigenie.com/Epigenetics.html" which was helpful and seems an ok source.

What are you confused about?
Try reading these
nature.com/ng/journal/v33/n3s/full/ng1089.html
nature.com/scitable/topicpage/germ-cells-and-epigenetics-14426688

I also found an article on http://richarddawkins.net/articles/642737-is-epigenetics-a-revolution-in-evolution" that suggests epigenetics might be a bit over-hyped?

I was not aware that epigenetics was so hyped. The article is right though; while epigenetics might bring significant developments in the field of genetic and genmic research it won't certainly change the principles of evolution.

 

[atyy]

What prompted me to ask the question was http://www.scientificamerican.com/article.cfm?id=ductile-helix-jumping" paper), which appears to indicate that retrotransposon mutations influence brain function. My layman's read of this is that if experience affects RNA and DNA in brain cells through retrotransposition, and if these modifications made it into gamete cells by means of retroviruses, this would be a mechanism of evolution supplemental to natural selection?

Hmmm, although the paper is interesting, wrt to natural selection, I'd naively think it not relevant. Many organisms do not have as strong a distinction between somatic and germline cells, and this has not prevented the hypothesis of natural selection being suggested to apply to their species.

I've been reading on wikipedia and definitely getting myself confused. I found this http://www.epigenie.com/Epigenetics.html" that suggests epigenetics might be a bit over-hyped?

Epgenetics is important. "Revolution in evolution"? - of course not. But that does not mean that the neo-Darwinian synthesis (especially if Coyne is talking about Dawkins's selfish gene ideas when he says "gene centered view of evolution") is orthodoxy either (or well defined in the first place).

 

[adamq1]

What are you confused about?
Try reading these
nature.com/ng/journal/v33/n3s/full/ng1089.html
http://nature.com/scitable/topicpage...etics-14426688

...

I was not aware that epigenetics was so hyped. The article is right though; while epigenetics might bring significant developments in the field of genetic and genmic research it won't certainly change the principles of evolution.

I hadn't come across the idea before and so maybe it's just me that's overhyping it.

Hmmm, although the paper is interesting, wrt to natural selection, I'd naively think it not relevant. Many organisms do not have as strong a distinction between somatic and germline cells, and this has not prevented the hypothesis of natural selection being suggested to apply to their species.

In my defence , i didn't say it was relevant to natural selection or would affect natural selection in any way. I just said it would be a mechanism of evolution that wasn't natural selection. I guess the outcomes of such a mechanism would be subject to natural selection, which is why i said 'supplemental'.

Epgenetics is important. "Revolution in evolution"? - of course not. But that does not mean that the neo-Darwinian synthesis (especially if Coyne is talking about Dawkins's selfish gene ideas when he says "gene centered view of evolution") is orthodoxy either (or well defined in the first place).

TBH, i thought neo-darwin stuff was orthodoxy.

 

[atyy]

TBH, i thought neo-darwin stuff was orthodoxy.

Well, it's not clear what the term means. Stephen Jay Gould, for example, defines and argues that neo-Darwinism is not a major force in evolution. Eg. p 208 of http://books.google.com/books?id=yf...jay+gould+neo-darwinism&source=gbs_navlinks_s.

A summary of the debate of the gene as a "unit of selection" can be found in Ridley's http://books.google.com/books?id=HCh7n6zuWeUC&source=gbs_navlinks_s, chapter 11.

 

[Ryan_m_b]

Personally I would avoid using terms like "neo-Darwinism" and "orthodoxy". When evolution was a new field there were a lot of theories and ideas floating around that weren't entirely correct and didn't fully account for how evolution could occur. Once things started picking up and more facts were obtained from a myriad of different fields all these ideas from before were able to be brought together to make the modern evolutionary synthesis (sometimes called neo-Darwinism). This paradigm has essentially continues until today although it has been vastly expanded with a wealth of new data. We've really got to the point where nothing is going to change our fundamental view of evolution, we may find new processes and there could be some surprises but overall we have it nailed.

 

[atyy]

Fee article: http://www.ncbi.nlm.nih.gov/pubmed/18256243

As far as I can tell, the cases discussed involve elements that "hop" within the cell. Thus for a mutation caused by a hop to be inherited, it must hop in a germline cell.

The resulting mutation would be acted on by natural selection (or not) in the same way as a mutation arising any other way.

http://www.ncbi.nlm.nih.gov/pubmed/2831458" (unfortunately not free) discuss two patients with diseases caused by hopping mutations. Anyway, they say at the end "Because we do not know when these L1 insertion events occur, whether in the sperm or ovum, after fertilization, or during early stages of embryogenesis, the proportion of such insertions that are heritable is unknown."

 

[mes314159]

Argh, wrote a lengthy reply which I did not save as a separate document, and the system kept kicking me out when I tried to post it, to the point where it was lost. I try again, with apologies if the earlier attempt shows up eventually in this forum. I do research in human genetics, and we often see results of germ-line mutations, i.e. mutations in the genomes of individuals where both parents are somatically apparently unmutated. According to embryology textbooks, at some point in early embryogenesis about 50 or so cells are set aside to become the eventual germ, yielding the gametes (sperm and eggs). It is believed that these cells do not have any somatic descendants themselves. It can easily be imagined that a spontaneous mutation arising early enough in an embryo could show up in somatic and germ line descendants. However it seems very unlikely that any somatic cells in adults (I refer to humans throughout) have germ line descendants. So vertical transfer of somatic mutations (or epigenetic changes in chromatin for that matter) are pretty much ruled out. I see no theoretical problem with horizontal transfer of genetic information via a viral vector. That could in principle include transduction, so for example a virus infects a somatic cell, recombines to take up some human genomic DNA, which is transmitted to progeny that later infect germ line cells and potentially land that human DNA of somatic origin in the germ line genome which is then transmitted via reproduction to actual children. I don't know of any known virus with all these properties, but it could be imagined. Whether endogenous retroviruses could become active in a somatic cell, and then be horizontally transmitted to a germ line cell of the same actual person is also unclear to me biologically, but I suppose it is probably feasible though probably rare. In general the remnant endogenous retroviruses in our genomes are inactive, though perhaps not irrevocably. On the other hand some other repetitive elements in our genomes are active on evolutionary time lines, and sooner or later one of them could presumably find a way to get from a somatic to a germ line cell in the same individual. I think if these events were frequent geneticists would have seen the medical consequences already, so I guess they are rare if at all.

 

[bobze]

I don't know of any known virus with all these properties, but it could be imagined. Whether endogenous retroviruses could become active in a somatic cell, and then be horizontally transmitted to a germ line cell of the same actual person is also unclear to me biologically, but I suppose it is probably feasible though probably rare. In general the remnant endogenous retroviruses in our genomes are inactive, though perhaps not irrevocably. On the other hand some other repetitive elements in our genomes are active on evolutionary time lines, and sooner or later one of them could presumably find a way to get from a somatic to a germ line cell in the same individual. I think if these events were frequent geneticists would have seen the medical consequences already, so I guess they are rare if at all.

Interesting thoughts. That is something that would have had to have happened (fairly frequently in evolution apparently as well). All those ERVs had to cross from cells derived from the soma to gametic line at some point. The opposite is to suppose they are retroviruses which infect gametes preferentially, which I would find to be a dubious claim.

Whether or not they took somatic mutations with them, is a different story. I suspect though (given by the large amount of retroviral genome mixed up in ours) that they did take genes from the soma with them. Statistically, if that crossing has happened as much as all the artifact ERVs alludes to; one would expect some soma mutations to be carried across with them.

It appears to me though, that for a retrovirus to forfeit its ability to move through hosts and become integrated in host genome, it "looses" the ability produce anything "actively". In fact, I would suspect there would be a very strong selective pressure for a retrovirus making that crossing to "sit back, shut up and enjoy the free ride". So in that regard, any somatic mutation that crossed with it would likely be silent in my opinion. That isn't to say though that some act of genetic change couldn't plop that mutated gene of soma right into the correct mix of promoters to produce activity again. If that's happened, I have no idea, but what a wonderfully novel thing to study (*wink*, *wink* at all you young future aspiring molecular biologists who read the website )