But some say not all viruses are bad for us!? More importantly, some say there would be no humans if there were no viruses!! How stupid is that?!
Well to look at this question, let’s first consider what we are, mammals.
According to the Oxford dictionary a mammal is “any animal that gives birth to live young, not eggs, and feeds its young on milk”. The first important part of the definition is the feeding with milk which is from a mammary gland or breast and hence where the name mammal comes from (“mamma” means breast).
Another important part of being a mammal is that the young are born alive and not in eggs. When you think about it this is truly remarkable. Half of the foetal genetics are from the father so as far as the mother’s immune system is concerned this foetus is a foreign object. So why is the foetus allowed to grow inside a mother and thrive? After all, if you took any other organ from a father and implanted it in the mother it would be rejected within a few days! Hence the need for a cocktail of anti-rejection drugs after transplant surgery.
Hold on! This is supposed to be a micro-biology blog not a macro-biology blog. Bear with me, I’m getting there.
Okay, here we go…
The organ that allows the foetus to survive is the placenta [yes, yes we all know this!!!, ECIC]; it’s where the foetus gets oxygen, nutrition and also where waste products are removed. And it exists because of microbiology… or more specifically, virology! WHAT, I hear you cry!!
Evolution of the placenta
About 400 million years ago placentas started to appear in fish. Although fish placentas are still relatively uncommon, and most fish still lay eggs, some fish do have a placenta and give birth to live young e.g. blue sharks, rose fish and red banded rockfish (they’re still not mammals though as they don’t feed their young with milk!). Other animals keep their eggs internally until the young hatch e.g. adders, insects and other types of sharks; they do not have a placenta even though they appear to give birth to live young.
About 130 million or so years ago mammals developed placentas and started to give birth to live young, but this required a major change in reproductive physiology in order to lead to the formation of the placenta.
The placenta evolved due to the action of retroviruses that changed the genetic makeup of cells in early pregnancy back 130 million years ago (and probably ever since!)
Retroviruses are RNA viruses that convert to DNA inside infected cells which is then incorporated into the cell’s chromosome. If that cell is a germ cell, an early foetal cell giving rise to all of the future baby’s cells, then the retroviral DNA becomes a permanent part of that baby; natural genetic manipulation if you like.
The best-known retrovirus is Human Immunodeficiency Virus (HIV) which causes the clinical condition Acquired Immune Deficiency Syndrome (AIDS). But it turns out not all retroviruses are bad.
The most important aspect of placental evolution is the ability to produce a protein called “syncytin”. Syncytin is a viral protein that is used by mammals to help form a layer of the placenta called the “syncytiotrophoblast”. The syncytiotrophoblast is the layer of the placenta that invades the uterine wall fixing the placenta in place and allowing the transfer of nutrients and waste across the placental surface. It is also the only part of the placenta that is NOT recognised as abnormal by the mother’s immune system.
The gene that now exists in human cells and is responsible for the ability to produce syncytin originated from the envelope gene of Human Endogenous defective RetroVirus known as HERV-W.
Over time the placenta has been “improved” by further retroviral “infection” and incorporation of more effective and efficient viral versions, and no doubt it will be further improved in the future. The evidence that the placenta can be improved by further infections does make me wonder if the opposite can happen, and that perhaps infection with a less effective or outright damaging retrovirus could be the reason for some early foetal losses… I’m not sure if anyone has ever looked at that?
What about other animals?
Now we all know that lizards are reptiles, right? [Here he goes again with weird facts, ECIC!!] But what about a reptile with a placenta? About 25 million years ago a reptile called the Mabuya lizard, from the Caribbean, evolved a placenta and now gives birth to live young. Wow! I never knew there were so many “non-mammals” which gave birth to live young.
Are retroviruses involved in other body organs?
It turns out that retroviruses are involved in the evolution of other body organs as well. In particular the brains of land animals appear to be affected by a retroviral gene known as “Arc” which has led to more effective and efficient neuronal signalling as well as plasticity (where the brain tissue can adapt to change).
In fact it is now estimated that 8% of the human genome is actually made up of retroviral genes… that means we are at least 8% viral! However, a lot of that genetic material is actually redundant or “fossilised” having no “apparent purpose” anymore, but these fragments may have been important in the past and then been replaced by new improved “features”, who knows?
It does make me wonder if apparent jumps in evolution are more than just natural selection? My understanding has been that if a single member of a species has a particular trait that gives it a survival advantage then it is more likely to reproduce and therefore it’s characteristic will be increasingly represented within future generations. This is the basis to Darwin’s work on evolution, but apparently Darwin didn't know about viruses. However, mutations in genes occurring at random are usually small and slow to occur, and therefore would be unlikely to give much of an advantage or “quick change”. A retrovirus incorporating a lot of new genes though could cause a big shift in genetic make up of an organism which in turn could give a more significant “jump” in evolution and a major selective advantage… Wow, that makes my brain hurt!
Golly. Time to go and lie down somewhere quiet and let my brain recover… maybe there’s a virus that can help with that….
- Viruses and the placenta: the essential virus first view
- Identification of an ancient endogenous retrovirus, predating the divergence of placental mammals
- The placenta goes viral: Retroviruses control gene expression in pregnancy
P.S. Who thought of looking for a virus that "created" the placenta anyway, and how do these researchers get funding for that!?!