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  • Alex Cloherty

A philosophy of parasites

Today on Microbial Mondays, I'm answering a couple of intriguing, and somewhat philosophical, questions asked by Michael. After reading this post about the question of whether viruses are truly alive, Michael asked:


"If viruses can and do invade all kinds of cells then I'm wondering: 1) A sound reason for this kind of 'parasitic' invasive behaviour (lacking a different term for this)? This obviously favours a virus' replication and sustainability but... Are there other theories about this? 2) Are some kinds of invasive, parasitic, 'hijacking' behaviour by viruses more dangerous - to humans - than others?"

First off, 'parasitic' is the perfect word to use. In fact, scientists like to refer to viruses as "obligate intracellular parasites". This means that viruses are obligated to hijack a cell (i.e. get inside the cell - thus the 'intracellular') in order to survive - making them a type of parasite.


In fact, viruses are part of a group of so-called "genetic parasites". Genetic parasites straddle the boundary between living and non-living; they are pieces of genetic material (sometimes encapsulated in a protein sheath, as are viruses) that rely on other organisms to some degree for their own reproduction. There is an impressive diversity and abundance within these genetic parasites - and in fact, viruses seem to be the most abundant "biological entities" (thus referred to because scientists still don't agree if viruses are alive or not) on Earth. Nearly all forms of life, including viruses themselves, are infected by these genetic parasites.


But why do they even exist, as Michael asks? What is the reason for this parasitic behaviour?


Well, some would argue that parasitism is intrinsic to evolution, and to life itself. As these scientists put it, "The entire course of the evolution of life is a history of host-parasite co-evolution".


Let's dig in to that statement, shall we?


First off, let's look into why genetic parasites like viruses would even evolve in the first place. As suggested in this paper, you can think of the situation in terms of game theory. The parasites are the cheaters in the system: they take advantage of the available resources without contributing. In more biological terms, genetic parasites take advantage of the resources of their hosts, just because they can. But how does this parasitic relationship arise initially?


Life leaves room for parasites as a side-effect of evolution. Evolution is key for sustainability of life on earth: it allows us living organisms to adapt to changing conditions, and withstand challenges as a population. Evolution ensures variability, so that different individuals in a population have different strengths and different weaknesses - which makes sure that all together, we are strong. However, in order to evolve, and gain all of the diversity and strength that comes with evolution, there is a cost: we cannot make carbon copies of ourselves.


You can think of it this way: in a hypothetical unchanging world, where it is always sunny, dry, warm, and with unlimited food, loads of carbon copies would probably do pretty much fine as a population. The limitations of carbon-copy organisms only become clear under duress. If that hypothetical world suddenly became cold, dark, and barren of food, those carbon copies might all have the same weakness. Say for example, that our carbon copies were particularly vulnerable to hypothermia, having always lived in a warm world. The whole carbon copy population could then all die out in one winter storm. Evolution, by ensuring variability, would have given that population a better chance at survival: some small random mutations could have potentially made some individuals in the population less temperature sensitive, allowing them to carve out a new life in the colder, darker hypothetical world.


So, you can see why evolution is a pretty nice trick for a population to have up its sleeve. However, if we want to have evolution, we have to take the good with the bad. I can't put it more eloquently than this:


"By chance and the forces of evolution, as long as complex life exists, parasites will have a chance of evolving. If a stable population that harbours resources necessary for reproduction exists, a population of parasites that steals some of those resources for their own reproduction can comfortably emerge. In order to prevent the evolution of parasites, evolution would more or less have to be halted also for the cells."

This statement is evidenced by the fact that pretty much every single type of life on earth is parasitized in some way. New groups of parasites also emerge at different points in evolution, taking advantage of small and large changes in the way that lives are currently led on Earth. So, to answer Michael's first question, asking for "A sound reason for this kind of 'parasitic' invasive behaviour", all I can say is, where life can - it will.


Now, to get to the second question - "Are some kinds of invasive, parasitic, 'hijacking' behaviour by viruses more dangerous - to humans - than others?"


In a word, definitely.


Sticking with the theme of evolution, in the long-term, hosts and parasites go through processes of evolution together. And eventually, they can learn to co-exist. Actually, in the ideal scenario, a virus shouldn't want to kill its host. Think in terms of a macromolecular parasite for a moment. If you're harbouring a worm in your intestines, the ideal situation for that worm is for you to stay alive a long, long time so that it never has to move house to somebody else's gut. From its safe, warm lodge in your intestines, it can happily eat your pre-digested food and reproduce, sending its worm babies out into the great pearly toilet bowls of the world to go off and find new and unknown guts to colonize.


For example, in my opinion Herpes viruses have an amazing strategy. Humans have been dealing with herpes for a looooong time. Herpes viruses are an annoyance for sure (nobody likes cold sores), but definitely bring nothing close to SARS-COV-2 level fear. When you get a cold sore, which is caused by one specific type of Herpesvirus, you just put on some cream and get on with your life. This virus has evolved to live in a relative degree of harmony with us humans, which is totally beneficial to the virus, as reflected by the fact of so many humans being infected with it.


However, I don't think I could argue that being infected with a Herpesvirus is also beneficial to us humans. But, in the broader context of infection with genetic parasites, there still remains potential for mutualism - i.e. a mutually beneficial scenario. We can get some insight into this potential by looking at plant viruses.


Although plants can indeed be infected with highly parasitic viruses, which kill the plant host, there are also cases of mutualism. For example, some plant viruses can boost the fertility of infected plants. This is nice for the plant, because it can have more plant babies, but also nice for the virus - because it can spread with the plant's seeds and infect all of those plant babies.


It's been suggested theoretically, but also by experiments in the lab, that mutualistic relationships like this can eventually evolve out of parasitic relationships. The idea is, the longer that a host and its parasite co-evolve, the better they'll be able to live with each other.


Interestingly, one way in which we can learn to live with our genetic parasites is to actually have them become part of us. If you look at the genetic material of plants, up to 90% of their genome can be derived from parasites. The plants took on useful sequences from their genetic parasites and claimed it back for themselves! Strikingly, we're no different. In vertebrates like us humans, at least 50% of our genomes can be traced back to genetic parasites. Stealing back genetic information from parasites is more of a rule than an exception, and has been suggested to be responsible for key moments in evolutionary history - like the development of immune systems in the first place. More specifically in humans, genes derived from genetic parasites are involved in core biological processes that make us who we are - such as placenta formation, cognitive functioning, and even immune defense. If these parasites hadn't crept into and altered our genomes, we would not be who we are today.


So, to summarize my answer to Michael's second question, of whether some kinds of invasive, 'hijacking' behaviour by viruses is more dangerous to humans than others, I can confidently say yes. On one hand, we have the SARS-CoV-2s of the world, wreaking havoc on the human population. On the other hand, we have all the parasite-derived DNA inside of us, which is utterly necessary for making us who we are. As with many things in biology, our relationship with genetic parasites is a balance.


Until next week - happy evolving.


~Alex

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