The scientific discovery that viruses move frequently and unexpectedly from species to species is changing our understanding of their evolutionary history and could have troubling consequences in the form of new diseases.
When new species form, where do their viruses come from? Viruses, which are little more than a herd of free-grazing genetic material, desperately need their hosts' cellular structures and resources to reproduce over and over again. A virus without a host is nothing.
Because of this dependence, some viruses remain loyal to their hosts throughout evolution, mutating and changing slightly each time the host transforms into a new species. This process is called co-divergence. Humans and chimpanzees, for example, have slightly different hepatitis B viruses, both of which most likely mutated from the version that infected the common ancestor of humans and apes more than four million years ago.
Another option, called interspecies transition, occurs when a virus migrates to a completely new type of host that has nothing to do with the previous one. This type of viral evolution is associated with serious new diseases such as avian influenza, HIV, Ebola and SARS. And since such diseases are extremely dangerous, we are lucky that interspecies transition is a rather rare occurrence.
However, recently, when scientists in Australia conducted the first study of the long-term evolution of thousands of different viruses, they came to the startling conclusion that interspecies transition is much more important and occurs much more often than we imagined. Species change is the driving force behind most major evolutionary neoplasms in viruses. Meanwhile, co-divergence is less widespread than we expected, and it mainly causes gradual changes.
“They have shown very convincingly that co-divergence is the exception rather than the rule,” said evolutionary biologist Pleuni Pennings, an assistant professor at the University of San Francisco and not involved in the Australian study.
These findings by no means mean that new diseases arising from interspecies transition are a more serious and imminent threat than medicine assumed. However, they show that the evolutionary dynamics of viruses can be surprisingly complex. If scientists underestimated the frequency of the transition of viruses to new hosts, then in this case, it becomes a very important priority to study which viruses are most susceptible to this.
There are many reasons why interspecies leaps are unlikely to have a significant impact on the evolution of viruses. The obstacles preventing a virus from successfully passing to a host from another species are very serious and formidable. If the virus is unable to manipulate the host's genetic material and reproduce, then this is a dead end, the end of a branch. The virus may need many attempts to infect a new host, which it has been making for decades or even more, accumulating corresponding mutations at this time. He does this until he asserts himself and begins to multiply and spread.
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Last spring, for example, a group of biologists and biomedical researchers led by Susan VandeWoude, professor of comparative medicine at the University of Colorado, gave an example of what might be called incomplete interspecies transition. Vandewood researches lentiviruses. This is the type of retrovirus that HIV belongs to. Its carriers are cougars and red North American lynxes. The professor, along with her research team, constantly found a certain lentivirus of the red lynx in a cougar in California and Florida. But every time, genetic data indicated that this virus appeared as a result of contact of a cougar with an infected lynx, say, when the cougar ate a lynx, and not from another infected cougar that spread it. The concentration of the virus in cougars was also low, which indicates thatthat the virus is difficult to reproduce.
In short, the virus entered a new feline host, but the host's organism was not very suitable for the parasite, and it could not settle on it properly. “In many of the transitions, there was no evidence that the new virus was multiplying in cougars,” Vandewood notes. (In contrast, Vandewood's team found that a certain form of the lynx virus migrated to the Florida panthers, which transmitted the variant they had adapted.) Because lentivirus transitions from one feline species to another occurs so often, it can mutate quite strongly over time, after which the cougar will become a suitable habitat for him. But so far this has not happened, although there were plenty of such opportunities.
Moreover, when viruses successfully jump from one species to another, they can become a victim of their own success. This primarily applies to small isolated populations (this is how many new species were born). Dangerous viruses can very quickly destroy available hosts, after which they disappear on their own.
For this reason, virologists can say with a high degree of confidence that even if interspecies jumps over a wide time frame occur frequently, co-divergence of viruses and their hosts may be the norm. But there is little experimental evidence to support this assumption. “Ideal co-divergence is one of those phenomena that you can learn about. But if you try to find good examples of this kind of co-divergence, it turns out that they are very, very rare,”says Pennings.
Biology professor at the University of Sydney, Edward Holmes, and his Australian colleagues decided to solve this mystery. Using data on the viral genome, they reconstructed the evolutionary history of 19 major viral families, each of which contains from 23 to 142 viruses inhabiting a variety of hosts, from mammals to fish and plants. They created phylogenetic (evolutionary) schemes for virus families and for their host species, and then compared them. Scientists reasoned as follows: if a virus basically co-diverts with its host, evolving with it, then in this case the phylogenetic scheme of the virus should be similar to the scheme of its host, since the ancestors of the virus must have infected the ancestors of the host. But if the virus jumps from host to host,the evolutionary patterns of hosts and viruses will look different. How different is it? It depends on the number of interspecies transitions.
In their work, published in the journal PLOS Pathogens, they reported that in all 19 families of viruses, interspecies transitions were widespread. Holmes said it came as no surprise to him that every viral family they studied looked like it was making interspecies leaps. But he was surprised at how often they made such jumps throughout their history. “They all do it,” Holmes said. "And this is something out of the ordinary."
Concerning the question of why scientists had not previously realized how important interspecific transitions are for the evolution of the virus, Holmes explained that in the past, authors of phylogenetic studies have often considered the problem too narrowly, studying a rather small number of host species and viruses, and doing it in a small time frame … In 10 or 20 years, you may not get an interspecies leap. “And in a million years this has definitely happened,” Holmes said.
His innovative approach “provides insight into the long-term relationship between hosts and viruses,” said John Denn, associate professor of biology at Queens College, of the study.
Understanding how and why interspecies transitions occur was helped by Holmes and his colleagues' observation of RNA viruses (which use RNA as genetic material). They concluded that such viruses cross-species much more frequently than DNA viruses (which use DNA). “This is probably due to the fact that they have a higher mutation rate,” said Vandewood. With a combination of a smaller genome and a higher mutation rate, the RNA virus has a better chance of adapting to the environment of the new host.
In addition, Holmes explains this trend by the different life cycles of RNA and DNA viruses. Infections with the participation of RNA viruses are often difficult, but they are short-lived, that is, the disease comes and goes rather quickly, as is the case with the flu or the common cold. This transience leads to the fact that the virus may miss the opportunity to become part of the emerging host species. “In a dangerous virus, the damaging effect lasts for days or weeks,” says Holmes. “And on average, co-divergence in such cases is rare. It's just that the virus disappears pretty quickly."
But infections involving the DNA virus are often chronic. When a portion of the host population deviates from its typical shape to create a new species, it is more likely to take the virus with it, since many more hosts are infected. Thus, the likelihood of co-divergence between the virus and its new host increases.
The host's lifestyle also plays a role in the transition of viruses and in the co-divergence of these interspecies leaps. “We know that host population size and density are very important, and that factor determines how many viruses they carry,” Holmes says. He cites bats as an example. Bats tend to carry a large number of different viruses, but this is partly due to the fact that there are a huge number of bats. Such large populations are more likely to catch the virus. “There is a very simple ecological rule: the more hosts, the more dangerous viruses they can carry,” Holmes notes. "It's just that the virus has a better chance of finding a vulnerable host."
In 1975, Francis L. Black of Yale University wrote a research paper that provided an in-depth understanding of how host population dynamics affect human disease. Having studied the rather isolated and small communities of the Amazonian aborigines, scientists have found that chronic viral infections in these people occur quite often, but acute infections are mostly absent. Isolation protects these tribes from new viruses. Those few dangerous viruses that nevertheless got into the indigenous communities soon died out. They had few hosts to survive, and therefore the viruses disappeared quite quickly.
The finding that interspecific transitions occur frequently may cause considerable concern, as they are associated with dangerous new diseases. In the past, there were many jumps and they happened frequently. So what the future has in store for us - the same, but in large quantities?
Not necessary. “Statistics of interspecies transitions from the past do not always accurately predict the future, especially when it comes to humans,” says Pennings. Our lifestyle today is also different from how people lived only a few centuries ago, and therefore the risk of contracting new diseases seems to be different for us.
A person is also a carrier of a large number of viruses. Our populations are too large, and we are incredibly mobile, which means that we quite easily and simply transmit viruses to new susceptible hosts. “We do many things that increase the chances of virus transmission. We love to poke our nose in places where we shouldn't go, we take risks too often, we eat what we shouldn't eat,”says Vandewood. “We are probably the worst offenders of the rules, and therefore most often we become the objects of interspecies leaps - simply because we sometimes commit insane acts.”
Such insane acts often lead to collisions with other species. The more often we do this, the more we are exposed to new viruses. The species with which we come into contact most often endanger us. “We're more likely to get infected with something from mice than from tigers,” Pennings says.
However, further research into the history of the evolution of viruses will help scientists understand if there are species to which we should pay more attention as sources of new infections. (Epidemiologists are already closely monitoring viruses transmitted from poultry to humans, because they fear bird flu.) Viruses from plants, fish and mammals are probably just as dangerous to humans. It is equally possible that in research to predict the next epidemic, scientists will narrow their focus to a few high-risk groups.
Holmes has a different point of view. “I don’t think that forecasts in this case can be effective,” he says. “I understand why this is being done, but the number of new viruses that we are detecting is huge, and therefore forecasts in this case are simply unsuitable.”
Fortunately, this kind of analysis has become easier with the advent and development of metagenomics, as the branch of genomics is called, which studies not the genome of an individual organism, but the totality of genomic information obtained from the environment. As part of such research, Holmes and colleagues select genomic sequences from a variety of available databases. They do not need physical samples of viruses, and this in itself is an innovation in the field of research. "Virology is moving to a new stage where metagenomics can be used to sample massively to see what's there," Holmes says.
He also notes that new information about viruses is more available today, and therefore the phylogenetic schemes created by him and his colleagues in the near future will undergo major changes. “In three years, these schemes will be much more complete, because we will find so many new samples of these viruses,” Holmes promises.
Mallory Locklear