Red queens, tiny crabs and parasitic bacteria

Even in the relationship between parasite and victim, evolution acts like the Red Queen in Alice in Wonderland - you have to run to stay put

One of the most famous principles in evolutionary research is named after a scene from Lewis Carroll's book "Through the Looking Glass and What Alice Found There" - the sequel to "Alice in Wonderland". In the scene, the Queen of Hearts wins a race where everyone runs as fast as they can, but gets nowhere. When Alice asks her about it, the queen replies that this is exactly the idea - you have to run with all your might just to stay in the same place.

The "Red Queen" principle talks about the exact same thing - run not to get ahead, but to not fall behind. One example of this is trees in a tropical forest - in order to get the amount of sunlight they need, the trees in these dense forests began to grow higher and higher to pass the tops of the trees around them. Today, the trees invest a lot of resources, which could have been invested in creating more seeds or spreading them more efficiently, in developing trunks of 50 meters and more. But the amount of sunlight reaching them has not increased since this "arms race" began - that is, the trees "ran" faster and faster just to stay exactly at the starting point. Trees that did not participate in the race and did not develop long trunks, began to lag behind and receive less and less light.

Another situation in which the principle of the red queen is manifested is the parasite and host relationship. The host develops different ways to block the parasite, the parasite tries to overcome the host's defenses, and if both run fast enough, neither side overcomes the other - and the parasite's infection rates remain more or less constant.

One of the problems in evolutionary research of parasite and host is that the researchers are able to look at the situation between the parties at any given moment, but they have no way to draw significant conclusions about the situation in the past. A little over a year ago, an article was published in Nature magazine in which Belgian researchers found a way to do exactly that - to look back at the history of a parasite and a host.

Alan Decaestecker (Decaestecker) and his colleagues studied a tiny water crab called Daphnia (Daphnia) which is one millimeter long, and a parasitic bacterium called Pasteuria ramosa. When Daphnia is infected with a bacterium, it does not die immediately, but it is unable to reproduce, and it becomes larger and darker - so it is quite easy to identify the infected crabs. The advantage of studying these creatures is that both the daphnias and the parasitic bacteria form reproductive bodies that can go into a dormant state and remain that way for years, submerged in the mud at the bottom of the lake. Different layers in the sediments in the lake represent different times, and thus the researchers were able to dig and find daphnias - along with their parasitic bacteria - representing the population that was common in the lake years ago, and grow them in the laboratory.

In the lowest layer were daphnias from 39 years ago. The researchers collected samples from 8 different layers, and exposed daphnias from each layer to bacteria from that layer, from an earlier layer, and from a later layer. It turned out that the bacteria were more successful in infecting daphnias from their layer, compared to daphnias from a previous layer - that is, those that were common in the lake in earlier times. Daphnias from a later layer were also glued less successfully than those from the same layer. The researchers concluded that at each point in time, the parasite changed to adapt to the genetic profile of daphnia that was common at the time.

One of the common hypotheses regarding the relationship between parasite and host claims that the best strategy for the host is to produce offspring that are as genetically different from each other as possible - this way it will be more difficult for the parasite to adapt and infect all the offspring, and at least some of them will remain healthy and parasite-free. A more far-reaching (and much more controversial) hypothesis suggests that avoiding parasites was the primary motive for the shift from asexual reproduction to sexual reproduction, which produces much greater genetic variation. The results from these experiments testified that the genetic composition of the Daphnia population does change rapidly over time, and that of the parasite population - along with it. The fact that the daphnias were more resistant to bacteria from later layers testified that the infection ability of the bacteria did not improve over the years in absolute terms, but at any given time the bacteria improved their ability to infect the common daphnias in the lake along with them. The results corresponded to a computer model built by the researchers, which was based on the assumption that different frequencies of genotypes (all the genes of a certain individual) are common at different times in the two populations, and a certain genotype of a bacterium infects a certain genotype of Daphnia particularly well.

Perhaps the most important discovery of the researchers was that with all the genetic changes the two populations underwent, the ability of the bacteria to infect - to the daphnias that lived together with it - did not change much. This corresponds exactly to the "red queen" principle - after all the changes, neither the cancer nor the parasite progressed beyond the starting point. This is how sediments in a Belgian lake managed to bring back to life the history of Daphnia and its parasite - and obtain particularly strong evidence for one of the most well-known evolutionary principles.