Just over 35 million years ago, a determining and unusual genetic hijacking took place. Somehow, a self-defense gene from a plant passed into the cell interior of a whitefly. Once inside, it stayed there and from there it was inherited until it spread to all of them. Since then, the whitefly has used its function as a shield and is able to resist the attacks that many plants launch against all kinds of insects. Today, and by virtue of this gene, the whitefly is one of the most destructive pests in the world.
Kidnapping is a case of what is known in the jargon as horizontal gene transfer. This assumes that the passage of genetic material is not directed to descendants, as occurs in the traditional and vertical way, but to companions in the environment.
The evolutionary leap is understood with the example of Lynn Margulis: “It is something like going to a pool, entering with brown eyes and leaving with blue eyes, just because you have swallowed water”
In some cases the result is an almost immediate evolutionary leap and it is understood with the comparison made by the microbiologist Lynn Margulis: “It is something like going to a pool, entering with brown eyes and leaving with blue eyes, just because you have swallowed water. “. It is very common among microorganisms, but much rarer in cells such as those of plants or animals.
This is the first time that such a jump has been described and with such an obvious function between a plant and an animal. The inaugural study is published in the journal Cell Y, in the words of Charles Davis, an evolutionary biologist at Harvard University, “is really cool.”
The spear and the shield
“There has been a lot of debate in recent years about whether something like this was actually produced in eukaryotic cells (such as those of plants and animals)”, acknowledges Luis Boto, evolutionary biologist at the Museum of Natural Sciences of the CSIC in Madrid and what He has been researching this type of transfers for years. “The new study clearly and in detail shows that it can happen. In addition, for the first time the leap of a trait by horizontal transfer from a plant to an animal is verified, overcoming barriers and providing it with a defense against weapons material. of that”.
The researchers weren’t even looking for something like that. What they were trying to do was to find out in a general way what allowed the whitefly to roam freely, to feed without complexes on the sugary sap of hundreds of plants that other insects find more or less toxic. And cause, incidentally, pests that destroy a multitude of crops with enormous economic consequences.
The natural pesticides found in many plants are called phenolic glycosides, molecules that basically consist of a sugar linked to an acid. The researchers searched the fly’s genome for what might be helping them to resist their attack and that, at the same time, was not in other more or less close relatives. The gene they found that looked better is called BtPMaT1. And that’s where the surprise was.
The gene contains the instructions for making a protein that adds a chemical group to pesticides, rendering them harmless. But not only was it not in other insects, but they did not even have any other that resembled it and that evolution had been modifying. Where did it come from then?
Francis Mojica identified the viral origin of the CRISPR sequences on your computer, cutting and pasting letters in different databases. Those were the times before the internet explosion. Now, the researchers launched a much more powerful search for gene-like sequences across multiple genome libraries. They found them, but they were only on plants. Genomic archeology has allowed them to establish that the insect took the gene sometime between 35 and 80 million years ago and has spread thereafter. How and from which plant they did it they do not know yet, although it could well have been through a virus, they suspect.
From there they did a whole series of checks. They saw that the gene was integrated into the genome of the insect, as one more. Who worked, mostly in the gut, and it worked as predicted. Instead of waiting for evolution to generate and support desperately slow and gradual changes that it could not promise, the whitefly gobbled up a gene and dodged thousands of years in one bite. He went in and out of Margulis’s pool.
“It is a sign that evolution can incorporate genes from other organisms and that they can help them survive better”, said Ted Turlings, one of the co-authors. “This case is something extremely rare, but if we consider that we are talking about billions of insects and plants interacting for millions of years, becomes more possible“.
It is not actually the first case to be found. Just a few months before, Colombian researchers described another gene that he had jumped from a plant to the whitefly itself. The difference is that, for the moment, they have not been able to demonstrate that it is fulfilling a function.
BtPMaT1 it seems that it does. The researchers designed an original experiment to see how useful it was for the fly. They generated a transgenic tomato plant that produced a particular RNA, an RNA that would inactivate the gene when flies ate from it. Within a week of starting the experiment, almost 100% had died. At the same time, only 20% of those who ate a normal plant did so. The gene is crucial to them.
At the same time, the modification did not affect other insects and animals with whom they tested it or the plant itself, which uses the gene to defend itself against its own toxins. This is because the fly gene looks a lot like yours, but time has modified it to be different enough. Ecologist Jonathan Gersenzhon comments in the magazine Nature that it would open doors to new ways of controlling pests and that “it offers a huge opportunity. You could keep whiteflies away but without harming beneficial insects like pollinators.”
Authors are necessarily more cautious in their article. They launch this possibility, but acknowledge that the consequences of the modification in the plant itself and in other organisms should still be better investigated. And, amid technical comments on the experiments, six lines are reserved to compare their discovery with the story of the Chinese philosopher Han Fei, which gave rise to the term paradox (in Chinese: “spear-shield”). The story goes like this:
“In the Kingdom of Chu there lived a man who sold spears and shields.
“My shields are so solid,” he boasted, “that nothing can penetrate them.” My spears are so sharp that there is nothing they cannot penetrate.
“What would happen if one of your spears collides with one of your shields?” -They Asked.
The seller didn’t know what to say. ”
The whitefly’s response was to steal the shield.
Horizontal transfer and its impact
One of the first suspicions that horizontal transfer existed was in 1928, when in what is known as “Griffith’s experiment“It was found that a harmless strain of pneumococcus could become very dangerous when it came into contact with another virulent strain. Then, in the 1950s, it was shown that the bacteria antibiotic resistance genes could be passed on and the knowledge was accumulating, even getting to pose debates about the importance of the mechanism throughout the history of evolution and life.
In many microorganisms horizontal transfer occurs constantly and in various ways. Sequences can be exchanged for small antennas called ‘pili’ that put them in contact with each other, can collect them from the environment or use intermediaries such as viruses. In eukaryotic cells, such as those of plants and animals, it is much more difficult. This is so because “many barriers have to be overcome,” explains Boto. For example, “they must cross the membrane of the nucleus, where the DNA is (and that the bacteria do not have). And, in general, it must happen in the sex cells so that it can then pass to the offspring.”
Still, there are accepted examples that this has happened on occasion. Generally from bacteria, such as those that have given genes to many algae or such as the one that donated one to the coffee-boring insect and allowed it to better digest plant cells. But it was not only them: at some point, a mushroom gave him a gene to a variety of wild wheat and is now more resistant to certain infections. Other made the green pea aphid could be red.
If it happens in animals, for different reasons these are usually microscopic, non-sexual reproduction – like worms that can do it by fragmentation – or insects. “But not only,” says Boto, “has recently been described a case between fish of different species, probably facilitated because they release the eggs to the water and the fertilization is external “.
In humans, a study assured having found in our genome 145 genes from other species and cell types, all of them acquired before we were properly human, millions of years ago. However, the finding “is very controversial,” says Boto, and the work has been quite criticized. In any case, “it cannot be ruled out that a case may be confirmed in the future.”
What we have incorporated are virus fragments. At least 8% of our DNA is of viral origin, and although most of them are not genes as such, their role is not irrelevant. Is it then considered horizontal transfer? Although there are times when it is accepted, “it is almost a philosophical issue,” says Boto. “If we consider that it has to occur between living organisms, it is not clear that a virus is“.
What to expect then, from now on? Are we seeing the tip of the iceberg of horizontal transfer out of microorganisms or are there not many surprises left to emerge? “It is very possible that news will appear”, answers Boto. “Time and the generalized use of new techniques will give us a measure of their extension. But no matter how few cases they are, they can be very important, because they facilitate the acquisition of traits very quickly. We have just seen it with the example between a plant and a fly “.
Returning to the original article, evolutionary biologist Andrew Gloss launches in the magazine Nature and points to a truffled ecological history of crossovers: “That insects receive genes from the plants themselves is that last part of the arsenal that we had not yet found,” he says. “In the battle between plants and their insect pests or pathogens, there are genes that are drawn from the entire tree of life.”
** This article has been published correctly in the agency SINC.