Tardigrades resist (almost) everything, thanks to genes from extinct species
Tardigrades are small but tough: X-rays, extreme temperatures, gigantic pressure - they're incredibly resilient. A new study shows that, as they evolved, they acquired genes from other species, giving them these "superpowers".
Simon Galas, University of Montpellier and Myriam Richaud, University of Montpellier
Imagine a tiny organism all around us on our planet, carrying with it a lost genetic memory. Tardigrades are invertebrates ranging in size from 0.2 to 1.2 millimetres maximum, resembling miniature teddy bears with 4 pairs of legs, muscles, neurons and a microbiota. They can be found everywhere on our planet, from the ocean floor to the top of the Himalayas.
Tardigrades are also known as water bears, because they always evolve in an environment where water is present, such as oceans, glaciers, rivers or in the gutters of houses, but also in the mosses and lichens on trees or rocks. Nearly 1,500 species are already known, and they are the champions of our planet's survival and the undisputed kings of a very select club known as extremophiles, organisms capable of surviving the most extreme environments.
In fact, tardigrades are capable of withstanding the lowest temperature measured in the Universe (-272°C) or even temperatures close to those measured on the planet Mercury (+151°C). They even manage to survive at a temperature close to absolute zero (-273.16°C), which does not exist in the Universe but only in physics laboratories. In a recent physics experiment, one of three tardigrades of the species Ramazzotius varieornatus, pictured below, was successfully revived after exposure to a temperature close to absolute zero.
Tardigrades can also survive a ten-day stay in the vacuum of space exposed directly to cosmic rays, and have become a working model for astrobiology research. As for radiation, we know that they can survive doses of X-rays 1,000 times higher than those lethal to humans. Their resistance to gigantic pressures has also been tested for several hours and, surprise, they survive being crushed by the equivalent weight of a 60,000-storey building.
Cryptobiosis: life on hold
The first discovery of tardigrades dates back to the 18th century. After studying with the Jesuits in Reggio (Calabria, Italy), biologist and philosopher Lazzaro Spallanzani (1729-1799) published his first study of these small animals in 1776, in his "Opuscules de physique animale et végétale". He gave them the name tardigrade and observed their ability to dehydrate completely and then "resuscitate after death" in the presence of water, describing for the first time the phenomenon of cryptobiosis.
Cryptobiosis is a "suspended state of life" during which no life indicator is detectable. In this state, tardigrades collected in Antarctica have been successfully reawakened after thirty years. Other data have shown that tardigrades in cryptobiosis are not only in a "suspended state of life", but also "out of time". Indeed, the time spent in cryptobiosis is not deducted from their normal lifespan (the average lifespan of a tardigrade species in controlled rearing is around 60 days). In short, whether it enters cryptobiosis or not, a tardigrade will not see its normal active life expectancy modified. The Anglo-Saxons call this phenomenon "Sleeping Beauty", indicating that an organism stops aging as long as it remains in this state.
Our CNRS laboratory in Montpellier was the first to successfully observe what happens inside a species of tardigrade(Hypsibius exemplaris) when it enters cryptobiosis. In this state, the species miniaturizes, losing 38% of its volume, and builds a kind of visible rampart around each of the cells that make up its body. This structure gradually disappears during reanimation.
Different survival strategies for different species
Most surprising of all, however, is a recent study by our laboratory of a related species(Ramazzottius varieornatus), also from our farms. When it enters cryptobiosis, this species miniaturizes by only 32%. Even more surprisingly, it was impossible to observe the presence of the cryptobiosis-specific rampart that surrounded the cells of the previous species. These experiments indicate that different tardigrade species are capable of withstanding stresses that are lethal to other living species, but that they do so in different ways and using tools that are not all common to them.
As of 2016, this set of genetic tools enabling them to withstand extreme environments began to be identified on the occasion of the first sequencing of their genomes. These tools are already of interest to scientists for future revolutionary biomedical applications, such as preserving drugs and vaccines in dehydrated form, or protecting cells from lethal radiation, which would be useful for future space missions.
Geneticists believe that these genes were acquired by tardigrades to enable them to resist dehydration, but they also propose that it is these same genetic tools that enable them to resist all types of lethal environments. Studying their genetic equipment, the scientists were surprised to observe that almost 40% of tardigrade genes are unknown in other species currently living on our planet.
But where do these so-called "unique tardigrade genes" come from?
One explanation involves the mechanism of horizontal gene transfer (or HGT). As illustrated below, a living organism classically inherits genes vertically from its parents.
Acquiring your neighbors' genes
In the case of horizontal gene transfer, the organism has the additional option of acquiring genes from its neighbors and retaining them if they prove advantageous to the survival of its species. This has already been observed in a species of aphid in which the green individuals are eaten by ladybugs, while the red ones are parasitized by wasps. One aphid had the "good idea" of acquiring a fungus gene by horizontal gene transfer and adopting a yellow color that protects it very effectively against these two predators.
More recently, a new species of tardigrade identified in China revealed that it had acquired the gene from a bacterial species enabling it to protect itself against lethal doses of X-rays. For both these examples, the organism from which this genetic gift originated has been identified because it is still alive, but for the unique genes of tardigrades, this is not possible.
It would seem that tardigrades, which have inhabited our planet for around 600 million years, have had time to acquire numerous genes by horizontal transfer from species now extinct, building up a veritable library. This is all the more likely given that tardigrades have resisted the 5 major extinctions of living species that our planet has experienced in the course of its history. The most recent of these was the extinction of the dinosaurs. A small number of these unique tardigrade genes have already been identified and given bizarre names such as Dsup, TDR1, CAHS, SAHS, MAHS, TDPs, LEA, Doda1 and Trid1.
Placed in human cells or other laboratory organisms (Drosophila, bacteria, yeast, plants, etc.), these genes have been able to dramatically increase their resistance to normally lethal treatments such as X-rays, ultraviolet light or powerful oxidants. What's more, proteins derived from some of these genes have been able to protect drugs from dehydration, enabling them to be preserved at room temperature, revealing enormous potential for vaccine delivery without the need for costly freezers. The future use of these unique tardigrade genes in the biomedical field is already the subject of numerous patent applications, heralding revolutionary new biomedical technologies that could soon follow, ranging from the protection of astronauts' epidermis against cosmic rays to the possibility of preserving drugs, tissues or organs by dehydration pending their use.
A "DNA fragrance
But where does the DNA that can be incorporated by tardigrades come from? The answer lies all around us. We are constantly bathed in a "DNA perfume" released by all the living organisms around us. This DNA is called eDNA for environmental DNA. By sequencing the DNA contained in a soil sample, we can determine which living species inhabit a given area, even without having seen them. This is a highly effective technique for assessing the biodiversity of a terrestrial or marine environment. Recently, scientists were able to identify the DNA signatures of Asian elephants and giraffes from samples taken from a spider's web some 195 metres away at Perth Zoo in Australia.
Scientists have devised a possible scenario to explain how these pieces of eDNA can be found in tardigrade species, as well as in some worms and a few other invertebrates. These organisms all share the ability to survive dehydration for varying lengths of time.
When they are in cryptobiosis following dehydration, we observe the progressive appearance of chromosome breaks.
Tardigrades will be able to repair this damage as soon as they are rehydrated. Water is potentially capable of transporting fragments of eDNA to the cell nucleus, where the chromosomes are located. Their presence in the midst of fragmented tardigrade chromosomes means that they can be integrated at the same time as the repair mechanisms are at work.
With their ability to capture new genes from their environment, tardigrades have accumulated genes with exceptional properties from species that have long since disappeared from our planet. These unique tardigrade genes may hold the secrets of future biomedical revolutions, offering new possibilities for the protection and transport of drugs and fragile tissues, new protection for future missions already programmed by space agencies, or dermocosmetics to combat the effects of aging.
Simon Galas, Professor of Genetics and Molecular Biology of Aging, IBMM CNRS UMR 5247 - Faculty of Pharmacy, University of Montpellier and Myriam Richaud, Doctor in Genetics and Molecular Biology of Aging, Faculty of Pharmacy, University of Montpellier
This article is republished from The Conversation under a Creative Commons license. Read theoriginal article.