A secret about the most resilient animal in the world
The world’s most resilient animal is not an elephant who has large body size, not a penguin who can tolerate cold, nor a cockroach who is famous for its vitality. The real champion is a micro-animal: Tardigrades, also known as ‘water bears’.
From the high mountains to the endless deep sea, from the hot springs to the Antarctic ice layers, even the New York city, water bears can be found. They can enter an almost unbeatable state to cope with extreme environment. Their eight legs are curled up, rounded body becomes shrivelled and wrinkled. They expel almost all the water in the body and reversibly suspend their metabolism and enter a state called cryptobiosis. They are super difficult to be killed in this condition because they do not need water or food. They can easily to survive under the temperature near absolute zero temperature or up to 151℃. They can also tolerate the great pressure of the deep sea and withstand 1,000 times more radiation than other animals. Also, it is reported that they are tolerate high levels of environmental toxins. In addition, water bears are the first known animal to survive in outer space. Some researchers said that their only weakness is they can’t tolerate mechanical injury, in other words, you can squeeze them to death.
In order to uncover the secret of water bears’ tolerance mechanism, scientists have conducted many experiments. Last year, a group of geneticists have sequenced the genome of a freshwater Tardigrade (Hypsibius dujardini). They claimed that thee have discovered a huge number of exogenous genes from other organisms in the genome of this tardigrade and thereby they made a hypothesis that the water bears obtaining tolerance genes from other organisms in the environment via Horizontal Gene Transfer (HGT). But soon, some scientists questioned that this result is not reliable because they considered that this genome was contaminated with other micro-organisms.
However, recently, researchers from Japan have found a magic weapon – an anti-radiation protein that used by water bears to against the extreme environment though genome analysis. With this protein, water bears can significantly reduce the damage of their DNA under the exposure of X-ray radiation.
These researchers have chosen one of the most resilient water bear species – Ramazzottius varieornatus to run the genome sequencing and studied their gene expression at different stages. In order to avoid the microbial contamination during sequencing, they used hypochlorite and antibiotics for sterilization.
The sequencing result showed that the percentage of genes from other organisms in the environment in water bears is just 1.2%. Therefore, they concluded that the key genes to resist the extreme environment in water bears come from their own evolution, rather than other exotic organisms.
What is special about water bears’ genome? Compared with others, it has more superoxide dismutase genes, as well as more copies of the MRE11 gene (used to repair the break of DNA double-strand). These genes can help to reduce the oxidative damage, repair broken DNA and thereby playing a protective role in water bears. In addition, they also discovered that, the water bears also lack of some of the genes associated with stress response, which may also help them to overcome the extreme condition.
Through the study of genome and gene expression, scientists have also found a unique trick of water bears to cope with radiation damage – Damage suppressor (Dsup). The genes associated with these proteins begin to express themselves since the embryonic period of water bears. These proteins can combine with DNA and act as a ‘shield’ to against the radiation damage. These proteins can also play a protective role when being expressed in other cells. The human cells cultured in the laboratory by researchers also expressed Dsup. As a consequence, cells expressing the Dsuo proteins were still active and maintained a certain degree of cell activity and proliferation capacity when compared with the ordinary cells (control group). Also the DNA damage was reduced by 40% compared with control group.
With the further study of these resistance mechanism, we might someday improve ourselves’ or other animals’ ability to tolerate extreme radiation, or perhaps become more adaptive to the outer space environment. However, it is not promising. Researchers also mentioned that the Dsup protein is not a panacea. Although its effects are obvious, it only reduces about half of the DNA damage.
In order to have a full understanding of the function of water bears’ genome, further research is required. Follow the steps of the most resilient animal in the world, human may also learn more about how to survive in an extreme environment.