Exceptional sturdiness among tardigrades is linked to the fact that they have the highest concentration of foreign DNA across all of the species on Earth, research has shown.
The study, published on Monday, November 23 in the Proceedings of the National Academy of Sciences, was carried out by experts at the University of North Carolina, Chapel Hill.
The objective was to analyze the genetic makeup of tardigrades (also known as moss piglets or water bears). This eight-legged aquatic species is considered to be the most indestructible on Earth, given that it can remain alive even when being faced with extreme environmental stimuli.
More precisely, it can survive temperatures verging from absolute zero to well beyond the boiling point of water (212 degrees Fahrenheit). In addition, it can withstand radiation doses several hundreds of times higher than those which could kill a human being, as well as pressures six times higher than those encountered in the deepest oceans.
Moreover, it can’t be defeated by the vacuum of outer space, and it can stay without nutrition and water for over 10 years, to the point where its total body water percentage reaches just 3%.
Despite these exceptional qualities, the tardigrade doesn’t actually qualify as an extremophilic, given that it doesn’t actually thrive in life-threatening conditions, but simply manages to endure, without exploiting these radical changes.
By reviewing this incredible species’ genome, experts concluded that it actually contains around 6,000 genes from external sources.
Most of this foreign material comes from bacteria, while smaller percentages are represented by plants, fungi, viruses and Archaea (strangely shaped prokaryotes especially common in plankton).
It’s not the first time that researchers have encountered organisms which incorporate DNA through their interactions with others.
This process known as horizontal gene transfer has been identified for instance among bacteria, one species transmitting its antibiotic resistance to others, which eventually leads to the development of new, potentially lethal superbugs.
However, while the phenomenon may be extremely common among single-celled organisms, it’s highly unlikely when it comes to complex, multi-cellular ones. Specifically, foreign DNA among such species accounts for just 1% of their total genome.
In contrast, tardigrades rely on other organisms for around 17.5% of their genetic makeup, thus vanquishing rotifers, which have 9% foreign DNA, and had previously been thought to have the highest amount of genes from other species.
This may explain why water bears are virtually indestructible, and one of their more awe-inspiring properties could be the key in understanding how they manage to store so much genetic material from the outside.
Namely, during the long periods of time when they are left without water, they undergo a process of desiccation (drying out). Their cells become much more permeable, and when they re-hydrate they collect DNA material left behind by other organisms, whose genes had also broken apart.
During reproduction, this new genetic inheritance is passed to their offspring as well, and through several cycles of this kind, previously external genes become part of the population’s genome also.
This phenomenon suggests that the more tardigrades are exposed to environmental stress, the more they increase the acquisition of genes from outsiders, thus becoming sturdier.
In fact, beneficial foreign material could turn into an evolutionary advantage, allowing some organisms to thrive, while others with less useful DNA would become outnumbered.
As study author Bob Goldstein explains, this discovery is unprecedented, since until now scientists had been unaware that gene transfer could occur at such a significant rates, and that such a large chunk of a complex organism’s DNA could have foreign origins.
Also, this might mean that the entire tree of live might have to be re-evaluated, and possibly replaced by a “web of life”, to account for the complex interactions and interferences which take place between species.
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