New genome sequences have revealed exciting new information about the origins of tardigrades as well as the genes that underlie their extraordinary ability to survive in extreme conditions.
A team of researchers led by Mark Blaxter and Kazuharu Arakawa from the universities of Edinburgh, Scotland and Keio, Japan respectively, have carefully stitched together the DNA code for two tardigrade species.
What are tardigrades?
Tardigrades (also known as water bears or moss piglets) are water-dwelling, eight-legged, segmented micro-animals.They were first discovered by the German zoologist Johann August Ephraim Goeze in 1773. The name Tardigrada derives from the Italian word for “slow stepper”, and was given three years later by the Italian biologist Lazzaro Spallanzani. Tardigrades are famous for their amazing ability to withstand complete dehydration, resurrecting years later when water is again available. They can be desiccated, frozen, exposed to radiation or sent into space, and still they cling to life. Tardigrades have been found everywhere: from mountaintops to the deep sea and mud volcanoes, from tropical rain forests to the Antarctic.
Tardigrades hit the news recently when it was suggested that their DNA was a mix of animal and bacterial segments, rendering them “Frankenstein” hybrids. However this study disputes the Frankenstein theory by arguing that tardigrade DNA looks “normal,” with no evidence that these special animals use extraordinary means to survive.
It was also previously thought that tardigrades take up large numbers of foreign genes from bacteria. However, these mixed results were proved to be simply the results of contamination.
These “normal” gene sequences are still exciting. Indeed, the tardigrade as an animal is an incredibly enigmatic and fascinating creature. Measuring less than a millimetre in length, tardigrades are too small to leave fossils. However, using the new genomes, the research team was able to explore what the DNA could tell them about where tardigrades sit in the tree of animal life.
Tardigrades are a distinct type of animal whose closest relatives are nematodes (roundworms) and arthropods (insects, spiders and their allies). It was previously unknown as to which group was more closely related to tardigrades. While the accepted view is that their four pairs of stubby legs make them more closely related to arthropods, the DNA evidence surprisingly strongly favoured a closer kinship with nematodes.
The researchers then looked at a set of genes — the so-called HOX genes. Hox genes (a subset of homeotic genes) are a group of related genes that control the body plan of an embryo along the cranio-caudal (head-tail) axis. The Hox proteins determine the type of segment structures (e.g. legs, antennae, and wings in fruit flies or the different types of vertebrae in humans) that will form on a given segment, after the embryonic segments have formed.
There are usually about ten different HOX genes in animals, each involved with a different part of the nose-to-tail pattern. The researchers found that tardigrades were missing five HOX genes, and that most nematodes also were missing the same five genes. This may constitute further evidence that tardigrades and nematodes are closely related.
The research team was also able to identify the genes that tardigrades use to resist the adverse effects of desiccation. By examining which genes were turned on during the drying process, scientists could identify sets of proteins that appear to replace the water that their cells lose, helping to preserve the microscopic structure until water is available again. Other proteins look like they protect the tardigrades’ DNA from damage, and may explain why they can survive radiation.
“I have been fascinated by these tiny, endearing animals for two decades. It is wonderful to finally have their true genomes, and to begin to understand them. It has also been great to work with Kazuharu Arakawa and his Japanese colleagues on this – science is truly global, and together we achieved exciting things,” Professor Mark Blaxter said. “This is just the start – with the DNA blueprint we can now find out how tardigrades resist extremes, and perhaps use their special proteins in biotechnology and medical applications.”