A new champion in the crazy genome sweepstakes has been crowned. And it is a tardigrade.
A tardiwhat? A tardigrade, or better known as those lovable little “water bears” or “moss piglets.” These tiny (<1mm) animals have always held the fascination of biologists not just because they are so darn cute and photogenic but because they are quite possibly the toughest organisms on Earth. How many living things to you know that you can boil, cool to several hundred degrees below zero, put outside of the space shuttle, and slam with enough ionizing radiation to kill 99% of all living things and expect to just keep on going like nothing has happened to them?
Yeah, those are some tough critters and there are lots of them around if you know where to look. There are over 1100 species and as a group they are so unique they are placed in their own phylum.
Now, we find out that tardigrades have yet another amazing and unique attribute: their genome. Scientists have wondered, how do these little critters survive in such extreme environments. To learn more they have sequenced the DNA genome of one species of tardigrade. The most surprising thing they learned was that at least 17.5% of their genes are not animal genes but instead are from bacteria, plants, fungi and viruses. Specifically, they have identified a total of 38,145 genes in their genome. That is 10,000 more genes than our own genome contains which in itself is an interesting story. Of those genes they determined that 6663 of them had far less similarity to animal genes than they did to other major groups of life. Most (92%) matched sequences from bacteria but there were also genes with greatest similarity to 91 different fungal species, 45 plant species and 6 different viruses.
Most organisms, including humans, have large amounts of “foreign” DNA in their genomes, the result of transfer of DNA from one organism to another most likely though a viral intermediate but usually this extra DNA is usually not in the form of functionally active genes. What makes the tardigrade genome so special is that so many genes from other organisms are in their genome and are actually being used by the tardigrade. These little animals truly are the Frankensteins of the microscopic world. They are a combination of animal, fungus and plant with a bit of virus thrown in for good measure.
What are these genes good for? Some of “foreign” genes provide functions to the tardigrade that no other animal studied so far has. Many of the new genes replace the animal version of the gene possibly providing the same function but with a different DNA sequence. Other genes are additions to what are large gene families in animals. For example animals have large numbers of genes responsible for making proteins that are important in stress response activities such as DNA damage repair, antioxidant production, heat shock response and lipid production. Tardigrades appear to have added genes from many other organisms to the typical set of animals genes giving them a wider number of responses they can make to environmental extremes.
The ability for genomes to integrate foreign DNAs into themselves and find new uses for genes is becoming more widely appreciated as more genomes are sequenced and the mechanisms for transfer become better understood. However, widespread recombination of genetic material across different kingdoms is not thought to be a common process for many reasons. The tardigrade is probably very unusual among animals but can still teach geneticists a lot about what kinds of genome architectural changes are possible.
How has the lowly little water bear managed to abscond with so many genes from other organisms and make them their own? That is one of the questions that scientists will be looking at but they have an idea. In times of stress these little critters may dry up and their genomes are known to break into hundreds of thousands of little pieces. Upon rehydrating their DNA repair enzymes stick their DNA strands back together effectively resurrecting their genome. But during the rehydration process, along with water re-entering their cells, pieces of DNA from the environment which include pieces of bacteria, plants and fungi may also enter the cells. As the water bears stitch their DNAs back together once and a while they probably insert pieces of this foreign DNA into their genomes. If they are inserted in the right location, they may take on new function in the water bear and they will go merrily on their way.
“Evidence for extensive horizontal gene transfer from the draft genome of a tardigrade. Boothby et al. PNAS, November 2015 http://www.pnas.org/content/early/2015/11/18/1510461112.abstract
Another story about the remarkable genome of tardigrades can be read here: http://www.theatlantic.com/science/archive/2015/11/tardigrades-worlds-toughest-animals-borrowed-a-sixth-of-their-dna-from-microbes/417243/