But leaves phylogeny unclear. From ScienceDaily:
Tardigrades are microscopic animals, justly famous for their amazing ability to withstand complete dehydration, resurrecting years later when water is again available. Once desiccated, they have been frozen in ice, exposed to radiation, sent into space vacuum… and still they spring back to life.
Tardigrades became more famous recently when it was suggested that their DNA was a mix of animal and bacterial segments, making them “Frankenstein” hybrids. The new research has now laid the Frankenstein idea to rest by arguing that tardigrade DNA looks “normal,” with no evidence that these special animals use extraordinary means to survive. Previous ideas that they might have taken up large numbers of foreign genes from bacteria are shown to be due simply to contamination.
But what is “normal” to a tardigrade is still enigmatic and exciting. At less than a millimetre in length, tardigrades are too small to leave fossils, but using the new genomes, the scientists were 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 arthropods (insects, spiders and their allies) and nematodes (roundworms). But which is closest? 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.Paper. (public access) – Yuki Yoshida, Georgios Koutsovoulos, Dominik R. Laetsch, Lewis Stevens, Sujai Kumar, Daiki D. Horikawa, Kyoko Ishino, Shiori Komine, Takekazu Kunieda, Masaru Tomita, Mark Blaxter, Kazuharu Arakawa. Comparative genomics of the tardigrades Hypsibius dujardini and Ramazzottius varieornatus. PLOS Biology, 2017; 15 (7): e2002266 DOI: 10.1371/journal.pbio.2002266 More.
Also, from Abby Olena at the Scientist:
The genomes of two species of water bears reveal clues about how they persist in extreme conditions, yet don’t resolve the animals’ debated evolutionary story.
“The authors found that some genes associated with desiccation tolerance . . . look like they have been horizontally transferred from foreign sources, at least in some species of tardigrades,” Thomas Boothby, a postdoc at the University of North Carolina, Chapel Hill, writes in an email to The Scientist. Boothby did not participate in the work, but studies how tardigrades survive extremes. “One of the more interesting questions this study raises is how and when tardigrades evolved tolerances to different stresses,” he adds.
The most compelling open question for Blaxter focuses on phylogeny and the still unclear relationship between tardigrades, nematodes, and arthropods. “Understanding them is going to be very useful for sorting out the history of life in this bit of the animal tree,” Blaxter explains. “It might seem a bit like bookkeeping, but in fact if we can sort out these relationships,
So we really aren’t sure where they are from or how they can do what they do but at least we are ruling out wrong answers.
See also: Water bears do not have lots of foreign DNA
30 year old frozen water bears come back to life