Tardigrades Are One Giant Head

By Elizabeth Preston | January 26, 2016 5:38 pm


No one would argue that tardigrades got stiffed in the weirdness department. These teensy animals, also called water bears, look roly-poly under a microscope. Less than a millimeter long, they can survive extremes of heat, cold, pressure, and radiation that are deadly to most other lifeforms. Under duress, a tardigrade may curl itself into a dried-up ball called a tun, then stay in a state of suspended animation for years before returning to life. Now, researchers poring over the animal’s genes have found another oddity. The tardigrade, they say, is essentially one giant head.

Frank Smith, who’s a postdoc in Bob Goldstein’s lab at the University of North Carolina, Chapel Hill, and their colleagues studied the evolution of tardigrades by looking at their genes. Specifically, they looked at bits of DNA called “Hox genes.” These are master controllers that organize an animal’s body. During development, Hox genes make sure all the parts end up where they’re supposed to be. Mutations in Hox genes can cause unsettling problems like, say, legs growing out of the head.

The researchers looked for Hox genes in the genome of a tardigrade called Hypsibius dujardini. (Yes, there’s more than one tardigrade. There are actually more than 1,100 species, living in wet places all over the planet.) They compared H. dujardini‘s Hox genes to those of arthropods, the large group of animals that includes bugs of all sorts, plus crustaceans. Arthropods are cousins to tardigrades. The researchers also looked at the genomes of two tardigrades distantly related to H. dujardini.

What emerged was a kind of whodunnit. The researchers saw that as tardigrades evolved from the ancestors they shared with arthropods, four or five of their Hox genes had simply disappeared.

Next, Smith and his colleagues asked what exactly the surviving Hox genes were doing in the tardigrade. Looking at where those genes switch on during the tardigrade’s development, they saw a pattern “nearly identical” to how those genes are turned on in an arthropod’s head, Smith says. In other words, most of a tardigrade’s body is equivalent to just the head of an arthropod.

“Our findings were pretty surprising,” Smith says. Previously, scientists had thought that tardigrades evolved their stumpy bodies by fusing body segments together. Finding several totally absent Hox genes was an unexpected twist.

Smith says tardigrade ancestors, like many tardigrades alive today, probably lived in sediment on the ocean floor. His coauthor Lorena Rebecchi has speculated that a compact body would have been useful to an ancient tardigrade burrowing through ocean muck. So if a mutation lopped off part of its body, so much the better.

Here’s what the researchers think happened: The ancestors of tardigrades were longer, with lots of body segments. But mutations cropped up in the genes that made these segments, causing whole sections to disappear from the tardigrade ancestor’s middle. Once most of the tiny animal’s body was gone, the Hox genes that used to build those segments became unnecessary. Eventually, other mutations erased those genes from the tardigrade’s genome.

If a tardigrade is a giant head, why does it have so many legs—eight of them? “In fact, arthropod heads most likely also have many legs, evolutionarily speaking,” Smith says. There’s an idea that all the appendages sticking off an arthropod’s head—chewing mouthparts, antennae, and so on—evolved from legs. This fits with what Smith found.

“In our model, the many legs of a tardigrade correspond to the many head appendages of an arthropod,” he says. Speaking of unsettling.

Image: by Schokraie E, Warnken U, Hotz-Wagenblatt A, Grohme MA, Hengherr S, et al. (2012). Comparative proteome analysis of Milnesium tardigradum in early embryonic state versus adults in active and anhydrobiotic state. PLoS ONE 7(9): e45682. doi:10.1371/journal.pone.0045682

Smith, F., Boothby, T., Giovannini, I., Rebecchi, L., Jockusch, E., & Goldstein, B. (2016). The Compact Body Plan of Tardigrades Evolved by the Loss of a Large Body Region Current Biology, 26 (2), 224-229 DOI: 10.1016/j.cub.2015.11.059

  • Derek Winer

    Given that tardigrades can travel in space for unknown periods of time, the intergalactic seed theory just got some high five evidence. So… we’ve been intimately living with, and are likely carrier host continually to countless scores of Aliens. Their evolutionary ancestors, in the form of all insects and yummy crustaceans are essential to our wellbeing, yet a vector for misery throughout history. I have a few questions… do any of the 1,100 species have just six legs? Could we extrapolate the six appendages of our heads to these? I’m not an evolutionary biologist by any means, but this has got to be freaking people out. For the time being, I am never eating another bug or crustacean again. I may raise bees. If they send the mother ship I want to be on the right side of the fence…in the good books if you will.

  • Mike Shaw

    Thanks for a great tardigrade discussion which actually adds more to the dialogue (rather than a re-post of someone else’s post). I like the way you simplified it too, keeping the integrity of the facts, because this can be a complex discussion. Thank you, Elizabeth!
    Mike Shaw
    The Space Bear Hunter

  • http://www.mazepath.com/uncleal/qz4.htm Uncle Al

    The Faculty (1998), Youtube v=vE_xjjCJWng 1:40 Evolution is about optimization of available genetic resources given shifting priorities. Let’s invade Jupiter’s upper atmosphere – chemoautotrophs plus tardigrades for a survivable aggressive Terran kick-start.

  • jug

    Sounds like democrats to me!

    • http://www.bogleech.com/ Bogleech

      …..what? How does this joke make any sense? You’re obviously trying to call democrats stupid, but this is an article about how an organism is genetically one large head. How does that indicate stupidity?

    • Los Pachucos

      You know, everyone in the world is tired of anyone who interjects politics into every little conversation. It is NOT cute, NOT funny, and NOT smart. So, go home.

      • jug

        If the shoe fits, wear it!
        Nice try to be one of those “talking heads”, not!

        • Los Pachucos

          You are such an idiot. I have voted republican for 35 years. You say things when you don’t know what you are talking about and guess what that proves? if you are up to it, which I really doubt.

          • jug

            You sound a little unhinged now.
            Been drinking the same stuff the narcisstic twins, Obutthole and Trump have been drinking?

          • Elizabeth Preston

            Hello everyone! Please keep comments in this space relevant and/or civil.

          • Thin-ice

            I don’t come to Discovermagazine.com often, but when I do, I expect stimulating science discussions, not inane and ignorant political comments. If you editors manage to clean up your comment column, then I’ll be back.

          • Los Pachucos

            Told you that you weren’t up to it. HA! LOL!

  • polistra24

    Love your first sentence.

    I wonder if the genes for antifreeze are ‘heady’ genes, intended mainly to protect the central ganglion and senses? If you messed up the Hox genes in a more complex animal to favor its head, would it develop the antifreeze trick?



Like the wily and many-armed cephalopod, Inkfish reaches into the far corners of science news and brings you back surprises (and the occasional sea creature). The ink is virtual but the research is real.

About Elizabeth Preston

Elizabeth Preston is a science writer whose articles have appeared in publications including Slate, Nautilus, and National Geographic. She's also the former editor of the children's science magazine Muse, where she still writes in the voice of a know-it-all bovine. She lives in Massachusetts. Read more and see her other writing here.


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