Thursday, February 4, 2010

Rotifers and the Pausing of Time

If you're like most people, you've never heard of rotifers (right). I'm not going to go and say these little creatures determine our way of life or anything, but a mystery about them has recently been solved. They've been examined ever since the microscope was invented, and have a couple interesting traits. For one, they have little spinning arms that propel food into their mouth – see the two furry things in the picture. For another, as reported on Science Friday, they don't have sex.

How do we know there never comes a point when two rotifers love each other very much? Well, not only has it never been observed, but also in 300 years of watching rotifers through microscopes, no one has ever even seen a male. There is additional modern genetic evidence that there are only female rotifers.

What's the big with not doing it? Well, the genetic exchange that occurs during meiosis greatly enhances variability of individuals within a species, speeding up evolution. Eliminate sex and a species basically freezes form.

Why is a static genome a problem? Everyone else is evolving, including your enemies. Say there's something in your blood that gives your species a resistance to certain parasite. Well, those parasites are having sex all the time, and eventually one might come about that happens to have a resistance to your blood. Since your species hasn't been having sex all this time, you have almost no genetic variation, and no one has any resistance, and you all die out and go extinct.

So that's why sex is good. There are even species of animals that under favorable circumstances skip the males and just have female offspring and basically clone themselves. However, once the environment turns sour, there starts being males born who then have sex with females, increasing variability, with the evolutionary "hope" that some new individuals will happen to have a greater tolerance to dry weather, or smaller size, or whatever. These species will skip sex for several generations, but eventually do it.

But bdelloid rotifers haven't done it in 35 million years. How do they get away with such genetic similarity? One possibility is that they don't have enemies. Oh, but they do. As researchers at Cornell University showed, certain species of fungus will decimate rotifer populations. As in, kill every single active rotifer.

What, you ask, is an inactive rotifer? Well, that's what the Cornell scientists figured out. Rotifers have the ability to dehydrate themselves, completely expunging any water from their body. They go into a state of suspended animation, essentially stopping time for anywhere from a few weeks to ten years. But when the rotifer comes back from its stasis field, won't the infecting fungus also reanimate?

Rotifers have extremely robust DNA that is good at reassembling itself after it gets broken when the rotifer dries out. The fungus does not have the same ability and the drying period kills it. The Cornell scientists even determined how much of the fungus was killed after 2, 3, 4, and 5 weeks, and found that a month was all the rotifer needed to purge the fungus from itself.

So rotifers can isolate themselves from the fungus in time. They can also do it in space. When they become dehydrated, they become little dandelion seeds (from an aerodynamics point of view anyway). That means that they are easily picked up by breezes and carried to a new, happy, fungus-free home.

One thing I didn't understand is that they program mentioned there are several hundred species of rotifers. But isn't sex and genetic variation the driving force behind evolution and speciation? Any thoughts?

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