Mystery Of The Bdelloid Dry Spell Solved Bdelloid rotifers haven't had sex for 30 million years and that's puzzling. Most asexual animals are doomed to extinction. Reporting in the journal Science, Paul Sherman and Chris Wilson explain the extraordinary adaptations that allow these rotifers to persist in celibacy.
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Mystery Of The Bdelloid Dry Spell Solved

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Mystery Of The Bdelloid Dry Spell Solved

Mystery Of The Bdelloid Dry Spell Solved

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Youre listening to SCIENCE FRIDAY from NPR. Up next, our Video Pick of the Week with our digital producer, Flora Lichtman. Hi, Flora.


FLATOW: What have you got for us this week?

LICHTMAN: This is a good one, I think.

FLATOW: You say that every week, and youre always right.

(Soundbite of laughter)

LICHTMAN: They're great. But this one is actually is a really good one.

FLATOW: This is one of your really good ones.

LICHTMAN: I mean, this is a biological mystery that scientists are starting to solve. So let me set it up for you.

FLATOW: Go for it. I'm going to put my feet up on the table. Go for it.

(Soundbite of laughter)

LICHTMAN: Okay. Most plants and animals, almost all plants and animals reproduce by having sex. And it's not because sex is so great in some ways - I mean, it's not efficient, it's faster to clone yourself. Its not because it's safe, think STDs, but yet if an organisms stops having sex, becomes asexual, it's doomed. I mean, they all seem to go instinct, which is sort of puzzling, right?

FLATOW: You know the ones that do do that, they go extinct over time.

LICHTMAN: They go extinct over time. Right.

FLATOW: Mm-hmm.

LICHTMAN: But there is one notable exception, and it's the bdelloid rotifer.

FLATOW: Say that again.

LICHTMAN: The bdelloid rotifer.

FLATOW: Good Sunday puzzle word.

LICHTMAN: Yes. And no one can quite figure out why this rotifer has persisted for 30 to 50 million years without having sex. Why hasn't this organism gone extinct?

FLATOW: What is a rotifer?

LICHTMAN: Well, I think we should bring on an expert...


LICHTMAN: ...who can tell us exactly what a rotifer is. So Paul Sherman is the author of a study this week in the journal Science that may explain why these rotifers are sexless - don't have sex, right? And he's also a professor of animal behavior at Cornell University in Ithaca. Welcome to the program, Dr. Sherman.

Dr. PAUL SHERMAN (Cornell University) Thank you. And I just want to say how honored I am to be on this show. We have listened to SCIENCE FRIDAY, my whole family has, for years and have thoroughly enjoyed it. So thank you for inviting me.

FLATOW: Youre welcome, and heres your 15 minutes of fame counting down.

(Soundbite of laughter)

FLATOW: Thank you for joining us. (Unintelligible) Flora, take it over.

LICHTMAN: Dr. Sherman, I think for people who haven't had the pleasure of meeting a rotifer, how would you introduce them to..

Dr. SHERMAN: Now okay. First of all, you can't really see them without a microscope. They're you could put a couple of dozen on a grain of salt, so they are like a little spec of dust. But if you put them under a microscope, you would see a little creature that moves around kind of like an inchworm, and attaches to itself, moves forward, pulls itself forward and moves. And when it's attached, it feeds by rotating wheel organs on its head. It looks like a little Norelco shaver, swinging around and around and around. And in the process, it gets its food sucked into its body with that head. And that's why they're known as rotifers, the rotating organs.

LICHTMAN: Because of those wheels.

Dr. SHERMAN: Yes, because of the wheels.

FLATOW: Mm-hmm. Youre listening to SCIENCE FRIDAY from NPR. 1-800-989-8255. We're talking about a rotifer with Flora Lichtman and Paul Sherman. Okay.

LICHTMAN: Dr. Sherman, how do we know this is something that confused me about the story when I first have heard about it. How do you know that they haven't had sex for this long?

Dr. SHERMAN: Well, there are several avenues of several kinds of answers to that. The first is that in nearly 300 years of looking through a microscope at these little animals, they were some of the first animals ever identified, when microscopes were invented - no one has ever seen a male, and there's never been any evidence of meiosis. In addition to that, these little animals have four chromosomes, but the chromosome that would have been homologous in a sexual organism have drifted apart through time so that they're no longer even homologous.

So mutations have come in and have not been corrected, so the chromosomes can no longer even line up with each other, which would be which they would be able to do if they were sexual. So in other words, there is molecular evidence suggesting that these animals have not had sex and there's no physical evidence that they have no meiosis, no males. And so they are the leading candidate for what's called ancient asexuality. In other words, they've been asexual, as you said in the opening, between 30 and 50 million years.

LICHTMAN: And they've done okay without male rotifers?

Dr. SHERMAN: They have not only done okay, but they've done remarkably. They have speciated. This is an entire class of organisms, and they've speciated into 400 more than 450 species in that time. Now, most organisms, when they become asexual, are extinguished so quickly they don't even have time to speciate. And they don't lead to higher taxonomic groups. But they little bdelloids have been able to speciate and to proliferate even as ancient asexuals, and all the speciation and proliferation has gone on without any sexuality.

LICHTMAN: So whats the problem with not having sex? And if theyve done it so well, why do other species seem to go extinct?

Dr. SHERMAN: Well, apparently, the leading hypothesis is that any organism that stops having sex, in essence, crystallizes its genome. It becomes - the same genome gets passed generation to generation to generation.

Now, every organism on the earth is afflicted by biotic enemies, especially parasites and pathogens which can evolve very, very rapidly. And any organism that stops having sex, that crystallizes its genome, very quickly gets overrun by the parasites and pathogens which afflict it. And they very quickly, through their new genetic combinations, come up with a new key to the lock that the host species had and overwhelm it and wipe it out.

LICHTMAN: And this is called the - this is the Red Queen hypothesis, right?

Dr. SHERMAN: Yes. This is known as the Red Queen hypothesis because it comes from a character in Lewis Carrolls Through the Looking Glass, where the Red Queen and Alice are running. And the Alice looks around and she notices theyre not going anyplace, despite the fact that theyre running as hard as they can. And she says to the Red Queen: Whats going on here? And the Red Queen says, well, now, here, you see, in this place, it takes all of the running you can do just to stay in the same spot.

And the idea being that every organism on the earth has to keep reproducing sexually in order to keep up with its biotic enemies, parasites, pathogens, competitors, predators, everything that afflicts it, which are all sexual, anybody who stops - anybody who gets off the train and stops running with the Red Queen, immediate - or not immediately, but very quickly gets overwhelmed and extinguished.

LICHTMAN: And yet these little bdelloid rotifers have made it. They've beat the Red Queen.

Dr. SHERMAN: Yes. Yes. And this was...

LICHTMAN: So - and thats what your study is about this week, right?

Dr. SHERMAN: This is what our study is about, is the question of how they do it. And for years, bdelloids have been called an evolutionary scandal because the question is: What are they doing? How do they get around this frightful problem? And one possibility would be maybe they dont have any parasites or pathogens. Maybe they just, for some reason, dont. But that is manifestly not true.

We and others have discovered that there are many species of very deadly pathogens, in particular fungi, that eliminate these bdelloids. And they do it very quickly. So it isnt that they dont have any. So then thats - then the questions becomes: Well, how do they avoid this problem?

And our experiment built on some knowledge of some unusual characteristics of these bdelloid rotifers. And the first is that they can undergo a physiological change which is called anhydrobiosis which is the lack of cellular water. They can dry up into a tiny, little, round ovule which is called tun(ph). And this one around the ovule has no water in it whatsoever.

LICHTMAN: Like a bdelloid jerky?

Dr. SHERMAN: Yeah. Like bdelloid jerky. And not...

(Soundbite of laughter)

FLATOW: Glad you said that and not me, Flora.

Dr. SHERMAN: Yeah. And not only are they bdelloid jerky, but they can stay in this in this state of suspended animation with no cellular water for weeks and months and even up to nine or 10 years. And then when you rehydrate them -poof! They come back to life again at the same stage they were when they went in. So they can go into anhydrobiosis at any life stage and sit there in suspended animation, and then pop out again.

LICHTMAN: And their DNA even - it breaks up into tiny, little pieces, right, when they go into this hydrated process.

Dr. SHERMAN: Yes. But they have remarkable capacities for repairing DNA, and also remarkable capabilities for protecting themselves from ionizing radiation. In other words, things that would destroy DNA mostly, they are able to handle as part of this process of drying up and then being able to rehydrate.

FLATOW: Now, what - but if they had this parasite in them when they dry up...

Dr. SHERMAN: Uh-huh. Thats...

FLATOW: ...and you throw the water back in, doesnt the parasite come back, also?

Dr. SHERMAN: Yes. Mr. Flatow, this is the key...

FLATOW: Call me Ira. Thats okay.

Dr. SHERMAN: All right, Ira. You call me Paul.

(Soundbite of laughter)

Dr. SHERMAN: This is the key thing. We discovered - we did an experiment to ask that very question. And we took the parasite-ridden populations of these bdelloids - of a species of bdelloid that we were studying. And my work is with my doctoral student Christopher Wilson, and Christopher Wilson is the lead author on this paper, and its very important to me give him full credit.

But we divide this experiment where we took infected bdelloids and we then subjected them to drying for various lengths of time - one week, two weeks, three weeks, four weeks and five weeks. We then rehydrated them to see what the effect on the fungus might be. If the fungus always stay with them, then your question, Ira, would be then all of them should be wiped out as soon as they came back.

Well, we found out that after one week, we rehydrated them, one week of dehydration. We rehydrated them and, sure enough, the fungus came back and killed every rotifer. After two weeks, we rehydrated them, and the fungus came back and killed every rotifer.

But after three weeks, there was a dramatic difference. And 60 percent of the rotifer populations came back without any fungus. And longer periods of time, 28 days and 38 - 35 days of dehydration almost completely eliminated the fungus down to where only - where 90 to 95 percent of the bdelloid populations came back fungus-free.

So what is happening here is that the fungus cant stand the dehydration as long as the bdelloid can. And the bdelloid, being able to handle it longer, uncoupled of itself in time from the fungus, which simply cant survive that long a period of time without cellular water.

Now, 35 days, we discovered, as I said, 95 percent of the populations were parasite-free. And in nature, these bdelloids can go much, much longer, as I was saying, weeks, months and even years in a dehydrated state, and then come back to life. And so the idea is that they can uncouple themselves from this deadly parasite and then, perhaps, move.

Now, the second part of our experiment had to do with the movement. When these bdelloids dry up and become these little tuns, they are very aerodynamically capable, and they can be swept up in wind and carried all around the world. In fact, genetically similar clones have been found as far as - far out as Africa, Australia and Italy, as if they were swept around the globe in a wind and then dropped in rain onto tiny, little ephemeral pools, little droplets on moss, little, tiny pools - it's been raining a lot up here in Ithaca, and little, tiny pools are all over our back hillside. And in those little pools, the bdelloids could land and then proliferate.

I'd like to say that you could catch bdelloids. If you're interested in these animals, you or any of the listeners, you can catch them by putting a wet sponge on your roof, and in the morning - or a little dish of water. In the morning, you'll have them. And you can take them out and look at them in a microscope. There they will be.

FLATOW: That's something for this weekend.

Dr. SHERMAN: Yeah. Exactly.

(Soundbite of laughter)

FLATOW: You're listening to SCIENCE FRIDAY from NPR.

I'm Ira Flatow, talking with Flora Lichtman and Paul Sherman. Paul is a professor of animal behavior at Cornell in Ithaca. And Flora, you created - you took a video, right?

LICHTMAN: Right. I mean, this - one of the amazing things about this is just watching this fungus devastate these rotifers - which you end up feeling pretty sorry for by the end of this video, I think.

(Soundbite of laughter)

LICHTMAN: But I think it's worth checking out the footage just to see how this fungus works.

Dr. SHERMAN: Yes. This is footage that Chris Wilson took, and it's very dramatic. What happens is that the fungus almost grows a tree out of the rotifer. It looks like a little branching tree. And on the tips of the tree branches are little spores, which then are shed into the water. And then the next poor rotifer, with its little wheel organ going around, eats - ingests one of these spores, and it lodges in it throat, where it immediately sporulates and then produces this awful mass of a tree-like structure to do it again and again.

Now, the interesting thing for our study is that these fungi are extremely good at dispersing in water. So they're very good in water. And if you put the bdelloids in with a fungus in a water solution, the bdelloids will all be dead in about two weeks. But the fungi - the fungal spores are not very good at dispersing in air. So our second experiment involved revealing this by taking an infected source dish and putting it in a wind box, and then using little fans to blow at the rate of a light breeze in nature, blow the material around the box. And what we discover is that about 60 percent of the population - then we had source dishes, where the material could blow into. About 60 percent of the dishes ended up containing only bdelloids. The fungal spores didn't go with them in space because the fungal spores are not as aerodynamic and as capable of moving as are the bdelloids themselves.


LICHTMAN: So they have this escape, both in time, they can - they can outlast the fungus in dryness, and then they can blow away to a new fungus-free place, right?

Dr. SHERMAN: Exactly. And the fungus can't follow them. See, this gets back to Ira's original point, which is the key point of the whole thing, is that the fungus can't go with them. If a bdelloid is infected, it's absolutely dead. But we're talking about - in any population, there'll be a few that aren't killed by the fungus immediately, or attacked. And those are the ones that can get out. And they get out and then dry out and stay dried out, and then they float around and land in a new place where the fungus hasn't gotten yet. The reason the fungus isn't there is it's a little ephemeral pool of water, and there's nothing for the fungus to eat.

FLATOW: Wow. Wow. We're going to get out our sponges this weekend...

Dr. SHERMAN: Oh, you've got to.

FLATOW: ...and go collect.

Dr. SHERMAN: You have to see these little animals, (unintelligible) little animals.

(Soundbite of laughter)

FLATOW: And you can see them on our Web site at, where Flora has made this terrific video with the help of Paul Sherman, showing you the whole lifecycle here and then the infection and these branching - all kinds of interesting stuff. And Flora's made little animations to go with them. Flora, a great piece of work...

LICHTMAN: Thank you, Ira.

FLATOW: ...on this video. Thank you, Dr. Sherman.

Dr. SHERMAN: Well, thank you very much, and I really appreciate your interest in showing this to a wider audience. Thank you again.

FLATOW: You're welcome. Paul Sherman is professor of animal behavior at Cornell University. Flora Lichtman is our digital media producer. Great piece, this week, Flora, as always.


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