Studies Reveal Mechanisms of Monarch Migration What drives monarch butterflies to travel en masse thousands of miles to pine groves in Mexico? New research sheds light on the complex circadian clock mechanism in the butterfly brain, and how monarchs can use the position of the sun for navigation.
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Studies Reveal Mechanisms of Monarch Migration

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Studies Reveal Mechanisms of Monarch Migration

Studies Reveal Mechanisms of Monarch Migration

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A little bit later in the hour, Hank Azaria and Jimmi Simpson, and Des McAnuff are here to talk about their play, "The Farnsworth Invention."

But up first, think you've got an uncanny sense of direction? You go somewhere once and you can find your way easily back, no map, no GPS? Well, you've got nothing on monarch butterflies. These butterflies that live in eastern North America migrate each winter several thousand miles to the same 70-square-mile plot of pine forest outside of Mexico City. Just how the monarchs manage to that every year without losing their way is a mystery scientists have been trying to solve for decades.

And this week, a paper out on the journal Public Library of Science Biology provides some new and tantalizing clues. Researchers know that the butterflies use the sun for navigation, but they aren't sure how the butterflies calibrate that sun compass. You know, the sun moves across - back and forth across the sky, how do they calibrate the compass to account for it being that moving target, the sun's changing position throughout the day?

Well, we're now joined by a scientist who has some answers. He's the lead author of this week's paper on monarch butterfly navigation. Steven Reppert is a professor and chair of neurobiology at the University of Massachusetts Medical School in Worcester. He joins me from his office. Welcome to the program.

Dr. STEVEN REPPERT (Chair of Neurobiology, University of Massachusetts Medical School): Hello, Ira.

FLATOW: Why study the butterflies? What got you interested in monarchs?

Dr. REPPERT: Oh, this goes back to my childhood, an interest in butterflies and moths. And then, after my training in science - I'm a physician, I went to medical school, and then got interested in doing science.

FLATOW: Mm-hmm.

Dr. REPPERT: And after studying circadian clocks for many years, I decided to get back to my real love, which was studying moths and butterflies. So, there seem to be no better lepidopteron species to examine from our vantage point than the monarch butterfly, again, because of its spectacular biology.

FLATOW: Yeah, yeah. And let's talk about the migration, this mystery. What aspect of this migration are you most interested in?

Dr. REPPERT: So, we're most interested in its navigation. You know, how does the butterfly navigate this incredible distance and find the same pine forest every year. And so, we're interested in that process and particularly, we're interested in how the butterflies use what is called a time-compensated sun compass. And that is believed to be one of the primary mechanisms that the butterflies use to navigate.

FLATOW: A time-compensated sun compass.

Dr. REPPERT: Right.

FLATOW: It sounds like - almost, like, one of those naval navigation devices.

Dr. REPPERT: Right.

FLATOW: What am I thinking of? We have to take a bearing on the sun?

Dr. REPPERT: Yes, yes. So, this mechanism really has two components that one has to understand. It has a compass, so the sun compass and all of it resides in the brain of the monarch, I must say, that is no bigger than the head of a pin. So, it's really remarkable stuff.

But, anyway, the butterflies sense the skylight information that they're going to use for direction through their sun compass. And then the dilemma is how do they compensate for the movement of the sun, as you mentioned, across the horizon over the course of the day. And the way they do that is to use their circadian clock. So, there is a communication, if you will, between the clock and the compass to recalibrate both systems so that the animals can maintain a fixed bearing, in this instance, going south over the course of the day and the course of many weeks and months, to get to their overwintering grounds.

FLATOW: So, they're like taking little sextant readings…


FLATOW: …of the position of the sun in their eyes, their brains. Where is this happening?

Dr. REPPERT: Well - so the compass information is processed in the eye and then through neural circuitry, essentially ends up at the sun compass integration station, which is in the central brain. Much more is known about this in other insects than in the monarch, at least we know where its at.

FLATOW: Mm-hmm.

Dr. REPPERT: But what we've really focused on is coming at it from the other vantage point, that once the information has been gathered by the sun compass, you know, how does the circadian clock communicate time-of-day information.

FLATOW: Mm-hmm.

Dr. REPPERT: And we've shown that if you destroy the clock by sort of using environmental lighting approaches a few years ago, that the monarch butterfly loses its way. It now follows the sun and it doesn't compensate for the movement of the sun, and therefore…

FLATOW: Mm-hmm.

Dr. REPPERT: …cannot go in the southerly direction.

FLATOW: Mm-hmm. That's very interesting, you know? And how - they must do a lot of compensation many times a day because they're aiming for this 70, what, a 70-acre patch?


FLATOW: Flying thousands of miles to hit this one little spot.

Dr. REPPERT: Well…

FLATOW: They must be very good at it.

Dr. REPPERT: Yeah. I think obviously they are. But I think the calibration is continuous.

FLATOW: Mm-hmm.

Dr. REPPERT: So, there's a continuous cross-talk between these two systems to allow for this process to take place.

FLATOW: Mm-hmm. And the circadian clockworks, what's inside the clockworks, if we were to smash open that clock?

Dr. REPPERT: Okay. So, what we know about the clock is based - the animal clock is based primarily on studies in two animals, the fruit fly and the mouse. And what we know in those animals is that there are clock genes, which give rise to RNA and protein levels that oscillate over the course of the day. And the way this works is that there is actually - this system is put together as a negative feedback loop. So what you have are clock proteins that are produced then after a delay, they feed back and shut down their own production…

FLATOW: Mm-hmm.

Dr. REPPERT: …and then this whole process starts over again. So, this feedback loop is really the critical gear of the molecular clock.

FLATOW: It's like a timer that's going on.

Dr. REPPERT: Exactly. This timer continues to go on and on and on. So, that even in the absence of environmental influences, this clock will continue to tick and keep time.

FLATOW: Kelly(ph) in "Second Life" asked an interesting question, one that I was wondering also. How similar is this to the way pigeons navigate? Do they navigate a different way? I've heard that they have little magnets in their heads, or something like that.

Dr. REPPERT: Well, they do. Pigeons are more complex than monarch butterflies and probably use magnetic fields as well as a sun compass. So, they use a variety of mechanisms and it's one of the reasons why we focused on the monarch butterfly because this appears to be genetically programmed, and one can really, you know, take the system apart at the most fundamental level in a simpler organism. And the simpler you can get things, the more likely you are to understand them.

FLATOW: Mm-hmm.

Dr. REPPERT: Then you can build on top of that.

FLATOW: Yeah. You had thought, I understand, that the circadian clock in monarchs would resemble the insects' circadian clock, but it doesn't, it turns out that it doesn't.

Dr. REPPERT: Right. So, this was a key finding of the study was that we got this feedback loop that's operational. It's very similar in both the mouse and in the fly. But there's one protein that we focused on and this is a protein called cryptochrome. And cryptochromes were discovered in plants many years ago where they function as blue-light photoreceptors. And what we have in the Drosophila, the fruit fly, is a cryptochrome that functions as a blue-light photoreceptor in the fly also.

So, the way this works is that light is sensed through this protein, this protein then interacts with proteins in this feedback loop and actually resets or recalibrates this loop so that the feedback loop is in tune with the ambient lighting conditions. So that's what happens in the fly.

In the mouse, on the other hand, on the mammalian clock, cryptochromes have taken on a different role. And what they do is they are part of the actual guts of the clock, if you will. They are part of the feedback loop itself. So what you have is this cryptochrome protein which in a fly has a photo receptive function and in the mammal has a light independent, totally different function with the clockwork itself.

FLATOW: That's…

Dr. REPPERT: And then if we get to the butterfly, what we found, which was really interesting because we thought it would be like the fly, we were able to identify the light-sensitive cryptochrome like occurs in the fly. And it has a similar function in the butterfly clock by sensing light and communicating that information to this molecular loop. But we were missing a component. And what we discovered was that there is a second cryptochrome in the butterfly.


Dr. REPPERT: And this cryptochrome is much more similar to the cryptochrome that's in mammals, like the one that we have. And, in fact, it functions in the central clock in a very similar way as it does in the mouse.

So here, we have a really cool situation wherein the butterfly, we have a clock that has sort of the best of both worlds. It's sensing light like the fly does, but its central clockwork is more like our clock than the fruit fly clock.

FLATOW: So you feel like you have satisfied that itch now about monarchs that you've always…


FLATOW: …questioning.

Dr. REPPERT: No, no, no. We're far from completing, you know, to even getting close to answering the questions that we want to address.

FLATOW: What question - what's the next question for you that you like to…

Dr. REPPERT: So the next question that we're looking at, we're trying to see how the clock actually talks to the sun compass and try to - also plays an important part in that. The other thing that we are interested in is the entire genetic program that the butterflies need to use to navigate.

So we have actually also reported a collection of genes we think represent over 50 percent of the genes in the butterfly genome that are expressed in the brain. And we can now go look in - at gene expression patterns between migratory and non-migratory butterflies. We're very excited about that. And, in fact, that will give us half the information; the other half will come from cloning the entire - or sequencing the entire genome, which we are in the process of doing.

FLATOW: Looks like you have some work to do here.

Dr. REPPERT: A lot of work.

FLATOW: Well - and you sound just like the guy for it.

Dr. REPPERT: Okay. Thanks a lot.

FLATOW: You're welcome. Good luck to you.

Dr. REPPERT: Okay. Thank you, Ira.

FLATOW: Steve Reppert, doctor of M.D. Steve Reppert is professor and chair of neurobiology at the University of Massachusetts Medical School in Western Massachusetts.

We're going to take a short break. When we come back, we're going to talk about the new Broadway play, "The Farnsworth Invention." So many of you know about Philo Farnsworth, we've all talked about it over the years, about him. We've all read all kinds of different stuff about it. There's a new play on Broadway.

And we're also going to come back and talk about how to diagnose psychotic illnesses in teenagers.

So more to come this hour on SCIENCE FRIDAY. Stay with us. We'll be right back.

I'm Ira Flatow. This is TALK OF THE NATION: SCIENCE FRIDAY from NPR News.

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