A Tiny Fern Has The Largest Genome Ever Discovered

IRA FLATOW: This is Science Friday. I’m Ira Flatow. Scientists just unearthed the largest genome of any living thing on Earth. That means if you split open one of its cells, unwound the DNA that’s coiled up in the nucleus, it would stretch out more than 300 feet. That’s taller than the Statue of Liberty.

Now, any guesses as to whom this giant genome belongs? You might be tempted to say maybe a complex being, like a person, a human, or a behemoth, like a blue whale, or a giant squid. Or maybe your mind went to a fancy fungus. Nope. A study in the journal iScience says that the new record holder is a fern– yes, a fern– found on the island of New Caledonia in the Southwest Pacific.

To put it in perspective, one of this fern’s cells contains more than 50 times more DNA than one of ours does. Wow. So how did this tiny fern end up with a giant genome? And what cost? Let’s talk about it. Joining me is a lead author on the study, Dr. Jaume Pellicer, evolutionary biologist at the Botanical Institute of Barcelona. Welcome to Science Friday, Dr. Pellicer.

JAUME PELLICER: Thank you, Ira. It’s a pleasure for me to be here.

IRA FLATOW: How excited were you by this discovery?

JAUME PELLICER: Well, we were absolutely astonished when we found out how big this genome was. Actually, scientists have been working on this field for a long time, to expand our understanding of plant genome sizes across the tree of life. But I mean, this discovery has really, really shocked us because we weren’t expecting something that big.

IRA FLATOW: So how big is the fern? Describe it for us.

JAUME PELLICER: Well, this fern is very small. It’s about 10 to 15 centimeters. I don’t know exactly how in inches it would be because–

IRA FLATOW: It’s like 4 to 6 inches. Yeah.

JAUME PELLICER: Yeah. Very, very small plant that you would probably, if you were just walking in the woods, not focusing on finding this plant, you probably would step over it because it’s like nothing that would catch your eye. It has no flowers. It’s all very greenish. It’s like a fishbone structure. It doesn’t even look like a traditional fern that you might have in mind.

IRA FLATOW: So how did it catch your eye? I mean, what made you not step on it?

JAUME PELLICER: Yeah. Well, I’ve been always interested in plant genome size diversity and what are the consequences of this trait in the evolution of plants. So we are interested in analyzing giant genomes because they are the exception rather than the rule. And that makes them very interesting to me.

Most plants have very small genomes. And only a very few groups of plants have giant genomes. One of them is Tmesipteris and its sister genus, Psilotum.

IRA FLATOW: So how did this small fern end up with so much DNA?

JAUME PELLICER: Well, that’s a great question. That’s the $1 million question. We don’t know yet. It’s still actually an unresolved question. What is exactly the biological meaning of this astounding plant genome size diversity?

And this extends into how exactly plants expand their genomes. At a first glance, for example, we cannot see any particularity or any need for this plant to accumulate such dramatic amounts of DNA in the cells, at least not from a functional point of view.

IRA FLATOW: You mean it doesn’t need all that DNA, is what you’re saying?

JAUME PELLICER: No, it doesn’t, because the actual functional DNA, which is the one that contains the coding protein genes, is very small. And it’s comparable to plants with very small genomes. So the rest is repetitive DNA, which for a long time, scientists called junk DNA because it apparently had no function. Now we know it has some roles to play. But it’s very, very repetitive. And it has no– it’s not the main function.

IRA FLATOW: Right. Well, having such a big genome, that’s sort of a bad thing for a plant, isn’t it?

JAUME PELLICER: Yeah, it’s mostly a bad thing. And this is because there are several costs that increase and are associated with maintaining and functional large genome. And this is, for example, the requirement for nutrients– for example, nitrogen and phosphorus, which are the main essential contributors to DNA. A plant with a large genome requires lots of these elements. And sometimes they are not available in the environment.

And also, for example, every time a cell divides, it needs to copy the whole strand of DNA. So this is a lot of work to replicate every time the cell. So that slows down their life cycles.

IRA FLATOW: So this is a puzzle then about why this has so much DNA?

JAUME PELLICER: It is, indeed. Yeah. We know that most plants are very efficient to remove it. So all these repetitive DNA sequences that populate the genome have– some of them have the ability to move around and replicate themselves. So the plant, even if it doesn’t have a brain, is very clever. It has a very efficient machinery that as soon as these elements amplify, they are detected. They are labeled. And they are targeted and removed from the genome. But we don’t know yet why in some plant groups these processes are not as efficient.

IRA FLATOW: Do we know why then this plant has more DNA than animals, let’s say?

JAUME PELLICER: Well, we don’t know yet. We don’t know yet. It might be that there is some sort of selective advantage for this fern that lives in a very particular stable environment restricted to it. And it has found the right conditions to cope with having such a big genome.

IRA FLATOW: Does this discovery challenge anything we know about genomes or plant DNA?

JAUME PELLICER: Well, it will definitely. I mean, not just this discovery, but it will challenge how do we see the structure of the DNA in the nuclei, because from a DNA sequence point of view, we have the technology to produce massive amounts of DNA sequences.

We have the potential, the computational power to analyze and assemble probably these genomes. But we don’t know yet how the 3D structure of the nuclei stands up. What are the intimate relationships between all the molecules that enable the integrity of these nuclei to be maintained, given the vast amount of DNA? And for that, we will need high microscopy technologies that probably will help us understand a bit more, because right now, we are pretty ignorant about the overall structure. How is this maintained and regulated?

IRA FLATOW: Are you just amazed that you could have stepped on this fern that you didn’t and missed the whole discovery?

JAUME PELLICER: If I’m honest with you, if I had been walking in the woods without looking for it, it would have gone missing. And this is something I have to acknowledge to our New Caledonian colleagues, because they were critical contributors to this work, because they showed us where these plants grow and helped us to make this story successful. Otherwise, it would have been unknown.

IRA FLATOW: Nobelist physicist Richard Feynman once talked about the beauty of flowers and plants and how you might look at the outside and love it. But there’s also a complicated beauty that goes on inside the plant that needs to be discovered and amazed at also.

JAUME PELLICER: Yeah. And this is the case. These very humble plant hides a very, very powerful and shocking secret in its genome. Yeah.

IRA FLATOW: Well, thank you very much. And congratulations on finding this.

JAUME PELLICER: Well, thank you very much for reaching out. And it’s been an absolute pleasure talking to you today.

IRA FLATOW: Thank you, Dr. Jaume Pellicer, evolutionary biologist at the Botanical Institute of Barcelona.

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