A Delicious But Invasive Mushroom Could Affect Fungal Diversity

IRA FLATOW: I’m Ira Flatow, and you’re listening to Science Friday.

Today on the podcast, a trip down the produce aisle including how mushrooms from some of those popular mushroom kits escaped into American forests.

AISHWARYA VEERABAHU: The tough thing is that we will never truly know exactly where they came from, but it started in general when they were brought over here to North America for commercial cultivation.

IRA FLATOW: But first, I love a good tomato plant. And while I knew that both tomatoes and potatoes are members of the nightshade family, which by the way includes peppers and eggplants, it was still a bit of a surprise to read new research that found that the edible part of the potato plant, the tubers at the bottom of the plant, came from another plant crossbreeding with a tomato. Yes, millions of years ago.

Joining me now to talk about this seemingly bizarre relationship is Dr. Sandra Knapp. She’s a merit researcher at the Natural History Museum in London and one of the authors on that paper. Welcome to Science Friday.

SANDRA KNAPP: Thank you, Ira. It’s great to be here.

IRA FLATOW: When we say that potatoes may have arisen from tomatoes or potatoes, potato, what do we mean by that?

SANDRA KNAPP: Well, it’s really hard to exactly what happened millions of years ago. One of the things that we’ve known for a really long time is when we reconstruct the phylogenetic tree of the whole of the genus Solanum, which by the way has more than 1,000 species in it so it’s no trivial undertaking, one of the places where there’s always been a lot of discordance, which means we never really what the branching pattern is, is where tomatoes and potatoes and this lineage called etuberosum is that relationship. You never really know whether potatoes were more closely related to tomatoes than they were to the other ones or vice versa or whether potatoes and etuberosum were most closely related to one another. So that’s what we call discordance.

IRA FLATOW: So how do we get a potato arising from a tomato?

SANDRA KNAPP: Well we don’t. We don’t. That’s the key thing is that one of the things about studying evolutionary biology is we can only study what we have today, and then we make inferences about what happened in the past.

So today’s tomatoes or tomatoes, whatever we want to say, are nothing like what the ancestors of tomatoes were probably. So we can’t say that one thing came from another. What we can say is that they share a common ancestor, and these three lineages all share a common ancestor.

IRA FLATOW: So do we have a bit of tomato in the potato and a bit of potato in the tomato?

SANDRA KNAPP: No, we have a bit of tomato and a bit of etuberosum in potato.

IRA FLATOW: I see. So how did we get either one?

SANDRA KNAPP: Well, through change, by evolution, by natural selection. As time passes, things change and things adapt to environments, and we get the species that we have today.

Now there are 17 species of tomatoes, only one of which we cultivate and have in our supermarkets. The rest of them are wild and live in places like Peru and Chile. And there are 104 wild species of potatoes.

IRA FLATOW: Wow, so this is a random event. No caveman doing a Gregor Mendel sort of thing.

SANDRA KNAPP: No, no. This is also 8 to 9 million years ago, which is–

IRA FLATOW: Yeah, yeah.

SANDRA KNAPP: A long time before any people were in South America.

IRA FLATOW: So did cross-pollination happen that gave us a tomato or a potato?

SANDRA KNAPP: Well, so what we think happened is we start out with this discordance, which was something we didn’t know who was most closely related to another. And where there’s this discordance, where things don’t neatly fall out, that’s actually where the exciting biology is. That’s where you think, ooh, there be monsters. Something’s interesting there. And we were interested in this, and our colleagues in China were interested as well because they’re really interested in potato breeding and creating new varieties of potatoes which can withstand climate change and all kinds of environmental challenges which crops have today.

And so we looked at this further, and what we found previously is if you look at one set of genes, potatoes and tomatoes were most closely related to each other. If you look at another set of genes, etuberosum and potatoes are most closely related to each other. But actually when you look across the whole genome, what you find is that potatoes are a mix of genetic material between the lineage that came to be tomatoes and the lineage that came to be etuberosum.

So we think what happened is that there was a hybridization event sometime in the past, which we reckon is 8 to 9 million years ago, that created plants that then were able to carry on and invade new habitats which were happening in the Andes at that time because 8 to 9 million years ago was one of the times when the Andes were actively rising. That rising of the Andes created a whole bunch of new habitats, and the coming together of these two lineages to have a hybrid brought together sets of genes that allowed the development of tubers, which is the bit of the potato that we eat.

But actually for wild potatoes, it’s really important because it allows them to persist through adverse conditions in these new habitats, which are both high, dry, and cold. And a tuber for a plant is a way to store starch and persist through adverse environmental conditions.

IRA FLATOW: Do we know what biological machinery that hybridization used to make a potato tuber because that’s what we eat?

SANDRA KNAPP: Well, exactly. And that’s the really cool thing that my Chinese colleagues did. They did all the very fancy work here. We did the evolutionary work and thought about who’s related to who and what we should sample.

But what they did is they looked across the genome, and they looked at particular genes that are involved in tuberization in potatoes. And they found that across the whole genome of potatoes, they had half their genes came from tomatoes and half of them came from etuberosum.

And interestingly two genes that together allow tubers to form, one of those genes comes from tomatoes, and the other of those genes comes from etuberosum. So putting those two genes together, this is what we think, allowed the ability to create these tubers.

Now there’s a ton of stuff left to do. This is a great hypothesis, and now it’s out there for people to test and really look at by knocking out genes. And my Chinese colleagues did some extraordinary work in knocking out some of these genes. So you can use genetic engineering to knock out particular genes.

And what they did is they knocked out the genes in potatoes from– I think they did it from the tomato– the tomato gene. And they knocked that out, and those plants didn’t produce tubers. So that’s pretty good evidence that potatoes need both of these genes to create tubers.

IRA FLATOW: So the genes are used to make tubers in all kinds of plants?

SANDRA KNAPP: Well we don’t. This is what we think is happening in potatoes. A tuber is a starchy storage organ which is formed on an underground stem. So all those– when you harvest potatoes– we see potatoes in the supermarket, and there they are. They’re just little round brown things. There they are–

IRA FLATOW: Right.

SANDRA KNAPP: Waiting for us to cook them. But in plants in the field, they’re on underground stems, and they’re– and what they are is starch storage organs. Tubers are not for us.

IRA FLATOW: They’re not?

SANDRA KNAPP: We’re lucky that they’re there because we’ve managed to exploit them. But they weren’t there for us.

IRA FLATOW: They were there to store the energy for the plant?

SANDRA KNAPP: Right. They’re there for the plant. And then human beings, peoples in the Andes exploited that, exploited that characteristic and created the– through plant breeding and through selecting plants that perhaps made bigger tubers than made the potato that we today because all the different kinds of potatoes that you find in your supermarket, those are all the same species. It’s all one species.

IRA FLATOW: Wow.

SANDRA KNAPP: And all the tons of different kinds of potatoes that they have in the Andes are all still a single species.

IRA FLATOW: Well, let’s– speaking of new potatoes, you mentioned that your colleagues were trying to improve potato breeding. Would they ever want to make a plant that had both tomatoes on top and those tubers down below?

SANDRA KNAPP: Well, you probably do that. You probably could do that. Maybe.

What they’re really interested in is improving potatoes so that we get better potatoes which are disease resistant and better for the environment. Because if you made a plant that had tomatoes on the top and potatoes on the bottom, it might not do either of them particularly well. It might be quite cool but–

IRA FLATOW: That’s what I’m thinking.

SANDRA KNAPP: Yeah, it could be cool, but it– you might not– it might not be commercially successful if you see what I mean. I don’t think it would make a dent in the world economic importance of actual potatoes.

[LAUGHING]

IRA FLATOW: I’ll buy that. Do you have a favorite potato or tomato?

SANDRA KNAPP: Oof, I try to grow potatoes. I grow potatoes in bags in my little tiny London garden, and I grow different ones every year. And I love growing the old heirloom varieties, but they often are susceptible to disease.

But actually what I’m always blown away with is when I go to the Andes, when I go to Peru and do field work is the numbers of different kinds of potatoes and how different they taste. They do taste completely different, each one. I would urge you boil lots of different kinds of potatoes and then just sit down and do a taste test, and you’ll find that they actually taste different.

Also nutritionally, if you look at the four big crops that– 80% of human calories come from four big crops– wheat, rice, corn, and potatoes. And of those four, potatoes are the nutritionally probably the best. They have vitamin C. They have vitamin A. They have a lot of fiber. They’re really, really good for you.

IRA FLATOW: Well, this is so fascinating, Dr. Knapp. Thank you for taking time to talk with us today.

SANDRA KNAPP: Well, thank you very much for inviting me. Dr. Sandra Knapp is a merit researcher at the Natural History Museum in London.

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After the break, the story of the invasive golden oyster mushroom. Good to eat but how it could be affecting fungal diversity.

AISHWARYA VEERABAHU: I guess I don’t about there for good, but they are there.

IRA FLATOW: Stay with us.

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It all started in a harmless enough way. People bought kits to grow mushrooms in home. It was simple, tasty, and an easy birthday present. But then scientists in the upper Midwest noticed something strange. The golden oyster mushroom, which is not native to the US, was spotted thriving in our forests. Those homegrown mushrooms escaped our basements into the wild.

So what impact do invasive mushrooms have on the ecosystem, and are fungal foragers in for a new pleasant adventure? Aishwarya Veerabahu is a fungal ecologist and PhD candidate at the University of Wisconsin in Madison. Welcome to Science Friday.

AISHWARYA VEERABAHU: Hi. Thank you so much for having me.

IRA FLATOW: You’re welcome. You published a study that looked into the effects of these invasive golden oyster mushrooms. Tell us what you learned.

AISHWARYA VEERABAHU: Yes, that’s right. We found that dead trees that are colonized by golden oyster mushrooms have about half the biodiversity. So they have lost so many of the native fungal species that would have been there, And? We saw that compared to trees that have not been colonized by golden oyster mushrooms.

IRA FLATOW: So why is that a bad thing if these other mushrooms are just taking their place?

AISHWARYA VEERABAHU: Well, native fungal species do a lot of different things in the forest. They might be interacting with other organisms. They might have some sort of mutualism with wasps or birds. They might be in charge of decaying wood and making that process go by slowly so that deadwood can still stay standing and provide habitat for tree seedlings or small mammals. There are so many different things that fungi do in forests, and so any time that native community gets thrown out of balance, it can be a potentially very dangerous thing.

Another part of that is fungi contain a lot of compounds, potentially therapeutic compounds that haven’t been discovered yet like, for example, penicillin, a hugely important medicine to us, was discovered from fungi. And so fungal biodiversity is incredibly important, first, to allow fungal populations to adapt to a constantly changing environment, and then it is also important because they might contain really important medicinal compounds that we haven’t even discovered yet.

IRA FLATOW: So tell me how far these invasive mushrooms have spread. Where can we find them now?

AISHWARYA VEERABAHU: They have spread to at least 25 states in the US and the province of Ontario, Canada. So they are across most of the Midwest and into the Northeast, and they are starting to spread South and West as well.

IRA FLATOW: So they’re outcompeting the native mushrooms then?

AISHWARYA VEERABAHU: Yeah, it seems so.

IRA FLATOW: Wow. What do they look like? Can people spot them?

AISHWARYA VEERABAHU: Yeah, you can spot them really easily. I’ve actually spotted them from hundreds of meters away because they are bright yellow. They will sometimes almost seem like a beacon in the forest.

They are very beautiful looking. They have these bright yellow caps and pure milky white gills and stems or stipes as we call them in mycology. So they’re pretty easy to spot, and it’s hard to confuse them with any other mushroom. There are not many other mushrooms that look like them.

IRA FLATOW: Are they good to eat? Should we go hunting for them?

AISHWARYA VEERABAHU: Yeah, they are edible, and they’re good to eat. A lot of people find them very tasty. It might even be their favorite. I personally am over them. I don’t care for them anymore.

IRA FLATOW: Tell us how they got– how did they crawled out of our compost bins or our basements or wherever we were growing these mushrooms and get out into the wild?

AISHWARYA VEERABAHU: Well, the tough thing is that we will never truly exactly where they came from. But it started in general when they were brought over here to North America for commercial cultivation. So there are hundreds of mushroom farms all over the country, all over the continent, and they grow a lot of these mushrooms.

And sometimes they put it in their own compost as part of their business operations. Maybe they’re venting their facilities to let spores out. And then, of course, other folks on the consumer side buy those mushrooms and like you’ve mentioned, in the form grow kits. And so that allows it to be efficiently dispersed into new parts of the country and introduced, and they can really get around that way.

IRA FLATOW: So is there anything we can do to control them, and I’m thinking if all of us go out and start to eat them, is that helpful enough or are there just too widespread. And what do we do about that?

AISHWARYA VEERABAHU: Gosh, I wish we could eat our way out of this problem. But no. There’s not really a way to control this. It’s hard enough to control invasive plants and animals. For example, if we think of carp or kudzu, at least plants and animals, their bodies begin and end at a single place, but with fungi and their mycelial networks that spread through wood or that spread through the soil, it is virtually impossible to get them out of the environment once they’re there.

IRA FLATOW: So their future is that they’re there for good now.

AISHWARYA VEERABAHU: Yeah, I guess I don’t about there for good, but they are there.

IRA FLATOW: Does this signal to you that we need to better take care of our mushroom farms to prevent spread of invasive mushrooms?

AISHWARYA VEERABAHU: I think so. I am talking to a number of stakeholders in this arena right now. So that includes mushroom growers and foragers, amateur mycologists, biodiversity advocates, all sorts of people in this mushroom space. And I think the collective consensus is that we do need to try and rethink what are methods of cultivation that could prevent another introduction like this in the future. And we’re really hoping to find have your mushroom and eat it too kind of compromise where we can still allow people to eat mushrooms and grow them and be fascinated by them but also do it in a way that prevents the introduction of invasives like the golden oyster.

IRA FLATOW: You sound very passionate.

AISHWARYA VEERABAHU: I am. This is an awesome project, and I feel very grateful to work on it. I’ve gotten to talk to a lot of amazing people out there because of it.

IRA FLATOW: Well, we wish you good luck and thank you for telling us all about this.

AISHWARYA VEERABAHU: Yeah, thank you so much.

IRA FLATOW: Aishwarya Veerabahu is a fungal ecologist and PhD candidate at the University of Wisconsin-Madison.

Before we wrap attention Bay Area listeners, we’ve got a special live show in Redwood City, Thursday, September 18th at the Fox Theater. You don’t want to miss it. There’s going to be a toddler robot on stage. Yeah. Tickets are going fast. So head to sciencefriday.com/bayarealive and snag yours today. See you next time. I’m Ira Flatow.