Science Friday - A Delicious But Invasive Mushroom Could Affect Fungal Diversity

Episode Date: September 11, 2025

It all started harmlessly enough: People bought kits to grow mushrooms at home. But then, scientists in the upper Midwest noticed something strange. The golden oyster mushroom, which is not native to the United States, was thriving in local forests. Those homegrown mushrooms escaped our basements into the wild. Fungal ecologist Aishwarya Veerabahu joins Host Ira Flatow to discuss what impact these invasive mushrooms might have on the ecosystem.Plus, nightshade expert Sandra Knapp describes the evolution of the potato plant, and how a lucky crossbreeding millions of years ago may have given rise to the starchy tubers we eat today.Guests:Aishwarya Veerabahu is a fungal ecologist and PhD candidate at the University of Wisconsin-Madison.Dr. Sandra Knapp is a Merit Researcher at the Natural History Museum in London.Transcripts for each episode are available within 1-3 days at sciencefriday.com. Subscribe to this podcast. Plus, to stay updated on all things science, sign up for Science Friday's newsletters.

I'm Ira Flato, and you're listening to Science Friday. Today on the podcast, they trip down the produce aisle, including how mushrooms from some of those popular mushroom kits escape into American forests. 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. 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, you know, that tubers at the bottom of the plant, came from another plant cross-breeding 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. Thank you, Iris. It's great to be here. When we say that potatoes may have arisen from tomatoes or potatoes, potato, you know, what do we mean by that? Well, I mean, it's really hard to know 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 Salanum, which, by the way, has more than a thousand 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 know what the branching pattern is, is where tomatoes and potatoes, and this lineage called etuborosum, 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 Etobrosin were most closely related to one another. So that's what we call discordance. So how do we get a potato arising from a tomato?

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. So do we have a bit of tomato and a potato and a bit of potato and the tomato? No, we have a bit of tomato and a bit of eti-uberosum in potato. I see. So how did we get either one? Well, through change by evolution by natural selection. As time passed,

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 chilling. And there are 104 wild species of potatoes. Wow. So this is a random event. No caveman doing a Gregor Mendel sort of thing. No, no. This is also 8 to 9 million years ago, which is. a long time before any people were in South America. So did cross-pollination happen that gave us a tomato or a potato?

Well, so what we think happened is, you know, 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, etuborosum 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 etuborosum. So we think what happened is that there was a hybridization event sometime in the past, which we reckon is sort of eight to nine 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 eight to nine million years ago was that one of the times when the Andes were actively rising, that kind of 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. Do we know what biological machinery that hybridization used to make a potato tuber because that's what we eat? 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 utoborosum. 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 etuborosum. So putting those two genes together, this is what we think, allow 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 the potatoes need both of these genes to create tubers. So the genes are used to make tubers and all kinds of plants? Well, we don't know. 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, I mean, we see potatoes in the supermarket and there they are. You know, they're just like little brown things. You know, there they are waiting for us to cook them. But in plants in the field, they're on underground stems. And what they are is starch storage organs. Tubers are not. for us. They're not.

We're lucky that they're there because we've managed to exploit them, but they weren't there for us. They were there to store the energy for the plant. Right. They're there for the plant. And then human beings, you know, peoples in the Andes

exploited that characteristic and created the through plant breeding and through selecting plants that perhaps made bigger tubers than then made the potato that we know 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. Wow. And all the tons of different kinds of potatoes that they have in the Andes are all still a single species. Well, 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? Well, you could probably do that. I mean, you probably could do that, maybe.

What they're really interesting 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. That's what I'm thinking. Yeah, it could be cool. But, you know, you might not be commercially,

successful, if you see what I mean? I don't think it would make a dent in the kind of world economic importance of hotchal potatoes. I'll buy that. Do you have a favorite potato or tomato? 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 kind of old hairling 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 fieldwork, is the numbers of different kinds of potatoes and how different they taste. I mean, they do taste completely different, each one. You know, 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, I mean, if you look at the four big crops that, I mean, 80 percent 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, you know, they have a lot of fiber. They're really, really good for you. Well, this is so fascinating, Dr. Knapp. Thank you for taking time to talk with us today. Well, thank you very much for inviting me. Dr. Sandra Knapp is a merit researcher at the Natural History Museum in London. After the break, the story of the invasive golden oyster mushroom. Good to eat, but how it could be affecting fungal diversity.

I mean, I guess I don't know about there for good, but they are there. Stay with us. Cephalopod Week, rocks on. Do you love books and octopuses? Check out many things under a rock, a surprising story from marine biologist David Sheel. This is also a sci-fry book club pick, nerd out with our discussion questions, Plus, middle schoolers can dive into the Young Reader's edition, too. Swim out of your sea cave and visit ScienceRiday.com.

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 U.S., 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?

Ishwarya Verabahu is a fungal ecologist and PhD candidate at the University of Wisconsin in Madison. Welcome to Science Friday. Hi, thank you so much for having me. You're welcome. You published a study that looked into the effects of these invasive golden oyster mushrooms. Tell us what you learned. 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. So why is that a bad thing if these other mushrooms are just taking their place? Well, native fungal species do a lot of different things in the forest. They might be interacting with other organisms, right? They might have some sort of mutualism with wasps or birds. They might be in charge of decaying wood and kind of making that process go by slowly so that that dead wood can still stay standing and provide habitat for tree seedlings or small mammals. There are so many different. different things that fungi do in forests. And so anytime 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, like 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. So tell me how far these invasive mushrooms have spread. Where can we find them now? They have spread to at least 25 states in the U.S. 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. So they're out competing the native mushrooms then? Yeah, it seems so.

Wow. What do they look like? Can people spot them? Yeah, you can spot them really easily. I've actually spotted them from like hundreds of meters away because they are bright yellow. They will sometimes almost seem like a beacon in the forest. They are very, like, beautiful looking. They have these bright yellow caps and pure, like, 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. Are they good to eat or, you know, should we go hunting for them?

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. Tell us how they got, had they crawled out of our compost bins or our basements or wherever we were growing these mushrooms

and get out into the wild. Well, 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. So there are, you know, hundreds of mushroom farms all over the country, all over the continent, and they grow a lot of these mushrooms.

And, you know, 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 of grow kits. And so that kind of allows it to be efficiently dispersed into new parts of the country and introduced, and they can really get around that way. 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 they just too widespread? And what do we do about that? 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, right? 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. So their future is that they're there for good now? Yeah, I mean, I guess I don't know about there for good, but they are there. Does this signal to you that we need to better take care of our mushroom farms to prevent spread of invasive mushrooms? 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 a, you know, have your mushroom and eat it to kind of a 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 that prevents the introduction of invasives like the golden oyster. You sound very passionate. 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. Well, we wish you good luck, and thank you for telling us all about this. Yeah, thank you so much. Ishwarya Verabahu is a fungal ecologist and Ph.D. candidate at the University of Wisconsin in medicine. 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 Science Friday.com slash Bay Area Live and snag yours today. See you next time. I'm Ira Flato.