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Daniel and Kelly talk about the symbioses! We talk about the symbiotic interaction that made us who we are, a symbiosis that produces soil, and symbioses that are many layers deep.
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Today we're talking about symbioses, which are when organisms coexist. Yeah, it's a broad term, and it does encompass parasitism, but today I'm going to do my best to keep the parasite examples to a minimum. We're going to be talking about the symbioses that made us who we are, the symbioses that turn rock into soil, and the symbioses that have a surprising number of layers. We're going to be talking about how symbioses aren't always what they seem, and sometimes good interactions can turn bad. Welcome to Daniel and Kelly's Symbiotic Universe.
Hi, I'm Daniel.
I'm a particle physicist who likes to think about aliens. And I have a proposed new alternative title for the podcast, which is It's Complicated with Daniel and Kelly.
Hi, I'm Kelly Wienersmith. I study parasites and space. And that sounds, yeah, that's great. Let's go ahead and change it right now. I think either it's complicated or it depends. Or the gray areas are sometimes the most interesting, although that's a little wordy. Yeah, exactly. The acronym doesn't flow off the tongue quite as nicely, you know, as DKU.
Yeah.
So this episode was inspired by a question from Discord user Quantum Cat.
Love Quantum Cat. Yep.
Yep. Yep.
And just like always, I forgot to ask their first IRL name before we started recording. But here is Quantum Cat's question.
Hi, Daniel and Kelly. This is Brandon, also known as Quantum Cat. I have a question about nested symbionts. I was reading about mealybugs, which suck sap from plants. And I read that they contain a bacteria to help them digest the sap. But the unusual thing is that that bacteria is not enough. And that bacteria contains even smaller bacteria to help it digest the sap. So that's pretty unusual.
Can you tell me more about the mealybug and its two bacterial helpers? Are there any other critters like that? Thank you.
OK, so we are going to dig into the example that Quantum Cat refers to later in the show. Hang on for the second segment, but it's coming.
So, Kelly, we all learned about symbiosis in elementary school. It's pretty straightforward. It's pretty simple. People helping each other, organisms helping each other out. Aren't you basically on a symbiosis on your farm with all your goats and sheeps or are they parasites on your time?
Well, I was going to say that sometimes they feel like parasites. But but OK, so this is an interesting point. Let's jump right in here. What how would you define symbiosis?
A relationship where organisms help each other. That's my Daniel fourth grade understanding of biology.
Well, so I'm a little bit embarrassed to say that that was my grad school understanding when I went in. And so I was talking about a symbiosis as two organisms coexisting and interacting in a positive way.
Yeah.
And I was.
Reschooled.
I was schooled, man. I definitely got a bit of a look from the professor I was with, and they're like, symbiosis is a word that sort of covers a variety of interactions. It just means organisms coexisting and interacting together. And then you split it further into different interactions depending on whether or not those interactions are beneficial, negative, or not really doing anything. And so what you and I...
thought was symbiosis is actually a mutualism.
Oh, I see.
And a mutualism is when both players benefit. And I don't know if you were just being nice and playing along with me to set me up there, but I really didn't in grad school. And this isn't entirely our fault because symbiosis has come to have a positive connotation. A lot of people think of it as a positive interaction, but it really encompasses a lot more.
So you're telling me symbiosis is not just the positive version of It's the name of the whole category. And the positive version is called mutualism. Wow. I'm going to go look up my fourth grade teacher and let them know.
Well, you know, I think it's possible that your fourth grade teacher got it right. And over time.
Impossible.
No.
No, that hypothesis will be ruled out. Thank you very much.
I was a.
Very attentive fourth grade student. I'll have you know.
I can imagine you sitting on the edge of your seat waiting for the information to come to you. But let's talk about mutualisms.
Yeah. All right.
Do you have an example of a mutualism in your head when you think about mutualisms? Because to me, there's a classic example, but I don't know if it's classic to everyone else.
Example of a mutualism. What about like horses eat an apple and they get yummy apple and then they poo out the seeds in their poo, which is good for the seeds. So everybody benefits.
Okay, yeah, so pollinators and seed dispersers, those are good mutualisms.
Amazing.
And you know what? That's definitely way more common than the mutualism that I'm thinking of.
And it involves poo, so.
Amazing, right?
No, I'm totally on board right from the beginning. But the one that I think of as like the classic example, I guess I specifically like to think of animals interacting, is cleaner fish in the Great Barrier Reef. Have you heard of these?
No.
No, tell me.
So these are fish where they will set up cleaning stations and bigger fish like sharks and groupers and stuff like that. They will come in. They'll open their mouth really big. And the cleaner fish will go inside and like pick out parasites and sort of clean their teeth. And then they'll like go along the exterior of the animal and like pick off all of the ectoparasites that are on there. And so they get a meal and whoever visited their cleaning station gets gets parasites removed and stuff like that. And so this is, in my world, one of the, like, classic examples of mutualisms.
Wow.
Yeah, well, very cool. Everybody definitely benefits. It's amazing, though, that fish basically set up, like, dental hygiene stations. That's cool.
I mean, that's super cute, right? I think that's awesome.
It's basically civilization.
That's right. That's right. That's right. And, you know, I have seen people have some pretty negative interactions with their dentists. They might be more civilized than we are in the fish world.
Well, I brought my son to one of those places where you dunk your feet in his vat and then the fish like nibble on it.
No.
I wasn't going to do it, but he thought, ooh, this is ticklish.
Yeah, right. I have heard examples of people getting real bad infections from those fish tubs. And so I don't know that I would, I will not be recommending that on the show. But I can say that I used to work at this lake in Wisconsin, and when I would go in barefoot, the minnows would come by and pick the dead skin off my feet, which I'm sure is really going to gross a bunch of people out.
But I remember feeling, oh!
That's amazingly grosser than poo, yeah.
Yeah, okay.
Well, you know, we're always working on one-upping our prior accomplishments.
So let's get into it. Let's make sure we have the definitions here. So symbiosis is two species coexisting. Mutualism is they both benefit. Is parasitism still when one of them is hurt and one of them is benefiting?
That's right.
Or is that more complicated?
I mean, yeah. So there's lots of complicated examples. But yes, that is, you are correct. And so this is like tapeworms and trematodes and stuff like that. And so I won't go into too much detail because anyone who listens to DKEU is familiar with the idea of parasitism. And then there is commensalism, which is when one benefits and the other, it's just kind of like, meh, they're not helped.
They're not harmed. And so examples here are cattle egrets. They will hang out in fields with cows. And as a cow walks through, it kicks a bunch of bugs up into the air that try to get away as the cow is moving through the grass. And then the cattle egrets pick those bugs out of the air and eat them for themselves.
Wow, very cool.
The cow doesn't care, but the cattle egret benefits.
What about situations where one is hurt and the other one isn't helped or harmed?
One is hurt... And the other isn't helped.
Oh, spite.
I think that's what you're talking about, right?
Well, I was thinking about the cows and the bugs. The cows are kicking up the bugs. The egrets are benefiting. The cows don't care, but the bugs are definitely not happy about it. So what's the relationship between the cows and the bugs? The cows are casually destroying the lives of the bugs without caring about it at all.
Interesting. Yeah, I mean, that's just an accident. I guess we could just call that a symbiosis. There's an interaction. Although, so that's more of a, like, short-term interaction. So the egrets and the cows are, like, coexisting together for a while. The egrets are, like, hanging out with the cows. Whereas the cow just has, like, a short-term interaction where it accidentally kicks off that bug. But I hadn't thought of that.
I don't know. We've got to get you into ecology.
There's a new category we're creating here. And the other question I have is like, how do we define these categories? Like, according to whom is this a benefit or a detriment? Is that fuzzy? Is that a judgment call? Or do we have like really crisp, clean definitions like we always do in biology?
Yeah, we don't usually have crisp, clean definitions in biology, unfortunately. But, you know, you look at the two players in the symbiotic relationship and you do your best to calculate what's happening to them. So, you know, it's pretty intuitive that if you had a bunch of like blood sucking ectoparasites stuck to the outside of you and someone came along and picked those off, that's good for you. You've got, yeah, you've got fewer blood sucking ectoparasites.
Right.
And the cleaner fish ate them and that's their food. And so they got a meal. You brought the dinner to them. And so like, I feel like in that case, you can pretty clearly say, well, that probably benefits everybody, but... You know, this. is ecology, right? And so this stuff gets complicated.
And.
That's the name of the podcast.
That's right. It's complicated with Daniel and Kelly.
And so.
These interactions can change either over time or depending on the environmental context or depending on the individual. So, for example, sometimes those cleaner fish, if they're hungry and there's not a lot of parasites on the fish they're cleaning, sometimes they'll grab a little bit of fin tissue or they'll grab a little bit of like the mucus that's a protective layer on the fish, but also could be a snack for the cleaner fish. And sometimes other cleaner fish will step in to punish the cleaner fish that's misbehaving.
And I think sometimes also the fish that's visiting the dental station will punish a cleaner fish if they get out of line. But every once in a while, these relationships will turn negative and there needs to be like punishment to get everybody back in line because interests are not always aligned.
Counselors. We call them counselors.
Counselors. That's right. Fishy counselors.
You guys need to go to therapy and figure this out.
That's right. That's right.
All right.
So let's dig into an example of symbiosis and tell us about how it's more complicated than Daniel's fourth grade understanding.
Well, I was thinking that today we could dig into some of the more epic symbiotic interactions.
Cool.
And one of the most epic.
Most epic.
Most epic. Like, you know, without this symbiotic interaction, we would not be here.
Okay.
Is the acquisition of mitochondria.
So. Whoa.
In the 1960s, Lynn Margulis pushed this idea of the endosymbiotic theory. Do you know of Lynn Margulis?
Lynn Margulis, is she connected to Carl Sagan somehow?
Yeah, and as a woman who's tired of having people say, oh, are you married to Zach Wienersmith? I hesitated, like, do we bring this up or not? But on the other hand, Carl Sagan is one of my favorite science communicators, and so I do bring it up. Yes, Lynn Margulis was, I think Sagan had two wives, and Lynn was the first.
And she was, I think, Lynn Sagan at the time this idea came out.
But did it end badly?
It ended. You know, some endings are good. I don't know the details.
Because I don't think anybody wants to be associated with, like, their ex that they're grumpy about. So context matters.
Yeah, no, fair enough. So, Lynn Margulis' reputation is mixed, let's say. So, this idea that we're about to talk about is this amazing thing that she pushed forward. She got a bunch of people to accept that this might be what's really happening. But she also believes some conspiracy theories. Like, she was pushing the idea that HIV isn't what caused AIDS. So, she kind of ended up with a bit of a mixed reputation.
But.
All right, let's stick with the cool thing that she pushed forward. So Lynn Margulis was pushing this idea that the mitochondria that you find in eukaryotes and the chloroplasts that you find in things like plants... Were actually free living bacteria that were essentially co-opted. And now they live with us forever.
I see. So they were out there in the wild and we basically made them tenants.
Yes.
Or like move in.
Right.
And so before this idea really got forwarded, people were thinking that like, oh, well, mitochondria and chloroplasts, they might just be like part of the cell membrane, sort of like invaginate. So it like kind of folds in and makes a little pocket and then it pinches off. And now you've got a mitochondria.
Yeah.
And so this idea had been, like, floated a couple times before, but Lynn Margulis really looked at the data, and she found... A couple of different observations that seemed to her to really say these mitochondria started as bacteria. And here are the observations.
And let's remind everybody a mitochondria is the little piece of the cell that like produces energy. It's like the little factory or the oven or something.
The powerhouse of the cell. I think that's what we. Yes, that's right. This helps produce ATP. These are the little energy sources for our bodies. And chloroplasts help plants convert sunlight into energy as well.
Right. OK.
Chloroplast for chlorophyll. All right. Got it.
Yes.
And so the idea is that we used to not be able to make ATP ourselves. We used to like work with an outside supplier and then we brought them in-house basically.
So the idea is that four billion years ago, there were prokaryotes. And reminder, prokaryotes are cells that don't have any like nucleus and they don't have membrane bound organelles. These are like bacteria.
Right.
And then 2 billion years ago, an archaea, which they look a lot like bacteria, they differ in a couple ways. For example, I think they have a different sort of membrane. An archaea acquired a bacteria, and we don't know how it acquired that bacteria. When I was younger, the story I heard was that it was like an amoeba sort of.
Yes, right.
Speaking biologically.
Exactly. Yes, right. And engulfing it as though it was going to take a meal. But then that bacteria just like survived and didn't die. But now we're not actually sure if it was parasitism or a predation. We don't actually know how it got there because amoeba are actually pretty advanced. And the kinds of organisms that we think acquired the bacteria, they don't do phagocytosis. They don't do this like thing that you were saying.
That's the biological word for slurping. Phagocytosis.
Phagocytosis, yes.
I see.
So anyway, we don't know how one got inside of the other, but about 2 billion years ago, that happened.
So I guess I can get behind the idea that we used to collaborate with this thing, and then we shooped it up into the cell, and we can more closely collaborate with it. All cool. But then how do you get new cells to get one? Do they all have to find it themselves?
Yes.
Or do you now have the machinery inside you to make them? Or like when the cell divides, does this thing also divide? How does that get coordinated if it's like a foreign body? I'm used to the idea that there's like a single set of instructions in the nucleus for making the whole cell. But if you slip the thing from the outside, you don't have instructions for making it.
So how do you pass it on?
Yeah, great question. So first, I want to start with you. You said something about how the two organisms were like collaborating outside and then One, the other.
You're going to correct Daniel's fourth grade knowledge again?
Would you prefer if I didn't?
Please do. That's what I'm here for.
All right.
Well, so we don't necessarily think that they were collaborating as outside partners initially. It could be that the bacteria was a pathogen and tried to invade the archaea and it just didn't take. And then over time, the nature of their relationship changed.
Well, that's a much cooler story with a twist in it.
Yeah, that's right. That's right. Sometimes your enemy becomes your friend. You should always have an open mind. But your question actually leads to part of how we figured out that this might have been a bacteria that was acquired because it was noted that mitochondria and chloroplasts divide in ways that look very similar to how bacteria divide. All right.
So you've got host bacteria and it Sort of splits and divides. And if you want to get the little inside bacteria also, that thing also needs to have split and divide and then separated the right way so that both of the new cells also have the new symbiotic inside cell. Does that make sense?
Yeah.
How do you get that to coordinate?
Well, that's a tough question.
Yeah.
I don't know how you get that to coordinate. I don't know how that worked over evolutionary time, but it was noted by like Lynn Margulis in her paper. She was like, look, these things are not being produced the way you would expect them to be produced if they were initially made by the organisms. The instructions are not in the DNA that's inside the nucleus. They seem to be doing their own replication. And like, I think in our liver cells, we have something like thousands of these mitochondria.
in our cells, right? And so they're replicating all the time. They're doing their own thing. And it was noted that you inherit from your mom mitochondria sort of separate from the instructions that's in the nucleus. It's not like when you're born, the nucleus says, okay, start making mitochondria, and then they pop up. The mitochondria come along with everything else.
I see. So this actually was a powerful piece of evidence. That they were once a separate organism because they're not included in the core DNA. They just sort of like replicate on their own. And we don't really understand that. But when the cell splits, you get some on one side and some on the other side and have their own DNA. I've heard of that mitochondrial DNA passed down through the mom.
Right. Right.
OK.
Yes.
And when you look at the sequences in mitochondria, you look at like the, you know, the genetic code inside of a mitochondria, that genetic code looks similar to genetic code that you find in bacteria. And it looks a lot more bacterial than it does like animal, for example. So that was more evidence that actually this used to be a bacteria. Does that make sense?
That does. And it's fascinating because it tells me that like moms are more closely related genetically to their kids than than the dads are. It's not a 50-50 split because the mom passes along the mitochondria because she creates the actual baby and fills it with her own mitochondria. She's not taking the dude's mitochondria and mashing them together.
You do always get mom's mitochondria. That's right. Yeah. Not dad's. Other pieces of information that Lynn Margulis used to make this argument is she noted that the membranes that surround things like mitochondria look kind of like the membranes you usually find in bacteria. So she really pushed this idea.
She pushed it a little bit further. She kind of overdid it. She was arguing that like lots and lots of organelles are probably from bacteria that were acquired and she was seeing endosymbiosis everywhere. But we do have really good evidence that mitochondria, for example, and chloroplasts used to be bacteria that lived on their own and then they got absorbed by another organism and that opened up a bunch of pathways because now they had all this extra energy.
They had these little dedicated energy sources and that appears to have opened up a lot of options for like diversity of size and function. And Nick Lane's book, The Vital Question, talks a lot about this. But yeah, we would not be here without this symbiotic relationship.
Now, clearly we live in symbiosis with a lot of bacteria. They're all over our guts. Katrina told us about our microbiome. So there's lots of bacteria living in us, on us, that participate in our lives in crucial ways. And your description of them earlier made it sound like, well, these are just more bacteria that live in us, but inside the cell. And they participate in crucial cell activities. They replicate on their own. They do all this stuff and have their own DNA, which makes me wonder, like, are they actually part of the human body or are they part of the microbiome that's living in and on us?
And I think you're saying that. They can't live on their own anymore. And they like become so specialized. They basically can only operate inside the human cell.
Yes.
Is that right?
Absolutely. And let's take a break. And when we get back, we're going to talk about examples in nature that appear to be somewhere along that process.
All right, we're back and we're talking about symbiosis. And I'm just now learning that there are bacteria living not only on me, but also inside me and inside my cells. But Kelly says they're kind of part of me now because they really, really made a home. But I feel like if they split on their own, they have their own mitochondria, and they're just passed down from the mom, I'm not sure they count as human bits.
Okay, so I think that they do because they are clearly dependents.
I don't think that's sufficient, though. Okay.
What would you need to be sufficient?
I don't know. I mean, there are definitely lots of organisms that need us to survive, but that doesn't mean that they're human, right? We're in symbiosis with them. For them to be part of us, I feel like they have to be part of the genetic code. Otherwise, who is it that's getting passed down, right? It's just part of the environment.
All right.
Well, so one, there is evidence that some of the genes that the bacteria used to have are now in us.
So- Cool.
Right.
So the bacteria have been like in us getting like, I don't know, a free home and a free meal. But while they're inside of us, that means that they no longer have the opportunity to like meet and mingle with other bacteria. So so there's no longer, you know, the bacterial equivalent of sex. Yeah, there's no horizontal gene transfer or anything like that. And so over time, actually, their genomes tend to get smaller. smaller and smaller and some of their genes end up getting incorporated into the host genome until they're a pretty small subset of genes that do the specific things that we need mitochondria to do in our body. And so they've definitely come to depend on us. We absolutely could not survive without mitochondria.
We would be goners without them. Whereas I think for the rest of your microbiome, like if you took antibiotics and you actually knocked out your microbiome, You'd be a mess. But like, I don't know. Do you think you could survive without most of the gut microbiome, for example?
I don't think so. I think they're pretty essential. But give me the alternative story. Say this hadn't happened. Where would life be on Earth without this?
I think that you could make the argument that we would still be at the prokaryotic stage of things like bacteria and archaea. It looks like. this extra energy that we were able to get by this symbiotic relationship sort of freed up a lot of the ability to, like, explore evolutionary space, as they would say. Like, we've got this energy to try new things. We can get bigger.
We can.
You don't think we would have figured it out ourselves? Like, we would have developed the same machinery? It's just biochemistry, right? It's not magic.
Well, I mean, it looks like You know, four billion years ago, you start to get bacteria and archaea. And then two billion years after that, you get this endosymbiosis where one ends up inside of the other. And that's when eukaryotic organisms, so organisms with like a nucleus and membrane bound organelles, that's when we really started getting going.
And so that does seem to be what kickstarted this like massive increase in diversity, but whether or not we could have figured out some other way.
To do it.
Impossible question, I know.
Impossible question. Yeah, hard to say. But again, Nick Lane's book, The Vital Question, I think would argue that that was like, that is what you need.
Fascinating. Well, I would love to see how this happened on alien planets so we could try to answer this question. Did something similar happen? Did they develop their own internal machinery? Is it just completely different biochemistry anyway? Can't wait to find out.
Yeah, no, I would love to know that as well. But let's talk about sort of like an intermediate stage in this process. And so there have been examples where people have found like bacteria inside of bacteria and they seem to be helping each other out in some way and living together for like long periods of time. But because I like to add complication, there's An additional layer to this example. So Quantum Cat referred to a paper where scientists had studied a mealybug and the endosymbionts in the mealybug. So mealybugs are insects that have these like mouth parts that they can stick into plants and they suck out the sap. There's a lot of in this episode.
Yeah.
And they have bacteria that provide them with amino acids and some vitamins. And these bacteria are beneficial for the mealybugs. But you know how we were talking about how our mitochondria over time, they sort of degraded. They lost some genes that now are in our nucleus and they just became simpler sort of overall. One of the problems with these long term interactions where one is trapped in the body of another is that they do tend to sort of degrade over time because they don't have like sex and reproduction to sort of refresh their genetic information. So they noticed that the mealybugs, they have this amino acid providing bacteria. And if you look at different mealybug species, these different amino acid providing bacteria have different species of
additional bacteria living inside of them. And so it looks like what's happening is that the main bacteria that was living inside the mealybug over time is sort of degrading and losing some of its awesome ability to provide amino acids and nutrients to the host.
Hmm.
And so it keeps picking up new bacteria that sort of replenish it and give it new abilities to be beneficial to the host. And so this is a symbiosis with three levels. It's a bug that has a bacteria that inside of it has a bacteria. And together, that unit helps the mealybug survive, which I thought was pretty awesome.
And which part keeps picking up new partners? The bug inside the bug or the bug?
Yeah.
You can't call bacteria bugs in this example, Daniel. That's too confusing.
All right.
So there's, there's a bug with a bacteria and inside that bacteria is another bacteria.
Right.
And that bacteria inside the bacteria, the mini bacteria, the mini bacteria is the one that keeps getting replaced.
Wow.
So that's like the bacteria keeps using up his relationship and then finding like a second spouse and then a third spouse. And it's just like a, It's like a serial monogamist.
Yeah, except that couple is living in service of some... I feel like this is kind of falling apart, but you need three players.
It's a cult, basically, yeah.
Well, there are relationships with more than two partners that aren't cults. We don't need to go too far down this. Anyway, I think the point is these three players together are needed to keep the bug going.
Wow.
And is that the only example of like these three level stories or is that a common thing to find in biology?
Oh, man.
OK, so this three level story, like I learned about when I was in grad school and it absolutely blew my mind when I learned about it. And I still think it's super cool today. So we're going to talk about it. OK, so tell me. P. aphids are these little bugs that like they eat plants. You often find them on like fava beans and stuff like that. And they can be infected by parasitoid wasps. This is not the symbiote. Well, this is a parasitic relationship we're talking about here. But in a second, we're going to be talking about bacteria living in the aphids.
And are the aphids good or bad? I got to know who to root for here. Aphids are bad, right? Because they eat your crops. We like set ladybugs out in your backyard to eat the aphids, right?
Exactly. Yeah.
So we're anti-aphid. All right.
Aphids are bad if you're a farmer. If you're an aphid, you probably think aphids aren't so bad.
Are you being paid by Big Aphid, Kelly?
Always.
You're weirdly pro-aphid today. I'm anti-aphid, so go ahead. Now I'm on the side of these wasps.
Okay.
All right. I'm never opposed to someone being on the side of the wasps. So you're on the side of the wasps. These parasitoid wasps, they lay eggs inside of the aphid. Their babies hatch from these eggs. They burst out of the aphids and they kill them.
Wow.
Okay. So as you can imagine, that's very bad for the aphids.
Yeah.
I'm starting to feel bad for the aphids, actually.
Oh, well, see, you know, nature is complicated. It's complicated with Daniel and Kelly. Okay, so P. aphids have a bacterial symbiont that defends them. And in fact, the name is called Hamatonella defensa. But I won't say it again. I use the scientific name just because defensa is in there. It's kind of cute.
All right, I'll give it to you.
All right, thank you. So it has this bacteria and cyanobacteria, noticed that when a parasitoid lays eggs in an aphid that has this bacteria, the egg laid by the parasitoid wasp was less likely to survive. And the aphid was more likely to be totally fine.
So the wasp is attacking the aphid by sticking its eggs in there, but the aphid has little microbes inside of it which attack the egg somehow.
Yes, which seem to be killing the egg in some way.
Wow. Yeah.
And so then they looked a little bit closer and they were like, hold on. It depends on the strain of the bacteria. So it's not like every aphid that has this bacteria in it is totally protected from parasitoid wasps. It looks like if you grow the bacteria in the lab for different amounts of time, you end up with these different strains. Some strains seem to do a better job of protecting than others. And they were like, well, what's different? And the thing that was different... was bacteriophages.
So Katrina came on the show to talk about phages, bacteriophages being viruses that infect bacteria. So some bacteria were infected by a virus and this virus codes for toxins. And so the bacteria that have this virus in it are able to produce a toxin that appears to be the thing that is killing the parasitoid wasps.
All right, hold on. I need a diagram.
Okay.
So we got a virus.
Yep.
Inside a bacteria.
Uh-huh.
Inside an aphid.
That's right.
That's producing a toxin that the aphid can use to attack the eggs that the wasp has also injected. inside this aphid. This aphid is a mess. Oh my gosh.
I know.
Well, I mean, you could think of it as a mess or you could think of it as having like an army on its side trying to help out, you know?
No, it's a battlefield. Like the battle is going on inside the aphid.
Yes.
Yeah.
It's amazing.
Wow.
But what's interesting is that these bacterial symbionts seem to get lost over time if there aren't parasitoid wasps around. So if you just grow aphids in a lab with no wasps around and Over time, you discover that fewer and fewer of the aphids still have this bacteria. But if you grow them in the lab with parasitoid wasps around and you watch them over many, many generations, more and more of them have this bacteria. And so that makes sense. The bacteria protects them from the parasitoid wasps.
So you would expect the ones that have this bacteria to be more likely to survive and have babies. But why would you lose it when the wasps are not around?
Right.
Yeah, you need to maintain them somehow and they're expensive or what?
Yeah, so they weren't able to figure out, like, what is the cost? But it does suggest that in environments where there aren't parasitoid wasps, there is some cost to this bacteria and the bacteriophage. And so that would suggest that in some environments, this is clearly a mutualism. This is clearly beneficial. But in other environments, you know, maybe the bacteria doesn't have a huge cost and it's a commensalism and it just gets lost through random processes.
Right.
Or maybe that bacteria actually does carry a cost and it's more like a parasite when there aren't parasitoid wasps around. Does that make sense?
Interesting.
Yeah.
So it's worth paying the cost when you need it. But if you don't, then it's just a drag on your internal finances.
Yes, exactly. And this is what I love about symbiotic relationships is that they are complicated. Complications in nature, I don't know, are interesting to me. But like which category you put this in, in terms of like what kind of symbiotic relationship is it? Depends a lot on what's in the environment, which I think is kind of fascinating.
It seems like the right way to think about this is from a sort of ecological perspective, like the whole network of critters working together and talking to each other and communicating. Katrina's always talking about how the right way to think about microbes is also ecological. That's sort of her big thing. Instead of thinking about individual pathogens or whatever, you've got to think about the whole system.
Yep, absolutely.
Which is interesting.
Really hard to do.
Yeah. It's complicated.
It's complicated. And that's why it's so important to, you know, do experiments that sort of build in their complications so you can look at all these different levels and then one day scale up to out in the field to see if you really understand what's going on there. Well, if you've been liking this conversation, you're going to love what comes next. All right, and we're back, and the last symbiotic relationship we're going to be talking about today are lichens. Daniel, let's continue today's tradition of putting you in the hot seat. What do you remember about lichens from, I don't know, kindergarten?
All right. Fourth grade Daniel understands that lichens are some weird combination. They're like a... Fungus or a moss plus an algae or something like that. And they're working together and they like break down rocks.
Yeah. Yeah.
That's done.
No, that was good.
That's it.
Yeah.
No, let's dig in a little more, but that's good.
Yeah.
So the fungus is usually the like bigger partner in this interaction and it provides like structure and protection and And inside of it, you usually have algae and the algae are photosynthesizing, they're providing energy. And these tend to be very slow growing organisms that can survive in super harsh environments.
All right.
So fungus is the major player.
Okay, yeah, fungus is the major player. And so, you know, as somebody who thinks about taxonomy pretty often, I know you're not a huge taxonomy fan, but what do you do? How do you characterize these organisms that like so clearly depend on one another, but like you've got two different players. Where does that fit in the tree of life? Very complicating.
Maybe it doesn't make sense to draw arbitrary dotted lines around things.
One of my favorite things in ecology and in evolution is when the tree of life has branches that sort of like come back together and touch each other or when it doesn't do what you would expect tree branches to do. That I always think is really fascinating. We have to think outside the box there. But so we tend to try to classify lichens based on the. big fungal partner that provides most of the structure. And I feel like this probably is because humans are very visual and probably initially it was just easier for us to be like, this is the part that I see. This one is green. This one is yellow.
Right.
But anyway, so we tend to classify based on what the fungal partner is doing. And there's something like 15 to 20,000 species of lichens. And it looks like this symbiotic relationship is about 400 million years old.
Wow.
And this relationship popped up, we think, something like nine to 11 times. So it's not like once a fungus and an algal partner were like, let's work together. This has happened multiple times. There's some species that we think used to be in a lichen relationship and then they got a divorce and now they both live separately.
You're saying they stopped liking each other?
Yeah.
Oh, why didn't I come up with that? Oh, my gosh. The hours that I spent researching this and I missed that. Anyway, I'm very disappointed. But what I'm excited about is, you know, it's complicated.
Hmm.
And it looks like the more people have spent time with lichens and the more we've been able to like, you know, this isn't very nice, but like grind up lichens and send them all through like genome sequencers to be like, okay, who is everybody that's in here? We've usually thought about lichens as fungus plus algae, but it turns out there's often like bacteria that are very consistent residents in lichens. And sometimes they do things like produce nitrogen, they're nitrogen fixing bacteria.
Yeah.
And so I think a lot of these lichens are not just two players. Like if you look at them more closely, there's probably bacteria or other kind, like multiple species of algae that are consistently showing up in the same species of lichen. And so lichen might actually be like not just two players, but some species might be like three players consistently found together or four players consistently found together so that they can provide everything that's needed for the lichen to persist.
Interesting.
What do you mean, interesting? That's fascinating, man. Imagine living in a triad where everybody had to play nice all the time. That's hard enough being in a dyad.
I'm surprised it doesn't fall apart more often, yeah.
Yeah, all right. Anyway, I think that's pretty cool. But one of my favorite things about lichens is that they help break rocks down into soil. And so they often stick to the side of things like rocks. And one of the things that they do is just very slowly grow into the like little nooks and crannies in the rock.
And over time, they're able to just like create extra stress in the rock to start like cracking it and breaking it apart.
Amazing.
I know, right? When I watch tree roots that have lifted up sidewalks, for example, just like the fact that slow, persistent growth can do something like that, like that needs to be a bumper sticker. That's inspirational.
Yeah.
Well, it's one of those great insights into deep time, right? To show you the things that you thought were static or were uninfluential over time can change everything. It's really incredible.
Yeah, it is totally inspirational. And additionally, some of these lichens are able to produce things like acids or other like mineral dissolving compounds that further break down through chemical means, not just like physical means. And that helps the lichen grow. And then one day the lichen will die and it will decay and become part of the soil. And through this process, you go from giant rocks to soil. And I just think that's absolutely amazing.
And did we not have soil before lichen or was it just slower?
There are multiple ways that you can get soil, and this is one of them. It's not like all of the soil on the planet we can tip our hats and say, thank you, lichen. But it's possible that there are some minerals in your body that were liberated from rocks by lichens. And I'd say probably even probable. And so, you know, thank you, lichens.
We've all been soiled by lichens.
Or spoiled.
Spoiled, yeah. Man, you're on a roll today. I'm loving it, Daniel. I'm loving it.
I'm just bringing my fourth grade sense of humor, you know, because I'm bringing my fourth grade level of expertise.
Highly appropriate. Highly appropriate. And, you know, I may or may not have sat for a second and thought, do I know who the two partners are in a lichen? And I had to think about it for a second because I hadn't accessed that information for a while. And so, you know, did some deep reading before this. But one of the cool things about lichens is that when they are dried out and desiccated... It's really hard to kill them.
Like when the fungus is all sort of like dried out, it does a really good job of protecting the algae inside. And so lichen have been like brought to space. And it turns out that they're super resistant to radiation when they're in this desiccated state. But when they are moist and active and like metabolically doing the stuff that they usually do, they're actually pretty darn sensitive to things like air pollution. And so they can be used as bioindicators.
Like if you go to an area and there's like a lot of diverse lichen, you can be like, oh, the air quality here is probably pretty good. But if you notice that you're losing lichen in an area, that could be an indicator that the air quality in your area is going down. And so some ecologists are pushing lichen as a way of like monitoring how good the air quality is in an area.
Whoa.
Yeah.
Are you liking the air or not?
I'm not sure I'd be liking living in Beijing, but I hear that it's getting better there. And I like caribou and lichen tend to be a major food source for caribou. So what's not to like? N.
All right.
So Kelly, tell us, is symbiosis underappreciated in the general community out there? Do people not realize the power and the awesome impact of symbiosis in general?
Gosh, I mean, I don't know. I think most of us appreciate symbioses, but we like appreciate them in a more fine grained way. Like, you know, I have dedicated my life to appreciating the category of symbioses, which is parasitism. You know, and so I think a lot of people are like, oh, I study mutualism. I study parasitism. I study commensalism or something. But like symbiosis is just such a broad term. But to be honest, I was kind of like, what am I going to do with this episode?
This could be like, you could pick any species and there's like thousands of symbiotic interactions you could talk about.
Because nobody lives in isolation, which means that everybody's in symbiosis all the time.
Yeah, except symbiosis is meant to focus a little bit more in on like long-term interactions. And so like if you pass a stranger on the street, you wouldn't say you had a symbiotic relationship with them. But you do have a symbiotic relationship with your wife, for example.
Yes.
Definitely not parasitic.
That's right. That's right. Here's hoping. And so anyway, none of us would be here if it wasn't for a symbiotic interaction that happened something like two billion years ago. Thank you, symbioses, for making us what we are.
Hi, Daniel and Kelly.
This is Brandon, otherwise known as Quantum Cat. That was another excellent episode. I found two things especially interesting.
One is that for the mealybugs, the bacteria that it contains that have a bacteria inside of it, that bacteria inside of it can vary over time, and it's not just one species.
And the other thing I thought was fascinating was the, um, aphid, that for defense against the wasp that predates upon it, it contains a bacteria that makes a toxin that goes after the wasp's eggs. And that if it's not predated by that.
Wasp, the level of bacteria drops.
That just is fascinating how nature works that way. So thank you again for an excellent episode.
Thanks, everybody, for listening. Please go and do us a favor and rate the show on whatever podcast app you're using. It really helps people find us.
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