Emily Elias: Once upon a time, mistletoe was seen as a symbol of fertility, because even in the winter it was still green and growing on branches. And that’s why at Christmas there’s a tradition of kissing under it. But seasonal kissing aside, mistletoe is actually a parasite, and researchers want to know where it is. On this episode of the Oxford Sparks Big Questions Podcast, we’re asking “why track mistletoe?”

Hello, I’m Emily Elias and this is the show where we seek out the brightest minds at the University of Oxford and we ask them the big questions. And for this one we have reached a researcher who wants to find where all that mistletoe is growing.

Ollie Spacey: I’m Ollie Spacey, I’m a third year DPhil student at the University of Oxford in the Department of Biology and I study mistletoe for a living.

Emily: Mistletoe, I mean that’s not one that I would… jump out at me as something to study. Why are you studying mistletoe?

Ollie: Yeah. So mistletoe, a lot of people know it for its Christmas connotations, but it’s actually a really interesting plant, biologically, for lots of reasons. So just a couple. It is a parasite of trees. It grows on these clumps, on trees in tree branches and it has a really interesting relationship with the tree host. And so even though we often associate it with romantic, lovely, festive times, it’s actually, it can be quite damaging to the trees that it grows on, and quite invasive actually in the trees when it grows into them. So, yeah, that’s what I’m… I’m studying it from the point of view as a parasite, rather than as something to be kissed under.

Emily: Now, okay, people might be a bit confused of what mistletoe exactly is, so can you just paint a picture for me? What does mistletoe look like?

Ollie: Yes, for sure. So worldwide there are actually hundreds of different species of mistletoe and the majority of which are actually found in the tropics. They look quite different to the mistletoe we have in Europe and in North America. But over here, we have one species of mistletoe in the UK. It’s called European mistletoe, Viscum album if you like the Latin. And it’s a green plant. It’s a green fleshy plant with sort of long, slender, smooth leaves. And it grows in this bifurcated pattern. So the stems kind of split in half every year. And this eventually forms essentially a ball of mistletoe in the tree. So because it’s not constrained by having to have roots like a normal plant, it can just grow outwards in all directions. So this makes it into a green ball shape. And then once they get to about three or five years old, the female mistletoes – because there are males and females – they start producing berries, and they have these very iconic white berries. So white berries, long slender green leaves as characteristic of the mistletoe we have here.

Emily: And usually you notice it more in the winter, right, cause the leaves of the trees have all fallen down and it’s like, kind of like, a big nest of branches up at the top. How does it get there?

Ollie: Exactly, yes. So this has puzzled people for hundreds of years. Sort of during the early times it was considered a very sacred plant. Cause it seemed to appear from nowhere. It would just never touch the ground. And it’s there up high in the tree canopy, and it actually gets there through birds spreading the seeds. So birds like mistle thrushes, which get their name from mistletoe, will pick up the berries – these white berries that are produced by the mistletoe. They will spread them onto branches of trees that they like to perch on. And the mistle thrushes will do this by eating the berries and then pooping out the seeds onto branches lower down. But other birds, like black caps, are a lot pickier, they don’t actually want to eat the seed. So they’ll eat the external parts of the berry and then wipe the seed onto the branch. So it’s birds which are doing the transmission. But how that transmission between trees happens depends on the bird.

Emily: So you say it’s a parasite. Are all parasites bad?

Ollie: No. So this is quite an interesting, controversial topic right now in biology, about what the role of parasites are, and whether they’re always bad. Now we actually, we define a parasite as having a negative effect on the host that it grows on. So it has to use a host to survive, reproduce at some point in its life cycle. And the thing that makes it a parasite, as opposed to a mutualist, is that it will hurt that host in some way. However, that doesn’t mean that all parasites are bad for all things. So mistletoe, for example, is actually very good for the rest of the ecosystem. As I say, it provides berries for the birds, but it also encourages nutrient cycling. So each year, old leaves fall down once new leaves of the mistletoe have been produced, and this cycles nutrients very readily back through the ecosystem. So this is good for other components of the community, the organisms living around that tree. And it also houses invertebrates that aren’t found anywhere else. It is really good at helping other trees to get ahead because they’re suppressing the growth of the tree that it’s growing on. So actually, whether we consider it bad overall is quite, is not really true. And there’s been lots of work done in Australia to show that actually mistletoes are a keystone species, are really important for the functioning of ecosystems. And if we see them go away, if we don’t see them as often, then we might have negative effects on our ecosystems.

Emily: How much mistletoe is actually out there, then?

Ollie: This is something that we’re trying to improve our records on, and trying to understand where mistletoe is today. It’s generally quite a common species in, European mistletoe is, across Europe, but it’s also thought to be changing in its distribution and how that’s happening, we’re trying to understand. The last major survey of mistletoe in the UK was done in the 1990s. The records were quite patchy and low quality. So we’re wanting to improve on that.

Emily: Sure. That’s like 30 years ago at this point. Like, that’s – in plant land – that’s got to be like generations!

Ollie: Yeah, yeah, exactly, exactly. Particularly because the mistletoes, as I say, they can be spread by mistle thrushes into new areas, and once they do, their populations really take off. So we’ve probably seen mistletoe introduced into new areas. Anecdotally, I’ve had people telling me, “oh, we never used to have mistletoe in my town and now it seems to be on all the trees”. And so understanding how this change is happening, what might be driving it, is really interesting.

Emily: So what have you been working on, to try and keep track of all of this mistletoe?

Ollie: To keep track of all the mistletoe, we have launched a citizen science survey called MistleGo! Kind of a pun on Pokemon GO, because you’ll go out there, track the mistletoe, and it is a citizen science survey which you download through the app Survey123. So using this app, you can take a photo of a tree with mistletoe in. You can record how many clumps there are in that tree. If you know the species of tree, then you record that too. But that’s not a problem if you don’t know the species of tree. The focus here is to see where is the mistletoe and how much of there is it [sic]? So that data comes into us and we can use that to predict where the mistletoe is today and where it might be into the future.

Emily: Could I just back up to the Pokemon GO of it all? So is it like with your camera you take a picture and send you a picture of the mistletoe? Cause I imagine people think they know what mistletoe looks like and then you get like a weird picture of holly.

Ollie: Yes, yes, and actually if you type ‘mistletoe’ into the internet you get lots of pictures of holly, which is quite devastating! Um, no. So, in this case, there are example pictures of what mistletoe looks like in the app, so you can just compare – is this what I’m looking for? Is this actually a bird’s nest? You’re looking for a ball in the tree which is slightly green. It may or may not have berries on. Once you see it, it’s quite clear, I think generally. It’s quite a simple process. Take the photo of the tree, record roughly how many mistletoes there are, and then send it in, and you’d be doing us a massive favour.

Emily: I mean, it sounds pretty simple, but what kind of difference could this make in your research?

Ollie: Exactly. So getting an understanding of where mistletoe is today in the kind of areas that it likes to grow in, the kind of areas that it’s spread to, allows us to predict into the future where mistletoe might be headed. Under climate change, we expect lots of species to move northwards because the climate becomes more suitable, at least in the northern hemisphere. Mistletoe really struggles under cold winters, for example. So once it reaches below -7 degrees, mistletoe says, “nope, I’m not doing this, too cold for me!” – even though it’s a Christmas plant.

Emily: I respect mistletoe, I’m going to say…

Ollie: Yeah, exactly, just too chilly. So it says, “I prefer slightly warmer temperatures”. Now, as the climate warms, we expect that winters might be milder for further north. So the mistletoe says, “actually, I don’t mind this so much anymore”, and it can spread and survive there. Incorporating this data, with where the birds might be headed into the future, where might mistle thrushes and black caps be in 30 years time that they’re not now, that could also tell us where the mistletoe is heading. And that kind of analysis has not been done before. More broadly, this acts as a case study for how parasites in general and the species that they interact with, will change under climate change and how their dependency on one another might influence their ability to survive and reproduce into the future.

Emily: So a lot of really big questions just centering around something that I guess a lot of people have taken for granted, you know, you go on a walk, you see some mistletoe, you don’t think much of it.

Ollie: No, no. So, a lot of times people will walk right by it. I’ve had people that live here in Oxford for years and it only, it took them to speaking to me, before they realized, “oh, that’s mistletoe? I always wondered what that was!” And I say, “yeah, it’s always been there, it’s doing some weird things”. Just another couple of fun facts. Biologically, it does some really strange things. It’s lost a big part of its respiration apparatus, so part of its mitochondria – if you’ve heard about mitochondria – it’s lost a really key component of that, and we don’t know why. It’s got a massive genome, a bigger genome than any other species in the UK!

Emily: Mistletoe?!

Ollie: Mistletoe – 30 times the size of the human genome. It’s about 90 gigabase pairs in genomics talk.

Emily: So I don’t know what a gigabase pair is, but that sounds terrifying.

Ollie: It’s a billion little letters of DNA, and so 90 billion letters of DNA making up its genome. And we, again, we’re still trying to understand why does it have all of this DNA, what’s it doing? And it’s probably linked to the fact that it is a parasite and it is doing biology in quite a bit of a different way to how a lot of other organisms, free living organisms, do it.

Emily: So I guess for Christmas this year, all you want is for people to download MistleGo!

Ollie: Exactly. Please make my Christmas wish come true. Type in MistleGO! into the internet, that’s ‘M I S T L E G O’ and you can have all the instructions you need to download the app and take part in our survey, and help understand what this fascinating plant is doing into the future.

Emily: This podcast was brought to you by Oxford Sparks from the University of Oxford, with music by John Lyons and a special thanks to Oliver Spacey. Tell us what you think about this podcast, we are on the internet @OxfordSparks. You can go to our website, oxfordsparks.ox.ac.uk and please, please, please go and download MistleGO! Think of it as like your Christmas walk activity. You can walk and track mistletoe. I’m Emily Elias. Bye for now.