How did plants evolve to attract insects?

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1:06We're doing a trek up the mountain today and we're standing in front of a map with lots of pins and bits of string so is this the route that we'll be taking? Yes Hello and welcome to CrowdScience from the BBC World Service I'm Anand Jagatia and this week we're in the Philippines where I'm about to set off on a long hike through the forest up to a peak called Mount McKilling We are now here Checkpoint office, OK. Yeah, checkpoint office. And then we will try to traverse up to here. So expect some slopey areas. Is it going to be... Muddy terrains, yeah. OK, it looks steep. Yeah, definitely. So expect that.

1:43And then the leeches. Leeches? Yes. You can probably hear in my voice that I'm not that thrilled about the prospect of encountering blood-sucking leeches. But where we're going, it is apparently a real possibility. They are like everywhere, so we try to be cautious regarding those leashes. The things we do for CrowdScience listeners, eh? Although, despite the alarming warnings about parasitic worms, we're actually here in search of a plant, and it's thanks to a question from one of you.

2:15Anand Jagatia:Hello, I'm Alice from the UK, and my question for CrowdScience is, how did plants evolve to attract insects and animals? It's just mind-blowing how many different tactics they go to, from colour change and shape of the plant and what time of day you want to attract your insects. I mean, it's like they know what they're doing and they know they've got to have assistance in getting germinated in order to produce the next generation. So what was the inspiration for this question, Alice? Were you sort of sitting in the garden and seeing a bumblebee feed on some nectar or something? Well, it's just observing plants and all these tactics, these different ways they go about doing it, and just the sheer diversity of it and the ingenuity of it.

3:07Anand Jagatia:And how do they know they're getting it right? Thanks, Alice. So this is a question about flowering plants. How did they develop their ingenious strategies to attract pollinators like insects? To find out, we're on the hunt for a particularly elusive flower called raflegia, which has evolved to attract flies by smelling like rotting meat. Seeing one bloom is a rare sight to behold, but JK, a researcher from the University of the Philippines Los Baños, is hoping we might be able to track one down in the wild. Hopefully, today we see at least one blooming rafflesia. That's our goal for today. Oh, nice.

3:48Have you ever seen a blooming rafflesia? I've been there seven times, but unfortunately, I failed to see a blooming rafflesia. So, yeah. So would that be your first time if we saw one today? Oh, wow. Definitely. Yeah. So I've been here for four years and then I've never seen a blooming rafflesia. What would it mean if you did see one? I don't know. I think it's just completely shocked. And then I'll try to smell it if it's really like having some pungent smell. We'll be returning to our hunt for the rafflesia flower later in the show. But to answer Alice's question, first we need to learn a bit more about flowers and what pollination actually is.

4:31My name is Dr. Kira Krakis. I am a professor at Maryville University in St. Louis, Missouri in the United States. and I hold a joint position as a research associate at the Missouri Botanical Garden. So I am both a professor and a researcher in plant evolution and specifically my specialty is plant sex. Excellent. I'm glad you used that word because we've been sort of thinking about pollination for this show because we wanted to understand how plants evolved to attract animals and pollination. But pollination is kind of a euphemism, right? It basically is sex. It is. And it's always a little startling to some people.

5:11They go, wait, I'm sorry, what? Plants have sex? I'm like, well, yes, and rather similar to us. There's sperms and eggs and you get babies and it gets a bit spicy. Well, could you give us a little bit of an intro into that then? So maybe some plant sex ed, just the basics of how plants actually have sex. Sure, sure. Plants have sperm, they have eggs, and those need to get together. and the hard part is, is they can't walk around and find each other. So if you're a flowering plant, you have this great trick of using a pollinator to move pollen to the female part of the plant, the stigma, and that pollen grain inside it is water and sperm.

5:51And then the pollen grain lands. It grows a tube down to where the ovary is that has an embryo in it. And it fires a couple of sperm down there and they hit and ta-da, you have a baby and it grows into a seed and then eventually to a new plant. Insects and other pollinators do an incredibly important job for plants. They allow them to reproduce by ferrying sex cells between flowers that are too far apart from each other. But they don't do all that hard work for free. Let's take your standard bee. So the bee is after either nectar or maybe some pollen or a mating site or a place to rest. And it lands on the flower for those things.

6:34And as it's getting what it wants, it gets pollen all over its little fuzzy body. And the pollen sticks to it. And then it flies to the next flower. And when it gets to the next flower, the pollen is transferred. It's that simple. So what did the earliest plants on Earth do before that? So the earliest plants on Earth, and I'm going to talk about land plants, because in the ocean, well, water takes care of everything, just like salmon spawning, right? Just release your sperm and it gets to the eggs that were laid. But once you get on land, it gets a little harder. You have to deal with the open air.

7:08Then plants used primarily wind. Most of your, what we call the Christmas tree family, gymnosperms, and a lot of flowering plants are wind pollinated as well. But the big trick of using an animal, an insect, a bird, etc that means that you're more efficient you're not just wasting a lot of sperm and energy it's cost right everything's a cost benefit analysis in in biology and so using a pollinator means that you get a delivery of your sperm to your eggs yeah okay so it's more targeted isn't it because with wind pollination you just sort of release everything and hope for the best exactly and so you have to make a lot of sperm so it's costly so when did this relationship between plants and animals begin.

7:51So do we know roughly when the first flowers and plants that were pollinated by pollinators emerged? Yes, we do. It's about 120, 150 million years ago that the first plants show up. So they do overlap with dinosaurs, but only right at the end. And then they really quickly take over the planet. Like in 10 to 30 million years, they, what we call diversify, become a lot of different species. But our earliest fossils, if you will, of proto-flowers are around about 120, 150 million years ago. And the hard part is, is that we're not sure where the jump was made from. There's no transitional fossil. So we're still on the hunt for that.

8:29Okay. And what did these early proto-flowers look like? What do we know about them? They were very small and they were most likely beetle pollinated. The earliest pollination interactions are with beetles. Because remember, in evolution, there's no altruism. There's no big plan. It's just here's something that led to success. So it keeps getting reinforced. So there's a variation in the plants and one has a little adaptation for a modification of leaves. Those petals are modified leaves. And maybe they're a little brighter than others. Or there was a mutation for a sweet scent that came with it.

9:07And the beetle went, ooh, that's kind of delicious. and went over there to nibble on something. And as it did so, it moved the plant's pollen. And so the plant that had that trait got to have its pollen moved and make more babies. Well, if you have a trait that results in you making more babies, then that trait gets passed on and then that population grows. And that's evolution. It's just two goals, really, for all living things. Survival first and sex second. So what's happening with insects then? When did insects evolve and how long did it take before the insects and the plants kind of hooked up, so to speak?

9:47So that is the abominable mystery, because if you talk to botanists, they say that plants diversified, became the flowering plants broke out in these 350 ,000-ish and counting species in response to different kinds of pollinators. But if you talk to, say, the entomologist folks, they say, oh, yes, the beetles diversified and the insects diversified in response to all the different new food sources with all the flowers. And the truth is they sort of jumped together and then developed new niches together. The quintessential example of co-evolution. So before around 150 million years ago, plants relied on wind to transport their pollen.

10:33But somewhere along the line, a hungry beetle accidentally took on that role for themselves. It was a recipe for success, and so any genes in plants or insects that reinforced this novel dynamic became more common. And so over millions of years and countless generations, a beautiful new relationship blossomed around the world. And today, we see all kinds of flowering plants, pollinated by all kinds of animals, from the bees that pollinate sunflowers to the bats that pollinate agave, and the flies that pollinate rafflesia.

11:10Back in the Philippines, our trek through the forests of Mount McKilling is underway. But it's not long before one of the team discovers that some of the forest's more slimy inhabitants have been hitching a ride on him. I've been leeched. Oh, there he is. Wow, that is kind of horrifying. They love you. You've got three on you at least. Oh, maybe three that I know of. They loved you so much. Luckily just on my trousers. That poor leech victim is Lewis Barrett, a plant researcher at Oxford Botanic Garden and Arboretum in the UK. He's visiting the Philippines as part of an international conservation project to protect Rafflesia.

11:53After de-leeching himself, Lewis points out a woody vine hanging down from the forest canopy. Oh, Tetrastigma. Oh, wow. So this is the host plant for Rafflesia. So all of the species of Rafflesia that grow in Southeast Asia are found on Tetrastigma vines. And so does that mean that Rafflesia is a parasite, it's parasitic? Yeah, so rafflesia is a parasitic plant and more so it's endophytic as well, so it grows completely within the tissue of the tetrasigma. So unless it has a bud or a flower, you wouldn't even know it's there. Wow, so the cells of the rafflesia plant are literally inside? Yeah, they're actually inside the tissue of the tetrasigma.

12:35So yeah, so it's quite difficult to find. The parasitic rafflesia plant literally grows inside the tetrasigma plant, leeching off it for energy and nutrients until it emerges from the vine as a flower. Without this host, rafflesia wouldn't be able to survive. And so tetrastigma is a key part of the project that Lewis is working on with colleagues around the world that make up the community for the conservation and research of rafflesia. The approach they're taking is to use grafting, essentially finding vines in the wild that are harbouring rafflesia inside them and physically attaching them to new host plants in the hope that they can transfer the rafflesia cells between the vines.

13:20So we're hoping to conserve rafflesia and one of the best ways that's been found to be able to do that is by grafting rafflesia because with a lot of tropical species the seeds are actually, they can't be stored in the traditional way of putting them in a seed bank. So the only way that's been really successful in being able to propagate rafflesia is by grafting the parasitised tetrastigma vine onto a non-parasitised tetrastigma vine. So you're basically transporting, you're almost like smuggling the rafflesia when it's inside the vine onto another plant and then hopefully the rafflesia will then move into the new plant.

13:56Yeah, and it can take... We're still unsure how long that process takes, but we just know from experience that it will usually take about three years before you would start to see signs of buds knowing if you were successful or not. So what's the ultimate aim of this project? With this particular species, we are hoping to graft onto a rootstock at the McKilling Botanic Garden, which is at the base of the mountain. It would just be great for their ecotourism there if they are able to have Rafflesia there and it will be a much more accessible site to view the Rafflesia than trekking up the mountain and battling leeches.

14:34So that's the initial aim. But the partnership with the University of Philippines Los Baños is we're trying to hopefully conserve all of the species of rafflesia, but kind of in situ in the country is our main priority because all of the species are threatened and most of them are critically, we believe they're critically endangered. Perhaps in a few years it'll be possible to see rafflesia flowers in bloom from the safety of a botanic garden. But for now, if you want to stand a chance, you've got to face the forest, which is where our quest continues next.

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15:42Anand Jagatia:And so on the Global Story podcast from the BBC, we're telling surprising tales of American influence on the world stage and in ordinary people's lives all across the globe. We have this ability to export our story and a lot of people have bought it. I feel like the American dream is alive but not well. From the BBC, it's the United States at 2.50. Listen on bbc.com or wherever you get your podcasts.

16:11You're listening to CrowdScience from the BBC World Service. I'm Anand Jagatia, and this week we're trying to understand the evolution of flowers. They come in all shapes and shades, sizes and smells. the product of 150 million odd years of co-evolution between plants and insects. And while we might find many of them pretty, some flowers, like Rafflesia, are, well, downright strange, as I'm beginning to see for myself. We've just stepped off of the path. What are we looking at here, Lewis? So these are the buds of the Rafflesia. They're about the size of a cricket ball, I suppose. Yeah, yeah. They look a bit like cricket balls as well.

16:52Yeah, and there's probably about 10 just within a space of kind of one metre of the host vine. Wow, okay, so this vine that we can see, which is stretching up around this tree trunk, that's the host plant. Yeah, and the host plant needs to be quite mature in order to be able to play host to this parasite, because as you can kind of see, it must be sapping quite a lot of energy from the host plant, so it needs to be at a certain size before it can be parasitised by the rafflesia. So I mean, yeah, there are loads here. And so we're just, I guess, what potentially may be just a week too early to see these ones bloom.

17:26Yeah, but it looks like these ones are quite a good size. So I'm hoping that quite a few of these will flower. And there's still more of this population that we can explore. So hopefully we can still see one in flower. These brown spherical buds are not much to look at. But I've seen pictures of what rephlegia is like once it opens up. Vivid red petals covered in white pimples. It seems more like a fleshy alien life form than a flower, which makes sense given it's evolved to mimic the look and smell of rotting meat. As we continue walking, we see some more buds that are tantalisingly close to blooming.

18:02You can start to see where it's starting to open on the top of the bud, and I think you can see that underside of the petals there. It's starting to be a little bit of a reddish colour. Yeah, you can see that distinctive sort of yellowy-pink colour on the inside. And then once it does properly unfurl, it will be like a really vivid red, which I still haven't seen it in person yet, but from the pictures it looks incredible. I'm starting to lose hope slightly that we're ever going to find a rafflesia in bloom, especially as neither Lewis nor JK have ever seen one, despite several attempts. But there's still a chance, so we keep going.

18:38What I'm really intrigued by is the way they smell, which is something you can't get a sense of from a photo. Those rafleges, they are notoriously bad smelling because they're pollinated by flies. Plant biologist Kira Krakus explains that scent is one of the ways plants adapt themselves to appeal to different animals. They smell a certain way and bring in a certain pollinator. You know, as humans, we just go, oh, the plant smells sweet. We love that. But scent is one of those pathways, biochemically, I'll get a little nerdy on here, that is very easy for a plant to shift. And so they can, even within the same 24 hours, shift their scent for different insects or birds to show up.

19:21How much can or do plants specialize for maybe just a specific type of pollinator? Like maybe it's a specific species of bee or maybe it's a bat or something else. It is all along a continuum. So you'll have something very, very specific, like a highly specialized orchid. And it can only use one pollinator. Or there's certain kinds of fig species. They can only use fig wasps. That's it. If the fig wasps go extinct, so does this kind of fig. It's a very tight relationship. But most of the flowers use several groups of pollinators. And then you have your particularly promiscuous things like anything in the sunflower family.

20:03They're just big flat landing pads and everybody comes in for some of the action. Funnily enough, we actually came across the fig and fig wasp relationship that Kira is talking about while we were in the Philippines. And as you'll hear in a minute, it's kind of gross. This plant is a perfect demonstration of insect-plant relationship. This is Professor Pastor Malabrigo Jr., a plant taxonomist at the University of the Philippines Los Baños, who's also working on the Rafflesia Conservation Project. During our visit, he showed us a conservation garden of native plants that he's been creating, which includes various fig species.

20:45I mean, these look like figs, is that what they are? Yeah, it's a fig. And all figs are pollinated by wasps. And according to the studies, each species has a different wasp pollinator. There's something quite sort of gruesome about the way that these are pollinated, right? Doesn't the fig wasp end up dying inside the fig? Yes, yes, because these plants do not bear flowers. The flowers is inside. So only wasps can penetrate. That's why you cannot see flowers of this unless you break it and look at the wasp inside. Oh, my God. So what, are they all wasps in there or are they ants? Yeah, they are wasps, young wasps.

21:27What is going on here? So you've just broken open that. I'm glad I didn't eat one. So you've just opened up that fig and there's just loads of little wasps inside. so they're baby wasps yeah baby wasps uh sometimes if you time it that the wasp is about to to come out when you break the the fruit then all of these wasps inside will fly outside so just so i've got this clear in my head so you've got a young fig and inside it has the flowers so the wasp flies inside and lays its eggs and then the parent wasp dies the baby wasps fly out of the fig and then they go on to pollinate other fig plants yes and then what remains becomes a mature fruit which has seeds inside and then that is eaten by animals that then disperse the seeds and then plant a new fig tree potentially and it's it's really amazing because in the philippines alone we have near to 100 species of figs.

22:27And therefore, there must be at least 100 species of wasps as well. Oh, so each fig species has its own specific fig wasp. Yes. So if the wasp went extinct, then the fig tree wouldn't be able to survive. And if the fig trees disappeared, the wasps wouldn't be able to survive. The funny thing is that most people in the Philippines, particularly the tribal communities, they eat the fruits of the figs. So it's really palatable, but I don't want to eat this because I know there are a lot of wasp inside. That's understandable. I actually really like figs, but yeah, I'm glad I didn't decide to eat one of these ones.

23:08I'll never look at a fig in the same way again. So while some plants, like figs, can only survive because of a single pollinator, others have evolved a much more flexible approach. Back in the hills of Mount McKilling, our trek through the forest is almost drawing to a close. And while we've seen some raflegia buds that seemed close to opening, we haven't yet managed to find one in bloom. If anyone can help us locate one, it's our guide, a ranger who knows this forest very well. My name is Marvin Reyes. I'm a forest ranger here in Mount McKilling Forest Reserve. Have you seen raflegia flowers here before?

23:49Every year I see something. Yeah. Do you think we'll see any today? Do you think we have a good chance? No. Well, perhaps today just isn't our day. As we start to think about turning back and walking down the mountain, I want to ask Marvin what it's actually like to see a rafflesia in full flower. Is the flower important to you? How do you feel about the... But this one, it's just this. Oh, there!

24:23Just when we'd given up hope, another researcher, Migs, looks up and spots a solitary rafflesia flower, blooming about five metres above our heads, right where we're standing. That was a really good spot. Oh, there he is! Sorry, I'm wrong. The team is ecstatic, especially JK, who, on his eighth attempt up the mountain, has finally found what he's been searching for. Oh, it's blooming! And you can see the tetrastigma vine climbing all the way up the tree. Oh, it's that vine there! So this is the vine, it's the same vine, and it's climbing up this tree. It's the same vine. Woohoo! Wow! Yay! As we all stare up at the plant in amazement, Lewis describes what it is we're looking at.

25:11It's a bright red colour and it has five petals with white dots on the petals. Yeah, I suppose it's about the size of a hand. Maybe a basketball player's hand. Yeah, a large hand. And then inside the flower, there's kind of a white disc. Then from inside the flower, this is where that kind of bad fetid smell emanates from to attract the flies, which is what pollinates the Rafflesia. And the flies are attracted inside where they will crawl around where the reproductive parts of the flower are to transfer pollen to, hopefully, to another Rafflesia flower. We were really unbelievably lucky to see this Rafflesia on our first try.

25:50For something that's so hard to spot, it's incredibly striking once you do. Sadly, we were a bit too far away to get a good smell, but we could see from the ground that the flower was definitely doing its job, as flies were buzzing in and out of it while we watched. It's a scene that encapsulates a remarkable evolutionary journey, as plant sex scientist Kira sums up rather nicely. A little bit of a wrap-up perhaps for Alice's question, which was marvellous and has so many parts to it, just that when you're looking out over a field of flowers and you think it looks so beautiful in springtime, Just remember that what you're really seeing is a giant sexual battlefield and that you're also seeing old evolutionary patterns and these relationships that have been around for so long and that it is much more complex than we realize.