The Making of a Parasitic Plant: Parasitism as a Life Strategy
Transcript of Part 1: The Making of a Parasitic Plant: Parasitism as a Life Strategy
00:00:06:16 Hi, I'm Dr. Caitlin Conn, 00:00:08:22 and I'm an assistant professor of biology 00:00:11:12 at Berry College in Georgia. 00:00:13:16 And today I'm going to introduce you 00:00:15:22 to some amazing plants that I've been studying 00:00:19:01 since I was in graduate school. 00:00:21:14 These plants are amazing because they're parasitic. 00:00:24:23 So they steal resources from other plants. 00:00:28:13 And here, you can see three examples of parasitic plants 00:00:32:05 but you'll soon be familiar with many more. 00:00:36:02 Throughout today's lecture, 00:00:37:11 we'll focus on three different learning objectives. 00:00:40:18 The first is to explore parasitic plants 00:00:43:20 within the broad context of parasitism. 00:00:47:00 Next, we'll identify common adaptations 00:00:50:16 among different groups of parasitic plants. 00:00:53:10 And then we'll discuss the range of host dependence 00:00:56:20 that we observe among different parasitic plants. 00:01:00:16 And we'll also have three key concepts today. 00:01:03:24 So the first is symbiotic relationships 00:01:06:24 or long-term interactions 00:01:09:04 between members of different species. 00:01:12:05 The second key concept is convergent evolution, 00:01:15:21 which happens when similarities evolve independently 00:01:19:22 in different groups of organisms. 00:01:22:10 And the third key concept is trait reduction, 00:01:25:22 which is the loss of genes, processes, 00:01:29:12 or structures that we commonly observe among parasites. 00:01:34:08 So let's dive in to that first key concept right away. 00:01:39:08 As you just heard, symbiotic relationships 00:01:41:28 are long-term interactions 00:01:43:25 between members of different species, 00:01:46:02 and these interactions persist across generations. 00:01:50:15 These symbiotic relationships can have different outcomes 00:01:54:05 for the organisms involved, 00:01:56:08 depending on the type of relationship 00:01:58:17 that we're talking about. 00:02:00:08 So here we have an example of a symbiotic relationship 00:02:04:18 that's beneficial for both members. 00:02:07:23 We have a plant, specifically a legume, 00:02:10:27 and we're looking at its roots. 00:02:12:20 And if you look where the white arrow is pointing, 00:02:15:13 you'll see a nodule and you can see other nodules 00:02:18:22 in the roots here as well. 00:02:20:10 And inside these nodules, there are bacteria 00:02:24:14 that take nitrogen from the atmosphere 00:02:27:10 and convert it into a form that the plants can use. 00:02:31:06 And in exchange for this service, 00:02:33:10 the bacteria get resources from the plant. 00:02:36:27 So again, this is a symbiotic relationship 00:02:39:27 in which both members benefit. 00:02:43:04 In some symbiotic relationships, one partner benefits 00:02:46:27 while the other may be neither helped nor harmed. 00:02:50:20 And a commonly cited example of this 00:02:53:07 is the relationship between bromeliads and frogs. 00:02:57:14 Bromeliads are mostly tropical plants 00:03:00:14 and they often hold water among their leaves. 00:03:03:28 And some frogs will lay their eggs 00:03:06:20 in the water held by bromeliads 00:03:08:28 and their tadpoles will hatch and mature there. 00:03:12:06 And it's often thought that this symbiosis 00:03:15:02 is beneficial for the frogs 00:03:17:16 but it doesn't really have an impact on the bromeliads. 00:03:22:06 And still other symbiotic relationships can be beneficial 00:03:26:05 for one member and harmful to the other. 00:03:29:14 So consider mistletoe. 00:03:31:14 In this picture, mistletoe looks like the clusters of green 00:03:35:09 up in the branches of the tree. 00:03:37:16 Mistletoe lives in trees and it physically attaches itself 00:03:42:00 to the trees and it steals resources. 00:03:45:10 So mistletoe is indeed a parasitic plant. 00:03:49:00 Mistletoe doesn't usually kill its hosts 00:03:51:28 but heavy infestation can be harmful to the health 00:03:55:16 and development of a tree. 00:03:58:14 So these three examples of symbiosis 00:04:01:06 that we just discussed fall into three different categories. 00:04:05:14 With mutualism, both partners benefit. 00:04:08:15 With commensalism, one partner benefits 00:04:11:11 and the other isn't impacted. 00:04:13:26 And with parasitism, one member benefits 00:04:16:25 at the expense of another. 00:04:19:26 Now, throughout this lecture, 00:04:21:15 there will be some questions written in blue. 00:04:24:22 And whenever you see one, I encourage you 00:04:27:10 to pause your video and take a few minutes 00:04:30:06 to consider the answer. 00:04:32:10 So the first question is this, 00:04:34:10 besides these three examples of symbiosis 00:04:37:28 that we just discussed, what other examples 00:04:41:00 can you think of of mutualism, commensalism and parasitism? 00:04:51:25 Now, let's move on to learning objective one, 00:04:54:22 which is to explore parasitic plants 00:04:57:20 within the broad context of parasitism. 00:05:00:28 Parasitism is found across the tree of life. 00:05:04:08 So bacteria are parasitic if they take resources from a host 00:05:09:19 and a well-known example is listeria monocytogenes, 00:05:13:19 which can cause illness in humans who ingest food 00:05:17:18 that's contaminated by it. 00:05:19:24 Animals can be parasitic. 00:05:21:18 So think of the worms that live 00:05:23:22 inside the gastrointestinal system of humans, 00:05:27:04 pets and farm animals. 00:05:29:14 There are many well-known examples of parasitic protists. 00:05:33:10 And one is leishmania, which is transmitted 00:05:36:21 by biting sandflies and can make people pretty sick. 00:05:41:09 Parasitic fungi include the smut fungus, 00:05:44:20 which uses important crops 00:05:46:23 namely grasses and sedges as a host. 00:05:50:12 And as you know by now, plants can be parasitic. 00:05:53:20 The parasitic plant that you see here 00:05:55:28 is called Triphysaria versicolor, 00:05:58:04 and it doesn't look that unusual for a plant, 00:06:01:22 but underground, it attaches itself to host plants 00:06:05:14 and steals resources. 00:06:08:12 So now let's focus specifically on parasites 00:06:11:24 in the plant kingdom, and let's discuss 00:06:14:02 a few remarkable examples of parasitic plants. 00:06:18:11 This parasitic plant is one of my favorites 00:06:20:25 and it's commonly called the Australian Christmas tree 00:06:23:28 because it produces these beautiful orange blooms 00:06:27:12 around December. 00:06:29:02 It attaches to host plants underground 00:06:31:14 using specialized invasive structures. 00:06:34:05 And sometimes those invasive structures 00:06:37:03 mistake inanimate objects for host plants. 00:06:40:24 For example, this parasitic tree 00:06:43:03 has been found attached to underground power cables. 00:06:48:25 Our next remarkable parasitic plant is Rafflesia arnoldii 00:06:52:21 or corpse flower and it produces 00:06:55:05 the largest flower of any plant species. 00:06:58:20 It's beautiful, but it's called corpse flower 00:07:01:12 because it smells like a rotting corpse. 00:07:04:03 And this smell attracts insect pollinators. 00:07:07:24 This huge flower doesn't last long. 00:07:10:03 And in fact, the corpse flower plant 00:07:12:22 spends most of its life inside of a host. 00:07:15:28 And when it finally emerges, 00:07:17:25 it lacks many of the structures that we typically associate 00:07:21:11 with plants like stems, leaves, and roots. 00:07:25:21 And the final remarkable parasitic plant 00:07:28:05 that we'll discuss here is unfortunately remarkable 00:07:31:26 in its negative impact on agriculture. 00:07:35:04 This is Striga hermonthica or witch weed. 00:07:38:07 And in this picture, it's the plant 00:07:40:12 with the pink flowers. 00:07:42:11 Striga species parasitize grass crops usually 00:07:46:20 like corn, which is what's shown in this picture, 00:07:49:24 or rice. And Striga species cause billions 00:07:53:12 of dollars of yield losses every year. 00:07:56:15 Striga outbreaks are very difficult to control 00:07:59:18 for a number of different reasons. 00:08:01:18 First, a single Striga plant 00:08:03:25 can produce tens of thousands of tiny, 00:08:06:12 easily dispersible seeds. 00:08:08:12 And those seeds can remain dormant in the soil 00:08:11:08 for years if necessary. 00:08:13:10 And they only germinate when they detect a nearby host. 00:08:17:25 Shortly after germination, they attach to that host, 00:08:21:09 and by the time they become visible above the soil surface, 00:08:25:14 damage has already been done. 00:08:27:27 Now, we call parasitic plants that attack crops 00:08:31:08 or other plants cultivated by humans, parasitic weeds. 00:08:35:24 And Striga species are some of the most destructive 00:08:39:03 parasitic weeds in the world. 00:08:42:20 Different groups of parasitic plants 00:08:44:26 have evolved different ways to attach to a host. 00:08:48:09 Most of the parasites that we'll talk about today 00:08:50:26 attach directly to a host. 00:08:53:13 But mycoheterotrophs are different 00:08:55:23 because they go through a fungal intermediate. 00:08:59:10 So mycoheterotrophs connect themselves 00:09:02:03 to a mycorrhizal fungus that's usually in a mutualism 00:09:06:20 with a non-parasitic plant. 00:09:08:28 So the fungus becomes an unwitting middleman, 00:09:12:05 involuntarily shuttling resources 00:09:14:22 from its mutualistic, non-parasitic plant partner 00:09:18:22 to the parasite. 00:09:20:10 The mycoheterotrophs shown here are all orchid species. 00:09:24:02 And in fact, many orchids and many other plant species 00:09:27:29 as well are mycoheterotrophic at some point in life. 00:09:33:10 But most of the parasites we'll talk about 00:09:35:16 connect directly to a host. 00:09:37:18 They're called haustorial parasites. 00:09:39:25 And that's because the specialized invasive structure 00:09:43:15 that they use to get inside of a host 00:09:46:01 is called a haustorium. 00:09:48:15 And haustorial plants are quite diverse. 00:09:51:20 They include the familiar mistletoe, 00:09:54:10 the bizarre looking plant in the middle, 00:09:56:24 known as jackal food, and the parasitic weeds 00:10:00:19 and their less destructive relatives, 00:10:03:03 one of which is shown on the right. 00:10:06:24 Now, here we have a phylogeny of angiosperms 00:10:10:16 or flowering plants. 00:10:11:29 And this phylogeny comes from Professor Dan Nickrent 00:10:15:16 who maintains The Parasitic Plant Connection website. 00:10:19:11 Check it out for more information on 00:10:21:28 and great photos of parasitic plants. 00:10:25:20 So in this phylogeny, we're seeing a tree-like structure 00:10:29:26 that shows us the evolutionary history 00:10:32:23 of the plant groups labeled on it. 00:10:35:14 And any plant groups labeled in red 00:10:38:07 include haustorial parasites. 00:10:41:04 You'll notice that some of these red groups 00:10:43:22 are not that closely related to other red groups. 00:10:47:18 So it's pretty intuitive 00:10:49:09 that in those distantly related groups of parasites, 00:10:52:29 parasitism would have evolved independently. 00:10:56:18 But in some other cases, these red groups 00:10:59:18 are pretty closely related to other red groups. 00:11:02:07 And it's a little harder to tell whether parasitism 00:11:05:19 evolved independently in those groups 00:11:08:02 or whether it was inherited from a common ancestor. 00:11:12:02 But scientists have taken a closer look 00:11:14:16 at each of the groups labeled in red 00:11:16:25 and determine that parasitism 00:11:19:05 did indeed evolve independently in each of those groups. 00:11:23:18 And when a similarity or a trait like parasitism 00:11:27:14 evolves independently in different groups, 00:11:30:14 that's called convergent evolution. 00:11:32:28 And that's our second key concept in this lecture. 00:11:37:20 But why has parasitism evolved so many times among plants? 00:11:42:13 Well, to start to address this question, 00:11:44:29 think about the community of plants 00:11:46:27 that you would typically find in a forest. 00:11:50:01 Most of them are in competition 00:11:52:07 with one another for access to resources. 00:11:56:05 So parasitic plants may have an advantage 00:11:59:11 because instead of battling it out with other plants, 00:12:03:01 they can just steal from somebody 00:12:05:02 who has already obtained resources. 00:12:07:24 So in that sense, at least for parasitic plants, 00:12:11:06 cheating is an alternative to competing. 00:12:15:00 So pause and ask yourself this question. 00:12:17:26 What are some of the resources 00:12:19:24 that plants are typically competing for? 00:12:24:02 And now focus on the parasitic plant 00:12:26:13 on the right side of this slide. 00:12:28:11 This is called Conopholis americana, and it's a small plant. 00:12:32:07 It stands less than a foot tall, but its hosts are huge. 00:12:36:22 They're oak trees. 00:12:38:11 So relative to non-parasitic understory plants, 00:12:42:19 what advantage might Conopholis gain 00:12:45:22 by parasitizing large hosts like oak trees? 00:12:51:11 Now, an answer to the question 00:12:53:10 of why parasitism has evolved so many times among plants 00:12:57:20 may be found in the ecological principle 00:13:00:18 of competitive exclusion. 00:13:02:21 And this principle tells us that two species 00:13:05:28 cannot coexist in the long run 00:13:08:20 if they have the exact same ecological niche. 00:13:12:07 So they have to evolve different ways 00:13:15:01 to interact with their environment, 00:13:17:04 different ways to obtain and utilize resources 00:13:20:26 if they're going to coexist in the long run. 00:13:24:09 So if parasitic plants are using a host 00:13:28:05 to get their resources and non parasitic plants 00:13:31:10 are getting resources for themselves, 00:13:33:22 then those species have indeed evolved different 00:13:36:21 ecological niches and can persist together 00:13:40:12 over the long run. 00:13:46:04 So we've talked about one major example 00:13:48:28 of convergent evolution, 00:13:50:21 and that's the evolution of parasitism 00:13:53:04 across the tree of life and in the plant kingdom. 00:13:56:20 And now let's talk about some common adaptations 00:14:00:08 that have evolved convergently 00:14:02:13 among different groups of parasitic plants. 00:14:05:14 This is our second learning objective. 00:14:08:05 So here, you see two images 00:14:10:11 and they represent parasitic plants 00:14:12:21 from groups that independently 00:14:14:27 or convergently evolved parasitism. 00:14:18:06 And one adaptation that has evolved convergently 00:14:21:24 in these groups is a way to sense a host. 00:14:24:28 So at the top we have a parasite called dodder 00:14:28:01 and it picks up on nearby hosts when it detects metabolites 00:14:32:20 that those hosts are releasing into the air. 00:14:36:03 At the bottom, we have a parasitic plant 00:14:38:20 from the Orobanchaceae family. 00:14:40:27 And for some parasites in this family, 00:14:43:14 host detection has to happen before seed germination. 00:14:47:09 So it's usually happening underground 00:14:50:03 when those parasites seeds pick up on hormones 00:14:53:08 released by host plants. 00:14:55:17 So these hosts detection mechanisms are different 00:14:58:24 in the two groups of parasites shown here 00:15:01:18 but they serve a similar purpose, 00:15:03:21 which is to connect the parasitic plant with its host. 00:15:08:24 And of course, if haustorial parasitism 00:15:11:13 has evolved independently among plants, 00:15:14:09 then haustoria themselves 00:15:16:05 have evolved independently as well. 00:15:18:25 So on the top, you see haustoria from dodder 00:15:22:02 and they are penetrating the stems of a host plant. 00:15:26:02 In the Orobanchaceae, the haustoria 00:15:28:20 connect the parasites to a host's roots. 00:15:32:04 So again, the haustoria are different 00:15:34:13 in these two groups but have evolved convergently. 00:15:39:09 So now let's take a look at an amazing video 00:15:42:13 of one of those parasites, dodder, 00:15:44:25 as it goes through its life cycle. 00:15:47:18 This video comes from the lab of Dr. Jim Westwood, 00:15:51:09 who is a professor at Virginia Tech. 00:15:54:04 And you'll see a time lapse 00:15:56:09 of dodder's entire life cycle, really. 00:15:59:18 So here, it's germinating and the parasite seedlings 00:16:04:13 are growing up above the surface of the soil. 00:16:09:10 Pretty soon, you'll see them seeking out a host. 00:16:12:21 And remember that for dodder, 00:16:15:04 host detection happens when the parasite picks up 00:16:18:13 on those metabolites that the host plant 00:16:21:00 is releasing into the air. 00:16:23:24 So once dodder finds a host, it's going to coil itself 00:16:28:08 around that host plant. 00:16:30:04 You can see that up close right here. 00:16:33:25 The host plant in this video is tomato. 00:16:36:27 So dodder is wrapping itself around the tomato 00:16:41:28 and now you can see it forming haustoria. 00:16:44:22 They look like swellings that are growing inward 00:16:47:17 from the dodder toward the stem of that tomato host. 00:16:51:03 And those haustoria will indeed invade 00:16:54:15 the host plant's tissues, 00:16:56:07 and set up the connection so that the dodder 00:16:59:03 can steal resources. 00:17:02:21 The dodder continues growing on its tomato host. 00:17:08:13 And pretty soon we're gonna zoom out 00:17:10:05 for a bigger picture view of dodder interacting 00:17:13:29 with its host. 00:17:17:16 So here we have the dodder plant, 00:17:19:22 seeking out more of its tomato host. 00:17:28:11 And by the end of the video, 00:17:30:02 you'll see that those potted tomato plants 00:17:32:23 are completely covered by the dodder. 00:17:36:29 So clearly the dodder has been quite successful 00:17:39:25 in its host seeking 00:17:41:15 and its attachment to and parasitism of the tomato hosts. 00:17:47:26 Now we've started to address this question already 00:17:50:15 but pause your video for a minute 00:17:52:06 and ask yourself what traits parasitic plants share? 00:17:56:23 Besides the ones we've discussed already, 00:17:58:29 can you make hypotheses about other traits 00:18:01:29 that parasitic plants might have in common? 00:18:06:24 Well, we've already talked about the invasive haustorium 00:18:10:16 and it turns out that parasitic plants 00:18:13:01 from different groups have, in some cases, 00:18:16:10 convergently evolved reduced or absent structures 00:18:20:10 like leaves and roots 00:18:21:26 and have convergently lost photosynthesis. 00:18:26:03 Let's look at the haustoria again, first. 00:18:28:19 So here we have the haustoria 00:18:30:10 from dodder going into the stem of a host plant. 00:18:34:03 And now we have mistletoe in the middle, 00:18:36:24 and a parasitic vine on the right side of this slide. 00:18:40:14 And the haustoria are marked with white arrows 00:18:43:11 and you can clearly see how those haustoria 00:18:46:17 are connecting the parasitic plant to its host. 00:18:51:25 Now, let's talk about structural reduction. 00:18:54:09 So take a look again at Conopholis americana. 00:18:58:08 So this is that small understory plant that uses oak hosts. 00:19:03:01 The cream colored projections 00:19:04:28 coming off of this plant are the flowers, 00:19:07:10 and the leaves aren't so easy to see 00:19:10:10 because they've been reduced to scales. 00:19:12:22 They're present underneath the flowers. 00:19:16:24 Other parasitic plants from different groups 00:19:19:05 have also lost structures like leaves. 00:19:21:23 So on the left, we have a parasitic plant 00:19:24:09 called jackal food, you've seen it before, 00:19:26:25 and it doesn't have leaves either 00:19:28:27 although jackal food and Conopholis 00:19:31:04 do have roots and they use them to connect to a host plant. 00:19:35:14 And on the right, we have dodder, 00:19:37:18 and dodder also doesn't have apparent leaves 00:19:40:10 because its leaves have been reduced to scales. 00:19:43:21 And once dodder gets established on a host plant, 00:19:47:06 its roots will rot away 00:19:49:02 because it doesn't need them anymore. 00:19:50:25 It's fully reliant upon a host. 00:19:54:08 Photosynthesis has also convergently been reduced 00:19:58:04 or lost by parasitic plants from these different groups. 00:20:01:28 And that's why you don't see green tissue 00:20:05:00 on these parasitic plants. 00:20:12:14 So that brings us to our third and final learning objective, 00:20:16:16 and that's to discuss the range of host dependence 00:20:19:17 that we observe among parasitic plants. 00:20:22:16 So the parasitic plants that can photosynthesize 00:20:25:21 for themselves are known as hemiparasites. 00:20:29:16 Some can live with or without a host, 00:20:31:29 those are facultative hemiparasites, 00:20:34:11 but some photosynthetic hemiparasites 00:20:37:09 do need a host to complete their life cycle, 00:20:40:07 and they're called obligate hemiparasites. 00:20:43:14 The parasites that have lost photosynthetic ability 00:20:46:20 are fully reliant upon a host 00:20:49:09 to get through their life cycle. 00:20:50:26 So they're called obligate holoparasites. 00:20:54:12 So pause for a minute and ask yourself this, 00:20:57:26 why would parasitic plants lose the ability 00:21:01:12 to carry out photosynthesis? 00:21:04:00 Photosynthesis generates sugars for the plant, 00:21:06:24 so wouldn't it be really bad 00:21:08:22 to lose the ability to photosynthesize? 00:21:12:27 And as a followup question, 00:21:14:24 what are some of the consequences of losing that ability 00:21:18:15 to carry out photosynthesis? 00:21:22:22 Well, it turns out that this loss of photosynthesis 00:21:26:05 and this reduction or loss of certain structures 00:21:29:19 fall into a phenomenon known as trait reduction. 00:21:33:17 And trait reduction happens in parasites 00:21:36:26 because if parasites can either carry out 00:21:40:18 physiological processes and obtain resources for themselves 00:21:45:20 or they can rely on a host's physiological processes, 00:21:49:19 and steal resources from the host, 00:21:52:10 well, then, natural selection often stops maintaining 00:21:56:18 those processes and structures in the parasite 00:22:00:09 if it can rely on a host to meet its needs. 00:22:04:01 And of course the consequence of this 00:22:06:05 is complete dependence upon a host. 00:22:08:28 And we see this in diverse parasites 00:22:11:24 from across the tree of life. 00:22:13:16 So for example, on the left, we have a chlamydia bacterium. 00:22:18:10 And these parasitic chlamydia bacteria 00:22:21:14 cause the sexually transmitted infection known as chlamydia. 00:22:25:11 And their genome, their entire collection of DNA 00:22:29:05 has likely been reduced by 3/4 or more, 00:22:32:19 relative to non-parasitic ancestors. 00:22:36:00 In the middle, we have a eukaryotic parasite 00:22:39:09 known as microsporidium, 00:22:41:14 and it has undergone genomic, structural, 00:22:44:24 and metabolic reduction. 00:22:47:06 And on the right, we have our familiar dodder plant. 00:22:51:00 Let's take a closer look at dodder. 00:22:53:03 And this time, it's just at the seedling stage. 00:22:57:17 So this dodder plant is emerging from a seed 00:23:00:20 and it lacks some of the structures that we typically see 00:23:04:06 in photosynthetic plants as they germinate. 00:23:07:14 So on the right, we have a cartoon of the germination 00:23:11:06 of a photosynthetic plant, and we can usually see 00:23:14:08 a root and embryonic leaves called cotyledons emerging. 00:23:19:14 But in dodder, those cotyledons are absent 00:23:22:12 and the root may be absent as well. 00:23:25:05 And remember that once dodder gets established 00:23:28:02 on a host plant, its root rots away. 00:23:33:01 So these parasites that are fully reliant upon a host 00:23:36:16 have to adapt to that host. 00:23:38:21 And the host can adapt in response. 00:23:42:08 Now, some obligate parasitic plants 00:23:44:24 like Striga on the left, 00:23:47:11 use hosts that are typically only around for a short time. 00:23:51:20 So Striga parasitizes corn or maize. 00:23:54:23 And it's usually only around for a single growing season. 00:23:58:23 But other obligate parasitic plants like Conopholis, 00:24:02:08 on the right, parasitize long-lived hosts like oak trees. 00:24:07:19 So what adaptations do you think a parasite of annual 00:24:12:11 or short-lived hosts would have? 00:24:14:16 And what adaptations do you think a parasite 00:24:17:12 of longer lived hosts might have? 00:24:20:04 Pause for a moment and consider that question. 00:24:24:22 With that, we'll wrap up this lecture 00:24:27:05 and we'll go through a quick recap 00:24:29:04 of everything we discussed. 00:24:30:29 So first of all, we got acquainted 00:24:33:17 with parasitic plants and we saw diverse 00:24:36:17 and amazing examples of parasitism in plants 00:24:40:16 in groups that independently evolved to be parasitic. 00:24:45:16 Then, we saw some different cases of convergent evolution. 00:24:49:23 Parasitism convergently evolved 00:24:52:01 across the tree of life and in plants. 00:24:54:21 And then we saw some adaptations 00:24:56:15 that convergently evolved among parasitic plants as well. 00:25:01:02 Then, we discussed the range of host dependence 00:25:03:26 that we see in parasitic plants. 00:25:05:24 And we noted that some have to have a host 00:25:08:26 to complete their life cycle. 00:25:10:28 So before we end this, I have three final questions 00:25:14:09 for you to consider. 00:25:16:08 First of all, what environmental conditions 00:25:19:04 might favor the evolution of parasitism in plants? 00:25:23:16 Think about things like climate, resource availability 00:25:27:04 and the other plants and non-plant members 00:25:30:04 of a particular community. 00:25:33:03 Next, what unique challenges might parasitic plants face? 00:25:37:23 Stealing from a host might seem 00:25:39:23 like an easy way to get through life, 00:25:41:25 but what are some of the difficulties 00:25:44:04 that are associated with this lifestyle? 00:25:47:15 And finally, what traits are parasites unlikely to lose? 00:25:52:05 Another way to think about this is what traits 00:25:55:08 could a host not compensate for 00:25:58:00 if a parasite were to lose them? 00:26:00:23 So that's it for today's lecture. 00:26:02:17 Thank you for tuning in. 00:26:03:27 And I hope that you enjoyed this exploration 00:26:06:25 of parasitism in plants.