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.