Wisconsin Public Television Wednesday Nite at the Lab  Why Plants are Weird Christopher Day  June 6, 2012 UP726 CC >> Welcome, everyone, to Wednesday Nite at the Lab. I'm Tom Zinnen. I work here at the UW Madison Biotechnology Center. I also work for UW Extension Cooperative Extension, and on behalf of those folks and our other sponsors, Wisconsin Public Radio, excuse me, Wisconsin Public Television, the Wisconsin Alumni Association, and the Science Alliance, thanks for coming to Wednesday Nite at the Lab. We do this every Wednesday night, 50 times a year. Tonight I'm delighted to be able to introduce to you Chris Day. He's a faculty member here in plant genetics at UW Madison in the Genetics Department. He got his undergraduate degree and his PhD in biochemistry and molecular biology at the University of Edinburgh. He did postdocs at Yale and another one at Berkeley. Both of those were in plant development. 

And now he's here at UW Madison. He gets to talk tonight on one of my more favorite topics because I'm a plant pathologist by training and a gardener by inclination, Why Plants are Weird: Polyploids, Mosaics, and Especially Gardeners. Plants are not like people, and one of the things when I was teaching biology is that was one of the hardest things to get across is students all thought the life cycle of plants was just like people with leaves, and they're not that way at all which makes them really fascinating; it also makes it great for us as gardeners, as farmers, as vintners, as orchardists, as people that enjoy plants. So please join me in welcoming Chris Day to Wednesday Nite at the Lab. [APPLAUSE] >> So, it's good to have a nice warm up act. That's very nice of you, Tom. Thanks. So, I hope I can get through everything I have here. I had to cut stuff out already. So I know, actually last week Tom gave you, did he give you all a handout? Yeah, some have even got it. 

Good thing I read it, right? So, this was one of the things that Tom was excited about. So actually it was a very timely article. And this is specifically focusing on just the gene counts in tomato, and what this says here is this, in fact, has 7,000 more genes than we do. Which isn't so much. It's about the same. It's not so much. But, actually, there are many examples of plants having way, way more genes than humans. I wouldn't say that's a weird thing. I wouldn't say plants are actually less complex than humans. So maybe it's not such a surprising thing, but this is one of the big things about this article at least. 

And what they were looking at a lot was, in that article, was looking at this idea as being this genome triplication. Well, first of all, I want to just go through and talk about what is a genome triplication because I think it's really not very clear from that newspaper article. In fact, it's actually very not clear from the paper write-up. And now you've got to start learning terminology. This is the problem with, the bane of biology students here is that it's like learning a foreign language. And they didn't sign up for that, but unfortunately that's what they have to do. So we have this terminology which describes genomes. Now, in a pollen or an egg, you get one of each chromosome. And this is actually an organism with three chromosomes, and we call that haploid. And that's true of a sperm; it's haploid. 

It has 23 chromosomes, human chromosomes. The egg also has 23. Every one of your cells actually has 46 chromosomes. And that's actually what we call diploid. And that's pretty similar in plants. There are some weird examples within the life cycle, but we're not going to go into those. So what we're just looking at here is we have three chromosomes, three different chromosomes in the sperm. In the germ line cells you get one of each, and in what we call somatic cells, which are just regular cells, you get two of each. And in our case, we have one from the mother, one from the father. And, actually, that's true in plants too. 

What is unusual in plants, unusual thing number one or weird thing, is that polyploids are actually tolerated quite well, and this is not true in humans. And we'll get to that in a little bit more detail. In this example, you'll notice that we have, instead of having one pair of the yellow, we actually have three pairs of the yellow. So, in fact, we actually have three copies of this genome. In fact, really, actually, you have six copies of the genome. And, in fact, we would call this hexaploid. And, in fact, that's basically what this tomato is. The tomato, actually I should have done this example for the tomato, the tomato, you'll notice this has nine chromosomes, and there's actually three unique chromosomes. This tomato that they're talking about in the article has 12 chromosomes of which there are four chromosomes and you have, basically, six of each of those four chromosomes. Now, what you can see there is, in fact, you have a massive amount of genetic redundancy. 

Many genes are many copies of the same gene. And, in fact, actually what has been shown here, actually Tom Osborn, who you may have heard speak here, I'm not sure, he actually had a project which was designed to look at what happens when you make polyploids. And one thing he found, actually, is you either use it or lose it. And, in fact, you get a lot of deletions start occurring very quickly, and it's kind of like a genomic shock. But, at the same time, you can actually have, there's a time window when you can actually develop more different functions with those genes. So any time you have a polyploid, it can actually lead to some very interesting new functions. So, in a way, this is one of the driving forces of evolution in plant biology. And, actually, one of the most common forms of new speciation, and we'll talk about that a little bit as we go through. But, basically, that's what genome triplication is, and what I would call it would be, basically, hexaploid. And you can get triploids and tetraploids and everything. 

So this is the figure from the actual paper. It's a little bit complex. This was a Nature article. It was published last week, this week I guess. And you have a lineage here. Now, these circles here represent duplication events where the genome is actually duplicated. So now we have one set of chromosomes, now we've gone to two. So, basically, we have two chromosomes, and now we have four actually. So it's basically going from diploid to tetraploid. You can see, if you look at the lineage, here's the time here in millions of years, and you can see that here's Arabidopsis, our favorite model system. 

Here's some grapes. We're going to talk about grapes in a bit. We've got sunflowers and we've got tomatoes and potatoes here. Now, you'll see that there's duplication events occurring all over here. We've got at least three here leading to Arabidopsis. The paper is discussing this T, the red T here, where, in fact, it's basically gone from one genome to three genomes. And they call it T1, T2, and T3. Now, we don't really know how that happened. What we can see, though, is the signature of that event occurring. And that's really what the paper was about and a lot of hypothesizing about how this might have happened. 

So it's pretty interesting. And then they looked to see which gene, and actually you can compare. Here they compared three chromosomes. And you can see, if you look at the comparison, there's a line here, and you can see that they're actually quite similar. There's a lot of similarities going down the whole length. And actually you can just pair all three of those up. But what you do see, there are differences. There's a gap here. That means there's deletions occurring. And this is very interesting from a genomic perspective because you can now, if you have all three of those sequences, you can start seeing what's happened since that event. 

And maybe that's important for evolution, maybe not. In fact, we often are not so sure. But what I want to do is think about humans before we go into plants because to know that plants are weird you first of all have to have a baseline of what's normal for us. What's normal for us is you either have two Xs or an XY. Looks about 50/50 in this room. You have 23 chromosomes of which you have a pair of each, or actually 22 with exact pairs. Females actually have an exact pair with the X, and males have the X and the Y. So we normally think of us as either 23 or 46. You'll see those two numbers a lot. If it was a genome triplication, we would actually have 138 chromosomes, and we would be dead. 

It would actually be a miscarriage if that was the case. Now, it doesn't mean that things don't happen; it just means that we don't see them. There are some things which are allowed, in the sense that they're viable. Down syndrome is the one you're all probably very familiar with. The terminology here is that we have 47 chromosomes instead of 46. In this case, we have an XX and we have an extra 21. And this is kind of standard genetic counseling nomenclature. We call this aneuploidy. That's when you have an extra chromosome, basically. Or maybe, actually, an extra pod of a chromosome. 

But we're talking about a small amount is repeated. In this case, we have a little extra 21. It's actually not much size-wise, and that's probably why that extra amount of genetic information can be tolerated because there's actually not many genes on that chromosome. It's fairly common. It increases, likelihood increases as you get older for the females. We have another thing, though, which we call global changes. That's where all chromosomes get repeated. And we call that euploidy or polyploidy. So I'm just now, again, teaching you the language. Aneuploidy is one extra or one small piece extra; euploidy is when we double everything. 

Now, there's another one which we call Edwards syndrome which is Trisomy 18. That's when we have three of 18 instead of two of 18. Actually, that normally leads to a lethal phenotype in the sense you get a stillborn child often, or maybe it will live for a year, no more than that. And there's actually another one called Patau syndrome which actually has a very strong history here. Klaus Patau was actually a genetics professor here in Madison. He actually identified this one for first time by looking at karyotypes. You can see there's a huge number of phenotypes here associated with this. This is a very pleiotropic disease because you just have so many extra genes which are misexpressing and messing things up. The babies don't survive. Here we've got the extra 13 here. 

And actually he discovered, actually, he discovered this at the Mendota Mental Health Institute. Now, I don't know if you are aware of this, but that was actually a lot of, basically people with very severe genetic diseases were kept there and looked after there. And, actually, Klaus Patau went up there and he would take DNA samples, and he actually thought that he was going to be able to get trisomies for every single one of the human chromosomes. That was his expectation. On the first day he got Down syndrome, he got Edwards syndrome, and his own new one, Patau, and, in fact, that was it. He never found any more. And the reason is because the other ones are lethal, and you never see them. They will always lead to a miscarriage. So that's kind of a little bit of local connection there. So here is kind of a summary of birth defects, and it actually looks very scary. 

Over thousands of conceptions, 150,000 are spontaneous abortions. >> [INAUDIBLE] >> Sorry? >> You said a thousand. You mean a million. >> Oh, sorry. A million. 150,000 are spontaneous abortions. 75,000 of those are chromosome abnormalities, and we've triploids, tetraploids. Here's some here which are allowed. Here's the three trisomies here, and there's a few sex chromosome aneuploidies as well which are allowed. So about 5,000 live births have some kind of chromosome abnormality. 

Now, actually, this is an underestimate. Probably about half of conceptions actually lead to miscarriages. Often the mother does not even know that a miscarriage has occurred. Now, they're not all due to triploidies and tetraploidies and things like this. But it's very common. The reason it doesn't seem very common is because people don't normally talk about these things. But, in fact, it's much more common than people realize. It's actually a blessing because, in fact, it is our biological system having check points and saying is this a normal fetus, is this a normal fetus? No? Okay, let's try this again. So it's a very sophisticated mechanism where the body or at least the mother and the baby are monitoring each other and trying to figure out is everything okay. And if it's not, let's start again. 

So it looks bad and it looks scary, but it's not. It's actually a good thing. Plants, though, they can tolerate all these trisomies. Now, here's actually a wild type 2N. Now, 2N just means it's diploid, means it has two every chromosome, and N is just the number of chromosomes. And, in this case, we have 12 different trisomies, one for each extra chromosome, and they all have subtly different phenotypes but nothing that actually leads to lethality, and actually nothing that even leads to them not being able to actually produce seed and progenate. So this is another unusual thing about plants that they can tolerate aneuploidies, not just polyploidies. And, actually, if you, now here's a polyploid here. Here's Brassica oleracea and Brassica rapa. You take the chromosomes from each and put them together in a new plant, and we call that napa, Brassica napa. 

And, in fact, one thing you'll notice here is, in fact, that everything is a little bit bigger. And, in fact, there's a lot more vigor. And it turns out it's actually quite a good thing for plants. You actually get this extra oomph which is why we actually see this in agriculture a lot. So almost all of the things we eat, or many of the things we eat, have some kind of polyploidy which has led to them having some advantageous trait with regard to us, the eaters. So there's the kind of things that we get. Again, this is a language. If we have three chromosomes instead of two, we're going to call that triploid. If we have four chromosomes instead of two, that's tetraploid. Here's the original diploid. 

When it's the plant just doubling its own chromosomes, we call it autopolyploidy. There's actually another very common form of this. In fact, actually, this was an example of this where you get two different species and mix those chromosomes together. So, in fact, it's not really like doubling your own genome. You're actually getting two already different species and bringing them together. And now, in fact, you get a new species. And we call that allopolyploidy, and we're bringing together two diploids, we force them to mate, and then we can create this tetraploid which is actually an allopolyploid. So how do we do this? Okay, so here's AA, 1 chromosome, 8 genomes. We double it. We have an autopolyploid. 

So now we have four chromosomes, and you imagine that for every single chromosome that that plant has. If we force a mating between two, and we call this a wide cross between species, it's very difficult to do these crosses. Normally they would not actually mate with each other. You can force it sometimes and you get the AB together. So now you get one chromosome of one species and the other chromosome of the other species. And, in fact, that will be sterile, but if you can double the genomes, so everything doubles, you now actually have a balanced tetraploid where the two As can pair up nicely and the two Bs can pair up. And, in fact, it's very important that the chromosomes can pair up because otherwise going through meiosis and forming gametes becomes problematic. And I'm going to come back to that when we talk about triploids at the end if we have time. So let's look at, here we have species one, and let's say we have three chromosomes in this species, and notice it's a diploid. And we're going to call that the A genome. 

Here's species two. We have two different chromosomes, and we're going to call that the B genome. And if we force it to cross, here's the gametes, one haploid, one chromosome each. Again, one of each chromosome, that's haploid. We force it to cross and we get this hybrid, and, in fact, this is always going to be a sterile hybrid, in fact. Now, you're familiar with hybrid because you probably already know about mules, a cross between a horse and a donkey which gives you a sterile animal which is nonetheless completely viable, and, in fact, actually pretty strong and so good that people actually do that on purpose because these are such sturdy animals. In fact, there's a whole bunch of these things. You can zorses, zonkeys, zonies. Here's a debra. Actually the debra's my favorite mainly because I have a cute picture of a debra. 

Here's a debra. So you can see the dad looking on, and then here's the mom donkey. And it's actually very interesting, and we're going to talk about chimera later on in the second half of this talk, but here you can kind of see this chimera effect. It's not really a true chimera because, in fact, every cell in there is exactly the same genotype, but it looks like a chimera when you see that. So there things are pretty common. It's just that they wouldn't normally mate, but you can force these things to happen, just as plant breeders can do that between their plants. So the key here is that here's our sterile hybrid. Now this is what you can't do in animals but you can do in plants. And it occurs naturally in plants at a very high frequency. Well, very high relative to evolutionary time frames of very high frequency. 

And actually, even better, plant breeders can actually do it with chemicals and force it to happen. So what you actually do is you go from the sterile hybrid and you do this chromosome doubling step, and now, suddenly, every chromosome is happy because they're paired together. And, in fact, what we've done is we've created the diploid, effectively a diploid, and actually it's called an anti-diploid because it's like a diploid but it's not really. It's kind of, again, terminology. Let's not worry about that, though The important thing is it's fertile. And so, in fact, what this has done is this has actually effectively created a new species. And often they can self-cross, or if you're at a hybrid zone, often in South America, you might get different species in a hybrid zone and those guys will mate with each other and, bang, you've got a new species just like that. Now, you may not know this, how many of you have heard of the triangle of U? Has anyone heard of this? Because I'm assuming there's a lot of gardeners in here, so you're all growing these things in you vegetable gardens, I suspect, and actually some of you are probably growing them as ornamentals as well. So it turns out that this is a great example of how over time breeders have just been working with the material and not really knowing exactly what's going on but these traits have been bred and have been selected for because they're advantageous to us for our crops. So we have three different species: Brassica nigra, oleracea, and rapa. 

Notice they have different chromosome numbers. So this is actually eight, nine, and ten. So these are very different species even though they're all Brassicas. And you are very familiar with these because the oleracea are cabbages and broccoli and cauliflower. That's all the same species. They all look different just the way dogs all look different. That's basically breeding can accelerate these differences very rapidly. So we can do that. Black mustard is actually nigra, and rapa is turnips which are not so popular here, but actually where I grew up turnips were a staple. Actually, Norwegians must have brought turnips here. 

Do people here eat turnips? Yeah, okay. Yeah, I can't imagine the Norwegians would come here without turnips. And Germans too, right? So, yeah, okay, so you have turnips here. I just haven't seen them that often. >> [INAUDIBLE] >> All right, so you guys need to educate me on them. So what we can do, though, is we can cause these new species, so here we have Brassica juncea. Notice it has 18 chromosomes. It's basically eight from nigra, ten from rapa. It's all doubled up. So we've got two As and two B genomes. 

And that gives us Indian mustard. That is actually what that species has given us. Napus, which is oleracea and rapa, gives us the rutabaga. And that has 19 chromosomes. It's a combination of these two joined together and doubled up. So these are tetraploids, effectively. And we have oil seed right there as well, which is a very big one in Europe, less so here I think, but very big there. And, in fact, this has not really lead to anything much, interestingly, but you can do that it's just that there are no crop plants which are really represented from that species. So that's kind of a nice mating triangle. It's a really neat example of showing how this allopolyploidy is so important for agriculture. 

Now, obviously, breeders figured out pretty quickly that, in fact, a lot of these plants were polyploid, and so they actually started actually making their own polyploids, not relying just on nature. These, in fact, just happened naturally over our agricultural history. So within the last, I guess, 10,000 years. This example here, actually, is wheat, and I'm giving you this example because it's actually probably very similar to this example in the tomato which is the T1, T2, and T3. So we're talking about a hexaploid, three genomes. Let's look about how it happened in wheat. Again, this is not done intentionally. This is totally just natural effect. This is not breeders going in and forcing these things to happen. They just happen naturally. 

The farmer will go through and see, whoa, that looks good. I like that. And they'll pick it and keep it. And that will then mean that that tetraploid will get kept. Often because the head of seed is actually much bigger, and so for better yields it's actually a good thing. And actually, so here's the Einkorn wheat, which is still used, it's the A genome. All these wheats actually have different, this is good for, actually I forget which is which. Emmer I think is good for biscuits. This is good for bread. Hexaploid wheat is the more common one now. 

So we start off with the Einkorn which is probably one of the older cultivars that was used. We have a wild diploid here. They both have seven chromosomes and they come together in a wide cross, and just by natural luck it duplicates and doubles. And now you have a fertile plant which leads to an AA BB genome which is 14 chromosomes. So we've doubled, okay? And so this is about 10,000 years ago this was bred into Emmer wheat, which is still used as a cultivar we use now. And then this then was a wide cross with another wild type wheat. And this is the D genome. Again comes together as a sterile hybrid but doubles, and now we have an AA BB DD. That is the triplication. Basically, we have a hexaploid. 

It's very important to realize that this hexaploid has not superseded these other crop plants. It has different qualities. So these are all still going. And this is actually part of what breeding is all about. You want to have as many different varieties out there, all of which have different qualities. So that is really very similar to what happened here probably with this tomato. Although, we don't know the exact story yet. And, of course, I suspect that what's going to happen now is we're going to see a lot of papers come out. So keep an eye on the New York Times because they are very good in the science section of keeping track of these kinds of general interest stories. So I suspect something will come up. 

So breeders realized polyploids. This is a good thing. So what they figured out, now, okay, this is getting a little cell biology here and genetics. So here is a cell which is actually replicated its whole genome, and the chromosomes are starting to condense. And that's actually the chromosomes you see when you look at karyotypes. So we have these condensed chromosomes which just happens at cell division. This is going to divide if it's under normal circumstances. It's at this point that the breeders add a chemical called colchicine. Now there's actually cables of protein which pull apart the chromosomes. Colchicine stops those cables functioning so the chromosomes just sit there. 

Now, cells are clever. They don't divide if something's gone wrong, and they have these things called checkpoints through the cell cycle. And if you fail a checkpoint, it stops the whole division process. Now, if you leave it long enough, it just gives up and just goes back down into its normal cell state. So, in fact, it actually avoids going through cell division. So it goes through replication but not through cell division. So, actually, what we've done is doubled the whole genome from a diploid to an autotetraploid. So this actually very easy to do, and you can do it on pollen. So then your pollen, instead of being haploid, now ends up being, instead of being N it ends up being 2N so when you mate your cross you actually have double of what you should have. And because of this, it's lead to a lot of improvements in our fruit. 

Strawberries here, not really a fruit, it's a stem, but you get the idea. So here we've got two pollen and an egg, 2N instead of one N or instead of just N. It's going to come together as a tetraploid, and what you see immediately is it's huge, double the size, and notice, also, it doesn't taste as good. So that's a trait which is not so good that comes with this, but maybe over time, maybe a thousand year's time, you're going to have big, sweet strawberries as well. But we do though, we do get this new trait of it being a lot bigger. So that's because of the tetraploid. So remember that when you're in the shop and going these don't taste as good as usual, it's probably because it's a tetraploid, not a diploid. Same with those grapes. You've seen those grapes out there, big grapes, way bigger than when I was even a kid. So you guys, I guess you had tiny, tiny grapes. 

[LAUGHTER] So I shouldn't, that's ageist, isn't it? I shouldn't be doing that. I'm sorry. [LAUGHTER] All right, but here's the interesting thing is that when you double the genome, so you have now twice as much DNA a cell, bingo, the cell becomes twice the size. And that's why you see this kind of increasing in size. And this is actually the surface of a leaf. These blue cells are the stomata. That's where the air goes in and out for photosynthesis, and respiration as well. And what you see here when you compare it to the tetraploid is that these stomata are twice the size or approximately twice the size. And you'll notice they're all, in general, most of the cells in this leaf epidermis are about double the size as well. And that's actually often why you see these increases in size. 

So increasing ploidy correlates with an increase in cell size. And, in fact, we don't even know exactly why that is right now, but it is something we see a lot. Now, actually, this is an experiment you can do at home. This is a diagram of the kind of thing you see when you look at a cell, the cells on a leaf. And you see this pavement of cells. Now, notice some of them are actually really large, and then some are really small. It actually turns out in plants that they actually have a natural process of doubling without dividing, and they do that again and again. And, in fact, they use it as a way to grow. So the leaves, when they expand, when you see those leaves expanding, it's not due to cell division. It's due to, the DNA is replicating, and you're getting bigger and bigger nuclei, and then the cells are just expanding and growing, and they get bigger and bigger. 

Now, if you're an, actually the orchid comes later, but that explains, also, when you have, in orchids, all those petal cells have actually undergone multiple rounds of replication, and they have massive nuclei compared to the rest of the cells in the plant. And it drives that huge growth of those petals when the flower suddenly opens up. And you're seeing that here in the epidermis. And here you can see there's some really massive cells here, and if you look at the nuclei there, you actually see that in fact the nuclei is maybe 10 times bigger than what you would see in a normal, a regular diploid. So this is quite weird because plants, now we all think of ourselves as every cell has the same number of chromosomes. In plants, that's not the case. It's normal for cells to have different numbers of chromosomes. So if you have four chromosomes, you're going to have eight, and then it's going to be 16, then it's going to be 32, then it's going to be 64. So you actually get this doubling again and again and again, and they get larger and larger and larger. Now, you can do that, if you're interested in this, just get some nail polish. 

Paint it on a leaf, and then just peel it off and stick it on a glass slide. If any of you have a microscope, then just look under the microscope. You'll be able to see the imprint of the cells very easily. Almost as good as an electron microscope, which is actually pretty amazing. You can even tell if the stomata are open or closed. So you do it at night and all the stomata will be closed. Do it in the day and they'll be open. And you can actually see that in your nail polish impression, and it's a great experiment to do with kids if you have a microscope. You don't need a good microscope, just enough that you  can actually kind of see this impression. So that's a good thing to try at home. 

And this is basically an example of that. So you can see a lot of detail. That's surprisingly good. Okay, I probably have a good microscope. [LAUGHTER] I admit it probably won't look as good on another smaller one, but it's still amazing to me. So that was a cheap way for us, instead of buying time on the microscope you just get your nail polish out. Quick and easy. And, actually, this is, just look at your plants in the garden and look for hair cells. Those cells are actually individual one cell, not multiple cells. And even though, this is actually called a trichome, not surprisingly, right? It has three spikes. 

So I guess Poseidon's down there somewhere. And he is sticking that up there, but isn't it amazing that one single cell can be so elaborate? You think of them as being bricks stuck in the organism. But, in fact, you could have these amazingly beautiful structures and it's all done by the subset of the cytoskeleton sculpting that. And, again, this is something we don't fully understand. It's very reproducible. And we don't know exactly how it's controlled. But it's just beautiful. Actually, when you're on the electron microscope and you're looking at this stuff, you can spend the whole day there just exploring. It's like being on a planet and you're just exploring, looking at all these amazing things. You could easily lose a day doing that. 

Oh, and here's the orchids. So, basically, imagine that. When those orchids are open, you basically got a bunch of replication going on and then those cells are getting bigger and bigger and bigger. But they're not dividing. Unlike humans where most of our growth comes from actual cell division. All right, so I'm going to indulge you in some of the research that I do here. Only for a little bit, so don't worry too much. So, one thing that we've been doing is actually adding a protein, which is part of the cell cycle checkpoint, and actually forcing cells to skip the cell division and just go right into another round of replication. And by doing that, we can create, artificially, these enormous nuclei in cells. So here you can see, here's the wild type. 

So here's a normal sized nucleus. This staining here is actually a fluorescent dye that will bind to the DNA. So you can see the size. And, actually, it's semi-quantitative in the sense of quantifying the fluorescence. Here's the cells here. This is actually on a petal cell, so you can see these beautiful array of petal cells. And, actually, the reflection off these actually gives you part of the colors that you see on the petals. And it's one of the reasons why they're so nice and neatly arrayed. These little cones reflecting light back at you. So here's the nuclei and here's the cells. 

Notice there's different magnification. That's why these things are further apart than the cells here. So we've got 10 microns here, 100 microns here. So here, this is not an enlargement. This is actually where we're expressing this protein where it shouldn't be, at the wrong time where it shouldn't be, and we can force it to, instead of going through a division, to keep replicating and enlarging. And then you get these larger cells, and here's just a crazy example. See how big this is? This is just one, and in fact, it's so big that the nucleus has stopped being round, and it's just got kind of a crazy structure. And the cells don't really know what to do, actually. They're just too big. Now, of course, what we were thinking is this would be great, we can actually artificially get bigger fruits or we can get bigger leaves so they'll have more photosynthesis. 

And, of course, when we actually did it, we looked, so here's the wild type petals. This is Arabidopsis. Don't worry about that too much. So this actually, this process is called endoreduplication. That just means this goes through cell divisions, sorry, goes through cell replication, DNA replication without going through cell division. So, actually, the wild type petals are actually nice and big, and then here's the weak one. And then here's the strong one with the enormous cells but the piddly petals. So we didn't quite do what an orchid does. So, actually, this is interesting because this tells you, and of course genetics is never simple, right? It tells you there's a lot, there are temporal things which are important, there are spatial things that are important. You can't just go in and say let's turn it on everywhere and see what happens. 

And this is true, actually, when you hear, you often hear this is going to be the wonder drug which is going to give us everlasting life. What they don't tell you is if you have it on all the time it's going to give you cancer. So, yes, things can give you longer life, but only if they're on for a certain amount of time at exactly the right time, and we have no way of controlling that, sophisticatedly. We have not sophisticated way of controlling it. So, basically, a lot of these gene therapy things you read about are really actually pie in the sky type things. And here you see a nice example of where gene therapy is completely useless. But we'll get better. All right, now you might think this is weird to plants, but it's actually not actually. And this was a surprise to me. It turns out that actually this happens in humans. 

The megakaryocytes which actually lead to the platelet cells undergo this endoreduplication process as well. If you're familiar with Celiac disease, you're going to know that that destroys the epithelial cells on your gut lining, and you get all those cells which are sticking out with a huge surface area which are basically in all the nutrients. And if you have Celiac disease, the wheats actually, it's actually the glutens, I think it's the glutens in the wheat actually destroy those, so you actually now don't have enough surface area to take the nutrients in. So it's actually really bad. Those cells there have also undergone endoreduplication, and part of the reason they're so big and have such a large surface area is, in fact, they, too, have actually done this. And this is one of these textbook things or things that are not in the textbooks, and it took me a while to realize that this was the case. So we're not totally, plants aren't totally weird. Actually, we're weird too. All right, so I want to get onto triploids if I can, but I also want to do mosaic. So I'm going to hold the triploids to the end, and I'm going to talk about this wine, which I can't pronounce. 

Anyone know how to pronounce this? Pinot meunier. >> Meunier. >> Meunier? Anyone drunk this? No? Okay, I guess, I've never heard of it either, but the way I was reading about it, it sounded like it was a very well-known cultivar. It's been around since the 1500s. It's actually grown in the Champagne area I think. So that's one thing about it. So this actually turns out to be a very interesting cultivar. And, actually, for ages people were trying to figure out where did this come from. What were its original parents, and what were the crosses which involve, which may get into this, polyploid, or at least everyone thought it was a polyploid. And we knew that pinot noir is one in there, and then there's another one we weren't so sure about. 

But before we go into this, oh, let's talk about chimera. So here we have some mosaics which are also known as chimeras. This is, again, going back to the language. And chimeras, of course, go back to Greek mythology. Lots of wonderful chimeras in their stories. And, actually, this little guy is a mouse, actually, that was created in the lab. It is a chimera between cells from a white mouse and cell from a black mouse, and they're injected into the egg, and they mix up randomly, and you get this mosaic chimera of a mouse. So this is an artificial chimera. Now, key thing here is that the cells which give you the brown fur are different genotype to the cells which give you the white. So we actually have two different genotypes together in the same animal. 

Now, this isn't so weird because, actually, if you think about it, any time you get a mutation, you yourself will have a new genotype in yourselves. And, in fact, all of us have about a hundred to two hundred new mutations which go through into our germ cells. And, actually, within our arms and things, we're probably going to have even more. So we're actually probably quite chimeric ourselves. But this is actually, obviously, another level of chimeras. So it turns out that this grape is actually a beautiful chimera, a wonderful example in plants. But to understand it, we're going to have to do some developmental biology, and I love this because it leads to all these ornamentals. Understanding the chimeras, so this is a chimera, this is a chimera, I've got some more down here. So we can look at those afterward. But they are all chimeras because they're different genotypes together in the same organism. 

So obviously you know that the leaves are always growing out of a plant meristem, and, actually, that's another difference between plants. We have these meristems, we, plants have these meristems which keep growing, and, of course, you keep having new shoots coming off. You can take cuttings and you can graft them, all these things, and the meristems are always growing. The roots have meristems which are always growing. We don't have that so much. We actually take our germ cells and we set them aside very early, and that then leads to the sperm and eggs. But plants, that's not so true. They can be generating new cells. Flowers come from new cells all the time. Nothing's set aside. 

So here's what it looks like. Here's the meristem which is round. Here's the leaves coming off. We have a central zone which is actually, these are like real stem cells that you get in embryos. They are undifferentiated cells. They really can turn into anything. And they also divide really slowly. We call it the quiescent center because it doesn't really do much. In fact, people thought it was really boring because it wasn't doing anything. But, in fact, it's the critical part of the meristem where all those cells are slowly coming out and then they come down onto what we call the peripheral zone. 

And this is very interesting, this zone, because this is where the cells start to differentiate and turn into the cells which lead to new flowers, new leaves, and all that. So this whole meristem, and actually this PRM is the root meristem, and that actually leads to the vascular tissues. So all the xylem and phloem, all of where the water comes up and the sugars go down, that's all from this central region. So, actually, everything that's exciting is happening just on the outside. That's where the leaves come from, and that's also where the flowers come from. And if you look at the actual meristem, there's something that's quite striking. You can see that there's very clear layers here, I hope. There's two layers there. There's actually probably about three, maybe four. And then it starts to get a little higglety-pigglety down here. 

And, in fact, those layers, that very strong layering there, is not by accident. In fact, the outer layers are not dividing randomly. They're always dividing in exactly the same orientation, which is periclinal to the surface. So it's actually perpendicular to the surface. So if you think about it, if you always divide in this direction, you just end up with a long sheet of cells. And, in fact, that's what happens. So these cells have all come from that one cell at the top there. And they go down there. Likewise, there's a cell here which is going down here. And we call these cell lineages, and we give them these very creative names of lineage 1, lineage 2, and lineage 3. 

So L1, L2, L3. You might have heard of that. And, in fact, we have these very strong lineages where they pretty much don't intermingle very much, and it's very stable throughout the whole plant. And they lead to, so here's the shoot apical meristem. This actually is leading off into flowers. So these cells come down, and we get a flower. Actually, there's a tiny flower bud right here. You can just see it's bumping out a little bit. That will be the next one. Here's a floral meristem. 

Here's a floral meristem. Here's the shoot apical meristem. So that's the top of your flower when it's going up, and those flowers on the inflorescence are coming off of that. Here's our traditional kind of histology regions that we see. But these are the actual lineages. So we've got the L1, the L2, and actually this is called the corpus here. And this is why I brought my lunch because I want you to think about this, about how this structure is. Because if you think about it, you've got, here's a meristem. All right, so you've got the meristem here, and the cells are coming down. When the flower comes out or the leaf comes out, it kind of comes out like you're pushing a sock out, right? And so here's your sock. 

It's like a sandwich. So you always have your sandwich in a plastic bag, right? That is the L1 layer. And, actually, this is a great analogy because, in fact, the plastic is see through, and, in fact, our L1 layer is also see through. Those cells in the L1 layer are not green. They are colorless, and you can't normally see them. What you do see on there, though, is all those trichomes and the stomata and stuff like that. So that's a layer. Our L2 layer is actually the bread. Good. And inside, cheddar cheese, that's my favorite, is the L3 layer. 

Now, this is interesting because actually, if you think of a leaf, what you've actually got is, here's our leaf, L1, top and bottom, L2, top and bottom, and cheese in the middle. The cheese is actually the vascular tissues. So when you look at a leaf, you can see the vascular, that's where the cheese is. These are kind of the photosynthetic cells. And then you have the epidermis. So when you look at these, you're actually able to see those three layers, just like a sandwich. So does it look or how does it start? So, let's say we have a single mutation. UV light, maybe it's too sunny that day and, bang, you've got a mutation. So here's the mutation, starts here. And in this case, I don't think you can see it, but there's this blue line that, as it grows, those mutant cells will stay in that L1 layer. 

Now, there's a leaf, right? There is a leaf and out of there is going to come, you know how if you chop the top of the plant off and suddenly you get extra flowers coming through? Well, it's going to come from that axil layer. There's going to be an axillary bud there, and that will come out, and now you're going to have the L1 is all going to be mutant. And you're going to end up then with what we call a periclinal where basically the whole of the L1 on the meristem is the mutated genotype. So now you have a chimera because you have the blue which is a different genotype to the underlying layers. Now, the blue could be an albino, like in this one. It could be something completely different. But albinos are the ones we see the most commonly. And here's an example of a mericlinal. Now, let's look at the nomenclature. We have a GWG. 

So L1 is green. L2 is white, albino. Actually, not quite white. And then the L3 is G. Now, this is the periclinal, so this means that the whole of the L2 here is going to be white. So that's the bread is white here. So that green, that's the L3, that's the cheese, but you are seeing through the colorless white. And you'll notice that the L2 goes right to the edge. And, in fact, on top of that is the L1 as well. So that's, actually, just like this one. 

Actually, that's exactly what this one is. Now, what you'll see if you look at these plants, though, is a lot of variability. And, actually, sometimes you get a green shoot coming out. Well, that's because you've lost those mean cells there, and, in fact, you've lost that chimera now. So if you're a gardener, you cut those off because that means you've lost the second genotype. Sometimes, actually, sometimes you don't get those nice regular periclinal divisions; you get an anticlinal division. Now, this is very interesting because in plants, if you get a division and it ends up in the wrong place, the cell goes, ah, okay, my neighbors are not the neighbors that I thought they were going to be. So what the cell does is it actually changes into what its neighbors are. So it's like a chameleon almost. Now, this is very unusual. 

This will not happen in animal cells, in animals. If you get an animal cell goes in the wrong place, it's still what it was. So plant cells are always sensing their surrounding and will adjust their function based on their surroundings. So here's the brilliant thing here, this colorless L1 goes inside, is surrounded by L2 and thinks, oh, hang on a second, I should be an L2, and so it becomes those photosynthetic cells. Now, the problem is it's not going to be green, though, because, in fact, you've lost, sorry, actually in this case it would be. What is this L1? In this case, we would actually get a white where it was L2. Let's look at a more interesting example. Let's say that we got green on the outside. L1 is green. Now, remember though, even though it's green, it's not making chloroplasts. 

So it's still colorless. But if the L1 pops down into the L2, it will become L2, and, actually, it will start making chloroplasts because, in fact, now it's the L2. So, in fact, that's exactly what you see where this arrow is. A little dark like that is where it's popped down. So, actually, what you're seeing here is the green here is due to the L3. And you can see that's pretty much where the vascular is. So, actually, not much L3 made it into this leaf. Mostly L2, which is white, and then every time you see a little dot like that, that's where, in fact, you've got a cellular invasion, and it's changed its cell fate. That is something that animals cannot do. Plants are very versatile that way, the plasticize there. 

Now, here's another beautiful example of when you see, these are not cellular invasions, though, but it's effectively the same chimeric effect. These are actually jumping genes, and you've probably seen these in gardens as well. These are actually transposons in this case where, in fact, the transposon is inside a pigmentation gene and it's stopping that gene functioning. When the transposon, this is a piece of DNA like a virus, jumps out, literally, jumps out and jumps somewhere else and now that gene can function again. And so horticulturalists have found these transposons in colored genes, and now they use them. And here you get these beautiful patterns, random events, jumping events, which are occurring. So every single one of those is a different genotype, effectively, because the transposon is changing. And, actually, this is another example of how we ourselves are actually chimeras because we have transposons jumping too. Hopefully it doesn't cause a mutation but sometimes it does. Given the time, I'm going to skip this, but I want to go back to this wine grape. 

It turns out that the L1 layer in the wine grape is completely different genotype to the L2 and L3. And what they did to discover this was that they took cells from the L1 and made them into plants, took cells from the L2 and they made them into plants, and they found the phenotype from the L1, cells derived from the L1 would dwarf and totally, well, actually dwarf and hairy. The L2 and L3 cells gave us a regular pinot noir. So, actually, this gives you a clue. In fact, this grape cultivar probably came from a pinot noir, and there was probably some mutation which basically turned into a periclinal chimera. This is very cool. If you look here, you've got the L1, you've got all these little grapes forming on what, actually in the pinot noir, actually would be tendrils, which I thought was pretty interesting. So that's what we call a homeotic transformation where, so that would be, the equivalent would be my arm turned into a leg or something like that. Or my eye turning into an arm. So here we have a tendril turning into a fluorescence. 

And, actually, as you'd expect, so here, actually, is the very hairy leaf. This is a terrible picture. This is from that paper. Here's a hairy leaf. And, actually, you can see here is a situation where, in fact, that L1, L2, L3 structure has not actually maintained itself. And, in fact, we do see these weird chimeras. So here's the thing, what is the genetic difference, and is there a selective advantage? And, in fact, we don't know. What they've done is they've grown the plant without that L1 layer. They've grown this one, and they don't see too much difference, advantages. I suspect, though, I wonder whether, in fact, this extra fruit is somehow something that's been selected for by breeders in the past. 

It has hairier leaves, so maybe it's something about pathogens. They've only been growing this version for three years now, so it's hard to tell in the time frame of 500 years compared to three years. So maybe it has a pathogen advantage, but for whatever reason, this L1, L2, L3 structure has been maintained for many, many generations, and it's been maintained by they take it, they chop it, they graft it. But you can keep that periclinal chimera by just completely grafting. And you have, actually, a really very different, two very different genotypes leading to very different phenotypes. Here's the interesting thing: it turns out the mutation is in the same gene that was important for the green revolution. The green revolution gene was, actually it's multiple different green revolution genes, but the thing it does is it caused dwarfism due to a defect in our sensing, in this case, and in this case, actually, biosynthesis of this hormone which is important for elongation which is why you'll dwarf if you don't have it. This is good, it turns out, because if it's windy and you're tall, everything falls down, and your yields are dramatically reduced because you can't actually get the seed out of the field. So, actually, having it dwarf and stocky turns out to be a huge advantage with regard to yields, not because you get extra seed, but because you can actually get that seed into your barn before it rots in the field. So, maybe, I don't know, actually it's kind of weird to me that that would actually be the same gene in that grape. 

But it kind of makes you wonder. This is where science actually, often is the case, generates more questions than answers, and I think that's what we got here. So this is a nice example of a chimera which is out there, and we don't even know why it's out there. And this weird kind of connection to what we know is a very important gene in agriculture. Now, I was going to go through, do you want to do triploids or do you want to, let's just do them quick. Here's a triploid. I ignored these before because triploids are tricky because triploids, if you have three chromosomes and you have to divide them in two, how do you do that? Do you just split it in half? One half in each, right? You can't do that. Sorry, this slide got messed up a little bit. Here's what you do. You have three. 

You can either have a gamete with two or gamete with one. Sometimes you get a gamete with none and a gamete with three. It really gets a little confused when there's three, but that's okay because triploids themselves are actually quite good. Now, if you have two chromosomes, it gets trickier because now you've got multiple, actually what happens is these pollen will be viable because it's all balanced. These will be viable because you've got one and one and two and two. But if you get one and two and two and three, plants can kind of tolerate that a little bit, but as the number of variations get higher, in fact, turns out they can't tolerate it. So, in fact, these actually often lead to non-viable. So here we have a two-chromosome plant, which is actually not often the cause, often you have 12, 15 chromosomes. And this guy has 12 chromosomes, right? So imagine a triploid of that. Now, okay, don't worry about the equation so much. 

Just look at the graph. You can just, from probability, figure out that, in fact, this is done for 10, so N here, 10 is the number of chromosomes. Basically, 1 in 500 gametes coming together will form a seed which can actually fall. So you'll get a fertilization event, and actually the fruit will grow because of the fertilization, but the seed will die. Actually, just like miscarriages that we saw right at the beginning. This turns out to be absolutely brilliant and a huge money-maker for the breeders. Now, bananas are triploid. Oh, here's my lunch again. I wanted to bring it. Now, think about bananas. 

You've got this beautiful fruit, right? Where are the seed? There are little seeds if you look real close. If you look at a wild type banana, something that is not triploid and sterile, there are massive seed in these bananas. Okay? So, in fact, what's happening here is that we have selected a nice triploid because you can eat it without breaking your teeth. [LAUGHTER] So that turns out to be a blessing. Okay, and I've got this guy here which is a seedless melon, watermelon. Now, in fact, it's not completely seedless. If you come up here, you'll see that, in fact, there are some seed in here. Okay? Now, let's think about this. Now, I don't actually know how many chromosomes watermelon have, but if you think that you 10 chromosomes, you have a 1 in 500 chance of getting a seed that works. That means you can reduce, if it's triploid, you can reduce the number of seeds from 500 to 1. 

Well, that's pretty good, unless you like spitting seed. But how do we do that? So this is the last thing I'm going to do. So this is how the breeders do it. They create a tetraploid watermelon which is completely fertile and can produce seed, and you see it's a little bit bigger, lots of nice seed. We have our original diploid. So the pollen from the diploid will be haploid, and the pollen from the tetraploid will be diploid. Now, if you bring a diploid and a haploid, actually, sorry, you've got to have an egg from one. So what they do is they have all these guys in the field, and they create seed. So this seed, this grew from a triploid, and that seed was made by the breeders, and they have the diploids and the tetraploids, they cross them together, and that is the seed that they sell you. And that seed, when it grows up, because it's a triploid, now you don't get any seed because every time you get a fertilization event, the fruit will grow but the seed will not develop. 

So here's a huge, very simple, all they need to do is create a tetraploid, and now they can create triploids. Seedless grapes, same thing. So, basically, anything that's seedless, you're probably eating a triploid, which is kind of cool, I think. And that is, oh, yeah, now I've got some other things here. I think I'm just going to end here, and I am sorry for overrunning. It's long overrun. I'm sorry about that. So, if you have questions, I'll be happy to answer them. There's tons of things to look at here as well. [APPLAUSE]