Sciency Answers: Variegation part 2. Stripes and splotches!

Last week I talked about the colored patterns on leaves created for some evolutionary adaptive reason. This week I'll be talking about the freaks of the plant world, the strange variegated mutants that we humans love and keep around to add color and excitement to our gardens.

Chimeras

Imagine, for a moment, that you had to get a kidney transplant. Your kidneys are failing, so a good friend donates you her kidney. It is a medical miracle, you owe your friend everything, and you are now a chimera.

The word Chimera comes originally from Greek mythology, and refers to a monster killed by Bellerophon, (who, poor fellow, no one remembers anymore, though everyone knows his cool winged horse Pegasus) which had the head of a lion, body of a goat and tail of a snake. Since the original chimera is one animal made up pieces of lots of different animals, scientists now use the term to mean any single organism with genetically different sections of cells. Like a person with a kidney transplant. All of your body has your DNA, except the the cells in the donated kidney, which have the DNA of the friend who saved your life. You wouldn't, however, pass on your donor's DNA to your children. Kidney cells only make other kidney cells, not eggs or sperm. Unlike, say, if you had an ovary transplant in which case the children you bore would be genetically the children of the ovary donor, not you.

We can become chimeras naturally as well. Imagine a developing embryo. It starts off as one single cell, which keeps dividing, and bit by bit different cells get designated to develop into your brain and skin and kidneys. If some mutation just so happened to happen in that cell that was going to become your kidney, that mutation would be in that kidney for the rest of your life, but not in the rest of your body. (For more on human chimeras, including an incredible story about a women who was not the genetic mother of her own son, check out this amazing (as always) episode of Radio Lab. Her story starts at about the 6 minute mark).

Plants, of course, are arrange a little differently, with new growth and organs coming from buds. But within the group of actively dividing cells in a plants' bud, the meristem, there are three (sometimes 2) layers of cells, called (very uncreatively) L1, L2, and L3. L1 is very outer layer of the plant, L3 the center, and L2 in between. Just as new kidney cells only come from other kidney cells in our body, new L1 cells come from other L1 cells. So, if there is a chance mutation interrupting normal chlorophyll production results in mutant white (or yellow) cells in, say, the L1 layer, you can get a plant which is a chimera – mutant albino L1 layer, but regular green for the other two.

You've seen this is hostas. Here's the cute miniature hosta, 'Blue Mouse Ears'

And here are some chimeral versions of the exact same plant. The only difference between these different varieties is which layer or layers are albino mutants:

So chimeras can create all kinds of lovely variegated plants. Seedlings from these plants won't be variegated, just as the children of a person with a kidney transplant won't have genetically different kidneys. Plant gametes (sex cells) are made by just one of the layers, so depending on which layer is variegated, the seedlings will either be all green (boring) or all white (dead).

So where do new variegated hostas come from? Well, in some cases, people just have to be patient and wait for a variegated mutant (aka sports) to pop up. But there is also a special trick that results in variegated plants that can pass on their variegation to the next generation.

Chloroplast mutants

Here is a cell.

Chloroplasts are where photosynthesis actually happens, what makes a plant green. They also, strangely enough, have a little of their own DNA. They actually  essentially little cells within a cell, doing their own dividing, reproducing, and of course, mutating. Sometimes those mutations cause them to stop making chlorophyll, and become white.

As long as a cell has mostly green chloroplasts, all is well. The white ones just hang out, dividing occasionally, doing their thing. But when a cell divides into two new cells, the chloroplasts get split up between the daughter cells. And if the parent cell has some white and some green chloroplasts, just by chance, sometimes it will make cells with all green chloroplasts, sometimes all white chloroplasts, and sometimes a mix of the two types.

 When this happens in a plant, it looks like what you see here in yet another variegated sport of the (apparently very mutation prone) hosta 'Blue Mouse Ears'

The white patches have all albino chloroplasts, while the green ones are either all green, or a mix of the two.

This type of variegation will often come true from seed, because as long as the individual cells that develop into the embryo in the seeds contain both green and white chloroplasts, the new seedling will show just the same streaky, blotchy variegated pattern.

Sometimes, though, just by chance, the cells with white chloroplasts end up isolated in one of the layers, and the green cells end up in the other layers – the irregular chloroplast mutant variegation becomes a tidy chimeral variegation. Nursery people refer to this as the variegation stabilizing. Hosta breeders make use of this all the time, using plants with unstable streaky variegation in their breeding programs to create seedlings which can stabilize into varieties with neat variegation on leaf margins or centers which they can then sell to you.

Transposons

Another, completely different way variegation can come about is through the so called jumping genes, transposons.

Genes are essentially little templates for making proteins. Proteins in cells can be incredibly complex, and act like little machines doing. They build stuff, take stuff apart, modify chemicals into other chemicals, and generally run the show. But transposons are genes that do something rather odd. Instead of making a protein that goes off and does something, the protein they make simply comes back, makes a copy of the gene that made it and sticks that copy somewhere else in the genome. Transposons are genes that can make copies of themselves, the chain letters of the genetics world.

Transposons are everywhere. In fact, almost half of the DNA in your body is actually transposons. They keep copying and copying and copying, filling up the genome. All those transposons jumping around can cause problems. As they move about the genome, they sometimes land in the middle of other genes, causing them not to work right anymore. It is like you opened your cook book to make brownies and the recipe read: “1 cup COPY ME flour COPY ME COPY ME 2 COPY ME cu COPY ME ps sug COPY ME ar.”And sometimes, the transposon moves in and out of a gene at different times in different cells as a flower or leaf develops. And then you can get this:

This poor morning glory has a transposon problem. The transposon keeps bouncing into the middle of a gene it needs to make the purple pigment for the flower. When it does, the gene stops working, so those cells are white. When the transposon moves out of the gene again, it starts working, and you get purple. Many striped flowers, and some striped leaves, are the lovely result of a poor plant with transposons moving about.

So next time you pick up a beautifully variegated plant, or admire the pattern of a striped flower, take a moment to appreciate that is happening. Two genetically different cell types living peacefully together, white and green chloroplasts getting shunted this way and that, or perhaps unruly transposons bouncing in and out of genes.

(Correction: In my original post, I described the plant as actively removing transposons from genes, which a friend who knows a LOT more about transposons corrected. Plants have no way to actively kick a transposon out of a gene, though there are various mechanisms by which transposons are inactivated so they stop moving around the genome.)

Have a question? Get a sciency answer! Just e-mail me: engeizuki at gmail dot com

Sciency Answers: Variegation Part 1

Henry has a question about variegation:

I would like to ask you a question about variegated Hippeastrum reticulatum. Usually variegated gene is recessive but why the variegated mid-rib of
hippeastrum reticulatum is dominant in all its F1 hybrids, and
successive breeding dilutes the character from white mid-rib to yellow
mid-rib?

Looking forward to your sciency answers !!!

Well! I'm going to have all sorts of fun with this. I hope you are all ready, because the world of variegated leaves is cool and crazy.

First, some pictures to illustrate what he's asking about. This is the beautiful foliage of Hippeastrum reticulatum with distinctive white stripes down the center of each leaf:

And here is an example of a more typical sort of variegation, as seen on a Clivia leaf:

The word variegation covers both these types of leaf patterns. In fact, the most basic defintion of the word courtesy of Miriam-Webster is “Having discreet markings of different colors.” That could be applied to bicolored flowers, spotted cows, or striped curtains. But when someone says a plant is variegated, they almost always mean that it has some kind of white or yellow (or rarely other colors like pink) pattern on the leaves. So though the dictionary would tell you you are correct if you called the blotch in the center of a pansy flower variegated, say that to another gardener, and they'll be looking for white on the leaves.

But even narrowing down the term to white (or yellow or maybe pink) patches on leaves there are still a lot of different sorts of things being called variegation. 

Most variegations in your garden are what I'd call non-adaptive variegations. Some random mutation that would never survive in the wild and only persists because we think it is pretty. A classic example would be hostas. Visit wild hostas in the forests of Japan, and you'll see solid green leaves. The various white and yellow patterns so familiar to us in our shade gardens are man-made (or at least, man-preserved) and if humans went extinct, would die out almost at once. My second picture, with the white and green steaked leaves would be another example of this type of variegation.

There are, however, some wild plants that have what I'd call adaptive variagation, white or other colored patches on their leaves that help them make a living in the world. I've talked before about the example of wild Caladiums which apparently use their white variegation to trick insects into not eating them.

The difference between adaptive and non-adapative variegation is like the difference between a very pale white person from Norway, and an albino from Africa. 

A blond Scandinavian

An albino african

Pale skin in Northern Europe is the result of adaptation over time to low UV light levels, allowing people to get sufficient vitamen D during nearly sunless winters, and is caused by many different genes interacting to lower skin pigmentation. Albinoism, on the other hand, is a simple mutation in a single gene that knocks out the production of Melanin, and isn't at all beneficial in a region with intense sunlight.

The two types act differently when it comes to the next generation as well. The children and grandchildren of an albino will either be completely albino, if they ended up with two copies of that gene, or dark if they don't. It is a recessive gene, like the recessive gene for most non-adaptive variegations described in the question.

The descendants of a marriage between a Norwegian and an African, on the other hand, will have many different skin colors. Dark skin tones tend to be dominate in the first generation (just as Henry sees the white midrib as dominate in the first generation of crosses with Hippeastrum reticulatum) but in subsequent generations, we don't see white or black, but a whole range of beautiful colors (just as Henry sees the yellow mibribs in subsequent generations) as the many different genes controlling skin tone get arranged into new combinations each generation.

But... this just scratches the surface of the fascinating world of variegation. After all, an albino human has a mutation that makes it impossible for their body to produce melanin, they aren't variegated with blotches of different colors. A plant with the equivalent mutation would produce no chlorophylle at all, which would quickly make it dead. So how do plants get white leaf edges and green centers, or an mix of green and white? The adaptive variagates do it through having a bunch of genes that carefully contol where to express pigment and where not to. For the non-adaptive variegation, it gets very strange and very cool... So tune in next week for Variegation Sciency Answers Part 2, featuring mythical beasts and jumping genes!

Sciency Unreliability

Winston Churchill once famously said “Democracy is the worst form of government except for all those others that have been tried.” I think the same could be said of science.

I was thinking about this after my recent “Sciency Answer” about variegated plants. After I posted it, I had a couple of conversations with Kelly Norris on the topic. He read the studies I cited to support my explanation, and wasn't impressed with their technique and reasoning. I (obviously) was. The upshot is that I'm pretty confident variegation in wild caladiums (and other plants) is a disguise to prevent insect damage, and Kelly is very skeptical. Which is totally normal. Scientists often disagree about what studies mean, because scientists know a very important thing about science: It is often wrong.

Since I'm now in the business of dispensing sciency answers, I thought I should talk about that. About why science is so often wrong, why some of my answers may prove to be wrong. There are lots of reasons, but I think one of the biggest problems is people like me -- Graduate students.

Graduate students like myself are the people in the lab (or field or hospital) doing the actual work of most of the scientific research going on these days, and we have a HUGE conflict of interest that effects any sort of research we do. We desperately want to graduate.

In order get my PhD, I do research. In my case, research on petunias. I perform experiments, write up my findings as a dissertation, and hey presto, you have to call me Doctor. Unless, of course, my research doesn't work and I don't find anything interesting. To use an extreme, completely made up example: if I was studying the effects of chewing gum, and found that it caused cancer, WOW! That's shocking! It gets published in a fancy journal, I get a degree, I get a job, and everything is wonderful. But if chewing gum doesn't have any effect at all... I'm screwed. No chance of a good publication, no job, and maybe even no degree unless I start over with a new line of research.

This isn't just true for graduate students. University faculty need to make tenure, they need grants, and to get all that, they need publications. Publications from research with big, interesting findings.

In response to all the pressure to find results, people do sometimes make stuff up, but most of the time they do something more subtle, perhaps even unconscious. They overlook alternative explanations, massage their data, or keep asking the same question a different way until they find something that passes the test of statistical significance. Chewing gum may not cause cancer but it must do something... gum disease? Jaw injury? Stress? Divorce rate? Ask enough questions, and even if just by chance, the numbers will tell you one is right.

Because of that, new scientific findings tend to overstate the case – they find big, dramatic effects that sometimes prove to be weaker, or nonexistent in future studies. Science does however, eventually, tend to correct its own mistakes. Since no one currently believes chewing gum is dangerous, a study finding it to be safe is boring and unpublishable. But if someone else had said it did cause cancer, debunking that finding would be very publishable. That's why in significant areas of research we get dueling studies (Eggs are good for you! Eggs are bad for you! No, they're good for you!) but over time, eventually, we can look back over all the studies, compare them, and finally (hopefully) come to a conclusions that is close to actual reality.

That is why, despite all its flaws, I believe in science as a powerful way to understand how the world really works. Just don't confuse science with the actual truth. Truth is something we strive for, but can never really, absolutely, know.

Sciency Answer: Variegated plants are liars

Dear Mister Greensparrow Gardens Person:

I have a SCIENCE QUESTION (dah duhduh DAAAH) about variegation. Isn't the point of a plant being green for good light wavelength absorbency? I mean, plants that are other colors besides green still absorb most of the spectrum because of different kinds of chlorophyll, etc. But what about the plants that are variegated to have mainly white leaves? What is up with that? Doesn't the color "white" mean that all the wavelengths are reflected back? So how would they photosynthesize properly if they couldn't trap light efficiently? With some of these plants, there is still some green or other color, but it doesn't seem like it would be enough to support such a big plant. 

-Hannah

You are absolutely right -- completely white sectors on leaves don't photosynthesize, and plants that produce whiter leaves are going to be inherently less vigorous than ones with green leaves. So why are they like that?

Most variegated plants are essentially man-made -- they are unhealthy, mutant freaks that would die if we didn't like them and keep them alive in our gardens. Sort of like chihuahuas (except chihuahuas are disgusting and variegated plants are delightful.) Surprisingly, however, some wild plants, like some caladium, begonia, and dieffenbachia naturally have white patches on their leaves. Breeding has increased the amount of white on the plants we grow, but still, the wild plant have distinct white patches on their leaves. Why?

Because they are liars.

Imagine for a moment that you are a expecting mommy-to-be leaf miner. You are flying about, looking for a good leaf on which to lay your eggs so your babies can happily start eating away at them. First you see a leaf like this:

photo credit

This leaf is already infested with leaf miners. Lay your eggs on that leaf, and your babies will starve, because there isn't enough leaf to go around. So you keep flying, and see a healthy, green leaf like this:

This looks perfect! You land, and lay some of your eggs, and then happily fly on to find a home for the rest of your brood. But the next leaf you see looks like this:

photo credit

This leaf looks TERRIBLE! There must be a million leaf miners and caterpillars already there, munching away for this leaf to have so little green on it. So you fly on... fooled by a sneaky, variegated plant. The plant has made a trade off: less efficient photosynthesis in exchange for not being eaten alive.

In other words, natural leaf variegation is the plant equivalent of pretending you have whiplash in order to get insurance money. This is a fact that should make those of you who don't like variegated plants because they look unhealthy rethink your position. That is just what those plants WANT you to think! You are being fooled, just like the little leaf miners. Go buy some today just to show those plants you are smarter than them.

If you want more of the science behind white leaves, here are some good papers (subscriptions required):
The history of research on white-green variegated plants
Ecology of a leaf color polymorphism in a tropical forest species
Leaf variegation in Caladium steudnerifolium (Araceae): a case of mimicry?

Have a question? Get a sciency answer! Just e-mail me: engeizuki at gmail dot com