John Doebley - The story of maize
Now it’s time to hear from John Doebley, the winner of this year’s Genetics Society Mendel Medal. As DJ mentioned earlier, he’s been finding out how thousands of years of selective breeding have changed teosinte - a plant with just a handful tiny, rock-like kernels - into the large seed-packed ears of maize, or corn, that we know today...
John - Maize is a crop plant that is native to the new world. It was grown by native American people, from Chile all the way up to Canada - in the Amazon jungle, it was grown in the desert, so the south western United States and the high mountains of the Andes and the high mountains of Mexico. So, it’s a very broadly adapted grain crop that was the major food source for many native American societies.
Kat - Tell me about the history of maize. Where did it first come from?
John - So, what we’ve learned over the past several decades is that maize was domesticated in south western Mexico, southwest of Mexico city. It probably was first domesticated about 10,000 years ago by native American peoples. After that initial domestication, it spread all the way south into south America, and all the way north as far as Canada.
Kat - So, this was farmers going, “Ooh! That looks good. Let’s grow that” or “That looks better. Let’s grow some more of those.”
John - I think that’s how the process worked. These were very observant people. They were excellent naturalists. They knew plants inside and out. The way I like to put it is if you took most modern people in highly industrial societies like the UK or the United States and you let them go in nature, they would starve within a week. But these people back then could probably collect in just a few weeks enough grain crop to last them all year.
Kat - Tell me about the ancestor of maize, this plant teosinte. What did that look like and what was the journey that took it to becoming maize?
John - Teosinte looks a bit like maize. It’s a very large plant. It can be 10 up to 15 feet tall. So, you can think of it like a wheat plant, but 5, 6 times bigger. However, it doesn’t have one big ear or two big ears. It has dozens and dozens of just tiny little ears, each of those tiny little ears having maybe only 10 kernels. And so, the architecture of the plant is rather different. It’s very branched and not like maize which typically has just one giant stalk.
Kat - What genetic changes do we know must've happened to turn those plants with their rock hard little seeds into lovely corn?
John - We know a bit of the story. Of course, we don’t know the whole story. One of the changes we’re aware of is controlled by a specific gene that takes the many little ears of teosinte and blocks them from forming and instead, replaces them with one rather large ear. And so, that change has a big effect on the number of ears on the plant. One large ear could have a lot more kernels than the many small ears in teosinte. The logic behind having that change is, if you think about it, if you are someone needing to harvest grain from a plant, would you want to pick 50 tiny little ears off the plant, each with 10 kernels, or would you rather pick just one ear with 500 kernels?
Kat - I'm kind of lazy so I'm going to go for the one.
John - Exactly. And so, they selected for that change to make the plant easier to harvest.
Kat - What about some of the other genetic changes that have happened? Are they all similar things that humans have said, “That looks useful. Let’s go with that”?
John - Yeah, it’s exactly like that. Another one is there's another gene that’s involved in the manufacture of the casing around the teosinte kernels. That gene was changed so that those casings no longer form and the kernels are naked or uncovered on the ear. And that makes the grain much easier to eat because it’s a different – it’s as if a walnut, instead of having a hard shell around it was just sitting right there ready to eat.
Kat - Are these just single genetic changes we’re talking about with some traits particularly in humans, it’s many, many changes that make one variation, and these are just single changes that have wrought this very big change in the plant?
John - So, it’s a mix. So, in turning teosinte into corn, there are some of these genes that have single, very large effects on the way the plant looks, but then there are also many other genes with smaller effects. So, it’s a combination of the two and it is in a way very similar to many traits that are different between different human populations. Things like eye colour have a few genes that have very large effects and then some other genes with smaller effects as well.
Kat - And would all these kind of changes have been just kicking about in the teosinte plants for a native American to spot and go, “There it is” and just start growing that one?
John - Yes. I think it probably largely works something like that. So, teosinte is a plant that grows in enormous numbers in Mexico. It’s just millions and millions of plants growing over the landscape. They're each genetically different from the next one, just the same as people are each genetically different. And so, they could spot one that was having some desirable traits and use that one to grow the next generation.
Kat - And so, this is just random variations, random fluctuations in the teosinte genome that we’ve managed to capture and keep going for many, many years.
John - That’s right. As in any genome, there's a process of mutation in which errors are introduced – each generation there are a few things that don’t work quite right. For the most part, those mutations are destructive and interfere with the ability of the plant to survive and do its job. But occasionally, one of those mutations is useful. What people were doing was spotting the useful mutations and encouraging them to make their crop.
Kat - Do you think people realise when they look at maize on their plate or a sweetcorn and think, “This is a pretty mutant plant”?
John - I'm hesitant to guess what people think. Probably, most people don’t think that deeply about the food that they eat, unfortunately. If you want a really mutant, think about cauliflower. If you look at cauliflower, that plant has a hard time surviving on its own and really, it’s highly dependent because it doesn’t actually make good flowers. It just makes this mass of tissue that wanted to be a flower, but was disrupted.
Kat - Where is your work taking you next?
John - So, I mentioned one of the experiments I've been trying to do and that is to see if we can, in a sense, redomesticate teosinte. So, we’ve started with just taking a large number of teosinte plants, growing them in a field, and just like the ancient agriculturalists, picking up the ones that look the best. Then using those, start the next generation and want to do that over several generations and see how far along we can move from teosinte in a maize-like direction by applying artificial selection the same way that ancient peoples may have done.
Kat - So, they did that over maybe 8,000 to 10,000 years? How long do you reckon?
John - I'm not going to be around that long and so, I’ll be happy to do it just for about 10 years and see how far I get. I've already identified a younger colleague who’s just starting his career. He and I are going to work together on this. And so, he can take on the project when I retire.
Kat - John Doebley, from the University of Wisconsin-Madison.