{♫Intro♫}.

If you went to school in North America, you were likely introduced to tales of Johnny. Appleseed—a well-intentioned, if slightly odd gentleman who traveled the continent planting apple seeds everywhere he went.

Which, if you know anything about apple genetics, might come across as a colossal waste of time. After all, every time you grow an apple from seed, you're actually rolling the dice — you don't know what's gonna grow. So it's not like Johnny was spreading tasty apples across the US of A.

Just… crabby, gross ones. But it turns out growing all those not-so-yummy apples was kind of a good thing, because it's ensured that apple growers have the tools to continue to cultivate delicious varieties today. Jonathan Chapman traveled hundreds of thousands of miles across what is now the American Midwest in the 19th century toting the fruit seeds that would earn him the nickname “Johnny.

Appleseed”. He made a living selling the trees that sprouted from those seeds. But here's the weird thing: he had no way of knowing what apples would come from those seeds.

And a modern apple grower couldn't tell you much better. Suppose you go to the grocery store and buy yourself some nice Fujis or Pink Ladies. You say to yourself, gosh.

That was the best apple I've ever eaten. So you plant the seeds in your backyard in hopes that once that tree matures, you can experience that delicious apple all over again. But you wait about a decade until that tree finally produces fruit and — surprise!

The apples are small, or sour, or just kind of ugly looking. Or all of the above. Well, if you'd talked to an apple grower first, you would have expected that.

Unlike planting seeds from your favorite store-bought tomatoes, the fruit of any apple tree you grow from seeds will /never/ look the fruit it came from. They don't grow true-to-type, as gardeners say. And that's because genes in those seeds are /always/ from two genetically distinct trees.

See, we can't just inbreed the trees to preserve the traits we like, like we do with dogs. Apples — and other species like pears and sweet cherries — won't let us do that. These species have a system called self-incompatibility, where they're capable of recognizing genetically similar individuals — and then not breeding with them.

Generally, for flowering plants, the process of seed production starts when a pollen grain falls on an organ within the flower called a pistil. That pollen grain then grows a long tube down to the flower's ovaries and delivers its genetic material. Plants are a bit odd, so there's more to it than that, but that's the gist.

Many flowering plants produce both male and female reproductive organs on the same flower. And if that's the case, they can often fertilize themselves. Like, the reason Mendel's pea plants were so great for studying genetics was because they are #self-fertilizing.

If he'd been studying apples, he never would have gotten as far as he did. But even though apples do have the necessary parts in place for self-fertilization, they also have really robust ways of telling their own pollen from that of a genetically distinct tree. The female reproductive organ produces an enzyme called an S-RNase.

That enzyme's job is to chop up RNA — which would be bad for a future seed, since cells need RNA to make proteins, and by extension, live. Still, these enzymes are transported into the growing pollen tube. Luckily, it has a defense: it can degrade the S-RNase before the enzyme can do any degrading of its own.

But it'll only do that if its genes and the RNase are a mismatch. If it recognizes the RNase as being from genetic stock similar to its own, the RNase gets to do its work unencumbered, and fertilization is stopped. This means apple blossoms won't pollinate themselves or other blossoms on their tree, even if the pollen happens to land in the right place.

It even reduces the odds that parent or sibling trees can breed with them. Most of the time, pollen from a totally different strain has to be carried by bees or the wind for flowers to produce fruit. That's good for the plant, because inbreeding can lead to a loss of resistance to pests and disease, as well as just being less healthy overall.

But it's bad for us, because it means we can't pick a tree we like and force it to produce offspring with very similar genes. Instead, growers have to find another type of apple tree that blooms at the same time, produces compatible pollen, and carries desirable genes in order to breed new trees. What all that means is that we've been essentially rolling the dice for literally thousands of years, hoping that two trees will mate and produce a really nice apple.

And it's not even, like, a six-sided die. It's more like a whole handful of d20s. That's because apples have remained almost as genetically diverse as their wild ancestors, starting from when they were first cultivated around 4000 years ago.

Normally, domestication really hurts the genetic diversity of a population. As humans select for desirable traits, gene variants get left behind, creating what's referred to as a domestication bottleneck. And more modern methods of cultivation can narrow the gene pool even further, creating a second improvement bottleneck.

Estimates vary, but improvement bottlenecks can remove as much as 25% of the wild genetic diversity. But that's not the case with apples. A 2014 paper surveyed the genetic diversity of modern cultivated apples and found it's basically equal to the very oldest varieties.

That means that any genes that contribute to sweetness, or color, or pest resistance, or ability to grow in cold climates are mixed in with all sorts of other genes throughout the apple gene pool. And that means once apple growers find an apple they like, they just can't risk letting it breed with other apple trees. So if they hit the genetic jackpot, they usually propagate that tree by cloning.

Not modern, molecular cloning, but a growing technique called grafting where you take the fruit-bearing part of one tree and fuse it with the root of another, creating a new, hybrid tree that produces genetically-identical fruit. It's a process so ancient we've had it about as long as we've had cultivated apples. And it means we can keep growing what's effectively the same tree for generations.

Like, Golden Delicious apples go back to 1890. There is still some room for genetic change even when you're cloning trees in this fashion, though. Like, sometimes a new branch will turn up with a chance mutation that makes the apples on it a little different—a deeper shade of red, perhaps.

Growers might select for that more appealing color, propagating the mutant branches over the older variety, even if the deeper color comes at the expense of flavor. You might see where I'm going with this. Yes, the reason Red Delicious apples taste like misery incarnate is probably because of selection for color — at least according to some food scientists.

By all accounts, they used to taste pretty good! Of course, good-tasting apples have only really been a goal of apple growers for the last century or two. Your Honeycrisps and your Galas are what the trade calls dessert apples.

They're sweeter than cider apples, and we tend to want them to be more consistent. Apples that go into hard cider don't have to be sweet, or perfectly firm, or… well, good, really. They basically just have to have enough sugar to ferment.

Which in the end is why our buddy Johnny C probably wasn't wasting his time. Sure, he didn't know what would grow from his seeds exactly, but at the time, most apples ended up as hard cider, so pretty much any apple worked. And genetic studies suggest he, or people like him, may actually have helped apples maintain their genetic diversity up to the present day.

The apples you see in the grocery store originate from the Tian Shan mountains in Central Asia. They traveled to Europe along the Silk Route, where they further interbred with European crab apples to produce the modern domesticated apple, Malus domestica. In fact, they've interbred so much that domesticated apples have more genetic material in common with the European apples than the Asian ones, and only modern genetic studies have been able to establish for certain where they came from.

Then, those European domesticated strains were introduced to North America. And somehow, they stayed super diverse. Some have suggested that's because Malus domestica interbred with North American species to adapt to the new climate.

But others think it had more to do with our dear pal Johnny and others like him. Because even though different apple varieties were often kept apart in their European orchards, with people running around planting them all over North America, some were bound to go wild. That let them get together.

And they were different enough from one another to overcome self-incompatibility, so they made new varieties of trees, called “chance seedlings”. That turned out to be a pretty good thing for us, because we've gotten more than a few delicious apples through these new offspring. Literally.

Red Delicious and Golden Delicious apples were both chance seedlings. And the McIntosh, an apple so popular it's got a certain type of computer named after it, was also a chance seedling that was discovered all the way back in 1811. So there's a lot to be said for planting apple seeds when you don't know what will sprout from them.

Genetic diversity isn't just valuable for its own sake; apple breeders rely on that huge gene pool to create new varieties. Though these days, we're lucky enough to have genetic sequencing to cut down on the guesswork. And apple growers aren't just looking for things that improve flavor.

Hiding amongst those genes are also the keys to resisting pests and diseases, growing in different climates, or making apples that are hardier and easier to transport. Or so breeders hope. In fact, there's some evidence that a gene for disease resistance made the jump from wild to domestic apples as recently as the 1970s.

And the need for resistance isn't just theoretical. Both pests and a changing climate have been making life harder for North American apples in recent years. That's why efforts are ongoing to preserve apple diversity.

See, apples as a whole are diverse, but as of 2008, 90% of apples produced in the US consisted of just 15 varieties. And if we want to keep creating new, tasty apple varieties that can survive whatever gets thrown at them, we'll need to do better than that. Fortunately, researchers are on it.

Much like Johnny once did, they're planting all sorts of seeds, and by doing so, they're ensuring that apples stay wonderfully diverse. The rest of us will just have to wait for the fruits of their labor. Thanks for watching this episode of SciShow.

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If you want in, head over to patreon.com/scishow. {♫Outro♫}.