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{♫Intro♫}
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If you went to school in North America, you were likely introduced to tales of Johnny
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Appleseed—a well-intentioned, if slightly odd gentleman who traveled the continent planting
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apple seeds everywhere he went.
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Which, if you know anything about apple genetics, might come across as a colossal waste of time.
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After all, every time you grow an apple from seed, you're actually rolling the dice — you
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don't know what's gonna grow.
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So it's not like Johnny was spreading tasty apples across the US of A. Just… crabby,
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gross ones.
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But it turns out growing all those not-so-yummy apples was kind of a good thing, because it's
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ensured that apple growers have the tools to continue to cultivate delicious varieties
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today.
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Jonathan Chapman traveled hundreds of thousands of miles across what is now the American Midwest
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in the 19th century toting the fruit seeds that would earn him the nickname “Johnny
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Appleseed”.
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He made a living selling the trees that sprouted from those seeds.
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But here's the weird thing: he had no way of knowing what apples would come from those
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seeds.
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And a modern apple grower couldn't tell you much better.
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Suppose you go to the grocery store and buy yourself some nice Fujis or Pink Ladies.
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You say to yourself, gosh.
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That was the best apple I've ever eaten.
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So you plant the seeds in your backyard in hopes that once that tree matures, you can
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experience that delicious apple all over again.
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But you wait about a decade until that tree finally produces fruit and — surprise!
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The apples are small, or sour, or just kind of ugly looking.
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Or all of the above.
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Well, if you'd talked to an apple grower first, you would have expected that.
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Unlike planting seeds from your favorite store-bought tomatoes, the fruit of any apple tree you
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grow from seeds will /never/ look the fruit it came from.
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They don't grow true-to-type, as gardeners say.
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And that's because genes in those seeds are /always/ from two genetically distinct
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trees.
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See, we can't just inbreed the trees to preserve the traits we like, like we do with
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dogs.
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Apples — and other species like pears and sweet cherries — won't let us do that.
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These species have a system called self-incompatibility, where they're capable of recognizing genetically
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similar individuals — and then not breeding with them.
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Generally, for flowering plants, the process of seed production starts when a pollen grain
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falls on an organ within the flower called a pistil.
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That pollen grain then grows a long tube down to the flower's ovaries and delivers its
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genetic material.
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Plants are a bit odd, so there's more to it than that, but that's the gist.
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Many flowering plants produce both male and female reproductive organs on the same flower.
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And if that's the case, they can often fertilize themselves.
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Like, the reason Mendel's pea plants were so great for studying genetics was because
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they are #self-fertilizing.
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If he'd been studying apples, he never would have gotten as far as he did.
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But even though apples do have the necessary parts in place for self-fertilization, they
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also have really robust ways of telling their own pollen from that of a genetically distinct
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tree.
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The female reproductive organ produces an enzyme called an S-RNase.
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That enzyme's job is to chop up RNA — which would be bad for a future seed, since cells
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need RNA to make proteins, and by extension, live.
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Still, these enzymes are transported into the growing pollen tube.
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Luckily, it has a defense: it can degrade the S-RNase before the enzyme can do any degrading
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of its own.
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But it'll only do that if its genes and the RNase are a mismatch.
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If it recognizes the RNase as being from genetic stock similar to its own, the RNase gets to
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do its work unencumbered, and fertilization is stopped.
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This means apple blossoms won't pollinate themselves or other blossoms on their tree,
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even if the pollen happens to land in the right place.
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It even reduces the odds that parent or sibling trees can breed with them.
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Most of the time, pollen from a totally different strain has to be carried by bees or the wind
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for flowers to produce fruit.
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That's good for the plant, because inbreeding can lead to a loss of resistance to pests
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and disease, as well as just being less healthy overall.
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But it's bad for us, because it means we can't pick a tree we like and force it to
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produce offspring with very similar genes.
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Instead, growers have to find another type of apple tree that blooms at the same time,
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produces compatible pollen, and carries desirable genes in order to breed new trees.
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What all that means is that we've been essentially rolling the dice for literally thousands of
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years, hoping that two trees will mate and produce a really nice apple.
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And it's not even, like, a six-sided die.
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It's more like a whole handful of d20s.
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That's because apples have remained almost as genetically diverse as their wild ancestors,
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starting from when they were first cultivated around 4000 years ago.
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Normally, domestication really hurts the genetic diversity of a population.
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As humans select for desirable traits, gene variants get left behind, creating what's
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referred to as a domestication bottleneck.
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And more modern methods of cultivation can narrow the gene pool even further, creating
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a second improvement bottleneck.
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Estimates vary, but improvement bottlenecks can remove as much as 25% of the wild genetic
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diversity.
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But that's not the case with apples.
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A 2014 paper surveyed the genetic diversity of modern cultivated apples and found it's
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basically equal to the very oldest varieties.
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That means that any genes that contribute to sweetness, or color, or pest resistance,
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or ability to grow in cold climates are mixed in with all sorts of other genes throughout
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the apple gene pool.
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And that means once apple growers find an apple they like, they just can't risk letting
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it breed with other apple trees.
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So if they hit the genetic jackpot, they usually propagate that tree by cloning.
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Not modern, molecular cloning, but a growing technique called grafting where you take the
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fruit-bearing part of one tree and fuse it with the root of another, creating a new,
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hybrid tree that produces genetically-identical fruit.
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It's a process so ancient we've had it about as long as we've had cultivated apples.
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And it means we can keep growing what's effectively the same tree for generations.
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Like, Golden Delicious apples go back to 1890.
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There is still some room for genetic change even when you're cloning trees in this fashion,
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though.
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Like, sometimes a new branch will turn up with a chance mutation that makes the apples
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on it a little different—a deeper shade of red, perhaps.
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Growers might select for that more appealing color, propagating the mutant branches over
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the older variety, even if the deeper color comes at the expense of flavor.
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You might see where I'm going with this.
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Yes, the reason Red Delicious apples taste like misery incarnate is probably because
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of selection for color — at least according to some food scientists.
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By all accounts, they used to taste pretty good!
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Of course, good-tasting apples have only really been a goal of apple growers for the last
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century or two.
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Your Honeycrisps and your Galas are what the trade calls dessert apples.
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They're sweeter than cider apples, and we tend to want them to be more consistent.
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Apples that go into hard cider don't have to be sweet, or perfectly firm, or… well,
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good, really.
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They basically just have to have enough sugar to ferment.
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Which in the end is why our buddy Johnny C probably wasn't wasting his time.
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Sure, he didn't know what would grow from his seeds exactly, but at the time, most apples
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ended up as hard cider, so pretty much any apple worked.
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And genetic studies suggest he, or people like him, may actually have helped apples
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maintain their genetic diversity up to the present day.
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The apples you see in the grocery store originate from the Tian Shan mountains in Central Asia.
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They traveled to Europe along the Silk Route, where they further interbred with European
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crab apples to produce the modern domesticated apple, Malus domestica.
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In fact, they've interbred so much that domesticated apples have more genetic material
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in common with the European apples than the Asian ones, and only modern genetic studies
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have been able to establish for certain where they came from.
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Then, those European domesticated strains were introduced to North America.
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And somehow, they stayed super diverse.
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Some have suggested that's because Malus domestica interbred with North American species
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to adapt to the new climate.
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But others think it had more to do with our dear pal Johnny and others like him.
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Because even though different apple varieties were often kept apart in their European orchards,
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with people running around planting them all over North America, some were bound to go
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wild.
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That let them get together.
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And they were different enough from one another to overcome self-incompatibility, so they
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made new varieties of trees, called “chance seedlings”.
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That turned out to be a pretty good thing for us, because we've gotten more than a
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few delicious apples through these new offspring.
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Literally.
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Red Delicious and Golden Delicious apples were both chance seedlings.
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And the McIntosh, an apple so popular it's got a certain type of computer named after
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it, was also a chance seedling that was discovered all the way back in 1811.
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So there's a lot to be said for planting apple seeds when you don't know what will
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sprout from them.
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Genetic diversity isn't just valuable for its own sake; apple breeders rely on that
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huge gene pool to create new varieties.
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Though these days, we're lucky enough to have genetic sequencing to cut down on the
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guesswork.
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And apple growers aren't just looking for things that improve flavor.
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Hiding amongst those genes are also the keys to resisting pests and diseases, growing in
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different climates, or making apples that are hardier and easier to transport.
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Or so breeders hope.
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In fact, there's some evidence that a gene for disease resistance made the jump from
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wild to domestic apples as recently as the 1970s.
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And the need for resistance isn't just theoretical.
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Both pests and a changing climate have been making life harder for North American apples
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in recent years.
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That's why efforts are ongoing to preserve apple diversity.
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See, apples as a whole are diverse, but as of 2008, 90% of apples produced in the US
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consisted of just 15 varieties.
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And if we want to keep creating new, tasty apple varieties that can survive whatever
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gets thrown at them, we'll need to do better than that.
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Fortunately, researchers are on it.
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Much like Johnny once did, they're planting all sorts of seeds, and by doing so, they're
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ensuring that apples stay wonderfully diverse.
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The rest of us will just have to wait for the fruits of their labor.
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{♫Outro♫}