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  • {♫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 diceyou

  • don't know what's gonna grow.

  • So it's not like Johnny was spreading tasty apples across the US of A. Justcrabby,

  • 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 nicknameJohnny

  • 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 andsurprise!

  • 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.

  • Applesand other species like pears and sweet cherrieswon't let us do that.

  • These species have a system called self-incompatibility, where they're capable of recognizing genetically

  • similar individualsand 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 RNAwhich 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 colorat 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, orwell,

  • 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, calledchance 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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  • Plus, they're just a super awesome group of humans who help us make super awesome stuff.

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  • {♫Outro♫}

{♫Intro♫}

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約翰尼-阿普萊西德是在浪費時間嗎? (Was Johnny Appleseed Wasting His Time?)

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    林宜悉 發佈於 2021 年 01 月 14 日
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