Name: 約翰尼-阿普萊西德是在浪費時間嗎？ (Was Johnny Appleseed Wasting His Time?)
Uploaded: 2021-01-14T10:44:39.000Z
Duration: 10 min 23 s
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基礎被動語態

If you went to school in North America, you were likely introduced to tales of Johnny

第二類條件句型

Appleseed—a well-intentioned, if slightly odd gentleman who traveled the continent planting

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

So it's not like Johnny was spreading tasty apples across the US of A. Just… crabby,

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

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

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

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.

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.

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

See, we can't just inbreed the trees to preserve the traits we like, like we do with

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

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

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

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

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

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.

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

Estimates vary, but improvement bottlenecks can remove as much as 25% of the wild genetic

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

And that means once apple growers find an apple they like, they just can't risk letting

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,

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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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