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The only thing better than a nice, juicy bite
of ripe watermelon is the same bite
without all those annoying seeds.
Which is why seedless watermelons
are basically the best thing ever.
But they’re also a delicious paradox.
Like, seeds are kind of a key part
of plant reproduction.
So how do you breed a plant
that doesn’t make any seeds?
Well, it takes some genome-altering chemicals
and simultaneously growing
three genetically-distinct watermelon plants.
And similar methods can produce
all sorts of seedless fruits.
Five thousand years ago,
watermelon’s wild ancestor
was a small, hard, bitter African fruit.
Centuries of selective breeding
for desirable traits
led to sweeter, juicier, red melons
with shiny black seeds.
Then, about 80 years ago,
Japanese scientists tinkered with
the plants’ genome more directly
to create seedless varieties.
They used a process known as
mutation breeding:
basically, plants are exposed to something
that induces changes to their genomes,
and then scientists look for ones
that have some desirable trait.
The scientists that made seedless watermelons
in 1939 used a chemical called colchicine.
It messes with the proteins that make up microtubules,
filamentous structural proteins
in cells that help shuttle things around.
In people, that makes it a treatment for gout
because it tones down inflammation.
But in plants, it makes cells fail to separate]
duplicated DNA properly when dividing,
resulting in what are called ploidy changes:
alterations to the number of complete sets
of chromosomes in each cell.
Animals with more DNA than they should have
usually don't survive.
but plants seem to be totally chill with
multiple copies of their genomes.
Like humans, watermelons evolved to be diploid—
they have two complete sets of chromosomes
in their cells.
But in 1939, a professor at Kyoto University
used colchicine to double the number of chromosomes
in one of his watermelon lines,
creating a tetraploid watermelon.
Then, when he bred the two plants together,
the offspring were triploid.
Triploid watermelon plants can’t make viable seeds
because the cellular process that makes
reproductive cells
in plants requires matching chromosome sets.
But planting these genetic weirdos
isn’t the last step to delicious, seedless
fruit.
So to get seedless watermelons,
you have to pollinate female flowers
on a triploid plant with the pollen
from male flowers from a diploid plant.
And since the triploid plants don’t produce seeds,
you have to keep mating diploid
and tetraploid plants to make them—
so you end up having to grow at least three
watermelon plants to get seedless crop
every year.
If that sounds like a lot of work, well, it is,
but people are so fond of the seedless
varieties that it’s worth it to farmers.
And researchers have tried to make the
process a little smoother.
They’ve developed a diploid line
which doesn’t make female flowers,
so farmers can plant it for its pollen
without accidentally making any seeded watermelons,
for example.
Watermelon isn’t the only seedless fruit
that we enjoy because of ploidy changes—
seedless grapes, citrus fruits, and bananas
were all created with ploidy manipulation.
But counterintuitive as it might sound,
none of them are considered GMOs—
even though they’re very much genetically modified—
because only crops created with
modern transgenic technologies
fall under that label...for some reason.
Something to chew on the next time
you’re enjoying a sweet slice
from the offspring of a
chemically-treated melon mom.
Thanks for watching this episode of SciShow!
And a special thanks to our patrons on Patreon.
Kind of like farming seedless watermelons,
it takes a lot of work to make SciShow videos,
and we wouldn’t be able to cultivate
all of the talented people that work on them if it
weren’t for our patrons.
If you want to learn more about our
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