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Seedless watermelons are basically the best thing ever. But they’re also a delicious paradox. Seeds are a key part of plant reproduction. So how do you breed a plant that doesn’t make any seeds?

Hosted by: Hank Green

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Sources:
https://naldc.nal.usda.gov/download/CAT71326739/PDF
https://news.nationalgeographic.com/2015/08/150821-watermelon-fruit-history-agriculture/
http://extensionpublications.unl.edu/assets/pdf/g1755.pdf
https://learn.genetics.utah.edu/content/cotton/crop/
https://www.crops.org/about-crop-science/crop-breeding
https://www2.palomar.edu/users/warmstrong/hybrids1.htm
https://link.springer.com/article/10.1023/B:EUPH.0000014914.85465.4f
https://www.tandfonline.com/doi/full/10.1080/13102818.2015.1087333
http://www-naweb.iaea.org/nafa/pbg/
https://link.springer.com/chapter/10.1007%2F978-94-015-7271-2_12
https://aggie-horticulture.tamu.edu/newsletters/hortupdate/hortupdate_archives/2000/may00/h5may00.html
https://www.uaex.edu/yard-garden/resource-library/plant-week/seedless-watermelon.aspx
https://www.ncbi.nlm.nih.gov/pubmed/25151572
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4656054/

Image Sources:
https://commons.wikimedia.org/wiki/File:Kalahari_Tsamma_melon,_Namibia_(5182549010).jpg
https://en.wikipedia.org/wiki/File:Colchicine-3D-balls.png
https://www.flickr.com/photos/saeba/43366057712
{ ♪INTRO }.

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. Patreon community and how you can keep us doing what we do best, you can go on over to Patreon.com/SciShow { ♪OUTRO }.