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 watermelons 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 plant's 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 is 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 cant 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 a 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 its 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.
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