I am so old that I remember when watermelons had seeds. There was a time when this tasty fruit could only be politely eaten outdoors, where you could spit out the seeds easily, without offending anyone. I never liked having to deal with the seeds since I always ended up crunching and eating a few. My mother warned me not to eat the seeds, or I would end up with watermelons growing in my belly! Of course she also told me that if I did not wash out my ears, I would have potatoes growing in them.
So, “How do farmers grow seedless watermelons since they don’t have seeds to begin with?” Well, seedless watermelons are the result of cross breeding diploid plants with tetraploid plants. Diploid and tetraploid refer to the number of chromosomes living creatures have in each of their cells. Eggs and sperm are what are known as “haploid” cells meaning they contain half the number of chromosomes that the fertilized adult form will have. Humans get half their chromosomes from Mom in her egg and half from Dad, in his sperm. For some reason unknown to me the numbers of chromosomes are referred to as “n.” So haploid cells are “n”, diploid cells are “2n”, triploid cells are “3n” tetraploid cells are “4n” and so on.
In nature, it is not all that uncommon for eggs and sperm cells to have multiple numbers of chromosomes. Sometimes egg or sperm cells do not split their normal diploid 2n chromosome sets into 1n haploid cells. Sometimes eggs and sperm will end up as entirely 2n and when a 2n egg is fertilized by a 2n sperm, the result is a 4n tetraploid. The 4n creatures are often fertile and can reproduce, since they will usually produce 2n eggs and sperm consistently.
When a 2n egg is fertilized by a 4n sperm however, the result is a 3n triploid. Triploid cells cannot neatly divide 3n eggs or sperm into 1.5n sets of chromosomes, so the end result is a seedless watermelon, which is sterile. So, 3n watermelon seeds may be planted, but in order for them to make fruit, the flowers must be pollinated by either a 2n or 4n fruit growing nearby.
Fruit development is stimulated by the pollination process but all that will be found inside, the soft white “seeds” that you can eat. You can buy seeds to grow seedless watermelons, but the seed pack tells you that you need to grow a “normal” variety nearby. Bananas also contain little, black colored specks that are really just undeveloped seeds. Luckily for banana lovers, these tropical plants can make bananas without ever being fertilized once they reach a certain stage of development.
Oftentimes, multiple sets of chromosomes result in bigger individuals that have more desirable traits. Plant breeders use chemicals such as colchicine, which prevents pairs of chromosomes from splitting, to intentionally breed more vigorous or showy individuals. Even amateurs can do this by buying colchicine from chemical supply companies and dipping seeds or young plants into solutions made with it.
Actually, you can do this yourself by buying bulbs of the autumn crocus, appropriately named “Colchicum autumale” and using a garlic press to squeeze out the juice. The juice will induce the mutations as well and will result in some weird looking offspring! Be very careful when handling this chemical because it causes mutations that can be dangerous! People of child bearing age should never touch it. You might want to throw away that garlic press after using it to crush crocus autumn crocus bulbs!
You have probably eaten some other triploid fruit besides watermelons. My favorite apple variety is “Jonagold” a triploid cross between “Jonathan” and “Golden Delicious” and there are many other examples. Flower breeders really enjoy creating bigger and better flower cultivars. Most of the large flowered day lilies have multiple sets of chromosomes as do Iris and many other ornamentals.
My favorite plant, American ginseng, Panax quinquefolius, is a naturally occurring tetraploid that arose from the diploid dwarf ginseng, Panax trifolius. Cannabis indica” marijuana is also a tetraploid of Cannabis sativa. Both plants may contain THC, which gets people high, as well as CBD, which does not. Hemp plants, which I see being widely cultivated these days, are supposed to contain only miniscule amounts of THC by law, but they are visually indistinguishable from pot plants, that do contain lots of THC!