How can watermelons be seedless?

What seeds are used to grow seedless grapes?

The trick is: you don't need a seed at all. There is a whole range of plants that can also be reproduced asexually - so you don't sow them, but you propagate them by cuttings, or - hobby gardeners know this - by grafting. And you do the same with grapes. One takes the shoot of an existing plant and grafts it onto an existing vine. This then creates a completely new plant. Genetically, this does not change anything at all - an offshoot plant is always a clone of the original plant. This generally works quite well with grapes.

So the seedless grapes are simply special varieties and not high-tech varieties?

Of course, the way they are sold, they are cultivars. Apart from a few wild berries, there is hardly a type of fruit that man has not optimized over the millennia. But the seedless grapes are absolutely nothing new or even high-tech. The seedless Sultana grapes have probably been around since ancient times - this is the most common variety from which the sultanas are made. Another variety are the Korinthiaki grapes - as the name suggests, they are used to make currants. So also a kind of raisin. In contrast to sultanas, currants are quite small (which is why people who constantly ride around on small things are called accordingly).

And what applies to seedless grapes also applies to seedless oranges or melons?

It's a little different there. The older ones will remember: In the past, seedless oranges or melons were practically non-existent. They are a recent development. These plants are obtained from hybrid seeds. That is, the seedless melon plant or the orange tree have two parents. But the chromosome numbers are so different that they are sterile themselves. More precisely: They are “fruitful” in the sense of “fruit-bearing”, but the fruits do not contain any seeds. And unlike grapes, these hybrid plants have to be grown anew in each generation.