My daughter loves watermelon and I was thinking she was getting big enough to have a seed spitting contest. But watermelon seeds seem to be vanishing. I'm used to seedless grapes but only recently became aware of this watermelon situation. Seedless watermelons have been growing in popularity since 1990. From the standpoint of a plant, the whole point of fruit is produce seeds, so I wondered what kind of hanky panky was going on to produce seedless watermelons. Turns out that they are like mules, self-sterile hybrids and involve a lot of work.
Diploid
Watermelon plants are usually diploid, like us, meaning they have two sets of chromosomes, the packages of DNA with instructions for life.
Triploid
Seedless watermelons are triploid. They have three sets of chromosomes. This odd number results in them being sterile and not producing seeds. The way they become triploid is by mating a diploid male with a tetraploid female. Tetraploids have four sets of chromosomes.
Tetraploid
The way you get tetraploids is by applying a chemical called colchicine which messes with cells as they are dividing. You add it to diploid seedlings and then some cells become tetraploid. You have to cultivate these over several generations to get enough that produce enough viable seeds with suitable traits.
Mixing It Up
Watermelon plants have male flowers and female flowers. The female flowers have a little pea-sized melon behind it. You remove the male flowers on the tetraploid plants because the female tetraploid flowers produce triploid fruit. It doesn't work with a male tetraploid and female diploid. Pollination can be done by hand or using bees.
Go Time
Seeds from the triploid fruit grow into triploid plants. They don't produce much pollen, so you plant some diploid plants. This pollen stimulates the triploid female flowers to produce fruit. Because the number of chromosomes is not compatible, they don't have seeds.
The seedless watermelons are smaller and rounder. They are supposed to be sweeter and last longer. But do you think it's really worth all the extra work?