Seedless watermelon is triploid. Because the triploid cells have synapsis disorder during meiosis, triploid watermelon can't form seeds.

The parents of seedless watermelon are diploid watermelon and tetraploid watermelon.

Ordinary watermelon is a diploid plant, that is, it has two sets of chromosomes (2N=22) in its body. When its seedlings are treated with colchicine, the cell chromosomes of diploid watermelon plants become tetraploid (4N=44), and this tetraploid watermelon can blossom and bear fruit normally, and its seeds can germinate and grow normally.

Seedless watermelon is planted with seeds, but this seed is not the seed in seedless watermelon, but the seed in triploid watermelon formed by crossing natural diploid watermelon with tetraploid produced by mutation.

Seedless watermelon is triploid, so it has no reproductive ability and no seeds.