Seedless watermelons aren’t seedless. They have those little white seeds that don’t have hard black shells, like the seeds in seeded watermelons. How do they do that? Glad you asked!
In seeded watermelons, the seed develops its hard seed coat (or testa) once it is fertilized. But, because seeds in seedless watermelons cannot be fertilized, they never develop the testa. So, the question now is: why can’t the seeds in seedless watermelons be fertilized?
Seedless watermelons are the result of controlled manipulation of watermelon chromosomes. In a traditional, seeded watermelon, the egg cells in the female flowers contain one set of chromosomes (1n). Pollen, from a male flower also contains one set of chromosomes (1n, again). When the egg is fertilized with pollen (or pollinated), the fertilized egg has two sets of chromosomes (1n + 1n = 2n). Because the fertilized egg has 2n, it is called a diploid cell.
To create seedless watermelons, young watermelon plants are treated with colchicine. This causes the eggs in the flowers to develop with two sets of chromosomes (2n), instead of one. When the eggs are pollinated, they create triploid cells (because 2n + 1n = 3n). These cells are capable of maturing into fruit, but the seeds in that fruit are not genetically viable – so they can’t be fertilized and develop the hard, black testa.
You can now amaze friends and family at your next picnic.