Seedless watermelons are a summer staple in the US and can be found in almost all grocery stores, but how does a plant without seeds reproduce? Like humans, standard-seeded watermelons are diploids, however, seedless watermelons are triploids. A triploid watermelon is created by hybridizing a diploid and a tetraploid. In order to get a tetraploid watermelon a chemical called colchicine is used to double the chromosomes of a diploid watermelon. The resulting offspring of the diploid tetraploid cross will grow, but is sterile and will not produce fruit on its own.
Due to the watermelon's triploidy nature, they are sterile plants, unable to fruit on their own. In order to stimulate fruit production, farmers must initiate parthenocarpy. Parthenocarpy is the process of fruit formation without fertilization. This is achieved by interspersing diploid pollinizer watermelons among the sterile triploid watermelons. These pollinator varieties are specialty bred to be shrub-like with baseball-sized fruits and a higher pollen content.
The seeds used to grow the triploid variety are created by planting the diploid plants in a row next to the tetraploid plants. Male flowers of the female parent are then removed to prevent self-fertilization. Once these steps are complete, the plants are enclosed and bees are released to cross-pollinate the two varieties yielding the sterile triploid seeds. The “seeds” you may find within your seedless watermelons are not seeds at all, but the outer casing of a seed that was unable to develop.
It is fascinating to see how humans can create brand new hybridizations from plants with different numbers of chromosome sets and even manipulate the number of chromosomes using chemicals. Seedless watermelons are relatively inconsequential, however, similar growing techniques could be used to express all types of desirable traits. With large amounts of agricultural land affected by climate change each year, it will be interesting to see how various hybridization techniques, as well as GMOs, are used to solve the world's food deficiencies.