How Seedless Watermelon Seeds Are Created

Seedless watermelons are created by crossing a tetraploid female, four sets of chromosomes (2n = 4x = 44), to a diploid male, two sets of chromosomes (2n = 2x = 22), to yield a seedless (or sterile) triploid offspring, three chromosomes. These yield fruit with immature white seed coats, alongside some dark seeds if certain conditions are met.

Sterile triploid watermelons are not considered genetically modified organisms (GMOs), as they are produced through traditional plant breeding techniques rather than transgenic methods. To develop them, breeders use colchicine, a chemical originally used to treat gout, which functions as a mitotic inhibitor. Colchicine disrupts the mitotic spindle during cell division, leading to chromosome doubling and the formation of tetraploid (4x) plants from diploids (2x). These tetraploids are then crossed with diploid watermelons to produce triploid (3x) offspring. Since triploids have an uneven number of chromosome sets, they cannot undergo normal meiosis, resulting in sterile plants that produce no viable seeds. Once initial tetraploid individuals (T0) are identified, they are advanced through the breeding program. Each subsequent generation (e.g., T1) is screened to confirm tetraploidy before further selection and selfing. This process can take over a decade to produce stable tetraploid lines capable of generating sufficient seed for commercial triploid watermelon production.

Pollinator varieties are required for seedless watermelon production, although the fruit is sterile, a fertile diploid male must be present for fertilization. ‘Ace’ is an example of such melon; these are non-harvestable as they do not have any desirable consumer traits and do not create competition for the triploids.

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