Seedless watermelon is an example of:
Seedless watermelon is classic application of triploid sterility exploiting 3x block. Normal diploid watermelon has 2n=22. Using colchicine, breeders create tetraploid lines 4n=44 by doubling diploid seedlings and selecting fertile tetraploids. Tetraploid is then used as female parent crossed to diploid male, yielding triploid hybrid 3n=33. Triploid hybrid grows vigorously with larger leaves due to gigas effect of polyploidy but meiosis is chaotic because three homologs cannot segregate evenly, causing ovule and pollen abortion and highly sterile flowers. Fruit develops through stimulation by diploid pollinator pollen providing hormone signals auxin and gibberellin but without fertilization producing viable seeds; small white rudimentary ovules remain edible. Commercial fields interplant diploid pollinator rows for pollen supply. Vegetative vigor and inability to form black mature seeds give desired seedless phenotype. Same principle applied in banana, triploid citrus, and grape breeding for consumer preference. Plant hormones auxin, gibberellin, and cytokinin application can induce parthenocarpic fruit set in sterile triploids, while diploid pollinator induces pollen tube growth and auxin surge without double fertilization, mechanism underlying commercial seedless watermelon production that requires careful field arrangement of pollinizer and triploid hybrid rows.
Ref: Kihara H. 1951 Triploid watermelon history; Maynard DN. Watermelon breeding. USDA