Why Three Chromosome Sets Disrupt Seed Formation
Watermelon plants normally have two sets of chromosomes, one inherited from each parent. Colchicine can interfere with the machinery that separates chromosomes when cells divide, leaving some plants with four sets instead. Breeders cross these four-set plants with ordinary two-set plants; their offspring inherit three sets, a condition called triploidy.
The problem appears when a plant tries to make pollen or egg cells. Those cells are formed through meiosis, a kind of cell division that normally sorts matching chromosomes into balanced halves. With three copies of each chromosome, the copies cannot pair off evenly. The resulting reproductive cells usually have unbalanced chromosome combinations and cannot produce viable seeds.
The Fruit Can Grow Even When Seeds Cannot
Sterility does not prevent the plant from making a watermelon. Pollen from a compatible plant can still trigger fruit development, even though the triploid plant cannot make functional reproductive cells of its own. Growers therefore plant pollen-producing watermelons alongside the seedless crop. The fruit may contain small, pale remnants of seed development, but these do not mature like the hard seeds in a seeded watermelon.
Because the triploid plant cannot reliably reproduce itself, growers produce new seed by repeating the cross between four-set and two-set parent plants. The seedless watermelon on the vine is the harvest; the next generation begins with that carefully remade cross.
