Purdue News

July 23, 1999 If watermelons are seedless, how can you grow more? WEST LAFAYETTE, Ind. -- Watermelon seed-spitting contests are losing their ammunition. Nearly one-third of watermelons sold this year in the United States will be seedless, according to Purdue University vegetable specialist Dan Egel. But where do producers get the seeds to grow seedless watermelons? "As an Extension agent who deals with diseases of watermelon, that's one question I hear often," says Egel, who worked as a seed pathologist with a seedless watermelon company before joining the Purdue Cooperative Extension Service. "Let me use an analogy," Egel says. "Most everyone has heard of a mule. Mules are the product of the mating of a horse and a donkey. Although these parents are genetically different, they are similar enough to allow the parents to produce an offspring -- a mule. However, horses and donkeys are genetically different enough so that the mule offspring is sterile. To produce another mule, a donkey and a horse must be mated again." Vegetable breeders produce seedless, or sterile, watermelons in a similar way -- by starting with parents that are genetically different, Egel says. Normal watermelon plants have two sets of chromosomes. By adding a compound extracted from autumn crocus plants, breeders double the number of chromosomes in the female parent so that it has four sets of chromosomes. The breeders then cross a female plant that has four sets of chromosomes with a male plant that has two sets of chromosomes, Egel says, with each parent contributing half of its chromosomes to the offspring. That works because each parent still has an even number of chromosome sets. The male plant produces pollen with one set of chromosomes, and the female an egg with two sets. All the offspring of that cross have three sets of chromosomes. The plants with three sets of chromosomes grow normally. They also produce normal-looking watermelons, because the rind and flesh of the fruit itself grows from the parent plant's non-reproductive cells. To make reproductive cells in the flower and to produce seeds, however, the plant has to divide its normal chromosome number in half. The plant's pollen and egg cells have one-and-a-half sets of chromosomes each, can't pair up normally because they're not full sets, and so don't produce seeds, Egel says. As a result, the otherwise normal-looking watermelon is sterile and seedless. To get more seeds, growers must buy them from breeders. It's a complicated story, "but anyone can enjoy the crisp, sweet taste of seedless watermelon without knowing the genetics that make it all possible," Egel says. "Just ask any kid." Sources: Dan Egel, (812) 866-0198, egel@purdue.edu Writer: Rebecca Goetz, (765) 494-0461, rjg@aes.purdue.edu Purdue News Service: (765) 494-2096; purduenews@purdue.edu To the Purdue News and Photos Page