Benjamin Burr and Frances Burr, biologists at Brookhaven National Laboratory, offer this explanation:
Fruit development normally begins when one or more egg cells in the ovular compartment of the flower are fertilized by sperm nuclei from pollen. In some plants, however, fruit develops without fertilization, a phenomenon known as parthenocarpy. Parthenocarpic fruit has advantages over seeded fruit: longer shelf life and greater consumer appeal.
Image: VALENCIAN INSTITUTE FOR AGRICULTURAL RESEARCH
The most frequent reasons for lack of seed development are pollination failure, or nonfunctional eggs or sperm. In many plants, self-incompatibility genes limit successful fertilization to cross-pollination between genetically different male and female parents. This property is exploited by citrus farmers who grow seedless fruits, such as navel oranges and clementines. Because these cultivars are self-incompatible, they fail to set seed when they are planted in orchards of identical plants (clones). These plants have a high frequency of parthenocarpy, however, so they still produce fruit. Such trees do not require seed for propagation. In fact, propagation by seed would be disadvantageous because the progeny would differ from the parent. Instead nurserymen frequently propagate fruit trees asexually, usually by grafting.
Another frequent reason for lack of successful fertilization is chromosomal imbalance. For example, the common banana is triploid. In other words, it has three sets of chromosomes. Instead of having one set of chromosomes from each parent, it has two sets from one parent and one set from the other parent. Triploids seldom produce eggs or sperm that have a balanced set of chromosomes and so successful seed set is very rare. Bananas, too, are parthenocarpic and produce fruit in the absence of successful fertilization. These bananas are asexually propagated. After the stalk has flowered and borne fruit, it dies. But there are side shoots or suckers at the base of the main stalk, which can be removed and replanted to continue the cultivar. Growers also propagate bananas by tissue culture.
Seedless watermelons are particularly interesting because they must be propagated by seed, and yet growers can still exploit parthenocarpy. One way to make seedless watermelons is to produce triploid seed. As in the case of bananas, triploid watermelons cannot produce functional seed, but they still develop good fruit through parthenocarpy. Plant breeders produce triploid seed by crossing a normal diploid parent with a tetraploid parent, which itself is made by genetically manipulating diploids to double their chromosome number. In the case of watermelons, this manipulation has to be performed each generation, so it is a somewhat expensive proposition but still worthwhile.
Plant biologists have learned that if the plant hormone auxin is produced early in ovule development, parthenocarpic fruit can grow on plants that do not usually exhibit this property. Thus, genetic engineering will most likely give consumers parthenocarpic fruit in many other species in the near future.
Answer originally posted October 2, 2000.