genetics - How are oranges in the US or anywhere made seedless?

How are oranges in the US or anywhere made seedless? Please explain the broad principles and not the technicalities.

Answer

Oranges and other fruits are generally not actively made seedless. Rather, seeds may fail to develop due to either lack of fertilization (pollination) or a natural tendency. The natural production of unfertilized and thus seedless fruit is called Parthenocarpy.

To quote the Scientific American article (3) mentioned by Oreotrephes:

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.

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.

Parthenocarpic varieties may arise from a lack of pollinators. From the Wikipedia page on Parthenocarpy:

Plants moved from one area of the world to another may not always be accompanied by their pollinating partner and the lack of pollinators has spurred human cultivation of parthenocarpic varieties. Some parthenocarpic varieties have been developed as genetically modified organisms.

To preserve the seedless trait, parthenocarpic trees can be propagated by grafting. It is possible that more kinds of seedless fruits will be engineered in the future (3):

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.