Most of the high performing crops come from hybrid seed - that means that two very different parental lines are crossed to make seeds that have genetic material from both parents. In some cases, the hybrid exhibits hybrid vigor - desired qualities superior to either parent; maybe taller stalks, easier to dislodge fruit, or simply higher production. The hybrids, called an F1 cross in genetic parlance, make a relatively uniform population, with predictable production characteristics. But if it is crossed with itself (i.e., replanting the seed), the next generation will start segregating the genes, such that the original parental traits start emerging.
The uniformity of hybrid vigor is thus lost, as well as the predictability of the F1 generation. One could invest multiple generations recapturing the original parental lines to cross and make hybrid seed again - or just buy it ready made from the seed supplier. This is a property known of genetics since the time of Mendel, no fancy tricks or special tech required.
But hybridization can also be maintained in a different way.
|Seedless tomatoes and cucumber.|
Originally intended as a joke at a party, watermelons and other plants can be hybridized to exploit the triploidy phenomenon seen in bananas - resulting in F1 progeny that are not only superior in flavor and production, but also unable to produce fertile seed.