Most commercially produced seedless fruits have been developed from plants whose fruits normally contain numerous relatively large hard seeds distributed throughout the flesh of the fruit.
Common varieties of seedless fruits include watermelons, tomatoes, grapes (such as Termarina rossa), and bananas. Additionally, there are numerous seedless citrus fruits, such as oranges, lemons and limes.
Seedless fruits can develop in one of two ways: either the fruit develops without fertilization (parthenocarpy), or pollination triggers fruit development, but the ovules or embryos abort without producing mature seeds (stenospermocarpy). Seedless banana and watermelon fruits are produced on triploid plants, whose three sets of chromosomes make it very unlikely for meiosis to successfully produce spores and gametophytes. This is because one of the three copies of each chromosome can't pair with another appropriate chromosome before separating into daughter cells, so these extra third copies end up randomly distributed between the two daughter cells from meiosis 1, resulting in the (usually) swiftly lethal aneuploidy condition. Such plants can arise by spontaneous mutation or by hybridization between diploid and tetraploid individuals of the same or different species. Some species, such as tomato, pineapple, and cucumber, produce fruit in which there is no seed to be found if not pollinated but will produce seeded fruit if pollination occurs.
Lacking seeds, and thus the capacity to propagate via the fruit, the plants are generally propagated vegetatively from cuttings, by grafting, or in the case of bananas, from "pups" (offsets). In such cases, the resulting plants are genetically identical clones. By contrast, seedless watermelons are grown from seeds. These seeds are produced by crossing diploid and tetraploid lines of watermelon, with the resulting seeds producing sterile triploid plants. Fruit development is triggered by pollination, so these plants must be grown alongside a diploid strain to provide pollen. Triploid plants with seedless fruits can also be produced using endosperm culture for the regeneration of triploid plantlets from endosperm tissue via somatic embryogenesis.
One disadvantage of most seedless crops is a significant reduction in the amount of genetic diversity in the species. As genetically identical clones, a pest or disease that affects one individual is likely capable of affecting each of its clones. For example, the vast majority of commercially produced bananas are cloned from a single source, the Cavendish cultivar, and those plants are currently threatened worldwide by a newly discovered fungal disease to which they are highly susceptible.
When it comes to watermelons there are several ways one can modify them. One way is described here " Simply stated, the number of chromosomes (the threadlike bodies within cells that contain the inheritance units called genes) in a normal watermelon plant is doubled by the use of the chemical colchicine. Doubling a normal (diploid) watermelon results in a tetraploid plant (one having four sets of chromosomes). When the tetraploid plant is bred back, or pollinated, by a diploid or normal plant, the resulting seed produces a triploid plant that is basically a "mule" of the plant kingdom, and it produces seedless watermelons. Seed of seedless varieties are available from most major seed companies. " Texas Agricultural Extension Service.
Determining if a fruit, crop, etc. is classified as ¨'modified' depends upon the definition of 'gene modifications'. USDA (United States Department of Agriculture) apparently defines it as "a living organism must have had a gene from an “unrelated species” inserted into it. In other words, the USDA definition of GMO includes only the transgenic variety of GE organisms. Thus, according to the USDA, rearrangements, subtractions, or additions made within the chromosomes of one living organism do not prevent that organism from being organic." As f.ex a watermelon is modified without this 'outside gene' it then falls outside the USDA definition, and can then be argued to be 'organic', depending on definitions. A transgenic organism is modified by taking a gene from one species then adding it to another, while a non-transgenic is altering gene material inside one species without adding outside genes. Although not regulated those gene manipulations continue to bring the same problems with them as the older techniques.
" In fact, none of the many crop varieties created over the last 50 years through chemical or radiation mutation is considered a GMO, and they are not covered by the regulations that restrict the field-testing and sale of GM foods. In fact, they are not covered by any regulations at all, although many of the public’s concerns about GM crops—such as toxicity to humans or gene flow from modified crops to wild plants—apply to these crops as well. " Eberly College of Science
Then there exist other, newer modifications and techniques, that seems to avoid the USDA definition. " Regulators around the world are now grappling with whether these techniques are even considered genetic engineering and how, if at all, they should be regulated. “The technology is always one step ahead of the regulators,” said Michiel van Lookeren Campagne, head of biotechnology research at Syngenta, a seed and agricultural chemical company. The New York Times
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