Crops that rely on wind or insect pollination but are grown in a greenhouse may have to be pollinated by hand. Cultivated plants may be hand-pollinated with a small paintbrush. The horticulturist brushes the anther of the stamen to retrieve pollen grains and then deposits them on the stigma of the female flower on another plant. Hand pollination is also used to cross-pollinate plants to create new hybrids. Plants that belong to the same species, including all the subspecies (varieties or cultivars), will naturally interbreed with each other to form hybrids. The approach to classification developed by Linnaeus, described in the previous chapter, gave insight into the relationship between speciation and plant breeding that was developed further by the work of Charles Darwin in the 1800s.
Luther Burbank was a prolific plant breeder who was inspired by a book written by Darwin in 1868 called The Variation of Animals and Plants Under Domestication. Burbank bred hundreds of new cultivars; the first of these was the Russet (or Burbank) potato in 1873. He then imported berries, plums, and nuts to
California and experimented with wide crosses between domestic and foreign cultivars. Wide crosses do not naturally occur and have unpredictable results that often cause mutations that are harmful to the plant. Therefore, this method involves the propagation and screening of millions of plants to find one healthy cultivar with a desirable new trait.
Chemical- and radiation-induced mutations were introduced in the 1900s. The plant is exposed to certain chemicals, X-rays, or gamma radiation, and this treatment produces unpredictable mutations to the target plant. When the mutation occurs in a somatic (nonreproductive) cell, the mutation may affect the plant itself but not the offspring unless the somatic cells are used to clone new plants. When it occurs in a germ cell (sperm or egg), the mutation is passed to the offspring. Most of these mutations are not beneficial to the plant or to humans so this method also requires the propagation and screening of millions of plants to establish a useful new cultivar. Rio red grapefruit and wheat are commercial crops with radiation-induced mutations. A chemically mutated barley cultivar, introduced in 1995, is widely used in beer and a mutated commercial peanut cultivar was bred in 1959.
Transgenic plants are plants that have had genes from an unrelated organism inserted into the chromosome by genetic engineering for the purpose of breeding a new cultivar (Figure 3.5). Sources of these genes include other plants, microbes, and fish. The genes that have been used to transform plants code for traits such as resistance to specific microbial infections, resistance to cold temperatures, resistance to drought and salt, parthenocarpy, resistance to herbicides, and the creation of pesticides.
The most controversial transgenic plants are those that create pesticides. Government regulatory agencies classify the gene product as a pesticide and not a food additive; therefore, the FDA
(Food and Drug Administration) is not involved in the establishment of safety guidelines for pesticide residue in transformed crops.
Biologists cannot control where on the chromosome the integration of the inserted gene occurs, and the process will most likely cause a problem by interfering with the function of other important genes. Because of this, genetic engineering offers no time- or labor-saving advantage over wide crosses and chemical-or radiated-induced mutations, as this process also requires the propagation and screening of millions of plants to come up with a healthy new cultivar.
The unpredictable results obtained from wide crosses, chemical- and radiation-induced mutations, and genetic engineering may be a result of transposable elements. Transposable elements are naturally occurring, mobile segments of DNA (deoxyribonucleic acid). They play an important role in genetic engineering. Transposable elements can cause a whole segment of chromosome to be duplicated, moved to another chromosome, or even deleted. The genome can be radically altered and the result is offspring that are different from both parents. Transposable elements are found in all genomes that have been studied to date and are inhibited by naturally occurring modifications to the DNA. These modifications may be inherited and prevent the activity of the transposable element. Transposable elements can be triggered by X-rays, gamma rays, wide crosses, and tissue culture techniques, which explains the unpredictable results of breeding methods.
It has been suggested that a rise in the number of people susceptible to food allergies may be attributed to the introduction of new cultivars bred with methods that can trigger transposable elements. Peanuts, wheat, soy, and barley are examples of crops that may have acquired subtle mutations that trigger allergies. Extensive research would be required to confirm whether this is the case.
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