Plant Cloning And Artificial Hybridization
Genetically-identical plants, or clones, have been propagated from vegetative cuttings for thousands of years. Modern cloning techniques are used extensively to select for cultivars with particular characteristics, since there are limits to what can be achieved through direct hybridization. Some individual species or groups of cultivars cannot be genetically crossed. Sometimes this is because of natural polyploidy, when plant cells carry extra copies of some or all of the chromosomes, or because of inversions of DNA within chromosomes. In cases where cross-fertilization has occurred, "embryo rescue" may be used to remove hybrid embryos from the ovules and culture them on artificial media.
Pollen mother-cells in the anthers of some species have been treated with colchicine, to generate nuclei with double the haploid chromosome number, thus producing diploid plants that are genetically-identical to the haploid pollen. The use of colchicine to induce polyploidy in dividing vegetative cells first became popular in the 1940s, but tetraploids generated from diploids tend to mask recessive alleles. Generating diploids from haploids doubles all of the existing recessive alleles, and thereby guarantees the expression of the recessive characters of the pollen source.
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