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The procedures for selective breeding of food crops have come a long way since the beginning of agriculture, but the “big idea” is the same: to improve crops by selecting for desirable traits, thereby changing the genes responsible for these traits in a stable, heritable way. All cultivated plants were domesticated from wild relatives. Just as each person has a unique genotype (except identical twins), so does each wild plant for a particular species.

What sustains this genetic diversity? For sexually reproducing species, recombination, or crossing over, during meiosis is very important. Since the location(s) of DNA exchange between homologous chromosomes is unpredictable, each gamete contributes a unique genotype to the next generation. New gene variants resulting in altered protein activity can also be a result of rare, spontaneous, naturally occurring DNA mutations in gametes. Most of the time DNA damage is repaired during cell division. Rarely, damage is incorrectly repaired and the result is a change, or mutation, in the nucleotide sequence of the DNA. Gene mutations are usually detrimental to the organism, but sometimes they confer a survival advantage and persist through natural selection. For a conventional crop breeder, a new, agriculturally desirable trait is the starting point for artificial selection, or selective breeding.

Imagine a farmer starting with a field of wild soybean plants. Each plant would produce genetically unique gametes, pollen and ovules, and reproduce by pollination and seed production. Recombination during meiosis and spontaneous mutations guarantee that every so often, a soybean plant will grow that is agriculturally superior to all the others because of a genetic change. For this discussion, let’s imagine that two superior plants arise spontaneously. The first produces twice as many soybean pods as all the other plants and the second produces better beans inside the pods. The farmer wants to incorporate the high yield (HY) and high quality (HQ) traits into his crop. How can he do this, using conventional breeding techniques?

Some plants grown from the seeds of the original HY and HQ plants will have the same desirable trait. Generations of HY and HQ plants can be artificially self-pollinated to increase the frequency of each trait. If the farmer repeatedly selects HY and HQ plants and seeds over many generations of artificial self-pollination, he will end up with two stable genetically modified soybean varieties. When all alleles in the selectively bred genomes code for the new HY or HQ trait (homozygous for HY or HQ), then all progeny will have the trait and these pure lines are said to “breed true” (Figure 1).

What happens if the farmer wants to combine these desirable traits to create a crop with both high yield and high quality? One option is to create an F1 hybrid, where F1 stands for “first filial generation”. The two pure lines, HY and HQ, are artificially cross-pollinated to generate seeds that are HY+HQ. The plants grown from these seeds are the F1 hybrids, and they should have the combined traits of both parents (Figure 2). Hybrid seeds are used extensively in modern agriculture, especially for corn and rice, and can be used to combine multiple desirable traits. However, they have a few limitations. Pure parental lines take time to breed, sometimes as long as seven or eight years. They must also be continuously maintained and cross-pollinated to generate the F1 hybrid seeds, and the farmer must purchase new hybrid seeds from a crop breeder every year.

Another option to combine desirable traits found in different lines is backcrossing. In this technique, a trait from a donor parent line is stably incorporated into a recipient parent line so that the final line retains most of the genome of the recipient parent. Let’s imagine that our soybean farmer chooses his HY line as the donor and his HQ line and the recipient. He starts by cross-pollinating the two lines and selecting progeny with both the HY and HQ phenotype. These F1 hybrids are then backcrossed (mated) to the recipient parent HQ line and HY+HQ progeny are chosen again. Over many generations of backcrossing (often several years), the crop breeder will have a line in which the HY trait has been incorporated into the HQ line, with very few other genes from the HY donor (Figure 3). The process of gene incorporation is called introgression.

In all the examples so far, it is assumed that the desired trait arises spontaneously. For most of agriculture’s history, selective breeding capitalized on these very rare events and incorporated new traits by phenotypic selection, all without knowledge of the underlying genetic mechanisms.

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Krysta Levac

After an undergraduate degree at the University of Guelph, I earned a PhD in nutritional biochemistry from Cornell University in 2001. I spent 7 years as a post-doctoral fellow and research associate in stem cell biology at Robarts Research Institute at Western University in London, ON. I currently enjoy science writing, Let's Talk Science outreach, and volunteering at my son's school. I love sharing my passion for science with others, especially children and youth. I am also a bookworm, a yogi, a quilter, a Lego builder and an occasional "ninja spy" with my son.

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