THE LAST ONE HUNDRED YEARS – WHEN CONVENTIONAL CROP BREEDING MEETS BIOTECHNOLOGY
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In the 1920’s, scientists discovered that they could increase the frequency of mutations, and therefore new traits, by treating crop plants with radiation or certain chemicals. Even though the mechanisms of mutagenesis were unknown for decades, mutation breeding was very successful in creating new valuable traits for introgression into existing lines. Crop breeders now know that different mutagens result in different kinds of mutations. For example, gamma radiation often causes DNA double strand breaks and chemical mutagens often cause nucleotide excision or miss-pairing. If the damage is not repaired correctly, the resulting induced mutation may result in a desirable crop trait. Knowing the mechanism of damage and repair can guide a plant breeder’s choice of mutagen. Mutation breeding has generated more than 3000 different plant varieties from approximately 170 different species, including wheat, rice, barley, soybeans, potatoes and grapefruit. Since 1987, scientists in China have been using cosmic radiation for mutation breeding by sending seeds into space on satellites or spacecraft.
Another valuable tool in modern crop breeding is marker-assisted selection (MAS). Before MAS, backcrossing and introgression relied on phenotypic screening to identify plants with the desired trait, but this approach can be slow, difficult and inefficient. For example, if the desired trait is not obvious until late in the growing season, such as increased frost resistance, then many plants must be grown for the full season to select the best ones for each backcross generation. If the desired trait is environment-dependent, such as drought tolerance or insect resistance, it may be very difficult to ensure the right conditions for screening each crop generation. Marker-assisted selection is basically a molecular shortcut.
DNA markers are short nucleotide sequences that are easy to identify in the laboratory. If a marker is located on the same chromosome and close to (or even within) the desired gene, it can be used as a “flag” to indicate the presence of the gene without screening for the associated phenotype. When a marker is located close enough to a gene that they are almost always inherited together, this is called genetic linkage. A short nucleotide distance between linked genes or markers means it is unlikely that meiotic recombination will separate them. DNA markers are commonly detected by polymerase chain reaction (PCR), which is quick, easy, and suitable for high-throughput screening by using automated PCR machines that can analyze thousands of individual samples at once. Only a very small tissue sample is required for DNA marker screening, and taking a tiny sample doesn’t damage the plant, so seedlings can be screened at an early stage in each backcross generation to identify the plants worth growing (See Figure 1).
DNA markers can also make it easier to pyramid genes, or combine several traits together in a single genotype. Tomatoes are susceptible to many different pathogens, such as viruses, bacteria, fungi and nematodes. Several DNA markers associated with naturally occurring resistance genes have been identified in the tomato genome. Marker-assisted pyramiding of multiple resistance genes has made it easier to breed hardier tomatoes.
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