Lesson Media Objects
Backcross Breeding 2 - The Backcrossing Process
Identifying Transgenic Plants
How do breeders determine which plants express ECB resistance? The obvious option is to infest the plants with ECB and observe. However, this can be expensive and time consuming, especially in early stages of plant breeding when many lines are being tested. A quicker option is to utilize the selectable marker trait. If a selectable marker gene was inserted during transformation it has likely been passed on together with the ECB resistance trait.
A herbicide selectable marker can be used by spraying the plants with the herbicide. It can be assumed those that die do not have the herbicide resistance gene, and most likely do not have the Bt gene, (or other transgene), either. In rare cases, the selectable marker is not passed on with the other transgene. This happens when the two genes insert in different chromosomes. As a result, some progeny with the selectable marker gene may not have the transgene of interest.
Because the marker gene may be transmitted to the progeny independently of the transgene, it is desirable to have a way to test directly for the transgene in the plant. This can be done in two ways. The first is by testing for the presence of the protein encoded by the transgene in the plant tissues using an ELISA test (Enzyme-Linked-ImmunoSorbent-Assay). This test can be quickly done in the field using a kit and a small sample of tissue and will give a simple positive or negative response. The second method tests for the presence of the transgene itself. This laboratory method, called PCR (Polymerase Chain Reaction), is much more time consuming and expensive. However, it can be used to detect the presence of a transgene in tissues that are not expressing the gene. This may be critical if the event being tested is not expressed in all tissues or only at certain times of the year.
A fourth option sometimes used by major companies is to utilize genetic fingerprinting. This technique can be used to not only identify the presence of the transgene, but find which plants, through the natural variation in breeding, have obtained a greater percentage of the elite inbred genes. This can potentially shorten the number of generations required for backcrossing. Athough very advantageous, this technique is also very time consuming and expensive, and could not be performed on a large number of lines. All of these detection methods are important tools that help the plant breeder identify the plants they should be making crosses with.
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