Introduction - The Backcrossing Process
If the genetic engineer is successful in obtaining a transgenic plant that expresses the gene and passes it on to subsequent progeny in a normal, predictable fashion, s/he still does not have a product for the farmer. The event must be moved into an elite agronomic background. This is done by following the backcross breeding scheme outlined in this lesson. Why is it necessary to take the time and effort to develop a backcross line with the transgene? There are several reasons.
- In some crop species, the lines that can be put through tissue culture and will regenerate new plants are often genotypes that have poor agronomic characteristics.
- In all crop species, the transformation and tissue culture procedures are so labor intensive, time consuming, and expensive that they cannot be used to generate transgenic versions of large numbers of lines. Plant breeders use dozens of lines in their crossing programs each year and the lines change from year to year. Genetic engineers cannot keep pace with this cycle. Instead they choose a few lines to transform and then breed the transgene into the others.
- The tissue culture process can induce mutations in important genes. Thus even if the transformed line is an elite line, it may have some genetic changes that are not desirable. Backcrossing and selection can eliminate the deleterious genes.
Plant breeding is the final step of making a marketable transgenic line. Image by P. Hain
Consequently, plant breeding programs use backcrossing to incorporate transgenics into their hybrid or variety development. Many companies have taken advantage of genetic fingerprinting technology and year-round nurseries to maximize the efficiency and speed of backcross line development. Once the genetic engineer gives the transgenic plant to the plant breeder, it takes the breeder at least two or three years to derive a backcross line that is the genetic equivalent of the elite line plus the event. After that point the plant breeder can work with the genetically engineered line in the same manner they work with other parents in their breeding program.