Introduction to Backcross Breeding
With all of the advances in molecular biology, it may seem surprising to find out that traditional plant breeding methods are still needed in the development of plant varieties. Today, the backcross procedure is most often used to move a transgene from a good tissue culture variety that was used in transformation to an elite experimental line or variety. It turns out that for many crops, once the transgene is in the crop species crossing is more efficient than transformation procedures. Backcrossing is more efficient than transforming the elite line because most transformation protocols are optimized for a specific (often poorly adapted and lower yielding) laboratory line. Many elite lines (which are high yielding) are not amenable for transformation. Hence genetic engineers transform their lab line and breeders backcross the transgene from the lab line into the elite line.
In this lesson the focus will be on the backcross method, which is a form of recurrent hybridization (repeated crossing to a single variety; the recurrent parent) where a superior characteristic may be added to an otherwise desirable variety or elite line. In this method the breeder has considerable control of the genetic variation in the segregating population in which selections are to be made. The backcross method has been used extensively for transferring qualitative characters (characters with clear phenotypes that are easy to identify in cross progeny) such as disease resistance. It is effective in both self and cross pollinated crop species. To better understand the applications of backcrossing, the gene for leaf rust resistance in wheat will be used as an example. Figure 1 shows the visible symptoms of leaf rust in susceptible wheat. The left picture leaves are resistant as indicated by a small amount of rust. The plant on the right leaves are covered with rust, indicating it is susceptible.
Figure 2 illustrates how the backcross procedure can be used to move leaf rust resistance (RR, Rr) from one variety to a susceptible variety (rr).
The actual procedure for back crossing is almost self-explanatory. In back crossing you have a donor parent (has a gene of interest) and a recurrent parent (an elite line that could be made better by adding the gene of interest). The donor parent is crossed to the recurrent parent. The progeny of this cross is then crossed to the recurrent parent (it is 'crossed back' to the recurrent parent, hence the term back cross). The progeny of this cross is selected for the trait of interest and then crossed back to the recurrent parent. This process normally is repeated for as many back crosses as are needed to create a line that is the recurrent parent with the gene of interest from the donor parent. The goal of backcrossing is to obtain a line as identical as possible to the recurrent parent with the addition of the gene of interest that has been added through breeding.
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In the end, you want to keep only the individuals homozygous for the resistance gene. To obtain them, self Rr plants from BC4. The resulting offspring will be 1RR : 2Rr : 1rr. Progeny testing would be needed to identify RR from Rr plants. Progeny testing is where the genotype of a parent plant is determined by genotypes of the line’s progeny and often their phenotypic expression. In the case of an RR plant, the progeny will all be RR (no segregation for the gene/trait). However in the case of an Rr plant, the progeny will segregate 1/4 RR : 1/2 Rr : 1/4 rr. Therefore, the progeny of RR plants will be uniformly resistant to leaf rust, while the progeny of Rr plants will segregate for resistance and susceptibility.
In contrast, if the genes for rust resistance had been recessive (i.e., ss = resistant) rather than dominant, then the introduced resistant gene is only carried in the heterozygote and would not be detected throughout the backcross program. After each backcross, one would have to self the heterozygote (Ss) in order to produce resistant plants (ss) in the progeny. These resistant plants (ss) are then backcrossed to the recurrent parent (SS). See Figure 3.
When working with recessive traits, such as this example, Allard (1960) suggests advancing the 1st backcross to the F2 generation followed by selection for the desirable character from the donor parent (ss) and the general features of the recurrent parent. The 2nd and 3rd backcrosses are then made in succession after which the inbreeding with selection phase for ss is repeated. This is followed by the 4th and 5th backcrosses in succession. The BC5F2 that are resistant (ss) are crossed to recurrent parent (SS) for the BC6F1 which is Ss.
The above backcrossing protocols are using phenotypic selection for the trait of interest. With the advent of molecular markers, backcrossing can be facilitated by selecting for a molecular marker closely linked to the gene of interest. The molecular marker is usually dominant or codominant, hence the protocol in Figure 2 is used in marker assisted backcrossing.