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INTRODUCTION TO PLANT BREEDING

[BACK CROSS METHOD]

The backcross (bc) method is a form of recurrent hybridization by which a superior characteristic may be added to an otherwise desirable variety.

In this method the breeder has considerable control of the genetic variation in the segregating population in which selections are to be made.

The bc method has been extensively used for transferring qualitative characters, such as disease resistance, in both self and cross pollinated crop species.

Please click here to view a FLASH animation of backcross breeding.

Procedure:

DONOR PARENT                         RECURRENT PARENT Original cross                  Rust Resistant        X                Adapted Variety A                                     Variety                             rr (susceptible) RR (resistant) F1               X Variety A    1st backcross                                      Rr       rr                                                       BC1                 X Variety A    2nd backcross (Select rr)                       Rr : rr       rr                                                      BC2                    X Variety A    3rd backcross                                   Rr : rr         rr                                                      BC3                 X Variety A    4th backcross                                   Rr : rr      rr                                                      BC4               X Variety A

Rr : rr

Self Rr plants from BC to obtain plants homozygous for RR. Progeny testing would be needed identify RR from Rr plants.

1RR : 2Rr : 1rr

If the genes for rust resistance had been recessive (i.e., rr = resistant), then the introduced gene is only carried in the heterozygote and would not be detected. One would have to self in order to identify the resistant rr plants and then backcross these to the recurrent parent. (SELF AND ON — in crops.)

Allard suggests advancing the 1st backcross to the F2 generation with selection for the desirable character from the donor and the general features of the recurrent parent. The 2nd and 3rd bc's

are then made in succession after which the inbreeding with selection phase is repeated. This is followed by the 4th, 5th and 6th bc's in succession, with an F2 and F3 being grown after the 6th bc with intense selection for both the desired character and the recurrent parent. See Allard p. 156-157, for further description and rational for this approach.

GENETIC BASIS FOR THE BACKCROSS METHOD

In the F1 all loci differing in alleles between the parents will be heterozygous.

With backcrossing homozygosity for the recurrent alleles will increase at the same rate as the approach to homozygosity with inbreeding.

i.e., at a rate of where n = number of bc generations, and x = numbers of loci.

Aa self : F2 Aa X aa bc1

1/4AA : 1/2Aa : 1/4aa1/2Aa : 1/2aa

The amount of remaining genetic information, on the average, from the nonrecurrent (donor) parent is reduced by 50% with each bc.

i.e., at a rate of (1/2);n+1where n = number of bc.

However, the rate at which genes entering a hybrid from the nonrecurrent parent are eliminated during backcrossing will be influenced by linkage.

e.g., If b (undesirable allele) is linked to A, and selection is only for A, b tends to be brought along in the F1. By reintroducing A each bc, a number of opportunities for crossing over occur.

Probability of eliminating b = 1 - (1-p)nwhere, n = number of bc's.

(See Fehr p. 371, Table 28-2. ERROR)and p = recombination frequency.

Recombination frequency is determined by the map distance and factors such as centromere location and chromosome structural abnormalities that reduce crossover events.

GENERAL CONSIDERATIONS

• Backcrossing is most easily conducted if the character being added is. ..

Simply inherited (although the bc approach may be applied to quantitative traits); Dominant; Easily recognized in the hybrid plant. • Requirements for a successful bc program:

A satisfactory recurrent parent.

Selecting the recurrent parent — varieties or inbred lines that have maintained their importance for many years despite the release of numerous 'improved' varieties intended to replace them. It must be possible to retain a worthwhile intensity of the character under transfer through several backcrosses.

High heritability of the character being transferred is important. Transfer is easiest when the character can be identified readily in hybrid populations by visual selection or by simple tests. There has been some success in dealing with quantitative traits through backcrossing; however, like all other methods it depends on the ability of the breeder to distinguish between genetic and environmental variability, and to select those individuals that are desirable for genetic reasons. The general agricultural worth of the donor parent need not be of great concern. Selection of the donor, nonrecurrent parent is almost exclusively on the basis that it exhibits the character in a particularly intense form. This is particularly important since often the presence of modifier genes in the new genetic background cause some intensity to be lost even though the most stringent selection has been practiced throughout the backcrossing program.

Sufficient backcrosses must be used to reconstitute the recurrent parent to a high degree.

Recovery of the recurrent parent is a function of the number of backcrosses and the effectiveness of selection for recurrent parent type in the early generations. Usually 6 backcrosses with selection for type in the early generations has proved sufficient. However, in wide crosses and/or with undesirable linkages a greater number of backcrosses may be necessary. Remember that in procedure the objective is to improve the recurrent parent by one trait. • Influence of environmental conditions on a backcross program.

Provided the expression of the character being transferred is sufficient for selection, backcrossing can be conducted in any environment. e.g., Several generations may be grown per year in the greenhouse.

Also, since the recurrent parent is already a proven variety or line, it is not necessary to conduct extensive performance trials once satisfactory introduction of the desired character has been achieved.

IMPROVEMENT OF MORE THAN ONE CHARACTER

It is possible to introduce several characters in the course of a backcross program. Usually the other characters being added have already been introduced into the recurrent parent in other backcross programs; e.g., the development of rust and bunt resistant Bart 38 wheat.

•Introgression of germplasm through backcrossing

In interspecific crosses it has been found that cytological stability can be improved by backcrossing. e.g., in oats (Ladizinsky & Fainsten. C. J. Genet. Cytol. 19:59-66).

cultivated hexaploid oats X tetraploid wild oat species

pentaploid F1self sterile

Through backcrossing meiotic stability was obtained relatively quickly.

Properly executed, backcross breeding programs allow all the desirable characteristics of the recurrent parent to be recovered, except for the possibility that characters governed by genes tightly linked with the gene(s) being transferred will be modified inadvertently. This may be considered a strength. . . The backcross method provides a certain and precise way of making gains of predictable value with little possibility that uncontrolled segregation will produce subtle weaknesses which may be difficult to discover in a finite period of evaluation.

or a weakness. . . The method sets an upper limit that will often be lower than the progress that is possible when segregation is not rigidly controlled.

Currently, backcrossing is often used to develop improved gene pools or selection populations without trying to return completely to the recurrent parent's phenotype. In some cases, the breeder has good phenotype and wants to improve it without having a clearly defined trait (for example, yield). In order to meet this objective, the breeder can use different numbers of backcrosses (1 to 6) to add as much or as little of the recurrent parent as he/she thinks is necessary. A backcross one population (A x (A x B)) is preferred by many breeders to a simple three-way cross (A x (B x C), may have too much variation).