Backcross Method in Plant Breeding

Introduction

A backcross involves crossing a hybrid (F₁) with one of its parent plants. When the F₁ is crossed with a homozygous recessive parent, it is specifically called a test cross. Backcross breeding is a systematic approach where repeated backcrosses are performed to introduce a specific trait into a well-adapted variety that lacks that trait.

Main Features

This breeding method is widely used for both self-pollinating and cross-pollinating species. Its application in vegetatively propagated crops like sugarcane and potato is less common and typically requires modifications.

1. Application

The primary use of the backcross method is to enhance a specific characteristic—such as disease resistance—in an otherwise excellent variety. It is most effective for traits controlled by one or a few genes (monogenic or oligogenic) rather than many genes (polygenic), as these tend to have higher heritability. The method is applicable to self-pollinated, cross-pollinated, and asexually propagated crops.

2. Parental Material

Two types of parents are involved:

• Recurrent Parent: The high-quality, adapted variety that is to be improved. It is used repeatedly in backcrosses.
• Donor Parent: The source of the desirable trait. This parent is typically agronomically inferior and is used only once in the initial cross.

3. Genetic Constitution

The resulting new variety is genetically nearly identical to the original recurrent parent, differing only in the newly introduced trait.

4. Number of Backcrosses

Typically, 5 to 6 backcross generations are sufficient to recover the recurrent parent's genome while incorporating the new trait.

5. Basic Requirements

To initiate a backcross program, three elements are essential: a recurrent parent, a donor parent, and a trait with high heritability.

Genetic Basis of Backcrossing

Backcrossing increases the frequency of desirable genotypes more efficiently than selfing for specific genes. For example, while selfing an F₁ (Aa) produces only 1/4 homozygous dominant (AA) plants in the F₂, backcrossing to the recessive parent (aa) yields 1/2 heterozygous (Aa) plants in each backcross generation, which can be selected for the dominant trait.

The population progressively becomes more like the recurrent parent. The rate of achieving homozygosity is similar to selfing. A key advantage is the increased opportunity to break linkages between desirable and undesirable genes through recombination over successive backcrosses.

Breeding Procedure

The strategy differs depending on whether the target gene is dominant or recessive.

1. Transfer of a Dominant Gene

If a trait like wilt resistance in cotton is controlled by a dominant gene (R), the procedure is straightforward. The F₁ (Rr) is backcrossed to the susceptible recurrent parent (rr). In each backcross generation, resistant plants (Rr) are identified (e.g., by growing in a diseased field) and used for the next backcross. After several backcrosses (e.g., 6), the resistant plants are self-pollinated to produce homozygous (RR) true-breeding lines.

2. Transfer of a Recessive Gene

If the desirable trait is recessive (e.g., rr for resistance), plants carrying the gene cannot be directly identified after a backcross because both heterozygous (Rr) and homozygous dominant (RR) plants look the same. Therefore, after each backcross, the population must be selfed to produce a segregating generation where the homozygous recessive (rr) individuals can be identified and selected for the next backcross cycle.

3. Transfer of Quantitative (Polygenic) Traits

While challenging due to low heritability and environmental influence, backcrossing can be used for polygenic traits. Success requires selecting a donor parent with an extreme phenotype for the trait (e.g., very high protein content), using larger population sizes, and often incorporating selection during selfing generations after backcrosses to achieve the desired genetic combination.

To transfer multiple traits, breeders can either conduct separate backcross programs and later combine the genes or attempt simultaneous transfer, which requires even larger populations to recover a genotype with all desired genes.

Merits and Demerits

Merits

• Preserves the superior genetic background of an adapted variety while adding a specific improvement.
• Highly effective for transferring simply-inherited traits like disease resistance or quality factors.
• Used to develop multi-disease resistant varieties, isogenic lines, and multiline varieties (mixtures of isogenic lines).
• Essential for transferring male sterility/fertility restorer genes and for interspecific gene transfer.
• The new variety requires less extensive testing as it is genetically similar to an already proven variety.

Demerits

• Only improves one or a few specific defects in an existing variety.
• Labor-intensive, requiring numerous crosses over many generations.
• There is a risk of transferring tightly linked undesirable genes from the donor parent along with the desirable gene ("linkage drag").

Achievements

The backcross method has been successfully employed to develop disease-resistant varieties in crops like wheat and cotton. For instance, in cotton, varieties such as V797, Digvijay, Vijalpa, and Kalyan (Gossypium herbaceum) have been developed using this technique. It is a cornerstone method for interspecific hybridization and the creation of multiline varieties.