Backcross Method – Procedure, Applications, Advantages, Disadvantages

What is Backcross Method?

• The backcross method is a breeding technique used to transfer specific traits from one plant variety to another while preserving the desirable characteristics of the recipient variety. This method involves crossing a hybrid or progeny from a segregating generation with one of its original parents. The primary objective of this technique is to improve one or two specific traits of a high-yielding variety that is already well adapted to its environment.
• In practice, the backcross method begins with a cross between a high-yielding, well-adapted variety, referred to as the recipient parent, and a donor parent that possesses the desired trait, such as disease resistance. The F1 hybrid, resulting from this initial cross, is then repeatedly backcrossed to the recipient parent over several generations. Each subsequent generation of progeny is tested for the presence of the desired trait while retaining the characteristics of the recipient parent.
• Typically, after 6 to 8 backcrosses, the progeny will closely resemble the recipient parent, except for the trait introduced from the donor parent. This trait transfer is achieved without altering the overall genotype of the recipient variety significantly, except for the new genes being incorporated. For example, if the objective is to introduce rust resistance into a variety like Malviya 12, the rust-resistant variety (donor parent) is crossed with Malviya 12, and the F1 hybrids are continually backcrossed to Malviya 12. Through selection, plants with the desired rust resistance are identified and self-pollinated to produce homozygous progeny.
• The result of a successful backcross program is a variety that closely resembles the original recipient parent in all aspects except for the improved traits, such as enhanced disease resistance. The recipient parent is thus known as the recurrent parent due to its repeated use in the breeding program, whereas the donor parent is referred to as the non-recurrent parent because its role is limited to the initial hybridization. This method allows for the efficient enhancement of specific traits while maintaining the overall genetic stability of the recipient variety.

Definition of Backcross Method

The backcross method is a breeding technique where a hybrid or progeny is repeatedly crossed with one of its original parent plants to transfer specific traits from the donor parent to the recipient parent while preserving the recipient’s original characteristics.

Requirements of a backcross programme

To effectively develop a new variety using the backcross method, several key requirements must be met:

1. Availability of a Suitable Recurrent Parent: The recurrent parent must be a well-adapted variety that lacks one or two specific traits. This parent is crucial because it provides the baseline genetic background into which new traits will be introduced.
2. Selection of an Appropriate Donor Parent: The donor parent should possess the desired trait(s) to be transferred, and these traits must be present in a highly intense form. The quality and strength of the traits in the donor parent are essential for successful integration into the recurrent parent.
3. High Heritability of the Desired Trait: The trait to be transferred must exhibit high heritability, ideally controlled by one or a few genes. High heritability ensures that the trait is consistently passed on through generations, making it easier to incorporate into the recurrent parent.
4. Adequate Number of Backcrosses: A sufficient number of backcrosses is necessary to recover the genotype of the recurrent parent fully. Typically, 6 to 7 backcrosses are required to achieve this goal, ensuring that the new variety closely resembles the recurrent parent except for the introduced traits.

Genetic consequences of repeated backcrossing

The genetic consequences of repeated backcrossing can be summarized as follows:

1. Increase in Homozygosity: Repeated backcrossing leads to a rapid increase in homozygosity, similar to the effects of selfing. For a single gene Aa, backcrossing the F1 hybrid (Aa) to the recurrent parent (AA) results in a 1:1 ratio of homozygotes (AA) to heterozygotes (Aa). With each successive backcross, the proportion of heterozygotes decreases by 50%, aligning with the patterns observed in selfing.
2. Decrease in Proportion of Heterozygotes: As backcrossing continues, the proportion of heterozygotes in the progeny decreases. This decrease occurs at a rate comparable to that seen in selfing, which means that the frequency of homozygotes increases progressively over successive backcross generations.
3. Effect on Multiple Genes: When multiple genes are involved, the proportion of homozygotes for all genes in a backcross generation follows a formula similar to that used in selfing, specifically (2^n – 1)/2^n, where nnn represents the number of segregating genes. This formula indicates that the rate of increase in homozygosity across all genes is consistent with that of selfing, but with backcrossing, the process can be more efficient in achieving a high frequency of homozygous desirable genotypes.
4. Rapid Replacement of Non-recurrent Parent Genes: With repeated backcrossing, genes from the non-recurrent parent are rapidly replaced by those from the recurrent parent. After approximately 6 to 7 backcrosses, more than 98% of the plants in the progeny will have the genotype of the recurrent parent. Therefore, the genotype of the backcross progeny becomes increasingly similar to that of the recurrent parent.
5. Frequency of Desirable Homozygotes: The frequency of desirable homozygotes, representing the recurrent parent type, increases rapidly with each backcross. For instance, in the sixth backcross progeny, a significant majority of plants will exhibit the recurrent parent genotype. This trend reflects the efficient replacement of non-recurrent parent genes with recurrent parent genes.
6. Selection and Maintenance of Traits: During backcrossing, it is crucial to maintain the gene of interest through selection. Without selection, the gene being transferred may be replaced by its allele from the recurrent parent. Consequently, genes linked to the transferred gene might also remain in a heterozygous state. Selection helps in preserving the desired trait while ensuring that the genotype of the backcross progeny closely resembles that of the recurrent parent.
7. Crossing Over and Linkage: The process of backcrossing provides opportunities for crossing over between the gene being transferred and genes tightly linked to it. This crossing over can lead to the formation of recombinant types. However, tightly linked genes may remain associated for several backcross generations, making it challenging to separate them.

Selection of parents

The selection of parents is a critical step in the backcross breeding method. The following points outline the essential considerations for choosing suitable parents:

1. Objective of Backcrossing: The backcross method aims to alter the genotype of the recurrent parent only for the gene of interest. This process focuses on removing specific defects from the recurrent parent while preserving its overall genotype. Therefore, the selection of parents must be aligned with this objective to ensure that only the intended traits are modified.
2. Selection of the Recurrent Parent: The recurrent parent should be a well-adapted variety that exhibits desirable characteristics, such as high yield and adaptability. However, it is important that this parent has one or two specific defects that need improvement, such as susceptibility to disease or undesirable seed traits. The recurrent parent’s primary role is to provide the genetic background into which the desired trait will be introduced.
3. Role of the Non-recurrent Parent: The non-recurrent parent is selected based on the specific trait that needs enhancement. The primary criterion for choosing this parent is that it must possess the desired trait in an intense form. For example, if improving disease resistance is the goal, the non-recurrent parent should exhibit a high level of resistance. Other characteristics of the non-recurrent parent, such as yield or general adaptability, are less critical in this context.
4. Intensity of the Trait: The trait being transferred from the non-recurrent parent should be present in a more intense form than what is desired in the recurrent parent. This consideration is important because some degree of intensity is often lost during the transfer process and the subsequent integration into the recurrent parent’s genetic background. Therefore, selecting a non-recurrent parent with superior trait intensity helps to ensure that the final progeny retains an adequate level of the desired trait.
5. Managing Linkage Effects: It is essential to recognize that genes tightly linked with the gene being transferred may also influence the resulting traits. While the primary focus is on the gene under transfer, unintended changes in other linked characteristics may occur. Therefore, the selection process should also account for these potential changes to mitigate any negative effects on the recurrent parent’s overall quality.

The procedure of backcross method

The backcross method is a systematic approach used to introduce a specific trait from a donor variety into a recurrent parent variety, while maintaining the overall genetic identity of the recurrent parent. The procedure varies depending on whether the trait being transferred is dominant or recessive. The following outlines the detailed steps for both scenarios.

Transfer of a Dominant Gene

The transfer of a dominant gene from a donor variety to a recurrent parent involves a series of systematic steps designed to introduce the desired trait while preserving the genetic integrity of the recurrent parent. This method is particularly effective when the trait of interest, such as disease resistance, is dominant. The following outlines the detailed procedure for the transfer of a dominant gene:

1. Hybridization:
   • Objective: Initiate the transfer of the dominant trait.
   • Procedure: Cross the recurrent parent variety A (which is susceptible to a condition such as stem rust) with the donor variety B (which has resistance to the condition). Typically, variety A is used as the female parent in this cross to facilitate the identification of selfed plants later in the process.
2. F1 Generation:
   • Objective: Establish initial hybrids and start the backcrossing process.
   • Procedure: Backcross the F1 hybrids (which are heterozygous for the dominant trait) to the recurrent parent variety A. Since the trait is dominant, all F1 plants will exhibit the trait, making the selection for this trait unnecessary at this stage.
3. First Backcross Generation (BC1):
   • Objective: Begin selection for the desired trait and further backcrossing.
   • Procedure: In the BC1 generation, plants will segregate into those exhibiting the dominant trait (rust-resistant) and those that do not. Select the rust-resistant plants and backcross them to variety A. Additionally, select BC1 plants that resemble variety A in overall plant type.
4. Subsequent Backcross Generations (BC2, BC3, etc.):
   • Objective: Continue selection and integration of the dominant trait into the recurrent parent’s genetic background.
   • Procedure: In each backcross generation, segregation will occur for the dominant trait. Continue selecting rust-resistant plants and backcrossing them to the recurrent parent variety A. Selection for plant type, similar to variety A, should be practiced, especially in the BC2 and BC3 generations. By the BC6 generation, approximately 98.4% of the genes will be from variety A.
5. BC7 Generation:
   • Objective: Develop a new variety with the desired dominant trait.
   • Procedure: Self-pollinate selected plants from the BC6 generation. Grow individual plant progenies from these selfed seeds and select those that are rust-resistant and similar in type to variety A. Harvest the seeds from these selected plants separately.
6. BC8 Generation:
   • Objective: Finalize the new variety and prepare for performance testing.
   • Procedure: Grow progenies from the selfed seeds and select those that are homozygous for the rust resistance trait and similar to the plant type of variety A. Bulk harvest the seeds from these similar, rust-resistant progenies to constitute the new variety.
7. Yield Testing:
   • Objective: Evaluate the performance of the new variety.
   • Procedure: Conduct replicated yield trials comparing the new variety with the original variety A. Assess traits such as plant type, flowering date, maturity, and overall quality. If the new variety performs similarly to variety A in these aspects, it may be considered for release.

Transfer of a Recessive Gene

Transferring a recessive gene requires a meticulous and iterative approach due to the nature of recessive inheritance, where the trait is expressed only in homozygous recessive individuals. The following outlines the systematic procedure for transferring a recessive gene, such as rust resistance, from a donor variety to a recurrent parent:

1. Hybridization:
   • Objective: Initiate the gene transfer process.
   • Procedure: Cross the recurrent parent variety A (which is susceptible to rust) with the donor variety B (which carries the recessive gene for rust resistance). Typically, variety A is used as the female parent to simplify the subsequent selection process.
2. F1 Generation:
   • Objective: Produce initial hybrids and prepare for backcrossing.
   • Procedure: Backcross the F1 hybrids (heterozygous for the recessive gene) to the recurrent parent variety A. As rust resistance is recessive, the F1 plants will not exhibit the trait, making it necessary to evaluate subsequent generations for the presence of the recessive gene.
3. BC1 Generation:
   • Objective: Start the selection process for the recessive trait.
   • Procedure: Self-pollinate all plants in the BC1 generation. Since rust resistance is recessive, all plants will be rust-susceptible, and thus, testing for rust resistance is not feasible at this stage.
4. BC2 F2 Generation:
   • Objective: Identify and select rust-resistant plants.
   • Procedure: Inoculate plants from the BC2 F2 generation with rust spores. Select plants that exhibit resistance and backcross these resistant plants to the recurrent parent variety A. During this stage, also select for traits resembling variety A in terms of plant type and other characteristics.
5. BC3 Generation:
   • Objective: Continue selection and integration of the recessive gene.
   • Procedure: Since there is no test for rust resistance, select plants based on their resemblance to the recurrent parent variety A. Backcross these plants with variety A to maintain the genetic background.
6. BC4 Generation:
   • Objective: Further refine the plant characteristics.
   • Procedure: Self-pollinate the plants to raise the F2 generation. Select plants based on their resemblance to variety A in terms of plant type. Continue to backcross with the recurrent parent.
7. BC5 Generation:
   • Objective: Continue the selection process and validate rust resistance.
   • Procedure: Inoculate plants with rust spores and select those showing resistance, while ensuring they resemble variety A in other characteristics. Backcross these selected plants to variety A.
8. BC6 Generation:
   • Objective: Finalize the gene transfer and develop a new variety.
   • Procedure: There is no test for rust resistance. Self-pollinate the plants to raise the F2 generation. Harvest seeds from rust-resistant plants that closely resemble variety A.
9. BC7 Generation:
   • Objective: Establish the new variety with the recessive trait.
   • Procedure: Grow individual plant progenies and expose them to a rust epidemic. Perform rigorous selection for both rust resistance and similarity to variety A. Bulk the seeds from selected, homogeneous progenies to create the new variety.
10. Yield Testing:
    • Objective: Evaluate the performance of the new variety.
    • Procedure: Conduct yield trials comparing the new variety with the original variety A. Assess traits such as plant type, flowering date, maturity, and overall quality. If the new variety matches the performance of variety A and shows the desired rust resistance, it may be considered for release.

Modifications of the Backcross Method

The backcross method is a versatile tool in plant breeding, allowing for the transfer of specific traits while maintaining the genetic integrity of the recurrent parent. To enhance the effectiveness of this method, several modifications have been developed. Below are three common modifications to the backcross method:

1. Production of F1 and F2 Generations

• Process:
  • After the initial backcross, F1 and F2 generations are produced following the first and third backcrosses, respectively. This strategy involves rigorous selection for both the trait being transferred and the characteristics of the recurrent parent in these generations.
  • In the backcross progenies, selection is not conducted for the trait or recurrent parent characteristics, focusing instead on the F1 and F2 generations. This method assumes that effective selection in these generations compensates for the absence of selection in subsequent backcross generations.
• Advantages:
  • This approach is believed to be equivalent to performing one or two additional backcrosses, thereby enhancing the efficiency of trait transfer. It can be used for transferring both dominant and recessive genes.
  • For the sixth backcross, a larger number of plants is utilized to ensure sufficient representation and selection.

2. Use of Different Recurrent Parents

• Process:
  • Multiple varieties with desirable quantitative traits may be employed as recurrent parents within the same backcross program. Each variety is used as a recurrent parent for one or two backcrosses.
  • This approach aims to combine advantageous genes from each recurrent parent with those from the non-recurrent parent to produce a new variety with a broad genetic base.
• Examples:
  • Sugarcane Breeding: Noble canes (S. officinarum) were crossed with Indian canes (S. barberi) and subsequently backcrossed to various noble cane varieties to develop commercially viable sugarcane cultivars.
  • Apple Scab Resistance: Scab resistance genes were transferred to apples using multiple recurrent parents.
  • Tomato Vitamin C Content: High vitamin C content was transferred from wild tomatoes (Lycopersicon peruvianum) to cultivated tomatoes (L. esculentum) using this approach.

3. Backcross-Pedigree Method

• Process:
  • This method involves performing one to two backcrosses of the hybrid to the recurrent parent, followed by handling the backcross progeny according to the pedigree method.
  • It is particularly useful when the recurrent parent is superior in multiple traits, but the non-recurrent parent is agronomically acceptable. The goal is to retain the superior traits of the recurrent parent while integrating the desired characteristics from the non-recurrent parent.
• Advantages:
  • The initial backcrosses ensure that the new variety inherits a majority of beneficial genes from the superior parent while maintaining sufficient heterozygosity to allow for the appearance of transgressive segregants.
  • Varieties developed through this method undergo yield trials similar to those developed by the pedigree method to ensure their effectiveness.

Merits of the Backcross Method

The backcross method is a strategic breeding technique used to introduce specific traits into a recurrent parent while preserving the parent’s overall genotype. The following outlines the key advantages of this method:

1. Minimal Genetic Alteration:
   • Advantage: The new variety generated through the backcross method maintains a genotype nearly identical to that of the recurrent parent, except for the introduced traits.
   • Implication: This predictability allows for the anticipation of outcomes and ensures that the process can be replicated in future breeding programs.
2. Reduced Need for Extensive Testing:
   • Advantage: The performance characteristics of the recurrent parent are well-established, reducing the necessity for extensive yield trials.
   • Implication: This can save up to five years and significantly cut costs, as the major traits of the new variety are already known.
3. Less Dependency on Environmental Conditions:
   • Advantage: The backcross method’s reliance on environmental conditions is minimal, focusing mainly on the selection of the specific trait being transferred.
   • Implication: This flexibility allows the use of off-season nurseries and greenhouses to grow multiple generations annually, thereby accelerating the breeding process.
4. Smaller Population Requirements:
   • Advantage: The backcross method requires smaller populations compared to other breeding methods, such as the pedigree method.
   • Implication: This efficiency in population size contributes to cost reduction and simplifies the management of breeding programs.
5. Retention of Performance and Adaptability:
   • Advantage: Defects such as disease susceptibility in well-adapted varieties can be corrected without compromising their performance and adaptability.
   • Implication: This approach is beneficial to farmers and industries who prefer improved versions of familiar varieties rather than completely new varieties.
6. Facilitation of Interspecific Gene Transfers:
   • Advantage: The backcross method is particularly effective for transferring genes between different species.
   • Implication: This capability expands the potential for genetic improvement across species boundaries, enhancing the diversity and adaptability of crops.
7. Potential for Transgressive Segregation:
   • Advantage: The method can be adapted to allow for transgressive segregation, where offspring exhibit traits that exceed those of either parent for quantitative characters.
   • Implication: This adaptation can lead to new varieties with enhanced characteristics beyond the original parent’s performance.

Demerits of the Backcross Method

While the backcross method offers several advantages in plant breeding, it also presents certain limitations and challenges. The following outlines the primary demerits associated with this technique:

1. Limited Improvement Beyond Transferred Trait:
   • Issue: The new variety produced through the backcross method generally cannot surpass the recurrent parent in traits other than the one being transferred.
   • Implication: This limitation restricts the potential for overall enhancement of the variety, confining improvements to the specific trait introduced from the nonrecurrent parent.
2. Transmission of Undesirable Genes:
   • Issue: Genes that are closely linked with the target trait in the nonrecurrent parent may be inadvertently transferred along with the desirable trait.
   • Implication: This can lead to the incorporation of undesirable characteristics into the new variety, complicating the selection process and potentially negating some benefits of the desired trait.
3. Repetitive Hybridization Requirements:
   • Issue: Each backcross generation requires a new hybridization event, which can be both time-consuming and expensive.
   • Implication: The need for repeated crosses adds to the overall cost and duration of the breeding program, making it less efficient compared to methods that may require fewer hybridization steps.
4. Obsolescence of Recurrent Parent:
   • Issue: By the time the backcross program completes its cycle, the recurrent parent variety may be outdated, having been surpassed by newer varieties with better yield and other desirable characteristics.
   • Implication: This can render the new variety less competitive or relevant, as it may not incorporate the latest advancements in breeding technology or agronomic practices.

Applications of backcross method

The backcross method has various applications in plant breeding, extending beyond simple trait transfers to more complex objectives. Here are the primary applications:

1. Transfer of Disease Resistance: The backcross method is extensively used to transfer disease resistance from one variety to another. This technique is highly effective for traits governed by a single or few major genes, such as resistance to specific pathogens. For instance, disease resistance genes from a donor parent are introduced into a recurrent parent, enhancing its resilience while preserving its other desirable traits.
2. Intervarietal Transfer of Quantitative Traits: Besides simple traits, the backcross method can also be applied to quantitative characters that are influenced by multiple genes. Traits such as plant height, seed size, and yield can be transferred if they exhibit high heritability. The success of such transfers depends on the heritability of the traits and the genetic background of both parent varieties.
3. Interspecific Transfer of Traits: This method is also used to transfer traits between related species, often involving the introduction of disease resistance from wild relatives to cultivated species. For example, resistance to diseases like black shank and rust has been transferred from wild species to cultivated crops such as tobacco and wheat. However, interspecific transfers can be challenging due to differences in chromosome structure and function, which may lead to the co-transfer of undesirable linked genes.
4. Transfer of Cytoplasm: In addition to nuclear genes, the backcross method can transfer cytoplasmic genes, which is particularly useful for traits related to cytoplasmic male sterility. For example, transferring cytoplasm from Triticum timopheevii to Triticum aestivum has created male-sterile lines that are useful in hybrid seed production. This process involves crossing the recurrent parent with the cytoplasmic donor and performing multiple backcrosses to achieve the desired cytoplasmic composition while retaining the nuclear genotype of the recurrent parent.
5. Production of Transgressive Segregants: The backcross method can be modified to produce transgressive segregants—progeny that exhibit traits beyond the range of the parents. This can be achieved by limiting the number of backcrosses or by using multiple recurrent parents to accumulate beneficial genes. Such modifications can lead to the development of new varieties with improved or novel trait combinations.
6. Development of Isogenic Lines: Isogenic lines, which are genetically identical except for a single gene or a small set of genes, are produced using the backcross method. These lines are useful for studying the effects of individual genes on traits such as yield and disease resistance, providing insights into gene function and interactions.
7. Germplasm Conversion: The backcross method can also be employed to convert photoperiod-sensitive germplasm into photoinsensitive forms. This process involves using the photoperiod-sensitive lines as recurrent parents in backcross programs to develop new lines that are adapted to different photoperiods. For example, sorghum land-races from Africa, which are valuable for their disease resistance and grain quality but are photoperiod-sensitive, have been converted into photoinsensitive lines, making them suitable for breeding programs in regions with different light conditions.

Achievements of the Backcross Method

The backcross method has proven to be a highly effective tool in plant breeding, leading to notable achievements in crop improvement. Below are some key accomplishments facilitated by this technique:

1. Gene and Chromosome Transfer Across Varieties and Species:
   • Achievement: The backcross method has been instrumental in transferring genes and chromosomes not only between different varieties but also across related species.
   • Example: A landmark example is the development of ‘Transfer,’ a commercial wheat variety where rust resistance was successfully transferred from a related species, showcasing the method’s utility in enhancing disease resistance through interspecific gene transfer.
2. Development of Cotton Varieties:
   • Achievement: The backcross method was used to develop two significant cotton varieties, 170-Co 2 and 134-Co 2M.
   • Process: The breeding involved crossing Gossypium hirsutum variety Dharwar-American 2-6-5 with Gossypium arboreum variety Gaorani 6. Despite initial sterility and the need for several backcrosses, progenies were successfully selected and improved for staple length and adaptability. These varieties gained widespread cultivation in Gujarat under both irrigated and rainfed conditions.
3. Disease Resistance Transfer:
   • Achievement: The method has been widely used to transfer disease resistance to popular and widely adapted varieties.
   • Examples:
     • Wheat: Rust resistance was successfully transferred to the Kalyan Sona variety from several diverse sources, leading to the development and release of varieties such as KSML 3, MIKS 1, and KML 7405.
     • Bajra (Pearl Millet): Male sterile line Tift 23A, originally susceptible to downy mildew, was backcrossed with resistant lines from India and Africa, resulting in the development of resistant male sterile lines like MS 521, MS 541A, and IMs 570A, now used in producing downy mildew-resistant hybrids.
4. Transfer of Non-Disease Related Traits:
   • Achievement: The backcross method has also been effective in transferring various non-disease related traits, such as improved fiber quality and agronomic characteristics.
   • Examples:
     • Cotton: Varieties like Vijapla, Vijay, Digvijay, and Kalyan were developed using the backcross method. Notably, BD 8, which had high spinning value but low ginning outturn, was backcrossed with Goghari A-26 to improve ginning efficiency. Further improvements led to the development of the Vijay and Digvijay varieties, featuring enhanced fiber length and early maturity.

Comparison Between Backcross and Pedigree Methods

Criteria	Pedigree Method	Backcross Method
Breeding Process	– Fi and subsequent generations are self-pollinated.	– Fi and subsequent generations are backcrossed to the recurrent parent.
Resulting Variety	– New variety differs from parents in agronomic characteristics.	– New variety is nearly identical to the recurrent parent, except for the transferred trait.
Testing Requirements	– Extensive testing required before release.	– Extensive testing often not necessary due to known performance of recurrent parent.
Objective of Improvement	– Aims to improve yielding ability and other characteristics of a well-adapted variety.	– Focuses on transferring specific traits from a donor to a well-adapted recurrent variety.
Type of Traits Improved	– Suitable for both qualitative and quantitative traits, provided they have high heritability.	– Effective for transferring specific traits, especially those with high heritability.
Suitability for Gene Transfer	– Not ideal for gene transfer from related species or for creating addition and substitution lines.	– Only method suitable for gene transfer from related species and for creating addition or substitution lines.
Hybridization Frequency	– Limited to producing the Fi generation.	– Required for each backcross generation.
Population Size	– Fi and subsequent generations are generally larger (20-100 plants per generation).	– Backcross generations are smaller in size.
Gene Transfer Handling	– Procedures for transferring dominant and recessive genes are similar.	– Procedures for transferring dominant and recessive genes differ.