Genetic consequences of repeated backcrossing.

What is Backcrossing?

Backcrossing is a breeding technique used to transfer a single or a few genes from a donor plant to an elite recurrent parent. The hybrid progeny resulting from the initial cross is repeatedly crossed with the recurrent parent. Ideally, the donor gene is transferred while the recurrent parent’s genotype is largely preserved. The goal is to create a plant with the desired trait(s) from the donor combined with the superior traits of the recurrent parent.

Genetic Consequences of Repeated Backcrossing

While seemingly straightforward, repeated backcrossing leads to several genetic consequences that breeders must carefully manage:

1. Loss of Desirable Alleles

• Genetic Drift: With each backcross generation, there’s a chance of losing desirable alleles present in the recurrent parent due to random segregation during meiosis. This is particularly concerning for traits controlled by multiple genes.
• Linkage Drag: The desired gene is often linked to undesirable alleles on the same chromosome. Backcrossing transfers these undesirable alleles along with the desired gene, which can negatively impact the overall quality of the new variety.

2. Accumulation of Undesirable Alleles

• Reversion Mutations: Although rare, reversion mutations can occur, bringing back undesirable traits that were initially eliminated.
• Negative Reciprocal Effects: Genes can interact in complex ways. Introducing a gene through backcrossing can unexpectedly affect other traits, leading to undesirable consequences.

3. Reduction in Genetic Vigor

• Heterosis Loss: The initial hybrid vigor (heterosis) observed in the first generation is gradually lost with each backcross generation. This is because the progeny become more and more similar to the recurrent parent, diminishing the contribution from the donor parent.

4. Impact on Molecular Markers

• Marker Distortion: Molecular markers used for selection can become less reliable as the backcrossing process progresses, leading to inaccurate selection and potential loss of genetic diversity.

Mitigation Strategies

• Selection Pressure: Rigorous selection at each backcross generation is critical to eliminate undesirable alleles and retain the recurrent parent’s desirable traits.
• Molecular Markers Assisted Selection (MAS): Using DNA markers linked to desirable and undesirable traits accelerates the process and increases selection accuracy.
• Controlling the Number of Backcrosses: Limiting the number of backcrosses can minimize the loss of genetic diversity and the accumulation of undesirable alleles. Typically, 5-7 backcrosses are performed.

Example: Durum Wheat Breeding

In durum wheat breeding, backcrossing is frequently used to introduce disease resistance genes from wild relatives into elite cultivars. However, breeders must carefully monitor for the introduction of undesirable traits like soft grain quality, which are often linked to the resistance genes.

Case Study: Bt Cotton Development in India

Title: Development of Bt Cotton in India

Description: The introduction of Bt cotton involved backcrossing the Cry genes (responsible for insecticidal properties) from a donor variety into high-yielding Indian cotton varieties. While successful in controlling bollworms, some reports suggest the unintentional introduction of traits impacting fiber quality in certain lines, highlighting the need for careful selection during backcrossing.

Outcome: The initial success of Bt cotton was tempered by the emergence of resistance in some insect populations and concerns about the environmental impact, demonstrating the complexities of genetic modification and the importance of long-term monitoring.