Model Answer
0 min readIntroduction
Plant breeding aims to improve the genetic makeup of crops to enhance desirable traits like yield, quality, and resistance to biotic and abiotic stresses. Disease and insect resistance are crucial for sustainable agriculture, reducing crop losses and minimizing pesticide use. The backcross method is a powerful breeding technique used to introduce one or a few desirable genes from a donor plant into a well-adapted elite variety (recurrent parent), while retaining the genetic background of the latter. This method is particularly useful when the donor parent possesses a single dominant gene for the desired trait.
Understanding the Backcross Method
The backcross method is a type of artificial selection used in plant breeding. It involves crossing a hybrid (F1 generation) with one of its parents (usually the recurrent parent) and then selecting for the desired trait in subsequent generations. The process is repeated multiple times to increase the proportion of the recurrent parent’s genome in the offspring, effectively ‘diluting’ the genes from the donor parent except for the desired gene(s).
Steps Involved in Backcross Breeding
- Initial Cross (F1 Generation): The first step involves crossing the elite variety (recurrent parent – A) with the donor parent (B) possessing the desired gene for disease or insect resistance. This produces the F1 hybrid (A x B).
- Backcrossing (BC1 Generation): The F1 hybrid is then crossed back to the recurrent parent (A). This results in the BC1 generation. The BC1 progeny will have approximately 50% of the genes from each parent.
- Selection in BC1 Generation: Individuals in the BC1 generation exhibiting the desired resistance gene are selected.
- Repeated Backcrossing (BC2, BC3… Generations): The selected BC1 plants are again crossed back to the recurrent parent (A), producing the BC2 generation. This process of backcrossing and selection is repeated for several generations (BC3, BC4, and so on).
- Selection and Evaluation: With each backcross, the proportion of genes from the recurrent parent increases, approaching 99%. In later backcross generations (BC4, BC5, etc.), plants are rigorously evaluated for the desired resistance trait and also for agronomic performance, ensuring that the elite characteristics of the recurrent parent are maintained.
- Release of the Variety: After several generations of backcrossing and selection, a plant with the desired resistance and acceptable agronomic traits is identified and released as a new variety.
Genetic Principles
The effectiveness of the backcross method relies on the principles of Mendelian inheritance. If the resistance gene is dominant, it will be easily identifiable in each backcross generation. However, if the resistance gene is recessive, it will be more challenging to identify in the early backcross generations, requiring larger populations and more rigorous selection. The goal is to achieve ‘homozygosity’ for the resistance gene while maintaining the genetic background of the recurrent parent.
Advantages of the Backcross Method
- Retention of Elite Traits: The primary advantage is the preservation of the desirable agronomic traits of the recurrent parent.
- Efficient Gene Transfer: It is an efficient method for transferring one or a few major genes.
- Relatively Quick: Compared to other breeding methods, it can be relatively quick, especially when dealing with dominant genes.
Disadvantages of the Backcross Method
- Linkage Drag: Undesirable genes linked to the resistance gene from the donor parent may also be transferred, a phenomenon known as ‘linkage drag’.
- Difficulty with Recessive Genes: Identifying plants carrying recessive resistance genes can be challenging.
- Loss of Genetic Diversity: Repeated backcrossing can lead to a reduction in genetic diversity in the improved variety.
Examples of Backcross Breeding
- Development of Blast-Resistant Rice: The backcross method has been extensively used to incorporate blast resistance genes (e.g., Pi-ta) from wild rice species into high-yielding rice varieties.
- Wheat Improvement for Rust Resistance: Rust resistance genes have been successfully transferred into wheat varieties using backcrossing. For example, Sr2 gene for stem rust resistance.
- Tomato Breeding for Disease Resistance: Backcrossing has been used to introduce resistance to various diseases like Fusarium wilt and Verticillium wilt in tomato.
| Crop | Resistance Trait | Donor Parent | Recurrent Parent |
|---|---|---|---|
| Rice | Blast Resistance | Wild Rice (Oryza nivara) | High-yielding Rice Variety |
| Wheat | Stem Rust Resistance | Wild Wheat Species | Improved Wheat Variety |
| Tomato | Fusarium Wilt Resistance | Wild Tomato Species | Commercial Tomato Variety |
Conclusion
The backcross method remains a cornerstone of plant breeding, particularly for incorporating single, major genes for disease and insect resistance. While it offers significant advantages in retaining the genetic background of elite varieties, breeders must be mindful of potential drawbacks like linkage drag and loss of genetic diversity. Continued advancements in molecular marker-assisted selection are further enhancing the efficiency and precision of backcross breeding, enabling the development of improved crop varieties that contribute to food security and sustainable agriculture.
Answer Length
This is a comprehensive model answer for learning purposes and may exceed the word limit. In the exam, always adhere to the prescribed word count.