Model Answer
0 min readIntroduction
Interspecific hybridization, the crossing of individuals from different species, is a powerful tool in plant breeding, offering the potential to combine desirable traits from divergent genetic backgrounds. However, reproductive isolation mechanisms often prevent successful hybridization. These barriers can be pre-zygotic (preventing gamete formation or fusion) or post-zygotic (resulting in inviable or sterile hybrids). Consequently, various techniques have been developed to circumvent these incompatibilities and raise viable hybrids, contributing significantly to crop improvement and understanding plant evolution. This answer will detail these mechanisms, categorizing them for clarity.
Overcoming Pre-Zygotic Barriers
Pre-zygotic barriers prevent the formation of a zygote. Techniques to overcome these focus on facilitating pollination and fertilization.
- Pollination Techniques:
- Hand Pollination: The most basic technique, involving manual transfer of pollen from the desired male parent to the stigma of the female parent. Effective when natural pollination is limited or incompatible.
- Assisted Pollination: Utilizing insects or other vectors to transfer pollen, particularly useful for wind-pollinated species.
- Removal of Incompatibility Factors: Some species exhibit self-incompatibility or gametophytic/sporophytic incompatibility. Removing anthers before pollen shedding (emasculation) and then pollinating with desired pollen avoids selfing and allows for cross-pollination.
- Breaking Self-Incompatibility:
- Chemical Treatments: Application of certain chemicals (e.g., gamma-aminobutyric acid - GABA) can temporarily overcome self-incompatibility in some species.
- Mutations: Identifying and utilizing recessive mutations that disrupt the self-incompatibility system.
- In Vitro Pollination: Pollen grains are germinated *in vitro* and then transferred to the stigma, bypassing some of the barriers related to pollen tube growth.
Overcoming Post-Zygotic Barriers
Post-zygotic barriers occur after zygote formation, leading to hybrid inviability, sterility, or breakdown. These are more challenging to overcome.
- Embryo Rescue: A crucial technique where the hybrid embryo is excised from the endosperm at an early stage of development (often before it becomes inviable due to endosperm-embryo incompatibility) and cultured *in vitro* on a suitable nutrient medium. This allows the embryo to develop into a plantlet.
- Protoplast Fusion:
- Isolation of Protoplasts: Plant cells are treated with enzymes (cellulase, pectinase) to remove the cell wall, yielding protoplasts.
- Fusion Induction: Protoplasts from different species are fused using polyethylene glycol (PEG) or electrofusion.
- Selection of Hybrid Cells: Hybrid cells are selected based on complementation or marker genes.
- Regeneration of Plants: Hybrid cells are cultured to regenerate whole plants through callus formation and subsequent differentiation.
- Chromosome Doubling (Allopolyploidy): Hybrid plants often exhibit chromosome imbalance, leading to sterility. Treating these plants with colchicine, a chemical that inhibits microtubule formation during cell division, can induce chromosome doubling, resulting in a stable, fertile allopolyploid.
- Grafting: While not creating a hybrid *per se*, grafting can combine the rootstock of one species with the scion of another, allowing for the expression of traits from both. This is useful when incompatibility affects vegetative growth.
Specific Techniques & Examples
The choice of technique depends on the specific species involved and the nature of the incompatibility.
| Technique | Species Example | Barrier Overcome |
|---|---|---|
| Embryo Rescue | Rice x Wild Rice (Oryza sativa x Oryza nivara) | Endosperm-embryo incompatibility |
| Protoplast Fusion | Potato x Tomato (Solanum tuberosum x Solanum lycopersicum) | Reproductive isolation, creating ‘Pomato’ |
| Chromosome Doubling | Wheat (Triticum aestivum) – an allohexaploid | Genome imbalance in early hybrids |
| Hand Pollination & Emasculation | Various Orchid species | Self-incompatibility and preventing unwanted pollination |
Conclusion
Raising hybrids between incompatible species is a complex endeavor requiring a nuanced understanding of reproductive barriers and the application of sophisticated techniques. While pre-zygotic barriers can be overcome with relatively straightforward methods like hand pollination, post-zygotic barriers necessitate more advanced approaches such as embryo rescue, protoplast fusion, and chromosome doubling. These techniques have been instrumental in broadening the genetic base of crops, creating novel plant varieties, and furthering our knowledge of plant evolution and reproductive biology. Continued research in this area promises to unlock even greater potential for crop improvement and adaptation to changing environmental conditions.
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.