Model Answer
0 min readIntroduction
Agamospermy, derived from the Greek words ‘a’ (without), ‘gamos’ (marriage), and ‘spermy’ (seed), refers to the development of seeds without fertilization. It’s a form of asexual reproduction in plants, resulting in offspring genetically identical to the mother plant. This phenomenon is a component of the broader process of apomixis, which encompasses various mechanisms of asexual reproduction through seeds or other reproductive structures. The increasing interest in apomixis stems from its potential to revolutionize crop breeding by fixing hybrid vigor and enabling the clonal propagation of superior genotypes.
Types of Agamospermy
Agamospermy can be broadly classified into three main types:
1. Sporophytic Agamospermy
In this type, the embryo develops directly from a nucellar cell surrounding the embryo sac, without the formation of a zygote. The nucellar cells proliferate and form an embryo. This is commonly observed in citrus (e.g., oranges) and mango. The resulting embryo is diploid and genetically identical to the maternal plant.
2. Gametophytic Agamospermy
This is the most common type of agamospermy. Here, an unfertilized egg cell develops into an embryo. This can occur in two ways:
- Diplospory: The embryo sac develops from a megaspore mother cell without meiosis, resulting in an unreduced embryo sac with the same chromosome number as the parent plant. This is found in species of Poa and Hieracium.
- Apospory: The embryo sac develops from a nucellar cell directly, bypassing meiosis and fertilization. This is observed in Ranunculus and Haworthia.
In both diplospory and apospory, the resulting embryo is genetically identical to the maternal plant.
3. Adventitious Embryony
In this type, embryos develop directly from somatic cells of the nucellus or integuments, independent of the embryo sac. Multiple embryos can develop within a single seed (polyembryony). This is commonly seen in citrus and Allium species. Each embryo is genetically identical to the maternal plant.
Apomixis as a Tool of Practical Importance
Apomixis, encompassing agamospermy, is now considered a valuable tool in plant breeding due to several reasons:
- Fixing Hybrid Vigor: Hybrid vigor (heterosis) is often lost in subsequent generations due to segregation. Apomixis allows the maintenance of superior hybrid combinations indefinitely by producing seeds that are genetically identical to the hybrid parent.
- Clonal Propagation through Seeds: Apomixis enables the propagation of desirable genotypes through seeds, eliminating the need for labor-intensive vegetative propagation methods like grafting or cuttings.
- Rapid Multiplication of Elite Lines: Apomixis facilitates the rapid multiplication of genetically superior plants, accelerating the breeding process.
- Bypassing Sexual Reproduction Barriers: Apomixis can overcome barriers to sexual reproduction, such as self-incompatibility or sterility, allowing for the propagation of otherwise difficult-to-breed plants.
- Cost-Effectiveness: Seed propagation is generally more cost-effective than vegetative propagation, making apomixis a potentially valuable tool for resource-poor farmers.
Research efforts are focused on transferring apomixis genes into important crop species like wheat, rice, and maize. While achieving complete apomixis in these crops is challenging, significant progress has been made in understanding the genetic and molecular mechanisms underlying apomixis, paving the way for its potential application in crop improvement.
| Type of Agamospermy | Origin of Embryo | Chromosome Number | Examples |
|---|---|---|---|
| Sporophytic | Nucellar cell | Diploid | Citrus, Mango |
| Gametophytic (Diplospory) | Unreduced embryo sac from megaspore mother cell | Diploid | Poa, Hieracium |
| Gametophytic (Apospory) | Nucellar cell | Diploid | Ranunculus, Haworthia |
| Adventitious | Nucellus or Integuments | Diploid | Citrus, Allium |
Conclusion
In conclusion, agamospermy, a key component of apomixis, manifests in diverse forms – sporophytic, gametophytic, and adventitious – each characterized by the development of seeds without fertilization. The growing recognition of apomixis as a practical tool stems from its ability to fix hybrid vigor, facilitate clonal propagation through seeds, and accelerate crop breeding programs. Continued research into the genetic basis of apomixis holds immense promise for enhancing crop productivity and ensuring food security in the future.
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.