Model Answer
0 min readIntroduction
Flowering plants exhibit remarkable diversity in their reproductive strategies. While sexual reproduction is the dominant mode, asexual reproduction offers advantages like maintaining desirable traits and bypassing the limitations of pollination. Two significant asexual reproduction mechanisms in angiosperms are polyembryony and apomixis. Polyembryony refers to the development of multiple embryos within a single seed, while apomixis encompasses the formation of seeds without prior fertilization. Both phenomena have implications for plant breeding and evolutionary biology, offering potential for clonal propagation and the preservation of superior genotypes.
Polyembryony in Flowering Plants
Polyembryony, literally meaning ‘many embryos’, is the occurrence of more than one embryo in a seed. It is relatively common in several plant families, including Citrus, Mangifera (mango), and Allium (onion). The extra embryos can arise from different sources:
- From the egg cell: This is the most common type, resulting in embryos genetically similar to the mother plant.
- From synergids or antipodal cells: These are accessory cells within the embryo sac, and embryos arising from them are also genetically similar to the mother plant.
- From the nucellus or integuments: Embryos originating from these tissues are genetically different from the mother plant and represent adventive embryos.
In Citrus, polyembryony is particularly prominent, with multiple embryos developing from the nucellar tissue. This leads to a high degree of genetic uniformity among the seedlings, making it valuable for vegetative propagation through seeds. Mango also exhibits polyembryony, though the viability of the extra embryos is often lower.
Apomixis in Flowering Plants
Apomixis is a form of asexual reproduction in plants that results in seeds without the fusion of gametes. It bypasses meiosis and fertilization, leading to the production of seeds that are genetically identical to the mother plant. Several types of apomixis exist:
- Gametophytic apomixis: The embryo develops from an unfertilized egg cell within an unreduced (2n) embryo sac.
- Sporophytic apomixis (Adventitious embryony): The embryo develops directly from somatic cells of the nucellus or integuments, as seen in Citrus and Mango. This is often confused with polyembryony, but differs in origin.
- Diplospory: The embryo sac develops from a megaspore mother cell without meiosis, resulting in an unreduced embryo sac.
- Apospory: The embryo sac develops from a nucellar cell without meiosis.
Poa pratensis (Kentucky bluegrass) is a classic example of a plant exhibiting gametophytic apomixis. Hieracium species also demonstrate apomixis, allowing them to spread rapidly and colonize new areas. The development of apomixis in crops like wheat and maize is a major goal of plant breeders, as it would allow for the fixation of hybrid vigor without the need for repeated crossing.
Polyembryony vs. Apomixis: A Comparative Table
| Feature | Polyembryony | Apomixis |
|---|---|---|
| Definition | Multiple embryos in a seed | Seed formation without fertilization |
| Mechanism | Development from egg, synergids, antipodals, or nucellus | Bypassing meiosis and fertilization; development from unfertilized egg or somatic cells |
| Genetic Similarity | Embryos can be genetically similar or different from the mother plant | Seeds are genetically identical to the mother plant |
| Examples | Citrus, Mango, Onion | Poa pratensis, Hieracium, some Citrus varieties |
| Fertilization Required? | Fertilization may or may not be required depending on the source of the embryo | Fertilization is not required |
Significance: Both polyembryony and apomixis are crucial for plant propagation. Polyembryony, particularly nucellar polyembryony, allows for the clonal propagation of desirable genotypes in citrus. Apomixis, if successfully introduced into crop plants, could revolutionize breeding programs by fixing hybrid vigor and eliminating the need for recurrent hybridization. However, the genetic and molecular mechanisms controlling apomixis are complex, making its transfer to crops challenging.
Conclusion
In conclusion, polyembryony and apomixis represent fascinating adaptations in flowering plants that allow for asexual reproduction via seeds. While polyembryony results in multiple embryos, potentially of varying genetic backgrounds, apomixis ensures the production of genetically identical offspring without fertilization. Both phenomena have significant implications for plant breeding and evolutionary biology, offering opportunities for clonal propagation and the preservation of desirable traits. Further research into the genetic control of apomixis holds immense promise for improving crop yields and stability.
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.