Introduction
Reproduction is essential for continuity of plant species. In flowering plants, the normal mode is sexual, involving meiosis and fertilization. However, in many species, seeds can form without fertilization. This phenomenon is called Apomixis, coined by Winkler (1908) from Greek words "apo" (away) and "mixis" (mingling). It is a type of asexual reproduction through seeds where offspring are genetically identical to the mother plant.
Historical Background
- Early observations of polyembryony were made in Citrus and Mango in the 17th–18th century.
- 1908: Winkler introduced the term Apomixis.
- 20th century: Cytological studies revealed embryos can arise from unreduced gametes or somatic tissues of the ovule.
Types of Apomixis
1. Agamospermy (Seed Apomixis)
Seeds are formed without fertilization. It is classified into:
- Adventive Embryony (Sporophytic Apomixis): Embryos arise directly from somatic cells of nucellus or integuments. Leads to polyembryony.
Examples: Citrus, Mango, Opuntia. - Apospory (Gametophytic Apomixis): Embryo sac develops from a somatic nucellar cell without meiosis. Embryo arises from diploid egg cell without fertilization.
Examples: Hieracium, Pennisetum, Ranunculus. - Diplospory (Gametophytic Apomixis): The megaspore mother cell does not undergo meiosis but develops into a diploid embryo sac. Embryo arises from unreduced egg cell.
Examples: Taraxacum, Rubus, Erigeron.
2. Vegetative Apomixis
Vegetative structures such as bulbils, tubers, or plantlets replace flowers or seeds.
Examples: Onion (bulbils), Agave, Kalanchoe (leaf buds).
3. Classification Based on Origin
- Sporophytic Apomixis: Embryo originates from somatic tissues (nucellus/integruments).
- Gametophytic Apomixis: Embryo arises from unreduced embryo sac (apospory/diplospory).
Mechanism of Apomixis
- Bypasses meiosis, so the embryo sac remains diploid.
- Fertilization is unnecessary; embryos may arise from diploid egg cells or somatic tissues.
- Offspring are genetically identical to the mother plant.
Examples of Apomictic Plants
- Adventive embryony: Citrus, Mango, Opuntia
- Apospory: Pennisetum, Hieracium
- Diplospory: Taraxacum, Rubus
- Vegetative apomixis: Agave, Kalanchoe, Onion
Significance of Apomixis
- Fixation of hybrid vigour (heterosis) for indefinite generations.
- Clonal propagation via seeds; elite genotypes can be preserved.
- Low-cost seed production; farmers can reuse seeds without yield loss.
- Preservation of rare and desirable genotypes.
- Useful tool in genetics and reproductive biology research.
Limitations of Apomixis
- Lack of genetic variation limits adaptability.
- Occurs naturally only in few species.
- Controlled by complex genetics, making transfer to crops difficult.
- Breeding limitations since it fixes but does not create variability.
Applications in Plant Breeding
- Revolution in hybrid seed industry – hybrids can be maintained without repeated crossing.
- Conservation of elite lines (high-yielding, stress-tolerant genotypes).
- Cost-effective technology for farmers.
- Ensures uniformity in crops, contributing to food security.
Differences Between Apomixis and Sexual Reproduction
| Feature | Sexual Reproduction | Apomixis |
|---|---|---|
| Meiosis | Present | Absent/Modified |
| Fertilization | Essential | Not required |
| Genetic Variation | High | Absent (offspring identical) |
| Seed Origin | From zygote | From somatic tissue or unreduced gamete |
| Breeding Use | Creates variability | Fixes superior genotypes |
| Examples | Wheat, Rice, Maize | Citrus, Taraxacum, Mango |
Future Prospects of Apomixis
- Introduction into cereals (rice, wheat, maize) could revolutionize seed industry.
- Reduces seed cost and stabilizes high-yielding lines.
- Enhances food security and climate resilience.
- Biotechnology and gene editing approaches are being explored to engineer apomixis in major crops.
Conclusion
Apomixis is an asexual mode of reproduction through seeds where fertilization and meiosis are bypassed. It offers immense potential in agriculture by fixing hybrid vigour, lowering seed cost, and conserving elite genotypes. Although its natural occurrence is limited, future biotechnological interventions may help introduce apomixis into major food crops, transforming modern plant breeding and ensuring global food security.
