Apomixis: 

Definition: Apomixis is a form of asexual reproduction in plants where seeds are formed without the process of fertilization (fusion of gametes). The term is derived from the Greek words “apo” (away from) and “mixis” (mixing), indicating the absence of sexual reproduction. Apomixis is significant in plant breeding due to its potential to produce genetically uniform offspring and maintain desirable traits over generations.

Types of Apomixis

Apomixis can be broadly categorized into four main types based on the origin and development of the embryo:

1. Parthenogenesis
   • Development of the embryo from an unfertilized egg cell.
   • The egg cell in the embryo sac divides and develops into an embryo without undergoing fertilization.
   • Example: Some species of dandelions.
2. Apogamy
   • Embryo development occurs from other cells within the embryo sac, such as synergids or antipodal cells, instead of the egg cell.
   • Fertilization is bypassed, and these cells directly form the embryo.
3. Apospory
   • The diploid cells outside the embryo sac (like nucellus or integuments) differentiate into a new embryo sac.
   • The embryo then develops directly from these diploid cells without undergoing meiosis or fertilization.
   • Example: Hieracium and Ranunculus.
4. Adventive Embryony
   • The embryo originates from diploid somatic cells of the ovule, such as nucellus or integuments, rather than from the embryo sac.
   • Often leads to polyembryony, where multiple embryos are formed.
   • Example: Citrus, mango.

Classification Based on Mode of Reproduction

1. Obligate Apomixis
   • The only mode of reproduction in some plant species.
   • Sexual reproduction is absent.
   • Example: Certain species of Taraxacum (dandelions).
2. Facultative Apomixis
   • Both apomictic and sexual reproduction occur in the same species.
   • Example: Grasses like Poa pratensis.
3. Recurrent Apomixis
   • Embryo sacs form directly from somatic cells without meiosis.
   • The resulting embryo is diploid.
   • Example: Crepis, Paa (bluegrass).
4. Non-recurrent Apomixis
   • Embryo forms from a haploid egg cell without fertilization.
   • Resulting embryos are haploid and sterile.
   • Rare in nature and mostly of genetic interest.
   • Example: Corn (in research studies).

Significance of Apomixis in Plant Breeding

1. Fixation of Heterosis (Hybrid Vigour) Apomixis allows the perpetuation of hybrid vigor (heterosis) across generations without the need for repeated hybridization. This is especially valuable in hybrid crops like maize and wheat.
2. Genetic Uniformity Apomixis ensures the production of genetically identical progeny, which is crucial for maintaining desirable traits in crop varieties.
3. Rapid Propagation It enables the rapid multiplication of superior genotypes, particularly in horticultural crops like fruits and ornamentals.
4. Maternal Influence Traits linked to the maternal genotype can be perpetuated without interference from paternal genetic material. Beneficial in crops where maternal traits are desirable, such as disease resistance.
5. Cost-Effective Seed Production Apomictic seeds do not require repeated hybridization, reducing the cost and complexity of seed production for farmers.

Applications in Crop Improvement

1. Development of Apomictic Hybrids Researchers aim to incorporate apomixis into economically important crops like maize and rice to stabilize hybrid vigor and improve productivity.
2. Sustainable Agriculture Apomixis can minimize the dependence on hybrid seed production, which is often labor-intensive and costly.
3. Preservation of Germplasm Apomixis can be used to conserve rare or endangered plant species by ensuring the propagation of genetically identical individuals.

Detection and Maintenance of Apomixis

1. Detection Apomixis is identified by examining the development of the embryo sac and embryo using microscopic techniques. Screening large populations is required to identify apomictic plants.
2. Maintenance Once an apomictic plant is identified, its inheritance is studied through controlled breeding experiments. Apomictic plants are maintained by vegetative propagation or self-pollination to ensure the trait is preserved.
3. Challenges Incorporating apomixis into non-apomictic crops is complex due to the instability of apomictic traits and the genetic mechanisms involved.

Research in Apomixis

• CIMMYT Apomixis Project:
  • Launched in 1989, aimed at transferring apomixis into maize using its wild relative Tripsacum.
  • Hybridization and backcrossing techniques are used to integrate apomictic traits into maize.
  • Promising results suggest the potential for creating apomictic maize varieties.

Advantages of Apomixis

1. Conservation of Superior Traits Traits like disease resistance, drought tolerance, and high yield can be preserved over generations.
2. Reduction in Seed Production Costs Farmers can reuse seeds without losing vigor or productivity.
3. Adaptability Apomixis ensures adaptability and stability in changing environmental conditions.