Difference Between Apomixis and Polyembryony

Understanding plant reproductive strategies is crucial in NEET Biology, particularly in the chapter on sexual reproduction in flowering plants. Among the important concepts in this area are apomixis and polyembryony. Although both occur in the context of seed development, they are distinct and serve different evolutionary and biological functions.

1.0What Is Plant Reproduction?

Reproduction is a fundamental process that ensures the continuation of life. In plants, reproduction can occur in two main ways: sexual and asexual reproduction.

• Sexual reproduction involves the fusion of male and female gametes to form a zygote, leading to genetically diverse offspring.
• Asexual reproduction occurs without gametic fusion, producing offspring identical to the parent.
• Two unique reproductive phenomena in flowering plants—apomixis and polyembryony—show how plants can bypass or modify traditional reproductive processes. 
• Though both are related to seed formation, their mechanisms differ. 

What Is Apomixis?

• Apomixis is an asexual method of reproduction that leads to the formation of seeds without fertilisation. 
• In this process, an embryo develops from a cell other than the egg cell in the ovule, such as the nucellus or the integument. As a result, the offspring are genetically identical to the parent plant.
• The term “apomixis” was introduced by Hans Winkler in 1908 to describe seed formation that occurs without fertilisation.

Characteristics of Apomixis

1. No Fertilisation: Seeds form without the union of male and female gametes.
2. Absence of Meiosis: The cells forming the embryo often bypass meiosis, remaining diploid.
3. Genetic Uniformity: Progeny are exact genetic copies (clones) of the parent.
4. Seed Formation: Seeds are produced normally, but via asexual reproduction.
5. Occurrence: Common in families like Asteraceae (sunflowers) and Poaceae (grasses).

Types of Apomixis

Examples of Apomixis

• Citrus – Multiple embryos from nucellar cells.
• Mango – Embryos formed from nucellus (adventive embryony).
• Dandelion – Seed formation without fertilisation.

Biological Significance of Apomixis

• Maintains hybrid vigour and stable traits in crops.
• Ensures seed formation even in the absence of pollination.
• Reduces dependence on male gametes.
• Enables rapid propagation of genetically identical plants.

2.0What Is Polyembryony?

• Polyembryony is the phenomenon where more than one embryo develops within a single seed. 
• This means one ovule gives rise to two or more embryos, which may originate from either fertilised or unfertilised cells.
• Antony van Leeuwenhoek first observed this process in Citrus seeds.

Characteristics of Polyembryony

1. Multiple Embryos: Two or more embryos form in a single seed.
2. Origin of Embryos: They may arise from zygotic division or from somatic (non-reproductive) tissues.
3. Can Be Sexual or Asexual: Some embryos form through fertilisation, others through vegetative means.
4. Common in Certain Plants: Seen in Citrus, Onion, Pinus, and Mango.
5. Seed Viability: Often enhances survival chances because multiple embryos compete for development.

Types of Polyembryony

Examples of Polyembryony

• Citrus – Multiple embryos in a seed from zygotic and nucellar origins.
• Pinus – Several embryos from the cleavage of one zygote.
• Onion – Multiple embryos from zygotic cleavage.

Importance of Polyembryony

• Ensures better survival of seedlings.
• Provides a mixture of genetically identical and genetically diverse offspring.
• Enhances the chances of plant propagation.
• Helps in breeding experiments and understanding plant development.

3.0Difference Between Apomixis and Polyembryony

Though both apomixis and polyembryony involve seed formation and embryo development, they differ in their biological processes and outcomes. The main distinction lies in fertilisation and the number of embryos formed.

• Apomixis is the production of a single, genetically identical seed without fertilisation—a clone of the parent.
• Polyembryony is like having twins or triplets within a single seed, some identical, some genetically distinct.

4.0Applications in Plant Breeding

Both apomixis and polyembryony have significant roles in modern agricultural and botanical research:

Apomixis Applications

• Helps in maintaining hybrid vigour (heterosis) across generations.
• Reduces costs in hybrid seed production.
• Ensures uniformity in crop populations.

Polyembryony Applications

• Used in cloning genetically superior plants.
• Aids in developing disease-resistant and high-yield varieties.
• Assists in genetic studies of embryo competition and development.