APOMIXIS
Definition
Apomixis is a type of asexual reproduction in which seeds are produced without fertilisation. The embryo develops without the fusion of male and female gametes.
Standard Definition
Apomixis is the formation of seeds without fertilisation, thereby bypassing meiosis and syngamy.
It is commonly found in some members of the Asteraceae and Poaceae (Grasses) families.
Etymology
Apo = Away from
Mixis = Mixing
Therefore,
Apomixis = Reproduction without the fusion of gametes.
Why is Apomixis called Asexual Reproduction?
In normal sexual reproduction:
Meiosis occurs.
Gametes are formed.
Fertilisation occurs.
Zygote develops into embryo.
In apomixis:
Fertilisation does not occur.
Embryo develops directly.
Offspring are genetically similar to the parent.
Thus,
Apomixis is an asexual mode of reproduction that mimics sexual reproduction.
Characteristics of Apomixis
Seed formation occurs without fertilisation.
Male gamete is not required.
Female gamete may or may not undergo meiosis.
Embryo develops directly.
Offspring are genetically identical or nearly identical to the mother plant.
Helps preserve desirable characters.
Plants Showing Apomixis
Species of Asteraceae
Grasses (Poaceae)
Commercially important examples include:
Citrus
Mango
Dandelion
Kentucky bluegrass
Types (Mechanisms) of Apomixis
1. Development from Diploid Egg Cell
Process
The egg cell does not undergo meiosis.
A diploid (2n) egg is formed.
Fertilisation does not occur.
The diploid egg develops directly into an embryo.
Flow Chart
Mother plant (2n)
↓
No meiosis
↓
Diploid egg (2n)
↓
No fertilisation
↓
Embryo (2n)
↓
Seed
Features
No reduction division.
No syngamy.
Embryo is maternal in origin.
Genetically identical to the mother.
2. Adventive Embryony (Nucellar Embryony)
This is the most important mechanism for examinations.
Process
Nucellar cells surrounding the embryo sac begin dividing.
These cells protrude into the embryo sac.
They develop into embryos.
Fertilisation may or may not occur.
Several embryos are produced.
Examples
Citrus
Mango
Diagrammatic Flow
Nucellus
↓
Nucellar cells divide
↓
Cells enter embryo sac
↓
Each develops into embryo
↓
One seed contains many embryos
POLYEMBRYONY
Definition
Polyembryony is the occurrence of more than one embryo inside a single seed.
Standard Definition
Polyembryony is the phenomenon in which two or more embryos develop within a single seed.
Causes of Polyembryony
Division of zygote
Development from nucellar cells
Development from synergids
Development from antipodals
Development from nucellar cells (adventive embryony).
Examples
Orange
Lemon
Citrus
Mango
Observation
When orange seeds are squeezed,
Several embryos of different sizes are observed.
Each seed may contain:
2 embryos
5 embryos
10 embryos
Even more
Difference between Apomixis and Polyembryony
| Apomixis | Polyembryony |
|---|---|
| Seed forms without fertilisation | More than one embryo forms in one seed |
| Type of reproduction | Developmental phenomenon |
| Produces clonal embryos | Produces multiple embryos |
| Example: Asteraceae | Example: Citrus |
Are Apomictic Embryos Clones?
Yes.
Since
no meiosis,
no recombination,
no fertilisation,
the embryos carry the same genetic constitution as the mother plant.
Therefore,
Apomictic embryos are clones of the parent plant.
Genetic Nature of Apomictic Embryos
Diploid
Maternal origin
No genetic recombination
Genetically identical
Homozygosity maintained
What is Parthenocarpy?
This is a favourite examination question.
Definition
Parthenocarpy is the development of fruits without fertilisation.
Difference
| Apomixis | Parthenocarpy |
|---|---|
| Seed formation without fertilisation | Fruit formation without fertilisation |
| Seeds produced | Usually seedless fruits |
Hybrid Seeds
What is a Hybrid?
A hybrid is produced by crossing two genetically different parents possessing desirable traits.
Advantages
High yield
Disease resistance
Better quality
Uniform growth
Hybrid vigour (Heterosis)
Problem with Hybrid Seeds
This is extremely important.
Farmers cannot use hybrid seeds repeatedly because:
Hybrid plants produce seeds.
Next generation undergoes segregation.
Hybrid vigour disappears.
Characters are not maintained.
Therefore,
Hybrid seeds have to be produced every year.
Why are Hybrid Seeds Expensive?
Because
Controlled pollination
Emasculation
Bagging
Skilled labour
Annual production
increase production costs.
Role of Apomixis in Hybrid Seed Industry
This is one of the most important NEET questions.
If hybrid plants are converted into apomicts,
then
No segregation occurs.
Hybrid vigour remains unchanged.
Farmers can reuse seeds.
No need to buy seeds every year.
Cost of cultivation decreases.
Productivity remains high.
Importance of Apomixis
Agricultural Importance
Preserves hybrid vigour.
Fixes desirable genes.
Prevents segregation.
Reduces seed production cost.
Produces uniform crops.
Helps farmers save money.
Scientific Importance
Maintains pure lines.
Conserves elite genotypes.
Useful in plant breeding.
Used in crop improvement.
Active area of genetic research.
Advantages of Apomixis
Rapid multiplication
No fertilisation required
No pollinators needed
Uniform progeny
Maintains superior characters
Saves hybrid seed cost
Useful in commercial agriculture
Disadvantages
Little genetic variation
Reduced adaptability
No evolution through recombination
Disease susceptibility if genotype is affected
Comparison: Sexual Reproduction vs Apomixis
| Character | Sexual Reproduction | Apomixis |
|---|---|---|
| Meiosis | Present | Absent or bypassed |
| Fertilisation | Present | Absent |
| Genetic variation | Present | Absent |
| Recombination | Present | Absent |
| Offspring | Variable | Identical |
| Seed formation | After fertilisation | Without fertilisation |
Keywords (Must Remember)
Apomixis
Asexual reproduction
Mimics sexual reproduction
Diploid egg
No reduction division
No fertilisation
Nucellar embryony
Adventive embryony
Polyembryony
Citrus
Mango
Hybrid seeds
Segregation
Hybrid vigour
Clones
Plant breeding
Frequently Asked High-Level Questions with Answers
1. Define apomixis.
Answer: Apomixis is the formation of seeds without fertilisation. It is an asexual mode of reproduction that bypasses meiosis and syngamy.
2. Why is apomixis called asexual reproduction?
Answer: Because embryos develop without fertilisation, resulting in offspring genetically identical to the mother plant.
3. Explain two mechanisms of apomixis.
Answer:
(i) Diploid egg pathway: A diploid egg forms without meiosis and develops into an embryo without fertilisation.
(ii) Adventive embryony: Nucellar cells divide and develop into embryos inside the embryo sac, as seen in Citrus and Mango.
4. Define polyembryony with examples.
Answer: Polyembryony is the occurrence of more than one embryo in a single seed. Examples: Citrus, Mango, Orange.
5. Why are apomictic embryos called clones?
Answer: They develop without meiosis and fertilisation, so they are genetically identical to the maternal plant.
6. Differentiate between apomixis and parthenocarpy.
Answer: Apomixis is seed formation without fertilisation, whereas parthenocarpy is fruit formation without fertilisation.
7. Why do hybrid seeds have to be produced every year?
Answer: Seeds collected from hybrids undergo segregation in the next generation, leading to the loss of hybrid vigour and desirable traits.
8. Explain the importance of apomixis in agriculture.
Answer: Apomixis maintains hybrid vigour, prevents segregation, reduces seed costs, ensures uniform crops, and allows farmers to reuse hybrid seeds.
9. Explain nucellar embryony.
Answer: Nucellar cells surrounding the embryo sac divide, enter the embryo sac, and develop into embryos. This results in polyembryony.
10. What are the advantages and limitations of apomixis?
Answer:
Advantages: Maintains superior traits, preserves hybrid vigour, reduces seed cost, produces uniform progeny.
Limitations: Lacks genetic variation, reduces adaptability, and may increase susceptibility to diseases.
Multiple Choice Questions (MCQs)
1. Apomixis is:
A. Vegetative propagation
B. Formation of seeds without fertilisation
C. Formation of fruits without pollination
D. Double fertilisation
Answer: B
2. Apomixis is a form of:
A. Sexual reproduction
B. Asexual reproduction
C. Vegetative propagation only
D. Tissue culture
Answer: B
3. Polyembryony means:
A. Many seeds in one fruit
B. Many embryos in one seed
C. Many flowers in one inflorescence
D. Many ovules in one ovary
Answer: B
4. Which plant commonly shows nucellar embryony?
A. Wheat
B. Rice
C. Citrus
D. Pea
Answer: C
5. Apomictic embryos are genetically:
A. Different from the mother plant
B. Identical to the mother plant
C. Haploid
D. Polyploid
Answer: B
6. Fruit development without fertilisation is called:
A. Polyembryony
B. Apomixis
C. Parthenocarpy
D. Syngamy
Answer: C
7. In nucellar embryony, embryos arise from:
A. Egg cell
B. Synergid
C. Nucellar cells
D. Antipodal cells
Answer: C
8. Hybrid seeds cannot be reused because of:
A. Polyploidy
B. Segregation of characters in the progeny
C. Mutation
D. Self-incompatibility
Answer: B
9. Which process is bypassed in apomixis?
A. Pollination only
B. Meiosis and fertilisation
C. Fruit development only
D. Seed dispersal
Answer: B
10. The major significance of apomixis in agriculture is:
A. Production of seedless fruits
B. Maintenance of hybrid vigour across generations
C. Increased pollination efficiency
D. Prevention of flowering
Answer: B
Quick Revision Points (Exam Booster)
Apomixis = Seed formation without fertilisation.
Parthenocarpy = Fruit formation without fertilisation.
Polyembryony = More than one embryo in a single seed.
Apomictic embryos are clones of the mother plant.
Citrus and Mango commonly exhibit nucellar embryony.
Hybrid seeds lose their desirable traits in the next generation due to segregation.
Incorporating apomixis into hybrid crops can preserve hybrid vigour (heterosis) and allow farmers to reuse seeds, reducing production costs.
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