Question
A plant breeder finds that certain citrus varieties produce multiple seedlings from a single seed, all genetically identical to the parent. Explain the phenomena of polyembryony and apomixis, their types, and why they matter in agriculture.
Classification of Asexual Seed Production
flowchart TD
A["Seed formation without normal sexual reproduction"] --> B["Apomixis"]
A --> C["Polyembryony"]
B --> D["Recurrent Apomixis"]
B --> E["Non-recurrent Apomixis"]
D --> F["Diplospory: embryo sac from megaspore mother cell without meiosis"]
D --> G["Apospory: embryo sac from nucellus/integument cell"]
C --> H["Cleavage of zygote"]
C --> I["From synergids/antipodals"]
C --> J["From nucellus cells — adventive embryony"]
J --> K["Most common in Citrus"]Solution — Step by Step
Apomixis is the formation of seeds without fertilisation. The embryo develops from the egg cell or some other cell of the ovule, bypassing the normal sexual process. The resulting offspring are genetically identical to the mother plant — essentially clones packaged in seeds.
Two main types:
- Recurrent apomixis — the embryo sac forms from a diploid cell (no meiosis), so the egg is already 2n. It develops into an embryo without fertilisation. Subtypes include diplospory and apospory.
- Non-recurrent apomixis — the embryo sac forms normally (haploid egg), and the haploid egg develops without fertilisation. The offspring is haploid — this is rare and usually non-viable.
Polyembryony is the occurrence of more than one embryo in a single seed. This can happen through:
- Cleavage polyembryony — the zygote splits into multiple embryos (like identical twins)
- Adventive polyembryony — extra embryos develop from nucellus or integument cells surrounding the embryo sac. These are diploid and genetically identical to the parent.
In Citrus, adventive embryony is very common. When you plant a Citrus seed, you often get multiple seedlings, most of which are nucellar (clones of the mother) and one may be the actual sexual embryo.
Why do plant breeders care?
- Hybrid vigour preservation: Normally, hybrid seeds lose their vigour in the next generation (F2 segregation). Apomixis allows hybrids to produce seeds that are genetically identical — maintaining hybrid vigour indefinitely.
- Uniform crop stands: Apomictic seeds produce genetically identical plants, giving uniform fields.
- Cost reduction: Farmers would not need to buy new hybrid seeds every season.
- Citrus rootstock: Nucellar embryos from polyembryony give virus-free, true-to-type rootstocks used commercially.
Why This Works
Both apomixis and polyembryony bypass normal meiosis-fertilisation in different ways, producing offspring that are maternal clones. The agricultural value lies in capturing the best genotype in a seed that reproduces it faithfully. If apomixis could be engineered into major crops like rice and wheat, it would be one of the biggest breakthroughs in agriculture — because every farmer could save hybrid seeds.
For NEET, remember this chain: Apomixis = seeds without fertilisation, Polyembryony = multiple embryos per seed. They can overlap (apomictic embryos alongside the sexual embryo), but they are conceptually different. Citrus is the go-to example for both.
Alternative Method
Think of it by comparing the source of each embryo:
| Source | Type | Ploidy | Genetic Identity |
|---|---|---|---|
| Normal zygote | Sexual embryo | 2n | Hybrid of parents |
| Nucellus cell | Adventive embryo | 2n | Clone of mother |
| Synergid/antipodal | Polyembryony variant | n or 2n | Varies |
| Unreduced egg (no meiosis) | Apomixis | 2n | Clone of mother |
Common Mistake
Students often equate apomixis with vegetative propagation. They are different. Apomixis produces seeds (involves the ovule and embryo sac), while vegetative propagation produces new plants from stems, roots, or leaves — no seeds involved. NEET specifically tests whether you know apomixis is a form of asexual reproduction through seeds.