NCERT grounding
Section 1.5 of NCERT Class 12 Biology (Chapter 1: Sexual Reproduction in Flowering Plants) introduces apomixis as "a special mechanism to produce seeds without fertilisation" found in "some species of Asteraceae and grasses." NCERT uses the phrase apomixis is a form of asexual reproduction that mimics sexual reproduction — a formulation that appears nearly verbatim in PYQ answer explanations.
"Although seeds, in general, are the products of fertilisation, a few flowering plants such as some species of Asteraceae and grasses, have evolved a special mechanism, to produce seeds without fertilisation, called apomixis."
— NCERT Class 12 Biology, Section 1.5
NIOS Biology Chapter 19 supplements NCERT by defining apomixis in Greek etymology terms: apo (away from) and mixis (act of mixing) — "away from the act of mixing." It also explicitly lists polyembryony under two causes: adventive polyembryony and cleavage polyembryony. Both NCERT and NIOS agree that apomixis-produced embryos are genetically identical to the mother plant and that Citrus is the canonical example for polyembryony.
Apomixis — the mechanism
Apomixis is defined as the production of seeds without fertilisation. Critically, seeds are formed — so fruits develop normally — but the genetic input of a male gamete is bypassed entirely. This makes apomixis a form of asexual reproduction that produces seeds, genetically identical to the mother plant.
In normal sexual reproduction, the embryo arises from a zygote formed by the fusion of egg (n) and sperm (n), producing a diploid (2n) cell. In apomixis, the embryo bypasses this fusion: it develops from either the egg cell without fertilisation, or from somatic cells of the ovule (nucellus or integuments) that are already diploid. The resulting seed disperses, germinates, and produces a plant that is a clone of the mother.
Fertilisation events in apomixis
Seeds are formed and dispersed, but no male gamete fuses with the egg. The embryo is a clone — not a genetic recombinant. This is why NCERT calls it "asexual reproduction that mimics sexual reproduction."
Types of apomixis
NCERT identifies two main pathways: agamospermy (embryo development from cells of the embryo sac without fertilisation) and adventive embryony (embryo from somatic cells outside the embryo sac). The figure below maps these relationships.
Figure 1. Classification of apomixis routes. Agamospermy involves cells within the embryo sac; adventive embryony uses somatic cells outside it. Both produce viable seeds without fertilisation.
Agamospermy
Agamospermy literally means "seed without gametes." In these cases, the embryo arises from cells of the embryo sac — but fertilisation does not occur. Three sub-types are recognised in advanced botany (NCERT alludes to this pathway without naming all three):
| Sub-type | Origin of embryo | Ploidy of embryo | Meiosis? | Example |
|---|---|---|---|---|
| Parthenogenesis | Egg cell (n) develops without fertilisation | Haploid (n) unless egg bypasses meiosis | Meiosis occurs; egg is haploid | Dandelion (Taraxacum), Parthenium, grasses |
| Apospory | Nucellar or integument cell forms embryo sac — egg is 2n | Diploid (2n) | Meiosis bypassed; embryo sac from somatic cell | Many grasses, Hieracium |
| Diplospory | MMC forms embryo sac without meiosis; 2n egg develops | Diploid (2n) | Meiosis bypassed in MMC | Taraxacum (some species), Ixeris |
Parthenogenesis in botany (NEET 2019 tested this) means the egg cell itself develops into an embryo without fertilisation. The embryo is initially haploid because the egg was produced by meiosis. This is the most-tested definition in NEET — it is distinct from adventive embryony, which does not involve the egg at all.
Adventive embryony
Adventive embryony is the development of one or more embryos directly from cells of the nucellus (the main tissue of the ovule) or the integuments — somatic cells that are naturally diploid (2n). These cells have not undergone meiosis. No egg is involved. NCERT states explicitly: "As in many Citrus and Mango varieties some of the nucellar cells surrounding the embryo sac start dividing, protrude into the embryo sac, and develop into embryos."
Because the source cells are somatic and diploid, adventive embryos are:
- Diploid (2n) — same as the mother plant
- Genetically identical to the mother (clones)
- Disease-free (they carry no pathogen transmitted via the zygote)
In Citrus, adventive embryos develop alongside the zygotic embryo within the same ovule, leading directly to polyembryony.
Polyembryony
Polyembryony is the occurrence of more than one embryo in a single seed. NCERT defines it precisely: "Occurrence of more than one embryo in a seed is referred to as polyembryony." It is most dramatically seen in Citrus, where squeezing a single orange seed reveals multiple embryos of varying sizes.
Causes of polyembryony
Three mechanisms can produce multiple embryos in one seed. Adventive embryony (NCERT-defined) is by far the most common and most tested cause.
Adventive embryony
Most common cause. Nucellar or integument cells (2n) proliferate and form additional embryos inside the same ovule alongside the zygotic embryo.
Examples: Citrus (orange, lemon), Mangifera (some mango varieties), Opuntia
NCERT Section 1.5 — direct textCleavage polyembryony
The zygote splits at an early stage of development, each portion forming a separate embryo — analogous to identical twinning in animals.
Recognised in NIOS as a distinct cause. Rare in flowering plants; more common in gymnosperms and orchids.
NIOS Chapter 19.4.3 — cleavage polyembryonySynergid / antipodal development
Cells of the embryo sac other than the egg — synergids or antipodal cells — occasionally develop into embryos (NIOS refers to this as adventive polyembryony from embryo sac cells).
Very rare; typically produces degenerate accessory embryos.
Rare — distinguish from adventive embryonyA Citrus seed typically contains one zygotic embryo and 2–40 nucellar (adventive) embryos. How is this exploited?
The nucellar embryos are diploid clones of the mother tree — genetically identical, disease-free, and vigorous. Citrus breeders select nucellar seedlings as rootstocks because they breed true and are free of virus infections transmitted through zygotes. The zygotic embryo (from fertilisation) carries recombined genetics — useful for producing new hybrid cultivars. In practice, nucellar embryos are often larger and outcompete the zygotic embryo; the zygotic one is usually smaller. This biology underlies commercial Citrus nursery production worldwide.
Figure 2. Three-way comparison of sexual reproduction, apomixis, and parthenocarpy. The most tested distinction for NEET: apomixis produces seeds (with embryo) but no fertilisation; parthenocarpy produces fruits (no seed, no embryo, no fertilisation). Banana = parthenocarpy; Citrus = apomixis (with polyembryony).
Agricultural significance of apomixis
NCERT devotes an entire paragraph to this application — it is the "so what?" that NEET examiners test at the conceptual level. The argument runs as follows:
The hybrid seed problem — and how apomixis solves it
-
Step 1
F1 hybrid created
Cross-pollination between two inbred lines produces F1 hybrid with maximum heterosis (hybrid vigour). Yield is dramatically higher.
Annual process -
Step 2
F2 segregation problem
If farmers save and re-sow F1 seeds, F2 plants segregate. They lose hybrid vigour. Farmers must buy new F1 seeds every year — expensive.
Major cost -
Step 3
Introduce apomixis
If apomictic genes can be transferred into the hybrid variety, seeds produced will develop without fertilisation — identical to the F1 parent.
Research ongoing -
Step 4
Hybrid vigour perpetuated
Farmers can save and re-use seeds each year. No segregation. Hybrid vigour maintained indefinitely. Cost of seed production falls dramatically.
NEET 2016 tested
NCERT notes: "If these hybrids are made into apomicts, there is no segregation of characters in the hybrid progeny. Then the farmers can keep on using the hybrid seeds to raise new crop year after year and he does not have to buy hybrid seeds every year." This is the key paragraph for NEET MCQs on apomixis significance.
"Apomixis is a form of asexual reproduction that mimics sexual reproduction."
NCERT Class 12 Biology, Section 1.5 — the most cited definition
Worked examples
A plant produces seeds without fertilisation. The embryo develops from the egg cell, which is haploid. What process is occurring, and how does it differ from adventive embryony?
Answer: This is parthenogenesis — a form of agamospermy in which the egg cell (n) develops into an embryo without fusion with a male gamete. The embryo is haploid (n). In adventive embryony, the embryo develops from somatic cells of the nucellus or integument, which are diploid (2n) — not from the egg cell. The embryo in adventive embryony is therefore diploid and genetically identical to the mother plant.
A seed of Citrus contains 12 embryos. A botanist observes that 11 of them are identical to each other and to the mother plant, while one differs genetically. Identify each type and explain their origin.
Answer: The 11 identical embryos are nucellar (adventive) embryos — they arose by adventive embryony from diploid nucellus cells surrounding the embryo sac. They are genetically identical to the mother (2n). The one genetically different embryo is the zygotic embryo — formed by normal fertilisation (egg + sperm). This is the hybrid embryo, and it carries a new genetic combination. The presence of multiple embryos in one seed is polyembryony.
Hybrid varieties of paddy are cultivated extensively. One major limitation is that hybrid seeds must be purchased every year. How would introducing apomixis into these hybrids solve this problem?
Answer: In normal sexual reproduction, F1 hybrid seeds produce F2 plants that undergo genetic segregation — hybrid vigour is lost. If apomixis genes are transferred into the hybrid, the plants would produce seeds without fertilisation. Each seed would develop an embryo genetically identical to the F1 parent. Farmers could save these seeds and re-sow them in subsequent seasons without any loss of hybrid characters. This would eliminate the need for annual hybrid seed purchase and dramatically reduce input costs — the agricultural rationale NCERT explicitly states.
Common confusion & NEET traps
Apomixis vs Parthenocarpy — both lack fertilisation, but the products differ completely
Both apomixis and parthenocarpy bypass fertilisation. NEET exploits this shared feature to confuse students. Apomixis produces a seed with an embryo (asexual reproduction through seeds). Parthenocarpy produces a fruit without seeds (the ovary develops without fertilisation, but no embryo forms). Banana is seedless and parthenocarpic. Citrus seeds contain multiple embryos — this is apomixis and polyembryony, not parthenocarpy.
Rule: Apomixis = SEED without fertilisation (embryo present). Parthenocarpy = FRUIT without fertilisation (no seed, no embryo).
Parthenogenesis (botany) vs Parthenogenesis (zoology) vs Parthenocarpy
Three terms share the Greek root partheno (virgin) and are frequently confused. In botany, parthenogenesis = egg cell (n) → embryo without fertilisation. In zoology, parthenogenesis = egg → new organism without fertilisation (same principle, animal context — e.g., bees). Parthenocarpy = fruit from unfertilised ovary. NEET 2019 explicitly tested that "female gamete develops into embryo without fertilisation" = parthenogenesis (NOT autogamy, NOT parthenocarpy).
Rule: Parthenogenesis (botany) = embryo from egg, no fertilisation. Parthenocarpy = fruit from ovary, no fertilisation, no embryo.
Adventive embryony vs Parthenogenesis — both are apomixis, but the source cell differs
Students frequently conflate these because both produce an embryo without fertilisation. Parthenogenesis: the egg cell (reproductive cell, haploid if meiosis occurred) develops into an embryo. Adventive embryony: a somatic cell of the nucellus or integument (2n, not the egg) develops into an embryo. In adventive embryony the egg cell plays no role. This is why adventive embryos are always diploid (2n), while parthenogenetic embryos may be haploid.
Rule: Adventive embryony = embryo from SOMATIC cells (nucellus/integument, 2n). Parthenogenesis = embryo from EGG CELL without fertilisation.
Apomixis ≠ vegetative reproduction — both are asexual and produce clones
Vegetative reproduction (runners, rhizomes, tubers) is asexual and produces clones. Apomixis is also asexual and produces clones. The key difference: apomixis produces seeds — the offspring are dispersed as seeds, benefiting from all the adaptive advantages of seed dispersal (dormancy, protective coat, nutritive tissue). Vegetative reproduction produces no seed.
Rule: Apomixis = asexual reproduction THROUGH seeds. Vegetative reproduction = asexual reproduction WITHOUT seeds.
Agamospermy
Embryo sac origin
- Embryo from cells inside the embryo sac (egg, synergids)
- Egg cell may develop without fertilisation (parthenogenesis)
- Meiosis may or may not occur (determines ploidy)
- Examples: Asteraceae, grasses, Taraxacum (dandelion)
Adventive embryony
Somatic cell origin
- Embryo from nucellus or integument — somatic (2n)
- Egg cell is NOT involved at all
- Always diploid (no meiosis in source cells)
- Examples: Citrus, Mangifera — leads to polyembryony