🔬 Special Reproductive Phenomena
Learn apomixis, polyembryony and parthenocarpy for CUET Agriculture. Adventive embryony in citrus, seedless fruits and clonal seeds.
Apomixis
- Seed formation without fertilization — no fusion of gametes occurs
- The embryo develops from maternal tissue (not from a zygote)
- Offspring are genetically identical to the parent plant — essentially clones
- Types: Adventive embryony (from nucellus/integuments), Diplospory (from unreduced MMC), Apospory (from somatic cells of ovule)
NOTE
Apomixis is agriculturally significant because it produces true-breeding offspring identical to the mother plant. Hybrid seed companies study apomixis to potentially "fix" hybrid vigor across generations without the need to re-cross parent lines each season.
Polyembryony
- Formation of more than one embryo in a single seed
- Can occur through multiple mechanisms:
- Multiple fertilization events
- Cleavage (splitting) of the zygote or proembryo
- Development of synergids or antipodals into embryos
- Adventive embryony (extra embryos from nucellus)
- Examples: Citrus (orange, lemon), mango, Opuntia
Parthenocarpy
- Development of fruit without fertilization — resulting in seedless fruit
- Can be induced by:
- Auxin (IAA, NAA)
- Gibberellin (GA₃)
- Natural examples: Banana, pineapple, grape
- Induced examples: Watermelon, tomato (treated with hormones)
TIP
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Apomixis
- Seed formation without fertilization — no fusion of gametes occurs
- The embryo develops from maternal tissue (not from a zygote)
- Offspring are genetically identical to the parent plant — essentially clones
- Types: Adventive embryony (from nucellus/integuments), Diplospory (from unreduced MMC), Apospory (from somatic cells of ovule)
NOTE
Apomixis is agriculturally significant because it produces true-breeding offspring identical to the mother plant. Hybrid seed companies study apomixis to potentially "fix" hybrid vigor across generations without the need to re-cross parent lines each season.
Polyembryony
- Formation of more than one embryo in a single seed
- Can occur through multiple mechanisms:
- Multiple fertilization events
- Cleavage (splitting) of the zygote or proembryo
- Development of synergids or antipodals into embryos
- Adventive embryony (extra embryos from nucellus)
- Examples: Citrus (orange, lemon), mango, Opuntia
Parthenocarpy
- Development of fruit without fertilization — resulting in seedless fruit
- Can be induced by:
- Auxin (IAA, NAA)
- Gibberellin (GA₃)
- Natural examples: Banana, pineapple, grape
- Induced examples: Watermelon, tomato (treated with hormones)
TIP
Parthenocarpy vs Apomixis: Parthenocarpy = seedless fruit (no seed at all). Apomixis = fruit with seed, but seed formed without fertilization. Both bypass fertilization but in very different ways.
Key Facts for Exam Revision
| Fact | Detail |
|---|---|
| Double fertilization discovered by | S.G. Nawaschin (1898) |
| Fertilization first seen by | Strasburger (1884) in Lilium |
| Syngamy produces | Zygote (2n) |
| Triple fusion produces | PEN (3n) → Endosperm |
| Endosperm types | Nuclear, Cellular, Helobial |
| Most common pollen tube entry | Porogamy (through micropyle) |
| Sporopollenin found in | Exine of pollen grain |
| Tapetum function | Nourishes developing pollen |
| Embryo sac structure | 7 cells, 8 nuclei (Polygonum type) |
| Number of megaspores from MMC | 4 (only 1 functional) |
| Male gametophyte | Pollen grain |
| Female gametophyte | Embryo sac |
| Apomixis | Seed without fertilization |
| Polyembryony | Multiple embryos per seed (Citrus) |
| Parthenocarpy | Seedless fruit formation |
| Insect pollination | Entomophily |
| Wind pollination | Anemophily |
| Water pollination | Hydrophily |
| Self-incompatibility examples | Brassica, pigeon pea |
| Cleistogamy example | Commelina, Viola |
| 80% of insect pollination | Done by bees |
| Protandry | Anther matures before stigma |
| Protogyny | Stigma matures before anther |
| Microsporangia per anther | 4 |
| Most common ovule type | Anatropous (80%) |
Summary Cheat Sheet
| Concept / Topic | Key Details / Explanation |
|---|---|
| Apomixis | Seed formation without fertilization; offspring = genetic clones of parent |
| Apomixis types | Adventive embryony (from nucellus/integuments), Diplospory (unreduced MMC), Apospory (somatic cells) |
| Apomixis significance | Can potentially fix hybrid vigor across generations without re-crossing |
| Polyembryony | More than one embryo in a single seed |
| Polyembryony mechanisms | Multiple fertilization, zygote cleavage, synergid/antipodal development, adventive embryony |
| Polyembryony examples | Citrus (orange, lemon), mango, Opuntia |
| Parthenocarpy | Seedless fruit development without fertilization |
| Parthenocarpy inducers | Auxin (IAA, NAA) and Gibberellin (GA₃) |
| Natural parthenocarpy | Banana, pineapple, grape |
| Induced parthenocarpy | Watermelon, tomato (hormone-treated) |
| Parthenocarpy vs Apomixis | Parthenocarpy = seedless fruit (no seed); Apomixis = fruit with seed but no fertilization |
| Double fertilization discoverer | S.G. Nawaschin (1898) |
| Syngamy product | Zygote (2n) |
| Triple fusion product | PEN (3n) → endosperm |
| Sporopollenin location | Exine of pollen grain |
| Tapetum | Nourishes developing pollen grains |
| Embryo sac | 7 cells, 8 nuclei (Polygonum type) |
| Functional megaspores from MMC | 4 produced, only 1 functional (closest to chalaza) |
| Male gametophyte | Pollen grain |
| Female gametophyte | Embryo sac |
| Entomophily | Insect pollination |
| Anemophily | Wind pollination |
| Hydrophily | Water pollination |
| Self-incompatibility | Brassica, pigeon pea |
| Cleistogamy | Commelina, Viola (flowers never open) |
| 80% of insect pollination | Done by bees |
| Protandry vs Protogyny | Anther matures before stigma vs Stigma matures before anther |
| Microsporangia per anther | 4 |
| Most common ovule type | Anatropous (80%) |
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