Apomixis and Polyembryony — Revision Notes

⚡ 30-Second Revision

Apomixis — Asexual reproduction mimicking sexual reproduction; seed formation without fertilization.
Genetic Result — Offspring are clones (genetically identical to parent).
Types of Apomixis

- Recurrent Apomixis: Diploid egg forms embryo. - Diplospory: MMC $ightarrow$ diploid embryo sac. - Apospory: Nucellar cell $ightarrow$ diploid embryo sac. - Adventitive Embryony: Embryos directly from nucellus/integuments (somatic cells).

Polyembryony — More than one embryo in a single seed.
Causes of Polyembryony — Cleavage of proembryo, multiple embryo sacs, synergids/antipodals, adventitive embryony.
Key Examples — Citrus, Mango (for polyembryony & adventitive embryony); Taraxacum (diplospory); Pennisetum (apospory).
Significance — Preserves hybrid vigor, uniform progeny.

2-Minute Revision

Apomixis is a fascinating reproductive strategy where plants produce seeds without the need for fertilization, essentially bypassing the fusion of male and female gametes. Despite forming seeds, it's a form of asexual reproduction because the offspring are genetically identical clones of the parent.

This is incredibly valuable in agriculture for 'fixing' hybrid vigor, meaning desirable traits of a hybrid can be passed on consistently through seeds without segregation. Key types include recurrent apomixis, where a diploid egg cell develops into an embryo (e.

g., diplospory from MMC, apospory from nucellar cells), and adventitive embryony, where embryos bud directly from somatic cells like the nucellus or integuments. Polyembryony, on the other hand, is the phenomenon of a single seed containing multiple embryos.

This can occur due to various reasons, such as the cleavage of a single zygote, the presence of multiple embryo sacs, or the development of embryos from other cells of the embryo sac. A significant cause of polyembryony, especially in plants like Citrus and Mango, is adventitive embryony, where nucellar cells form additional embryos alongside the main one.

These nucellar embryos are clones and provide uniform, vigorous seedlings, highly beneficial for commercial propagation.

5-Minute Revision

Let's consolidate our understanding of Apomixis and Polyembryony, two crucial deviations from typical sexual reproduction in flowering plants. Apomixis is defined as asexual reproduction that mimics sexual reproduction by forming seeds without fertilization.

The defining characteristic is the absence of gamete fusion, leading to offspring that are genetically identical to the parent – essentially clones. This has profound implications for plant breeding, as it allows for the indefinite preservation of 'hybrid vigor' (heterosis) in subsequent generations, which would otherwise be lost due to genetic segregation in sexual reproduction.

There are several key types of apomixis:

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Recurrent Apomixis — Involves the formation of a diploid embryo sac and a diploid egg cell that develops into an embryo without fertilization.

* Diplospory: The megaspore mother cell (MMC) directly forms a diploid embryo sac (e.g., *Taraxacum*). * Apospory: A diploid nucellar cell (or integumentary cell) develops into a diploid embryo sac (e.g., *Pennisetum*).

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Adventitive Embryony (Sporophytic Budding) — Embryos develop directly from diploid somatic cells of the ovule, such as the nucellus or integuments, bypassing the embryo sac stage entirely. This is a very common cause of polyembryony and is seen in *Citrus* and *Mango*.

Polyembryony refers to the presence of more than one embryo within a single seed. While a single zygote typically forms one embryo, polyembryony leads to multiple seedlings from one seed. Its causes are diverse:

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Cleavage Polyembryony — The zygote or early proembryo divides into multiple units, each forming an embryo (e.g., *Pinus*).
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Multiple Embryo Sacs — More than one embryo sac develops within an ovule, each potentially forming an embryo.
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Embryos from other cells — Synergids or antipodal cells within the embryo sac can sometimes develop into embryos.
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Adventitive Embryony — As mentioned, nucellar or integumentary cells form additional embryos alongside the main zygotic embryo. This is the most common cause in many fruit trees like *Citrus*.

Interrelationship: It's vital to note that adventitive embryony is a type of apomixis that *causes* polyembryony. Thus, many apomictic plants, especially those with adventitive embryony, are also polyembryonic.

The nucellar embryos formed are clones of the parent and are highly valued for producing uniform, vigorous, and disease-free rootstocks in horticulture. For NEET, remember the definitions, the specific types and their cellular origins, and the classic examples like Citrus and Mango, along with the agricultural significance of these phenomena.

Prelims Revision Notes

Apomixis (Asexual Seed Formation)

Definition — Asexual reproduction that mimics sexual reproduction by forming seeds without fertilization (fusion of gametes).
Genetic Outcome — Offspring are genetically identical to the parent (clones).
Significance — Preserves hybrid vigor indefinitely; ensures uniform progeny; enables seed production without pollinators.
Types of Apomixis

* Agamospermy: Apomictic seed formation. * Recurrent Apomixis: Diploid egg cell forms embryo. * Diplospory: Megaspore Mother Cell (MMC) directly forms unreduced (diploid) embryo sac. Egg cell is diploid.

Ex: *Taraxacum* (Dandelion). * Apospory: Nucellar cell (somatic) develops into unreduced (diploid) embryo sac. Egg cell is diploid. Ex: *Pennisetum*, some *Citrus*. * Non-recurrent Apomixis: Haploid egg cell develops without fertilization (parthenogenesis), resulting in haploid embryo.

Rare, often sterile. Ex: Some *Solanum* species. * Adventitive Embryony (Sporophytic Budding): Embryos develop directly from diploid sporophytic cells (nucellus or integuments) without forming an embryo sac.

Often leads to polyembryony. Ex: *Citrus*, *Mango*, *Opuntia*.

Polyembryony (Multiple Embryos in a Seed)

Definition — Presence of more than one embryo in a single seed.
Causes

1. Cleavage of Zygote/Proembryo: Zygote splits into multiple embryos. Ex: *Pinus* (Gymnosperm). 2. Multiple Embryo Sacs: More than one embryo sac develops in an ovule, each forming an embryo. 3. Embryos from Synergids/Antipodals: Other cells of the embryo sac develop into embryos (with or without fertilization). 4. Adventitive Embryony: Nucellar or integumentary cells form additional embryos (most common cause in *Citrus*, *Mango*).

Significance — Produces uniform, vigorous, disease-free seedlings (especially nucellar embryos); valuable for commercial propagation.
Key Examples — *Citrus*, *Mango*, *Onion*, *Pinus*.

Relationship: Adventitive embryony is a type of apomixis that directly causes polyembryony. Many apomictic plants are also polyembryonic.

Vyyuha Quick Recall

All Plants Often Make Identical Xeroxes (Apomixis: Asexual, Plants, Offspring, Make, Identical, Xeroxes/Clones).

For Apomixis types: Don't Always Ask Nice Apples: Diplospory: MMC Apospory: Nucellar cells Adventitive: Nucellus/Integuments