Polyembryony is a fascinating and valuable trait in plant
reproduction, especially in horticultural crops. It refers to the development
of multiple embryos within a single seed. While it may seem unusual,
polyembryony is quite common in nature and plays a significant role in plant
propagation and breeding.
What Is Polyembryony?
Polyembryony is the formation of two or more embryos from a
single ovule, resulting in multiple seedlings from one seed. These embryos may
arise from different tissues within the ovule, such as the nucellus,
integuments, or even the fertilized egg itself.
There are two main types:
- True
Polyembryony: Multiple embryos form within a
single embryo sac.
- False
Polyembryony: Multiple embryo sacs develop in
one ovule, each giving rise to an embryo.
In contrast, monoembryony refers to the production of
a single embryo from fertilization—common in most sexually reproducing plants.
How Polyembryony Happens: Key
Mechanisms
Polyembryony can occur through several biological pathways,
each involving different parts of the ovule:
1. Adventive Embryony (Nucellar
Embryony)
- Most
common and agriculturally important.
- Embryos
originate from somatic cells of the nucellus or integuments,
not the fertilized egg.
- These
embryos are genetic clones of the mother plant.
2. Cleavage Polyembryony
- A
fertilized egg (zygote) splits into multiple embryos.
- Seen
in families like Orchidaceae, Poaceae, and some gymnosperms.
3. Embryos from Other Cells in the
Embryo Sac
- Synergids,
antipodals, or even endosperm cells may develop into embryos.
4. Multiple Embryo Sacs
- More
than one embryo sac develops in a single ovule, each potentially forming
an embryo.
Polyembryony and Ovule Anatomy: Why
Structure Matters
Understanding ovule structure helps explain how multiple
embryos can form. Here’s a breakdown:
- Gynoecium:
The female reproductive organ of the flower.
- Carpel:
Functional unit of the gynoecium; contains the ovary.
- Ovule:
Found inside the ovary, it develops into a seed after fertilization.
A typical ovule includes:
- Embryo
sac: Where fertilization occurs.
- Nucellus:
Nutrient-rich tissue.
- Integuments:
Protective layers forming the seed coat.
- Micropyle:
Opening where pollen tubes enter.
- Funiculus:
The stalk attaching the ovule to the ovary wall.
Most horticultural plants have anatropous (inverted)
and bitegmic (two integuments) ovules.
Embryo Sac Structure and Double
Fertilization
Inside the embryo sac:
- 7
cells and 8 nuclei are arranged.
- Micropylar
end: Contains the egg apparatus (egg + two synergids).
- Chalazal
end: Has three antipodal cells.
- Center:
Two polar nuclei.
During double fertilization:
- One
sperm cell fuses with the egg → zygote (2n).
- Another
fuses with the polar nuclei → endosperm (3n).
In polyembryonic species, additional embryos can arise from:
- Synergids
or antipodals (n)
- Nucellar
or integumentary cells (2n)
Genetic Control of Polyembryony
Polyembryony is not random—it is genetically controlled.
- In
crops like citrus and mango, it’s usually governed by a dominant
gene with heterozygous alleles.
- Monoembryonic
plants typically carry homozygous recessive alleles, which suppress
extra embryo development.
- Minor
or modifier genes may influence the number and
viability of extra embryos.
Crosses between monoembryonic and polyembryonic varieties
show varying results, confirming genetic complexity.
Classifying
Polyembryony
By Frequency:
- Strictly
monoembryonic: <6% multiple embryos.
- Nearly
monoembryonic: 6–10%.
- Polyembryonic:
>10% occurrence.
By Embryo Source:
- Sporophytic
tissue (nucellus/integrument)
- Non-egg
cells in embryo sac
- Multiple
embryo sacs
- Cleavage
of fertilized egg
Why Polyembryony Matters in
Horticulture
Benefits:
- Produces
true-to-type, disease-free clones.
- Ensures
uniform crop quality.
- Speeds
up mass propagation.
Challenges:
- Hybridization
becomes difficult, as sexual (zygotic) seedlings
are outnumbered.
- Distinguishing
seedlings is hard during early growth
stages.
- Nucellar
embryos usually grow slower and are found
near the micropyle.
Only after fruiting can zygotic and nucellar seedlings be
accurately identified—this complicates breeding programs.
Polyembryony in Major Fruit Crops
Polyembryony is common in:
- Citrus
(especially rootstocks)
- Mango
- Syzygium
species
- Kiwi
- Almond
- Strawberry
(Fragaria spp.)
- Peach
The number and position of embryos vary by species and even
variety, affected by:
- Genetics
- Pollen
source
- Environment
- Position
of fruit on the tree
Molecular Insights and Recent Advances
Modern tools now allow better identification of embryo
origin:
- RAPD
markers and other DNA-based techniques
help distinguish between nucellar and zygotic seedlings.
- Genes
like msg-2 and SERK have been linked to somatic
embryogenesis, offering potential for targeted breeding and
manipulation.
In Citrus, research shows that adventive embryos can form
even before pollination, but their development depends heavily on endosperm
formation.
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