Polyembryony
பல் கருநிலை
Nature rarely settles for a single outcome. Polyembryony is the biological defiance of the 'one seed, one embryo' rule. If you confuse this with Apomixis, you lose four marks instantly.
The idea
Polyembryony is the occurrence of more than one embryo in a single seed. In species like Citrus and Mango, nucellar cells surrounding the embryo sac start dividing. These cells protrude into the embryo sac and develop into multiple embryos. Each seed thus contains several embryos of maternal origin.
Why it matters
It increases the chances of survival for the offspring and is a natural mechanism for clonal propagation in certain plant species.
NEET lens
NEET focuses on the origin of these embryos: they are diploid and maternal, not products of fertilization. Memorize 'Citrus' and 'Mango' as the primary NCERT examples. Do not mistake this for polyploidy; keep your terminology precise.
Clinical significance
In ecosystem dynamics, polyembryony ensures high genetic uniformity and survival rates in clonal propagation. If this mechanism fails in these species, the plant loses its ability to produce nucellar embryos, leading to a significant reduction in the propagation of superior parental traits.
Introduction
Polyembryony is the occurrence of more than one embryo in a single seed. It can arise from fertilization of multiple eggs, cleavage of a single zygote, or adventive embryony from nucellar/integument cells.
Core concept
Types: (1) Cleavage polyembryony: zygote splits → multiple embryos (like identical twins in animals). (2) Adventive embryony: nucellar cells or integument cells develop into additional embryos WITHOUT fertilization (apomixis-type) → these are nucellar embryos (2n, maternal genotype). Common in Citrus (orange, lemon, mango). (3) Simple polyembryony: multiple eggs fertilized independently → multiple embryos. Examples: Citrus (most studied) — usually 1 sexual embryo + several nucellar embryos. Mangifera indica (mango). Groundnut, onion. Competition: strongest embryo (usually nucellar) develops → others abort. Significance: multiple true-to-type seedlings from one seed (useful for citrus rootstocks).
Go deeper
Polyembryony in conifers: common in gymnosperms (pine) → cleavage polyembryony → usually one embryo survives (selective abortion). In Pinus: 4 archegonia each fertilized → 4 zygotes → up to 4 embryos begin → competition → 1 survives. This is normal in pines, not exceptional.
From Class 10
Normally one seed contains one embryo. But in some plants, multiple embryos develop in the same seed. This is polyembryony.