Lecture-19 Polyembryony: types and importance
Polyembryony:- Presence of more than one embryo inside the seed is known as polyembryony. It was first reported by A.V. Leuwenhoek (1719) in Citrusi Fam-Rutaceae).This phenomenon is very common in gymnosperm than angiosperm. Besides normal embryoe (develops from Zygote), other embryos are formed inside seed maybe as haploid (n) or Diploid (2n).
Types of Polyembryony:-
i. Induced Polyembryony:- When polyembryony is induced in plants and animals for experiments, it is referred to as induced polyembryony.
ii. Spontaneous Polyembryony:- When polyembryony occurs naturally in plants and animals, it is called natural or spontaneous polyembryony.
iii. True polyembryony:- Many embryos are developed inside single embryo sac.
iv. False polyembryony:- If the ovule carries more then one embryo sac & embryos develop in each embryo sac.
v. Cleavage polyembryony:- In cleavage polyembryony, a single fertilised egg can give rise to multiple embryos. When an embryo divides into several identical parts, each part can develop into a mature embryo. A common example of cleavage polyembryony is seen in Pinus.
vi. Simple polyembryony:- This type of polyembryony occurs when several archegonia are fertilised. When more than one egg is fertilised, it is called simple polyembryony. Adventive polyembryony is the formation of additional embryos through sporophytic budding. It is common in Cycas, lemon, groundnut, mango, Pinus, onion, and orange.
vii. Rosette polyembryony:- In rosette polyembryony, multiple additional embryos are produced from cells known as rosette cells.
Cause of polyembryony in angiosperm:- Polyembryony takes place due to:
i. Cleavage of proembryo:- It is simplest method of polyembryony where zygote divides into many units & each unit develops an embryo.
ii. Development of many embryo from synergid, antipodal cells, endosperm except egg.
iii. Development of many embryo due to presence of more than one embryo sac.
iv. Development of polyembryo from nucellus, integument (outside the embryo sac). According to Haberiandt (1921)” Stimulus for polyembryony is provided by degenerating cells nucelleus Necrohormone theory).
Importance of Polyembryony:-
i. Plant breeding and horticulture:- The embryos in fruit trees, such as citrus and mango, are often free from genetic variation, making them valuable for maintaining desirable traits in new generations of plants.
ii. Nucellar disease:- Polyembryony plays a role in the propagation of fruit trees, as the embryos produced are genetically uniform, helping to ensure consistent quality and characteristics in seedlings.
iii. Propagation:- Polyembryony offers a method for developing homozygous diploid plants, which can be advantageous in breeding programmes aimed at fixing desirable traits.
iv. Artificial production:- Polyembryony can be artificially induced to produce genetically uniform plant material from eggs or synergids, ensuring the quality and productivity of fruit tree crops.
