Polyembryony

  

Occurence of more than one embryo in a seed is called as polyembryony.
The first case of polyembryony was reported in certain orange seeds by Anton Von Leuwenhoek
Polyembryony in angiosperms may arise by :
1. Cleavage of Embryo
2. Formation of embryos by cells of Embryo Sac other than the egg
3. Development of more than one embryo sac within same ovule
4. Activation of some sporophytic cells of the ovule

I. Cleavage polyembryony
Cleavage and proliferation of zygote or it's derivatives leading to the establishment of seperate primordia is widespread among gymnosperms.
Swamy (1943) recorded three modes of supernumerary embryo formation:
1. Zygote divides irregularly to form a mass of cells of which those lying towards the chalazal end grow simultaneously and give rise to many embryos,
2. The proembryo gives out small buds and outgrowths which may themselves function as embryos,
3. The filamentous embryo becomes branched, and each branch gives rise to an embryo

II. Embryos from cells of the embryo sac other than the egg:
In this category the most common source of additional embryos are the synergids
Depending on whether it arises from fertilized synergid or unfertilized synergid, the embryo may be diploid or haploid.
In Aristolochia bracteata, Poa alpina besides the egg and the polar nuclei, one or both the synergids may be fertilized.
This can be brought about by the entry of more than one pollen tube into the embryo sac or by the presence of additional male gametes in the same pollen tube.
Embryos arising from unfertilized synergids are known in Argemone mexicana, Phaseolus vulgaris.
Formation of embryos from antipodals is rather rare. It has been observed in Paspalum, Ulmus.

III. More than one embryo sac in the same ovule
Multiple Embryo Sac in an ovule may arise from:
1. Derivatives of the same megaspore mother cell,
2. Derivatives of two or more megaspore mother cells,
3. Nucellar cells, eg., Casaurina, Poa, Citrus, Loranthus

IV. Activation of some sporophytic cells of the ovule or Adventive Embryony
The embryos arising from the maternal sporophytic tissue ( outside the embryo sac) are called adventive embryos.
The only maternal tissues which are known to form adventive embryos are the nucellus and the integuments. Eg., Citrus, Mangifera, Optunia, Trillium
Nucellar embryos can be distinguished from the zygotic embryos by their lateral position in the Embryo Sac, irregular shape and lack of suspensor.

Causes of polyembryony:
Many theories have been advanced to explain the occurrence of the polyembryony:
Haberlandt proposed the " necrohormone theory". He regards the degenerating cells of the nucellus as source of stimulus for the adjacent cells to divide and form adventive embryos.

Haberlandt attempted to induce adventive polyembryony in Oenothera by damaging cells by pricking the ovules with a fine needle and by gently squeezing the ovary.

Frusato etal showed the embryo number in Citrus seeds may be influenced by the following factors:

1.     Age of the tree; increasing in older trees,

2.     Fruit-set; being higher in years of higher fruit set,

3.     Nutritional status of the plant

4.     Orientation of the branch of the tree; being higher on Northern than on Southern branches.

Importance of Polyembryony:

Nucellar Adventive polyembryony is of great significance in Horticulture.

The adventive embryos provide uniform seedlings of the parental type, as obtained through vegetative propogation by cutting.

However, nucellar seedlings of Citrus furnish better clones than cutting; because:

1.     The nucellar seedlings have a tap root and, therefore develop a better root system than do the cuttings. The latter have only a small lateral root system.

2.     The nucellar seedling show a restoration of the vigour after repeated  propagation by cutting.

3.     Free from disease.