Polyembryony: Types, Significance and Its Occurrence in Gymnosperms

Polyembryony

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Polyembryony is the reason why humans have identical twins. It occurs in both plants and animals. A nine-banded Armadillo which gives birth to identical quadruplets is considered the best and classic example of this aspect.

Figure: Polyembryony - Formation of embryo from antipodal cell

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Types of Polyembryony

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 Polyembrony is broadly of two types i.e. Natural or sponateneous and Induced or artificial.

Induced Polyembryony

It is called so because it is induced experimentally. In this type, development of an embryo is managed in a culture medium(liquid or gel designed to support the growth of microorganisms).

Spontaneous Polyembryony

This type of polyembrony occurs naturally. Herbert John Webber, an American Plant Physiologist had classified Spontaneous Polyembryony into three types:

Cleavage Polyembryony

Cleavage Polyembryony occurs when a young embryo or zygote separates into two or more units. These units eventually develop into independent embryos. Sometimes, it is the result of proliferation of proembryo (the series of cells in the ovule of a flowering plant, after fertilisation but before the formation of embryo). It is common in gymnosperms and comparatively rare in angiosperms. Pinus is a popular example of this type. In it, zygote divides twice in order to form four nuclei.

Simple Polyembryony

Simple Polyembryony is the result of fertilisation of more than one egg or many of the archegonia (a haploid structure that produces female gamete) eggs. Pinus is also a suitable example of Simple Polyembryony.

Rosette Polyembryony

This occurs when additional embryos are developed in rosette cells in gymnosperms.

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Polyembryony in Gymnosperms

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Different gymnosperms shows different types of polyembryony.

Cycadales 

Although rarely, this group exhibits Simple Polyembryony. It was first noticed by A.N Rao in Cycas Circinalis. Two adjoining archegonia of the same ovule sometimes develop independently into two embryos and also rarely into two seedlings.

Coniferales

This category exhibits both Simple and Cleavage Polyembryony. Cleavage Polyembryony has been reported in Pine, Cedrus, Tsuga, Sciadopitys, Sequoia, Thuja, Juniperus and Podocarpus. However, both are common in Cupressus.

Texales 

In this group, Simple Polyembryony occurs due to the fertilisation of multiple archegonia eggs. Before walls form in Taxus, 16 nuclei are present in it. The terminal tier of the proembryo may contain several cells. However, only one cell cuts the apical cell segment thereby gaining ascendency over the others. Cleavage Polyembryony of suspensor cells also occurs here. Examples are, English yew,Taxus Cuspidats, Pacific Yew, etc.

Gnetales 

In this group, Polyembryony occurs in several ways. Each of the primary suspensor tubes may develop an embryo at its tip resulting in the formation of many embryos.The embryonal mass at the tip of the secondary suspensor may proliferate to give rise to additional embryos. Sometimes the cells of the secondary suspensor may become meristematic producing numerous embryos. In a primary suspensor tube, instead of one, two or more groups of cells may be produced forming many embryos at the top. Examples are, Gnetum, Ephedrales, Melinjo, Gnetum Oblongum, etc.

Ernst (1918) and Schnarf (1929) classified polyembryony into two kinds on the basis of development of embryos:

True Polyembryony

This occurs when an embryo is produced in a single embryo sac. The supplementary embryo(s) are formed either from cleavage of the egg or from the antipodal cells or synergids.

False Polyembryony

This is the phenomenon which occurs when two or more embryos develop in a single embryo sac.

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Significance of Polyembryony 

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Earlier, polyembryony was said to be an abnormal feature but now it is considered as a desirable character in citrus fruits, mango, jamun, rose, apple, almond, and so on to obtain true value products.

• Thus it is very important part of Horticulture and particularly, Nucellar adventive polyembryony is of great significance in it.
• It helps maintain the identicality between parent and progeny. Nucellar seedlings show restoration of vigour. Moreover, nucellar embryos are free from disease.

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Things to Remember

• Polyembrony is a phenomena where more then one embryo is fused together and developed into seed or ovum during fertilization.
• The polyembryony was first discovered by scientist Antonie van Leeuwenhoek. 
• Poembryony is broadly classified in two types- Natural and Induced.
• Identical twins are developed because of polyembrony.
• Natural or spontaneous polyembryony further classified in 3 types- cleavage, simple and rosette polyembryony.

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Previous Years Questions

1.  Meiosis takes place in...[NEET 2013]
2. The sexual reproduction is absent in….[NEET 1995]
3. Attractants and rewards are required for….[NEET 2017]
4. Functional megaspore in an angiosperm develops into ?...[NEET 2017]
5. Coconut water from a tender coconut is….[NEET 2015]
6. Double fertilization is exhibited by….[NEET 2017]
7. Pollen tablets are available in the market for...[NEET 2014]
8. Geitonogamy involves….[NEET 2014]
9. Which one of the following fruits is parthenocarpic?...[NEET 2015]
10. Seeds without fertilization is obtained from...[KCET 2015]
11. Identify the vegetative propagule M in the...[KCET 2020]
12. The process in which haploid embryo is formed from haploid egg without fertilization is called...[KEAM]
13. Gemmules are internal buds found in sponges and are….
14. Residual, persistent nucellus present in some seeds is known as...[AMUEEE 2014]
15. A typical dicotyledonous embryo consists of….[AMUEEE 2010]
16. Body of ovule is attached to funicle at a point called...[AMUEEE 2013]
17. Chalazogamy is shown by...[AMUEEE 2015]
18. Cleistogamous flower is found in...[JKCET 2011]
19. Which ones produce androgenic haploids in...[NEET 1990]