Polyembryony

Polyembryony refers to the growth of more than one embryo in a single ovary, seed, or complete fertilization of a fertilized egg. People have identical twins because of polyhedra. It's common in both animals and plants. The largest and most common example of this feature is the Armadillo with nine belts breeding identical twins

Vedantu’s Concept Explanation of Polyembryony

To define polyembryony, let us discuss fertilization of eggs in brief both in plants and animals.

In plants, male sperm cells fuse with an ovule which further develops into seed/seeds. Each seed contains an embryo (which is a very small undeveloped plant).

In animals, fertilization can be both internal or external. The male gamete fertilizes female organisms’ eggs forming the primary nucleus of an embryo. 

Polyembryony is understood as a kind of clonal development in which one egg goes on to produce two offspring which are genetically identical. It occurs both in plants and animals. 

Polyembryony in Plants

This condition was first observed in the orange seed by Antoni van Leeuwenhoek(1719).

(1719). Polyembryony is so common in gymnosperms that it may be considered a distinctive feature of this group. In most gymnosperms with polyembryony, the female gametophyte develops two or more arches. As each archegonium carries an egg, the presence of multiple archegonia results in two or more fertilized eggs, resulting in the formation of two or more possible embryos.

Different groups of gymnosperms exhibit polyembryony –

1. Cycadales 

Two adjacent archegonia within one ovule may independently develop into two embryos. 

2. Coniferales 

Simple polyembryony occurs with the number of embryos varying from two to numerous. 

3. Taxales 

Multiple archegonia are present within female gametes. Even though many eggs may be fertilized, only one embryo finally attains maturity. 

4. Gnetales 

In this case, polyembryony is a high order, and embryos multiply from each zygote.  

Causes in Polyembryony 

There are various theories that are put forward to explain polyembryony –

• Necrohormone Theory 

Cells witness nucellus degeneration in order to give rise to a stimulus for adjacent cells to undergo division. It leads to the formation of adventive embryos. 

• Hybridisation Theory 

The hybridisation process leads to the recombination of genes where a single unit is formed, which creates multiple embryos.

Types of Polyembryony

Polyembryony types can be spontaneous or naturally occurring as well as experimentally induced. Spontaneous polyembryony can be further categorized into three types –

• Cleavage Polyembryony- In cleavage polyembryony, one fertilized egg produces multiple embryos. 

• Made Polyembryony - Named because it is made by testing. In this case, embryonic growth is regulated in a traditional way (a liquid or gel designed to support the growth of microorganisms).

• Automatic Polyembryony - This type occurs naturally. Herbert John Webber, an American botanist, had divided Polybeyon into three types:

• Polyembryony in Coniferales: Cleavage Embryony occurs when a young embryo or zygote divides into two or more units. These units eventually grow into independent embryos. In some cases, it is the result of an increase in proembryo (a series of cells in the ovary of a flowering plant, after fertilization but before embryonic formation). It is common in gymnosperms and rarely compared to angiosperms. In it, the zygote divides into two nuclei to form four nuclei. ex-pinus

• Simple Polyembryony: Simple Polyembryony is the result of the fertilization of more than one egg or multiple archegonia (a haploid structure that produces female eggs). An example of Simple Polyembryony in Pinus.

• Rosette Polyembryony:  Rosette cells developed additional embryos in gymnosperms.

Polyembryony Examples 

• Cleavage Polyembryony – Mostly seen in gymnosperms. 

• Simple Polyembryony – Found in Brassica.

• Mixed Polyembryony – Found in Argemone Mexicana and Ulmus Americana. 

• Adventive Polyembryony – Polyembryony in citrus is most common along with Mangifera and Opuntia.

Different Polyembryony

1. True Polyembryony

True polyembryony is the common instance of the production of embryos with the projection into a single embryo sac. The additional embryos are created either from the cleavage of a zygote or from antipodal cells and synergids. 

2. False Polyembryony 

False polyembryony is the formation of two or more embryos with the development of a monosporic embryo sac. Here are two or more nucelli fuses for embryo sac development. 

3. Induced Polyembryony

Such polyembryony does not require initial sporophytic cells, proembryo or zygote. The development of an embryo can be made in a culture medium. These induced embryos are known as somatic embryos or adventitious embryos. 

Polyembryony with Different Types of Gymnosperms

• Cycadales (living organs and bones): Although rare, this group exhibits Simple Polyembryony. It was first spotted by A.N Rao at Cycas Circinalis. Two connected arches of the same ovule sometimes grow independently into two embryos and rarely have two seedlings.

• Coniferales (found in cooler regions and incorporating both extinct and living forms): This section presents both Polyembryony Simple and Cleavage. Cleavage embryos have been reported in Pine, Cedrus, Tsuga, Sciadopitys, Sequoia, Thuja, Juniperus and Podocarpus. However, both are common in the Pressressus.

• Texales: Fertilization of many archegonia eggs develops Simple Polyembryony. Prior to the construction of the walls at Taxus, there were 16 nuclei in it. The terminal stage of the proembryo can contain several cells. However, only one cell cuts off part of the apical cell thus gaining higher than the others. Cleavage Polyembryony of hanging cells also occurs here. 

• Gnetales (most flexible members of gymnosperms): In this group, Polyembryony occurs in many ways. Each primary suspension tube may grow a fetus at the top, resulting in the formation of multiple embryos. The fetal weight at the top of the second suspensor may increase to allow for more embryos. Sometimes cells of the second suspensor may be meristematic and produce multiple embryos. In the first suspensor tube, instead of one group, two or more cells may be formed to form more embryos at the top.

Significance of Polyembryony

In the past, polyembryony was considered a rare ingredient but is now regarded as a desirable ingredient in citrus fruits, mangoes, jams, roses, apples, almonds, and so on. to find products of real value. It is therefore a very important part of Horticulture and in particular, Nucellar adventive polyembryony is very important to it. It helps to maintain the similarity between parent and offspring. Nucellar plants show energy recovery. In addition, nucellar embryos are disease-free. There is a lot of practical importance in polyembryony. It is particularly useful for the propagation of certain species of plants. For instance, adventive embryos, which retain characteristics of the parent plant, are used to create genetically uniform seedlings of citrus and mango. Moreover, in the case of citrus, grafts from other types are also used in orchard stock. 

Test Yourself 

i. In which of the following species Nucellar polyembryony is found?

(a) Citrus 

(b) Brassica 

(c) Triticum   

(d) Gossypium 

Ans . (a) Citrus

ii.  Polyembryony was first noted in ______________

(a) Euphorbia   

(b) Citrus  

(c) Lilium   

(d) Mangifera

Ans.  (b) Citrus

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