Polyembryony: Do you have a twin? Have you ever thought about why twins are identical or why twins are born? Polyembryony is the reason why humans have identical twins. ‘Poly‘ means many or several, and ‘embryony‘ means the formation or development of an embryo. This article helps you understand the meaning of Polyembryony, its types, occurrences, and significance. Polyembryony is a very interesting topic, and to know more interesting facts, read the complete article.

What is Polyembryony?

Polyembryony refers to the development of more than one embryo in a single ovule, seed, or by complete cleavage of a fertilized ovum. As a result, supernumeraries of various frequencies, such as duplet, triplet, quadruplet, quintuplet, sextuplet, and so on, develop.

Humans have identical twins because of polyembryony. It may be found in both animals and plants. The greatest and most typical illustration of this feature is a nine-banded Armadillo that gives birth to identical quadruplets.

Polyembryony in Plants

This phenomenon was originally seen in orange seeds by Antoni van Leeuwenhoek \((1719).\) Polyembryony is so frequent in gymnosperms that it may be considered a distinguishing feature of this group. In the majority of gymnosperms with polyembryony, a female gametophyte develops two or more archegonia. As each archegonium carries an egg, the presence of multiple archegonia results in two or more eggs being fertilised, resulting in the creation of two or more possible embryos.

Different Polyembryony

There are two different types of polyembryony and are as follows:
1. Induced Polyembryony: This category contains examples of polyembryony that has been induced artificially.
2. Spontaneous Polyembryony: All cases of naturally occurring polyembryony are classified as spontaneous polyembryony.

According to Webber, polyembryony is classified into three different types:
1. Cleavage Polyembryony: Cleavage polyembryony occurs when a young embryo or zygote divides into two or more units. Hence, it is also called cleavage of zygote or budding. These units eventually develop or mature into independent embryos. It is common in gymnosperms and comparatively rare in angiosperms. Pinus is a well-known example of this kind. The zygote splits twice, resulting in four nuclei.
2. Simple Polyembryony: Simple polyembryony occurs when more than one egg or a large number of archegonia (a haploid structure that generates female gametes) eggs are fertilized. Pinus is a good example of simple polyembryony as well.
3. Rosette Polyembryony: Additional embryos emerge from the rosette cells in some gymnosperms (e.g., a few species of Pinus), and this kind of polyembryony is known as rosette polyembryony.

Polyembryony with Different Types of Gymnosperms

The different groups of Gymnosperms in Polyembryony are as follows:

Polyembryony in Cycadales:
1. Simple polyembryony is seen in this category in rare cases.
2. A.N Rao was the first to detect it in Cycas Circinalis.
3. Two adjacent archegonia from the same ovule can sometimes develop into two embryos and, in rare cases, two seedlings.

Polyembryony in Coniferales:
1. Simple polyembryony occurs in the majority of Coniferales members, with the number of embryos ranging from two to numerous.
2. Cleavage polyembryony has been identified in various families like Taxodiaceae, Cupressaceae, Pinaceae, and Podocarpaceae.
3. Both simple and cleavage polyembryony are common in Cupressus.

Polyembryony in Taxales:
1. The female gametophyte of Taxus has many archegonia. Many of their eggs might be fertilized, resulting in simple polyembryony. However, only one embryo matures at the end of the process.
2. Suspensor cell cleavage is observed in Taxus, according to Sterling (1948). The suspensors are separated from one another and may each contain one or more embryonal units.
3. At the base of the suspensor cells, clusters of meristematic cells are occasionally seen. The rosette embryos are these clusters of cells.

Polyembryony in Gnetales:
1. Polyembryony occurs in this group in a variety of ways.
2. Each of the primary suspensor tubes has the potential to produce an embryo at its tip, resulting in a large number of embryos.
3. The embryonal mass at the secondary suspensor’s tip may grow and give birth to more embryos.
4. The cells of the secondary suspensor can sometimes become meristematic, resulting in a large number of embryos.
5. Instead of one group of cells, two or more groups of cells may be generated in the main suspensor tube, resulting in a large number of embryos at the top.
6. Gnetum, Melinjo, Ephedrales, Oblongum, etc., are examples.

Polyembryony in Angiosperms

Angiosperms show the unique phenomenon of double fertilization.
1. Budding or Cleavage of the Zygote: In this kind, a single fertilized egg produces a number of embryos. Nicotiana, Orchids, Exocarpus, Nymphaea, and other plants are examples.
2. Fertilization of Synergids: In this type of polyembryony, a number of embryos are formed or produced as a result of the fertilization of eggs along with synergids. E.g., Anemone, Sagittaria, Aristolochia, etc.
3. Fertilization of Antipodals: In this type of polyembryony, a number of embryos are formed or produced as a result of the fertilization of eggs along with antipodal cells. E.g., Paspalum, Ulmus, etc.
4. Formation of many embryos from the structure outside the embryo-sac (adventive polyembryony) like nucellus and integuments, e.g., Citrus, mango, Opuntia.

Classification of Polyembryony by Ernst \((1918)\) and Schnarf \((1929)\)

1. True Polyembryony: When more than one embryo is formed in the same embryo-sac in the ovule, then it is known as true polyembryony. The antipodal cells or synergids create the additional embryo(s), which are generated either by cleavage of the egg or by the antipodal cells or synergids.
2. False Polyembryony: When more than one embryo is formed in different embryo-sacs inside the ovule, then it is considered false polyembryony.

Polyembryony in Animals

1. The fact that an egg may, under certain conditions, produce two or more embryos is perhaps no more remarkable than that an egg will form a single individual.
2. Most animals exhibit an accidental type of polyembryony, or twinning, in which a single egg splits into two or more embryos, but an obligatory form is very rare.
3. Twinning itself must be regarded as the simplest type of polyembryony.
4. Polyembryony occurs or is seen in a wide range of invertebrates, like cnidarians, insects and echinoderms.
5. Three types of polyembryony may be recognized; these are as follows:
a. Experimental polyembryony, or the production of multiple embryos by artificial means or artificial procedure.
b. Accidental or sporadic polyembryony, or the occasional production of multiple embryos in a species in which development is typically monoembryonic. This leads to the development of embryos that are genetically identical.
c. Specific polyembryony is the one where the development of numerous embryos in a particular species is on a regular basis.

Significance of Polyembryony 

1. In plant breeding and horticulture, this phenomenon is very significant. In horticulture, nucellar seedlings of Citrus have been shown to be superior clones of orchard stock versus cuttings.
2. Nucellar embryos, on the other hand, are intended to be disease-free, and nucellar seedlings are meant to restore the vigour that has been lost due to continual cutting propagation.
3. The adventive polyembryony is extremely beneficial in the growth of citrus and mango plants.
4. The use of adventive embryos is also crucial for producing genetically homogeneous fruit tree seedlings.
5. The haploids may be utilised to create homozygous diploids, which are extremely valuable.
6. Due to the practical significance of haploids in plant breeding, techniques for artificially producing these embryos from eggs or synergids have been recognised.

Summary

Polyembryony refers to the formation of several embryos in a single ovule, seed, or by complete cleavage of a fertilised ovum. Through this article, we understood the different types of polyembryony in plants. Polyembryony was once thought to be an undesirable trait, but it is now seen as a desirable trait in citrus fruits, rose, mango, apple, Jamun, almond, and other similar fruits to achieve genuine value items.

As a result, it is an essential element of horticulture, and nucellar adventive polyembryony is particularly significant in it. It aids in the preservation of parent-offspring resemblance. Nucellar seedlings retain their vitality and are disease-free.

Frequently Asked Questions on Polyembryony

Below are the frequently asked questions on Polyembryony:

Q.1: What is Polyembryony?
Ans: Polyembryony refers to the development of more than one embryo in a single ovule, seed, or by complete cleavage of a fertilized ovum.

Q.2: What are the types of Polyembryony?
Ans: The types of polyembryony are induced polyembryony, spontaneous polyembryony, true polyembryony, false polyembryony, etc.

Q.3: What are some examples of Polyembryony?
Ans: Polyembryony commonly occurs or is induced in Citrus fruits, Mango, Jamun, etc.

Q.4: What is the importance of Polyembryony?
Ans: In horticulture, polyembryony is quite important. It ensures that the parent and seedlings are genetically similar and that the seedlings are disease-free.

Q.5: What are the types of Polyembryony that Ernst and Schnarf classified?
Ans: There are two types of Polyembryony that Ernst and Schnarf classified, i.e., true polyembryony and false polyembryony.

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