Polyembryony

Introduction

Polyembryony is a common phenomenon that mostly occurs in plants when two or more embryos develop from a single fertilised egg. In case this happens in humans during reproduction, this leads to the birth of identical twins.

Polyembryony definition

Polyembryony occurs when two or more embryos develop from a single fertilised egg. This natural phenomenon mainly occurs in animals and plants. Nevertheless, the process also occurs in humans, resulting in identical twins. The most common example is the nine-banded armadillo found in the animal kingdom. Nine-banded armadillos are rare mammals that produce identical quadruplets. These are found in several parts of America.

Polyembryony in plants

Polyembryony in plants is the formation of more than one embryo from a single fertilised egg or a seed. In plants, it usually occurs because of the fertilisation of more than one embryonic sac or because of the origin of embryos out of the embryonic sac. In 1719, Antonie van Leeuwenhoek discovered polyembryony after observing two embryos forming in the seed of a citrus plant.

Polyembryony classification

There are mainly two types of polyembryony-

• Spontaneous ( or naturally occurring polyembryony)
• Induced (or experimentally induced polyembryony)

However, based on Weber’s theory, polyembryony has three types:

• Cleavage polyembryony: In cleavage polyembryony, only one fertilised egg can produce multiple embryos. When an embryo splits into several identical parts, it is capable of developing into a mature embryo. The most common example of Cleavage polyembryony is Pinus
• Simple polyembryony: This type of polyembryony results from the fertilisation of several archegonia. When the polyembryony as more than one egg is fertilised, it is simply termed Simple polyembryony. Adventive polyembryony can be defined as the formation of extra embryos through sporophytic budding. It is pretty common in Cycas, Lemon, Groundnut, Mango, Pinus, Onion, Orange
• Rosette polyembryony: In this type of polyembryony, several additional embryos are born from cells known as rosette cells

Polyembryony in different groups

• Polyembryony in Cycadales

Polyembryony is an uncommon phenomenon in Cycadales. However, the phenomenon of simple polyembryony in Cycas Circinalis was first reported in 1964 by Rao. In this species, the similar ovule of two adjacent archegonia starts to grow independently in two different embryos. However, this condition is extremely rare.

• Polyembryony in Coniferales

In the case of Polyembryony in Coniferales, simple polyembryony happens in the majority of its members. Here, the embryo numbers vary from two to several others. Several Coniferales have been reported to exhibit cleavage polyembryony, including Cupressaceae, Pinaceae, Taxodiaceae, and Podocarpaceae. Moreover, both cleavage and simple polyembryony appear to be common in Cupressus.

• Polyembryony in Taxales

In the female gametophyte, a number of archenemies are readily present. Simple polyembryony takes place because of the archegonia eggs fertilisation. Only a single egg manages to attain maturity out of all the eggs. In the process, cleavage of the suspensor cells occurs. Each suspensor is separated from another, and it may hold a single embryonal unit or more. At times, the meristematic cells groups form at the base of the suspensor cells. These cell groups constitute the rosette embryos. These embryos do not undergo further development.

• Polyembryony in Gnetales

Polyembryony is very common in Gnetales. Gnetum, a genus in the Gnetales, has a high level of polyembryony. There are several zygotes and prothalli present in each and every seed. However, the primary suspensors also branch out and produce multiple embryos from each zygote.

Differences between Apomixis and Polyembryony

Conclusion

Polyembryony is a common phenomenon that mostly occurs in plants when two or more embryos develop from a single fertilised egg. The nine-banded armadillo in the Animal kingdom is a common example of polyembryony. It is a medium-sized mammal found rarely in America. Polyembryony occurs in plants as well as some animals. In case it takes place in humans, those are referred to as identical twins. Polyembryony acts as a base for many upcoming chapters in biology; therefore, a thorough understanding of the concept is a must.