Polyembryony is the development of multiple embryos in a single ovule, seed, or fertilised ovum. It is found in both animals and plants. The most striking cases of polyembryony are seen in certain animals, such as parasitic Hymenoptera, where up to 2,000 embryos can develop from a single zygote.
This phenomenon was first observed in plants by Antoni van Leeuwenhoek (1719) in orange seeds. Polyembryony is so common in several gymnosperms that it may be considered an important characteristic of this group.
In the majority of gymnosperms that exhibit polyembryony, two or more archegonia develop in a female gametophyte. Because each archegonium contains an egg, two or more eggs may be fertilised, resulting in the formation of two or more potential embryos. Typically, only one embryo survives, and the others die during development.
Polyembryony
Polyembryony is the development of more than one embryo in a single ovule, seed, or by complete cleavage of a fertilised ovum. As a result, supernumeraries of various frequencies, such as duplet, triplet, quadruplet, quintuplet, sextuplet, and so on, emerge.
Polyembryony results in identical twins in humans. It can be found in both animals and plants. The best and most common example of this trait is a nine-banded Armadillo that gives birth to identical quadruplets.
Polyembryony in Plants
Antoni van Leeuwenhoek first observed this phenomenon in orange seeds (1719). Polyembryony is so common in gymnosperms that it may be considered a defining characteristic of this group. A female gametophyte develops two or more archegonia in the majority of gymnosperms with polyembryony. Because each archegonium carries an egg, the presence of multiple archegonium results in the fertilisation of two or more eggs, resulting in the formation of two or more possible embryos.
Types of Polyembryony
There are two types of polyembryony.
Induced Polyembryony
It includes cases of experimentally induced polyembryony.
Spontaneous Polyembryony
It includes all cases of polyembryony that occur naturally.
Webber (1940) classified Polyembryony into following Three Types
Cleavage Polyembryony
The single fertilised egg produces a large number of the embryos.
Simple Polyembryony
The fertilisation of several archegonia, a large number of embryos develop in this type.
Rosette Polyembryony
Additional embryos develop from the rosette cells in some gymnosperms (for example, a few Pinus species), and this type of polyembryony is known as rosette polyembryony.
Polyembryony in Various Groups of Gymnosperms
Polyembryony in Cycadales
The occurrence of polyembryony in Cycadales is uncommon. Rao (1964) found simple polyembryony in Cycas Circinalis. In this species, two adjacent archegonia of the same ovule sometimes develop independently into two embryos and, on rare occasions, two seedlings.
De Silva and Tambiah also discovered polyembryony in Cycas rumphii (1952). However, only one of the several embryos remains viable and survives in this species.
Polyembryony in Coniferales
The majority of Coniferal members exhibit imples polyembryony, with the number of embryos ranging from two to many. Cleavage polyembryony has also been observed in several Pinaceae (e.g. Pinus, Cedrus, and Tsuga) Taxodiaceae (e.g Sciadopitys and Sequoia) Cupressaceae (e.g.Thuja, Juniperus), and Podocarpaceae genera (e.g. Podocarpus). Cvpressus has both the simple and the cleavage polyembryony.
The zygote in Pinus divides twice to form four nuclei. These four nuclei divide again in the chalazal end of the archegonium to form two tiers of four cells each. Both tiers are divided once to create four tiers. These four tiers are referred to as embryonal tier, suspensor tier, rosette tier, and upper tier in ascending order.
The cells of the embryonal tier divide further into proximal secondary suspenders, which split apart, resulting in the formation of four distal embryos. This type of the polyembryony is known as cleavage polyembryony because of the splitting process. Some Pinus species also exhibit “simple polyembryony,” which is caused by the fertilisation of multiple archegonia.
A few Pinus species exhibit rosette polyembryony. Extra embryos develop from the cells of the rosette tier in this type.
The upper four cells of the embryonal suspensor, or E-tier, elongate in Cedrus, allowing the embryonal cells to penetrate quite deeply into the female gametophyte. Each lower embryonal cell divides multiple times to form a 4 to 5-celled structure. They separate from each other, forming four embryos, demonstrating cleavage polyembryony. Simple polyembryony occurs in Cedrus when more than one archegonia is fertilised.
Early embryonic divisions give rise to three tiers of four cells each, namely an upper ‘u’ tier, an’s’ tier, and a ‘e’ tier. Each cell in the ‘e’ tiers provides
Secondary suspenders and an embryo are formed. This pattern of embryogenesis is known as cleavage polyembryony of the most precocious kind. Sciadopitys has been reported to have very high order cleavage polyembryony, resulting in the formation of 12 to 28 embryos.
An advanced type of cleavage polyembryony has been observed in Podocarpus, and it “has an important bearing on an assessment of the evolutionary status of polyembryony in the conifers as a whole” (Sporne, 1965). In Podocarpus, each embryonal tetrad functions as an independent embryo.
Polyembryony in Taxales
Taxus’ female gametophyte contains several archeonies. Many of their eggs may be fertilised, resulting in simple polyembryony. However, only one embryo grows to maturity. Suspensor cell cleavage occurs in Taxus, according to Sterling (1948).
Suspenders separate from one another, and each one may contain one or more embryonal units. At the base of the suspensor cells, groups of meristematic cells can be seen on occasion. Rosette embryos are the name given to these groups of cells. However, further development does not occur in these embryos.
Polyembryony in Gnetales
Polyembryony occurs in all Gnetales (Gnetum, Ephedra, and Welwitschia). Polyembryony is of the highest order in Gnetum. “Not only are there several prothalli in each seed and several zygotes in each, but there is also multiplication of embryos from each zygote by the branching of the primary suspenders, and the further proliferation of the secondary suspenders can occur,” according to the description of this genus.
Despite all of this, only one embryo normally matures in each seed (Sporne, 1965). Vasil (1959) and Madhulata (1960) also reported polyembryony in Gnetum ula and Gnetum gnemon. Polyembryony occurs in these species either as a result of secondary suspensor budding or as a result of proliferation.
Each primary suspensor tube in Gnetum gnemon 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 grow to form more embryos.
In Ephedra, the zygote divides three times, resulting in the formation of eight products, all of which are capable of giving rise to embryos, though only three to five do so most of the time. Such early polyembryony is remarkable.
High degree of polyembryony is observed in Welwitschia, the third genus of Gnetales. This genus produces a large number of zygotes, almost all of which begin to develop embryos, but only one embryo develops at maturity in any one seed.
Conclusion
Polyembryony is the development of multiple embryos in a single ovule, seed, or fertilised ovum. It is found in both animals and plants. In the majority of gymnosperms that exhibit polyembryony, two or more archegonia develop in a female gametophyte. Because each archegonium contains an egg, two or more eggs may be fertilised, resulting in the formation of two or more potential embryos. . Because each archegonium carries an egg, the presence of multiple archegonium results in the fertilisation of two or more eggs, resulting in the formation of two or more possible embryos. The majority of a Coniferal members exhibit imples polyembryony, with the number of embryos ranging from two to many. A few Pinus species exhibit rosette polyembryony. Extra embryos develop from the cells of the rosette tier in this type. Each primary suspensor tube in Gnetum gnemon may develop an embryo at its tip, resulting in the formation of many embryos.