Polyembryony : Meaning and Types | Gymnosperms

In this article we will discuss about Polyembryony in Various Groups of Gymnosperms. After reading this article you will learn about: 1. Meaning of Polyembryony 2. Types of Polyembryony 3. Polyembryony in Various Groups of Gymnosperms.

Meaning of Polyembryony:

The phenomenon of the development of more than one embryo in one ovule, seed or fertilized ovum is called polyembryony. It occurs in both animals as well as plants. Most striking cases of polyembryony are seen in certain animals {e.g. parasitic Hymenoptera), where up to 2,000 embryos may spring from one zygote.

In plants, this phenomenon was first reported by Antoni van Leeuwenhoek (1719) in orange seeds. In several gymnosperms, the polyembryony is so common that it might be regarded as an important character of this group.

In majority of the gymnosperms showing polyembryony, usually two or more archegonia develop in a female gametophyte. And as each archegonium contains an egg, two or more eggs may be fertilized and thus two or more potential embryos may be created. Only one embryo, however, survives usually, and all the others perish during the course of the development.

Types of Polyembryony:

Polyembryony may be of following two types:

1. Induced Polyembryony : It includes cases of experimentally induced polyembryony.

2. Spontaneous Polyembryony: It includes all cases of naturally occurring polyembryony.

Webber (1940) classified polyembryony into following three types :

1. Cleavage Polyembryony: In this type a single fertilized egg gives rise to number of embryos.

2. Simple Polyembryony: In this type number of embryos develop as a result of the fertilization of several archegonia.

3. Rosette Polyembryony:

In some gymnosperms (e.g. a few species of Pinus), additional embryos develop from the rosette cells, and this type of polyembryony has been termed as rosette polyembryony.

Polyembryony in Various Groups of Gymnosperms:

(i) Polyembryony in Cycadales:

Polyembryony in Cycadales is not a common occurrence Rao (1964), however, reported simple polyembryony in Cycas Circinalis (Fig. 20.1 A). Two adjacent archegoia of the same ovule in this species sometimes develop independently into two embryos and also rarely into two seedlings.

Polyembryony has also been reported in Cycas rumphii by De Silva and Tambiah (1952). Only one out of the several embryos, however remains potential and persists in this species.

(ii) Polyembryony in Coniferales:

Simple polyembryony occurs in majority of the members of Coniferales and number of embryo varies from 2 to many. Cleavage polyembryony has also been reported in several genera of Pinaceae {e.g. Pinus, Cedrus and Tsuga), Taxodiaceae (e.g. Sciadopitys and Sequoia), Cupressaceae (e.g. Thuja, Juniperus) and Podocarpaceae (e.g. Podocarpus). Both simple and cleavage polyembryony are common in Cvpressus.

In Pinus, the zygote divides two times to form four nuclei. In the chalazal end of the archegonium these four nuclei divide again to form two tiers of four cells each. Both tiers divide once to form four tiers. From below upwards these four tiers are called embryonal tier, suspensor tier, rosette tier and upper tier.

The cells of the embryonal tier divide further into proximal secondary suspensors, which split apart, and four distal embryos are formed. Because of the splitting process this type of polyembryony is called “cleavage polyembryony”. Some species of Pinus also exhibit “simple polyembryony” which results from the fertilization of several archegonia.

A few species of Pinus also show “rosette polyembryony”. In this type, extra embryos develop from the cells of the rosette tier

In Cedrus, the upper four cells of the embryonal suspensor or E-tier elongate to push the embryonal cells quite deep into the female gametophyte. Each of the lower embryonal cells divides repeatedly to form a 4 to 5-celled structure. They separate from each other forming four embryos, showing an example of cleavage polyembryony. Simple polyembryony is also common in Cedrus as more than one archegonia get fertilized.

In Sequoia (Fig. 20.2 A) the early divisions during embryogenesis give rise to three tiers of four cells each, i.e. an upper ‘u’ tier, an ‘s’ tier and an ‘e’ tier. Each cell of the ‘e’ tier gives

rise to secondary suspensors and an embryo. This pattern of embryogenesis is regarded as cleavage polyembryony of a particularly precocious kind. Cleavage polyembryony of a very high order has been reported in Sciadopitys, and this results in the formation of 12 to 28 embryos.

In Podocarpus, an advance type of cleavage polyembryony has been observed, and it “has an important bearing on the assessment of the evolutionary status of polyembryony in the conifers as a whole” (Sporne, 1965). Each embryonal tetrad behaves as an independent embryo in Podocarpus (Fig. 20.2B).

(iii) Polyembryony in Taxales:

Several archegonia are present in the female gametophyte of Taxus. The eggs of many of them may be fertilized, resulting into simple polyembryony. Ultimately, however, only a single embryo attains maturity. According to Sterling (1948), cleavage of suspensor cells occurs in Taxus.

The suspensors separate from each other, and each of them may carry one or more embryonal units. Sometimes, groups of meristematic cells are observed at the base of the suspensor cells These groups of cells are called the rosette embryos. Further development, however, does not take place in these embryos.

(iv) Polyembryony in Gnetales:

All Gnetales (Gnetum, Ephedra and Welwitschia) exhibit polyembryony. In Gnetum, polyembryony is of very high order. In this genus, “not only are there several prothalli in each seed and several zygotes in each, there is also the multiplication of embryos from each zygote by branching of the primary suspensors, and further proliferation of the secondary suspensors can occur”.

In spite of all these, “only one embryo normally reaches maturity in each seed” (Sporne, 1965). Vasil (1959) and Madhulata (1960) have also reported some cases of polyembryony in Gnetum ula and G. gnemon. In these species, polyembryony occurs either due to budding of the secondary suspensor or due to proliferation (Fig. 20.1C).

In Gnetum gnemon, each of the primary suspensor tube 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 enlarge to develop into additional embryos.

In Ephedra, the zygote divides thrice resulting in the formation of eight products, of which all “are capable of giving rise to embryos, although more commonly only three to five do so. Such precocious polyembryony is remarkable”.

In Welwitschia, the third genus of Gnetales, “high degree of polyembryony” is seen. Many zygotes are produced in this genus, and almost all start producing embryo, “but only one embryo develops at maturity in any one seed”.