The phenomenon of polyembryony, i.e., the occurrence of more than one embryo in a seed, has attracted much attention ever since its initial discovery in the orange (Citrus sinensis) by Leeuwenhoek (1719).
Ernst (1918) and Schnarf (1929), who have reviewed the older literature, classify it into two types—”true” and “false”—depending on whether the embryos arise in the same embryo sac or in different embryo sacs in the ovule.
- True polyembryony: In which there is a development of multiple embryos within or by projection into a single embryo sac.
- False polyembryony: Multiple embryos developed from different embryos.
Methods of polyembryony: Braun (1859) divided origin of polyembryony into 4 types.
The simplest method of an increase in the number of embryos is a cleavage of the zygote or proembryo into two or more units. Although common in gymnosperms, its occurrence is only sporadic in the angiosperms.
Jeffrey (1895) gave a detailed account of cleavage polyembryony in Erythronium americanum. After fertilization the synergids degenerate and disappear, and the zygote divides to form a small group of cells, which do not show any definite order or arrangement. This group continues to increase in volume, and outgrowths arise at its lower end which eventually function as independent embryos. The production of two or three embryos from such an “embryogenic mass” was found to be a common feature, but sometimes as many as four were distinguishable.
Kausik and Subramanyam (1946) figure an embryo sac of Isotoma longiflora in which an additional embryo seems to have budded out from a suspensor cell.
It is only in the family Orchidaceae that cleavage polyembryony seems to be of more frequent occurrence. In Eulophia epidendraea, which may be cited as an example, Swamy (1943) records the following variations:
(1) the zygote divides irregularly to form a mass of cells, of which those lying towards the chalazal end grow simultaneously and give rise to multiple embryos.
(2) the filamentous proembryo becomes branched and each of the branches grows into an embryo.
(3) the proembryo gives out small buds or outgrowths which may themselves function as embryos.
Origin of Embryos from Cells of the Embryo Sac
Other than the Egg. Besides the zygotic embryo produced from the egg, embryos may also be produced from other parts of the embryo sac. The most common source is the synergids which frequently become egglike and may be fertilized by sperms from an additional pollen tube (Nazas minor, Poa alpina) or develop without fertilization (Phaseolus vulgaris).
Production of embryos from antipodals – Ulmus glabra, Sedum fabaria.
Embryos Arising from Cells outside Embryo Sac
Citrus, Eugenia, and Mangifera are well-known examples of this type of polyembryony. The embryos, although initiated outside the embryo sac, subsequently come to lie inside it and are nourished by the endosperm (Fig. 192).
Recent work indicates that the occurrence of adventive embryony may not be a constant feature of all the individuals of a species. Swamy (1948) has found that the orchid Spiranthes cernua comprises three races. The first shows normal sexual reproduction and a single zygotic embryo is produced in each seed. In the second race, which is apomictic, the male and female gametophytes are both functionless. Fertilization does not occur but the cells of the inner layer of the inner integument give rise to adventive embryos of which two to six may mature in a seed. In the third race, which is of an intermediate type, some ovules of an ovary follow the course outlined for the first race and others for the apomictic race.
Embryos Originating from Other Embryo Sacs in the Ovule
The polyembryonate condition is sometimes due to the occurrence of multiple embryo sacs within the ovule. These may arise
(1) either from the derivatives of the same megaspore mother cell, or
(2) from two or more megaspore mother cells, or
(3) from nucellar cells (apospory).
Examples: Citrus, Poa partensis.
Source: An Introduction to Embryology of Angiosperm by P. Maheshwari.
Importance of Polyembryony
(will be added soon with images)