To define polyembryony, let us discuss fertilisation of eggs in brief both in plants and animals.
To define polyembryony, let us discuss fertilisation of eggs in brief both in plants and animals. In plants, male sperm cells fuse with an ovule which further develops into seed/seeds. Each seed contains an embryo (which is a very small undeveloped plant). In animals, fertilisation can be both internal or external. The male gamete fertilises female organisms’ eggs forming the primary nucleus of an embryo. Polyembryony is understood as a kind of clonal development in which one egg goes on to produce two offspring which are genetically identical. It occurs both in plants and animals.
Different groups of gymnosperms exhibit polyembryony –
Two adjacent archegonia within one ovule may independently develop into two embryos.
Simple polyembryony occurs with the number of embryos varying from two to numerous.
Multiple archegonia are present within female gamete. Even though many eggs may be fertilised, only one embryo finally attains maturity.
In this case, polyembryony is a high order, and embryos multiply from each zygote.
There are various theories that are put forward to explain polyembryony –
Cells witness nucellus degeneration in order to give rise to a stimulus for adjacent cells to undergo division. It leads to the formation of adventive embryos.
Hybridisation process leads to recombination of genes where a single unit is formed, which creates multiple embryos.
Polyembryony types can be spontaneous or naturally occurring as well as experimentally induced. Spontaneous polyembryony can be further categorised into three types –
In cleavage polyembryony, one fertilised egg produces multiple embryos.
Here, a number of archegonia are fertilised to give rise to multiple embryos.
Some gymnosperms have rosette cells. The embryos develop from such cells.
True polyembryony is the common instance of the production of embryos with the projection into a single embryo sac. The additional embryos are created either from cleavage of a zygote or from antipodal cells and synergids.
False polyembryony is the formation of two or more embryos with the development of monosporic embryo sac. Here two or more nucelli fuses for embryo sac development.
Such polyembryony does not require initial sporophytic cells, proembryo or zygote. The development of an embryo can be made in a culture medium. These induced embryos are known as somatic embryos or adventitious embryos.
There is a lot of practical importance in polyembryony. It is particularly useful for the propagation of certain species of plants. For instance, adventive embryos, which retain characteristics of the parent plant, is used to create genetically uniform seedlings of citrus and mango. Moreover, in the case of citrus, grafts from other types are also used in orchard stock.
(a) Citrus (b) Brassica (c) Triticum (d) Gossypium Ans . (a) Citrus
(a) Euphorbia (b) Citrus (c) Lilium (d) Mangifera Ans. (b) Citrus
Ans. The condition of the development of two or more embryos from one fertilised egg is called polyembryony.
Ans. Polyembryony is common in citrus plants as well as mango and jamun where multiple embryos arise from sporophytic cells of ovules or zygote.
Ans. Polyembryony is important for horticulture and plant breeding along with producing genetically same seedlings.
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