Polyembryony in Ovule of Flower
When more than one embryo are produced in an ovule. In a seed with many embryos, usually one embryo matures and rest degenerate during course of development. According to Ernst (1918) and Schnarf (1929), ployembryony may be true or false depending upon whether the embryos arise in the same embryo sac or in different embryo sacs in the same ovule.
- Cleavage polyembryony
It results from the cleavage of the zygote or earlier stages of its development (proembryo) into two or more units e.g. Nicotiana rustica, Isotoma longiflora, Lobelia, Erythronium. Cleavage polyembryony is common in gymnosperms, but it is of rare occurrence in angiosperms.
In Erythronium americanum, first division of the zygote is normal. From embryonic mass, many cells at distal end form separate embryos.
In Isotoma and Exocarpus, additional embryos are formed from suspensor cells of proembryo.
- Embryos from cells of embryo sac other than egg
The embryo may appear from synergids and antipodal cells in the embroy sac. The synergids may be fertilized by sperms from an additional pollen tube or develop without such fusion. In Argemone mexicana and Phaseolus vulgaris, additional embryos may appear from unfertilized synergids and hence haploid in nature. Embryos from antipodal cells (Fig. 2.36) develop less frequently (e.g. Ulmus americana Allium odorum). All the antipodal embryos may not remain viable.
- Embryos arising from the cells outside embryo sac
When cells of the nucellus and integuments have also been observed to develop into embryos e.g. Citrus, Eugenia and Mangifera. In Spiranthes, additional embryos have been reported to be developing from inner cells of inner layer of integument (Swamy, 1948). Such embryos subsequently come to lie in the embryo sac and are nourished by the endosperm.
- Embryos from endosperm
Embryos developing from endosperm have been reported in Balanophora (Treu, 1898), Alnus (Woodworth, 1930). However, Ernst (1913) found that such embryos develop from egg, got embedded in cellular endosperm.
Spontaneous and induced polyembryony
Polyembryony may be spontaneous which includes instances of naturally occurring polyembryony. Induced polyembryony includes instances of experimentally used cases.
Arising from zygote, proembryo or initial sporophytic cells of the ovule (nucellus, integuments). Embryo development can also be made in culture medium (induced polyembryony). The embryos developed in culture medium are known as adventitious embryos, somatic embryos, supernumerary embryos or embryoids.
Practical value of polyembryony
Nucellar adventive polyembryony is of great significance in horticulture. The adventive embryos provide uniform seedlings of parental type. Nucellar seedling of Citrus provides better clones than cuttings. Cuttings form lateral roots and nucellar seedlings develop tap roots (better root system). Nucellar seedlings show restoration of vigour. Moreover, nucellar embryos are free from disease.
Apomixis (Gk. apo- without, mixis- mixing) is a mode of reproduction which does not involve formation of zygote through gametic fusion. It is, therefore, akin to asexual reproduction. In plants apomixis commonly mimics sexual reproduction but produces seeds without fertilisation, e.g., some species of Asteraceae and grasses.
There are several methods of apomictic development in seeds. The two common ones are recurrent agamospermy and adventive embryony.
- Recurrent Agamospermy
Agamospermy (Gk. a- without, gamos- marriage, sperma- seed) is the formation of seed that has an embryo formed without meiosis and syngamy. It is of two types, non-recurrrent and recurrent. In non-recurrent agamospermy, the embryo is haploid.
Therefore, the seed having it is non-viable. In recurrent agamospermy all the cells of embryo sac are diploid as it is formed directly either from a nucellar cell (apospory) or diploid megaspore mother cell (diplospory).
The diploid egg as well as other diploid cells of embryo sac can grow into normal embryos. Formation of embryo directly from diploid egg without fertilization is called diploid parthenogenesis, e.g., Rubus, Apple, Poa.
- Adventive Embryony (Sporophytic Budding)
When an embryo develops directly from a diploid cell other than egg like that of nucellus and integument, e.g., Citrus, Opuntia. There may be more than one egg cell in an embryo sac or more than one embryo sac in an ovule.
All the egg cells may get fertilised. Synergids and antipodal cells may also form embryos. In gymnosperms polyembryony can also occur due to cleavage of growing embryo. It is called cleavage polyembryony.
Occurrence of polyem-bryony due to fertilisation of more than one egg is called simple polyembryony. Formation of extra embryos through sporophytic budding is called adventive polyembryony. Polyembryony is quite common in Onion, Groundnut, Mango, Lemon, Orange.
(i) Hybrid varieties provide higher and better yield. They are, therefore, preferred. A number of cereals and vegetables are being raised by using hybrid seeds. However, there is one major drawback. Hybrid seeds have to be produced every year because seeds collected from hybrid plants, if sown subsequently, do not maintain hybrid characters due to segregation of traits.
Production of hybrid seeds every year is costly thus increasing the cost of crop production. This can be avoided if apomixis can be introduced in hybrid seeds. Apomixis is genetically controlled. Therefore, scientists are busy in identifying genes for apomixis so that they can be introduced in hybrid varieties.
(ii) Adventive embryos are better clones than cuttings.
(iii) Embryos formed through apomixis are generally free from infections.