Polyembryony, Apomixis, Embryo Culture

Polyembryony, Apomixis, Embryo Culture, 

Polyembryony

         

Normally, each seed contains a single embryo produced by the development of zygote. Sometimes, more than one embryo is present in a seed. This phenomenon of occurrence of more than one embryo per seed is called polyembryony (Poly = many). This is commonly seen in Citrus .But is rare in other plants. In these plants, if there is more than one embryo, only one of them matures and develops into the seedling. As spatial and nutritional considerations in the seed can not allow the development of more than one embryo.

         

It was discovered by Anton Van Leeuvenhoek in 1719 in the seeds of orange. Since, then it has been discovered in large number of taxa of angiosperms. It has been classified as true polyembryony and false polyembryony. When the embryos are formed by a single embryo-sac; it is called as true polyembryony, when the embryos arise from different embryo-sacs it is called as false polyembryony.

[A] True polyembryony-

It may be due to development of embryos

Within the embryo-sac either by a cleavage of the egg, or from the synergids or antipodal.

From the tissues lying outside the embryo-sac for e.g. cells of nucellus or the integuments.

 

[B] False polyembryony –

It includes fusion of two or more nucelli or the development       of two or more embryo-sacs within the same nucellus.

      

Cleavage polyembryony-

In this case the zygote undergoes splitting to form a number of embryos. It is common in gymnosperms but less common in angiosperms. When it does occur, it is by irregular divisions in zygote which forms a group of embryonic cells. Each of these cells may function as an independent embryo. E.g  Tuilpa, Primula, Cocos.

    

Embryos arising from the synergids –

Embryos may also be produced from cells of the embryo-sac other than egg. The most common source is synergids which frequently become egg-like and are capable of developing into embryos with or without fertilization. When they develop without fertilization the embryo contains haploid number of chromosomes. In some cases, during fertilization the two male gametes fertilize the egg and one of the synergids forming two identical twin embryos, but in these cases, there is no endosperm formation and such embryos generally abort. E.g. Argemone, Sagittaria, Aristolochia .

    

Embryos arising from the antipodal cells –

Usually the antipodal cells are ephemeral and degenerate shortly before or after fertilization. But, in few plants like, Ulmus, Nigella, Allium etc. the antipodal cells not only persist but also divide and produce embryos.

      

Adventive polyembryony-

Adventive embryos are formed from cells lying outside the embryo-sac i.e. cells of nucellus or the integuments. These cells originate from the nucellar tissue and grow at the expense of their surrounding cells and proliferate into the embryo-sac cavity where they complete their further development.

They can be identified from sexual embryos as they are lateral in position, irregular in shape and lack suspensor. Common examples are Citrus, Opuntia, Mangifera In such cases endosperm formation takes place as usual with exception of Orchidaceae and Opuntia. Adventive polyembryony is sometimes independent of the stimulus of pollination and fertilization e.g Euphorbia.

The adventive embryos of Eugenia jambosa are initiated without any external stimulus, but attain their full development only after pollination and fertilization without which no endosperm is formed. In most other plants pollination or pollination after fertilization is important even for the initiation of adventives embryos.

          

Apomixis

The organisms which follow the above mentioned pattern of life cycle are said to be amphimictic and the phenomenon is known as amphimixis. But, sometimes, this process is altered and organisms are borne without the fusion of two gametes this phenomenon is known as apomixis. Winkler (1908) coined the term apomixis and defined it as the substitution of sexual reproduction by a type of  reproduction  which does not involve meiosis and syngamy.

        

This phenomenon is classified into two categories- Vegetative propagation and Agamospory

        

Vegetative propagation-

In this case, sporophytic plants reproduce directly into another sporophyte using a part of their body. The vegetative propagation includes bulbils,   bulbs, suckers, offsets, runners etc.         

         

Agamospory-

In this case, sporophytic plants reproduce by producing a seed. But the seed may not be produced by the normal process of meiosis and syngamy (agamo  = without gametes, spermy = seed). Three types of  agamospermy have been recognized in  plants-

Adventive embryony :

In this type embryos arise from the sporophytic tissue of the ovule (nucellus or integuments) the sexual embryo sac may or may not produce an embryo.

Diplospory :

Here an archesporial cell differentiates into MMC but the MMC will not undergo meiosis but directly develops into an embryo sac. As a result all the cells of the embryo sac are diploid. The unfertilized egg develops into embryo (parthenogenesis) or as in some cases any cell other than egg cell may develop into embryo (apogamy).

Apospory : (Apo =without, spory = spore formation)

In this type any cell of the nucellus directly develops into an embryo sac without undergoing meiosis. Here also all the components of the embryo sac are diploid. Embryo might develop from the egg without fertilization.

Significance of apomixis-

·         It is advantageous due to

·         It shortens the time required to produce new individuals and

·         It assures complete transfer of parental combinations to the next generation.

Embryo culture

          

Hannig (1904) was the first person who tried to artificially culture embryos of angiosperms particularly of Raphanus and Cochlearia.   Dietrich (1925) reported that embryos grow very well on a semi solid medium containing minerals and sucrose. He further reported that the embryos from immature seeds grow directly into seedlings skipping some of the stages of the embryogenesis. This precious germination was called “kunstliche Fruhgeburt”. Liabach (1928) demonstrated that the dissected embryos can be used to break dormancy.

         

Fully matured embryos can be cultured on a basal medium supplemented with few minerals and sucrose. The embryonic cells have fully developed metabolic machinery and are able to synthesize all the requirements from a basic source.

         

In the embryo culture two stages may be recognized- a Heterotrophic phase and an Autotrophic phase. A young embryo requires specific nutrients which are provided to it by its own endosperm. Globular embryos can be cultured by adding to the basal medium substances such as coconut milk, yeast extract and growth regulators.  Embryos at the preglobular stage are very difficult to culture and even if they grow, mostly they form an undifferentiated mass of cells. This indicates that the maternal tissue surrounding the embryo not only provides the nutrition but also has a critical role on the process of differentiation.

         

In this technique, embryo of healthy seeds are dissected in sterilized condition and transferred to sterile containers with suitable medium. Conditions which should be considered during embryo culture are water, minerals like K, Ca, Mg, N, P, S, B, Cu, Mn, I, pH, osmotic value, vitamins, plant hormones, complex organic supplements, temperature and light. These embryos in favorable conditions develop into undifferentiated mass of cells the callus. This callus can be developed into a fully differentiated seedling by application of proper hormones.

    

Embryo culture has a number of practical applications.               

·         Crosses between plants of different ploidy within the same species have been prepared. E.g., Iris.

·         Interspecific crosses between plants of different species have been prepared. E.g., Gossypium, Datura.

·         It helps to break dormancy and hastens the process of germination.

·         It also helps in rapid testing of the seed viability, obtaining haploids and propagating rare plants etc.

·         It has also served in studies of the influence of specific substance on the morphology of embryos.