Polyploidy is the occurrence of multiple sets of homologous chromosomes in an organism/ cell. The economic significance of polyploidy especially in plants has been well established with the past research. Part of the credit goes to the tendency of polyploids to evade inbreeding depression.
Natural polyploidization has been known to be instrumental in altering gene expression, gene regulation etc and subsequently expressing increased vigor and resistance to diseases as phenotype qualities. More recently the importance of polyploidy in driving evolutionary processes is being discovered.
The term polyploidy was introduced by Winkler in 1916 and was instrumental in establishing artificial polyploidy. The vegetative grafts and chimeras of Solanum nigrum when allowed to regenerate formed tetraploids at the cut ends of stem explants. The occurrence of polyploidy was later confirmed by the works on Primula, Chrysnthemum, and Chenopodium.
Induced changes in chromosomes are referred to as Polyploidy breeding. The method of breeding is entirely dependent on the regulation of chromosome pairing and recombination. The strategies used for such breeding techniques depend on the origin. There are several different methods used for induction of polyploidy for commercial applications. They are
1. Physical agents
Polyploidy can be induced with sudden variations in physical and environmental factors such as sudden changes in temperature, dehydration, uv light, x-rays, infections, etc which can cause chromosome doubling.
a. Heat/cold shock - Sudden variations in temperature such as a heat or cold shock have been found to result in formation of polyploid cells. Examples include formation of tetraploidy in Datura due to cold shock and tetraploid origins in maize due to heat shocks. Heat treatment has been found to be effective in generation of polyploid offsprings in wheat and rye also.
b. X-rays - Normal diploid plant cells gives rise to polyploid generation on exposure to radio active substances such as radium.
c. Centrifugation of seedlings has been found to produce a higher incidence of polyploid cells in plants.
2. Chemical agents
Various chemical agents were used to induce polyploidy.
a. Colchicine - The colchicines method is perhaps the most widely used method. The process was developed by Blackslee and Avery in 1937. The alkaloid suppresses mitotic spindle fibre production during mitosis. The concentration of colchicine used depends on the nature of plant part used for regeneration.
Seeds- 0.001-1% (1-10 days)
Seedlings - 0.02-0.06% (3-24 hrs)
Shoot apices- 0.1-1.0 % (few days)
Shoot buds of woody plants - 0.05- 0.15% (4-36 hours)
Coclchicine is generally used as aqueous solutions at varying concentrations, in glycerine, or in agar gel. It can be used to generate fertile polyploids. For example, the triplicate triticale hybrid of wheat and rye can be made fertile by treatment with colchicines.
b. Other chemical agents: Polyploidy can also be induced by use of other chemicals such as acenaphthene, nitrous oxide, 8- hydroxyquinoline etc., which have polyploidizing effect. Others which yield similar results include chloral hydrate, veratrine, sulfanilamide, mercury chloride, ethyl, hexachrolocyclohexane etc.
These chemicals disrupt the formation of microtubules for mitotic spindles and thus caused non segregation of chromosome pairs eventually resulting in formation of tetraploid cells. These cells subsequently double their chromosomes to give higher numbers of polyploid offspring.
3. Cell generation
The method is used in bryophytes like mosses. The sporophytes are cut and kept in moist conditions. The cut ends develop callus having true protonema which produces diploid gametophytic generation and some polyploid cells. This results in production of autopolyploids. Callus and suspension cultures are widely used in production of polyploid cells in Nicotiana, Oryza and Datura.
Production of allopolyploid vs. autopolyploid
Autopolyploids are found to have more vegetative growth and reduced number of seeds. Hence they are more suitable for those plants which have economically important vegetative parts such as forage, root etc. Cross pollinated crops with more genetic variation function as better parental species for production of autopolyploids.
Production of synthetic allopolyploids involves production of F1 distant hybrids and chromosome doubling. Allopolyploids are used when direct cross between species is not permissible due to possibility of generation of sterility in F1. In such cases, first an amphidiploid is produced and then this is crossed with the diploid recipient species. Allopolyploidy is also useful in creation of new species and transfer of genes for desirable characters between species. Study of natural allopolyploidy has thrown sufficient light on the evolutionary history of plants.
However, the use of allopolyploidy is limited. The progeny may have defects and the whole process requires time, labor and resources. It is not possible to accurately predict the effect of allopolyploidy and the chances for synthesis of new successful species are also considerably low.
In spite of this, the research on polyploidy is expanding widely and polyploidy is found to be a promising approach in plant breeding.
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