Friday, September 25, 2009

Polyploidy, its Kinds and Significance

The term Polyploidy refers to an organism which contains more chromosomal sets than the normal diploid (2 haploid sets) number. Ploids refers to chromosome sets. Haploid means one set (also called genome). Diploid means two haploid sets (each chromosome is paired).

The cells or organisms containing, more than two sets of chromosomes are called “Polyploids” e.g. triploid, with three sets, tetraploid with four sets. Polyploid condition is produced due to extra duplication of the chromosomes. In plants Polyploids can be produced by treating the plants with colchicines which inhibits cell division but do not inhibit chromosomal replication.

Polyploids are often large more healthy, more strong and more vigorous than the normal diploids. It is common in plant but is rare in animals, although mammalian tissue culture cells sometimes become polyploid.

EUPLOIDY

If every chromosome of the set is represented in the same number of times, the condition is called Euploidy.

Epiploids are of two types.

(1) Autoploids (2) Alloploids

(1) Autoploids: If the chromosome sets of the same specie are in any multiple condition the condition is called Autoploidy.

The antophloids with odd number of chromosomes of each type (3, 5, 7 etc) are sterile because they npaired chromosome would be unable to undergo pairing during metaphase of meiosis. A type of wheat called Einkorn (T. monococcum) contains 7 pairs (14) of chromosomes. Plants are very small and yield little grains.

Another group of wheat variety contains 28 chromosomes. Commonly cultivated wheat or bread wheat contain 42 chromosomes.

(2) Alloploids or Amphiploids: If the chromosome sets of two different species are in multiples in the hybrids of two different species, the condition is called alloploid. The hybrids of two different species are sterile because the non homologous chromosomes cannot pair at metaphase of meiosis. If the chromosomes are some how doubled the hybrids become fertile because the homologous chromosomes would thus be able to pair at metaphase of meiosis.

Karpecheake crossed Radish with cabbage, the hybrids were vigorous but sterile. He repeated his crossing again and again unless he was able to get a few fertile hybrids. Chromosomal analysis of fertile hybrids revealed that they contained two sets of chromosomes of both the species.

The hybrids were unable to cross with both of their parental species, thus the polyploid hybrids were named Raphanobrasica species.

Anenploidy: If all the chromosomes of the haploid set are not represented in the same number of times i.e. some chromosomes are represented more times than the others, such a condition is called aneuploidy. Aneuploids are less viable than euploids.

Polysomic condition: If only one or a few chromosomes are represented more number of times, while the rest of the chromosomes of the set are normal diploid, the condition is called Polysomic.

SIGNIFICANCE OF POLYPLOIDY:

(1) Poluploids are usually more vigorous as compared with the diploids e.g.: triploid aspen tree have very large leaves, large stomatal cells and xylem cells.

(2) It often reduces sexual fertility.

(3) It has been estimated that one third species of flowering plants are polyploids. Therefore polyploidy is important in evolution. Polyploidy leads to sudden development new species in few steps.

(4) Alloploids contain new combination of characters and they may adapt themselves in a better way in case of a sudden change in the environment as compared with the previous ancestors.

(5) Recessive nutation which are usually harmful have the least change of expressing themselves in polyploids.

(6) Inherited variability is important in evolution. As the polyploids inhibits the chances of inherited variability; therefore it is disadvantageous in the long term evolutionary process.

(7) Accumulation of diverse genomes in alloploids lead to the ability to adapt a wide range of environmental tolerance.

(8) Alternations in genome structure not only affect fertility but also affect synapsis and crossing over.

(9) Polyploidy also results in modification in functioning of the genes as well as alter the dominant recessive relationship of the genes. Thus the physiology of the organism is modified.

(10) Unbalanced genome structure e.g.: triploid, pentaploidy and polysomy mostly results in invariability or infertility. Such unbalanced genomes are not advantageous.

(11) Reduplication of complete genome do not usually unbalance the organisms and synapsis and meiosis can occur regularly to continue sexual reproduction normally e.g. tetraploidy, hexaplooidy.

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