Each species has a characteristic number of chromosomes in its somatic cells. The chromosome number in gametic cell constitute the basic set of chromosomes of genome which called as haploid It becomes diploid in zygote on fertilization. Any change in number of basic chromosome set of variation in normal. Diploid (2n) chromosome number is called ploidy. The organism which bears three or more genomes (n) is called as polyploidy. Polyploidy is more commonly found in plants rather than animals. Polyploids may be triploid (3n), tetraploid (4n), hexaploid (6n) or) Octaploid (8n).
Origin of Polyploidy:
i) Polyploids are originated by failure of normal mitotic division in somatic cells.
ii) Polyploids may originate due to abnormal reduction divisions resulting
iii) It may naturally occur due to fertilization of egg by more than one male gamete.
iv) It may originated by artificial induction using colchicines or may be originated by cross hybridization between haploid and diploids.
Kinds of polyploidy:
i) Autopolyploids :
When more than two genomes developed by the multiplication of chromosome number of some individual is called Autopolyploids. On the other hand autopolyploids are the individuals of which body cells contains more than two identical set of chromosomes derived by self duplication. It arises due to failure of anaphase.
Autotriploids (3n) – Oenothera, Datura, Dahlia, Rose etc. autotriploids are more vigourous, shows more perenation of the organs of vegetative propagation and are highly sterile.
Autotetraploids : (4n) – it contain four identical genomes and arise by fasion of two diploid gametes. It may results from duplication of somatic chromosomes. Autotetraploids show great adaptability, disease registant, larger seeds, high vitamin-C content and low fertility. These are found in Apples, Grapes, Marigolds.
The individual in which the genomes makes up a multiple set of chromosome derived from two distinct individuals of two distinct species through hybridilization are called allopolyploids and the process of ploidy is called allopolyploidy.
These obtained from crosses made between two members of distinct taxonomic group in which chromosomes do not synapse during Zygotene stage of meiosis-I.
It help in origin of new species like Gossypium arboretum (A seatic cotton), Raphono brassica (Raphanus sativus x Brassica olaracea), Triticale (Triticum durum X Secal cereal) etc. Polyploidy has it significant role in crop improvement and evolution of new species in nature.