Principles of Plant Breeding (2+1)

Induction of polyploidy

Natural polyploids

• Polyploidy does take place in nature but at very low frequency. Some important crops like potato sweet potato and lucern are natural autopolyploids and other crops like wheat, brassica, cotton oat, tobacco sugarcane are natural allopolyploids.
  
• It is estimated that 70% grass species and 25% of legume species are polyploids.
  
• Polyploidy occurs either due to the mitotic failure or meiotic irregularities.
  
• During cell division the mitotic failure leads to a cell with doubled chromosome number.
  
• Generally such cells do not further divide and get lost but if present in the meristematic zones, these give rise to polyploid sector and occasionally the whole plant.
  
• The origin of polyploidy has been reviewed by Harlan and DeWet (1975) proposed that almost all polyploids arise by way of unreduced gametes and that all other.
Induction of polyploids

• Polyploids, in general, exhibit gigas characters which created interest in developing polyploids in otherwise diploid species. Polyploidy can be easily induced by using a chemical colchicine.
  
• The chemical prevents the spindle formation and thus, prevent the migration of the daughter chromosomes to opposite poles which leads to a polyploid cell and ultimately the plant.
  
• It is generally applied to the meristematic cells at the concentration ranging from 0.05-0.1 percent using cotton pads or by dipping the tissues in the colchicines solution for 2-10 hours.
  
• Treatment of presoaked seeds with 0.2 percent colchicines solution for 2-8 hours has also been reported to be very effective in several crops.
  
• In tissue culture during rapid cell divisions, endomitosis i.e. Chromosome doubling without cell division results into a high frequency of polyploid cells/plants.
  
• Although the poly ploids exhibit gigas characters but every species has got its optimum ploidy level at which it is more vigorous. For example corn at diploid level, banana at triploid level and potato at tetraploid level exhibit better performance and these constitute the optimum ploidy levels for these crops.
  
• All crops are not suitable for induction of polyploidy.
CONCLUSIONS

1. Greatest success is in diploids with low chromosome number. Increasing the chromosome level beyond hexaploid level is of little or no advantage.
2. Auto polyploidy is more useful in crops having vegetative parts of economic value.
3. Greater success lies in the annual species that must be grown for several years before they can be evaluated.
4. Vegetatively propagated crops are more suitable for induction of polyploidy as compared to seed propagated crops