Polyploids, characterization of polyploids and uses in plant breeding

Polyploidy:- Deviation from the normal chromosome numbers in an organism is called polyploidy. It is of 2 types -

1. Aneuploidy

2. Euploidy

1. Aneuploidy:-

•   Definition:- Generally, the number of chromosomes in the genome of each somatic cell of an organism is fixed. If there is a decrease or increase in the number of 1 or 2 chromosomes in the genome, it is called as aneuploidy. It is found mostly in animals. It is rarely seen in plants.

•   Many types:-

i. Nullysomy = (2n – 2)

ii. Monosomy = (2n – 1)

iii. Double monosomy = (2n – 1 – 1)

iv. Trisomy = (2n + 1)

v. Double trisomy = (2n + 1 + 1)

vi. Tetrasomy = (2n + 2)

2. Euploidy:-

•   Definition:- Normally, each somatic cell of organism has 2 copies of the genome. When more than 2 copies of the genome are found in the somatic cells of organism, it is called euploidy. It is found only in plants. It is completely absent in animals.

•   2 types:-

i. Autopolyploidy

ii. Allopolyploidy

i. Autopolyploidy:-

•   Definition:- When copies of the genome present in all the somatic cells of an organism belong to the same species, it is called autopolyploidy.

•   Examples:-

Triploid = 3x

Tetraploid = 4x

Pentaploid = 5x

Hexaploid = 6x

ii. Allopolyploidy:-

•   Definition:- When copies of the genome present in all somatic cells of an organism come from more than one species, it is called allopolyploidy.

•   Examples:-

Tetraploid = 2x₁ + 2x₂

Hexaploid = 2x₁ + 2x₂  + 2X₃

Reasons of producing autopolyploidy:-

•   2n gametes are produced due to abnormal meiosis.

•   Artificial duplication of chromosomes can be achieved using colchicine chemical. This chemical prevents the formation of spindle fibers during cell division.

• To produce autopolyploidy artificially, soak the seed or plantlet or shoot apex in 1% fresh colchicine solution for 1 to 10 days.

Effects of Polyploidy:-

i. Gigantism:- The size and vigour of the plant increases.

ii. The size of leaves, flowers, fruits, seeds etc. increases and their number decreases.

iii. Larger stomata and cells

iv. Larger pollens

v. Low fertility

vi. Low growth rate

vii. late flowering

viii. Higher fresh weight and lower dry mass due to increase in water content

ix. The suitable ploidy level varies from one species to another.

Examples:-

Sugar beet = 3x

Timothy grass = 8x – 10x

Uses in Plant Breeding:-

•   Autotriploid Plants:-

i. Banana:- Its fruits are seedless.

ii. Water melon:- It is found in Japan. Its fruits are also seedless.

iii. Sugar beet:- It is found in Japan and Europe. Its roots are bigger which produces more sugar.

iv. Tea:- Its TV-29 variety is drought resistant and high yielding.

•   Autotetraploid Plants:-

i. Maize:- It contains 43% more vitamins.

ii. Clover and Barseem:- Their fodder yield is high. Pusa Giant Barseem is a famous variety in India.

iii. Rye:- Its two varieties Double steel and Tetrapetkus have high yield.

iv. Hyoscymus niger:- Its HMT-1 variety has 15% more biomass and provides 36% more medicine.

v.  Vetiveria zizanoides:- Its vetiver variety yields 11% more fragrant oil.

•   Allotetraploid Plants:-

i. Cotton:- Gossypium hirsutum

ii. Mustard:-

Ø  Brassica juncea (AABB)

Ø  Brassica napus (AACC)

Ø  Brassica carinata (BBCC)

•   Allohexaploid Plants:-

i. Wheat:- Triticum aestivum (AABBDD)