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Variation in Chromosome Number

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					Variation in Chromosome
         Number
                  Chromosomal variation
Variation in chromosome may be of two types
1. Variation in chromosome number
      1.1. Euploidy/Polyploidy
      1.2. Aneuploidy

2. Variation in chromosome structure
     2A. Change in the amount of genetic information
         1. deletions
         2. duplications
      2B. Rearrangement of gene locations
         1. inversions
         2. translocations
                        1. Variation in Chromosome Number
     Genetic variability forms the basis of plant improvement and variation in chromosome
       number adds to genetic variability

1.1. EUPLOID: Chromosome number is changed to exact multiple of the basic set

Polyploids are euploids in multiple of basic set of chromosome

     –   Diploid             2x
     –   Triploid            3x
     –   Tetraploid          4x
     –   Pentaploid          5x
     –   Hexaploid           6x
     –   Septaploid          7x
     –   Octoploid            8x

•   EUPLOIDS may be
     – AUTOPLOIDS: Having Duplicate genome of same species
     – Autotetraploid: Having Duplicate genome of same diploid species

     – ALLOPLOIDS: Having Duplicate genome of different species
       Allotetraploid or amphidiploid: Having Duplicate genome of different species

1.2. ANEUPLOID: Chromosome number of basic chromosome set is changed by addition or deletion of
     specific chromosomes
                 Ploidy Levels in Different crops
Species                Crop       Basic     Haploid   Somatic
                               Chromosome (Gametic) (Diploid)
                                Number (x) Number (n) Chromosome
                                                      number (2n)
Avena strigosa         Oats        7           7      2n = 2x= 14

Avena barbata          Oats        7           14     2n = 4x= 28

Avena sativa           Oats        7           21     2n = 6x= 42

Gossypium arboreum    Cotton       13          13     2n = 2x= 26

Gossypium hirsutum    Cotton       13          26     2n = 4x= 26

Triticum monococum    Wheat        7           7      2n = 2x= 14

Triticum turgidum     Wheat        7           14     2n = 4x= 28

Triticum aestivum     Wheat        7           21     2n = 6x= 42
                          Induction of Ploidy
• Natural Induction:
    May arise from
    – Unreduced gametes: chromosome number is not reduced during
      meiosis
    – Natural wide crossing following chromosome doubling


• Artificial Induction:
    – Environmental Shock
    – Chemical
        • Colchicine: acts by dissociating the spindle and preventing migration of the
          daughter chromosomes to poles
        • It is applied to meristemetic tissue, germinating seed, young seedling, root
        • Its action is modified or affected by temperature, concentration, and duration
          of treatment
  Significance of Polyploidy in Plant Breeding
• Permits greater expression of existing genetic diversity
• Helps to change the character of a plant by altering number of
  genomes consequently changing dosage of alleles related to
  particular trait
• Ployploids with uneven number of genomes (Like Triploid and
  Pentaploids) may result in infertility. This loss of seed production can
  be used to produce seedless watermelons and banana


    – About 1/3 to ½ of angiosperms are polyploids
    – About 70% of wild species of grasses and 23% of legume family are
      polyploids
    – Most of the natural polyploids are alloploids
    – All species don't exhibit vigor with increase in ploidy
    – Optimum ploidy level for corn is diploid as compared to tetraploid
    – Optimum ploidy level of banana is triploid (Seedless)
    – Blackberry is insensitive to ploidy level
Artificially Induced Autoploids
•   Easy to produce: AA doubled to AAAA
•   Characterized with thicker vegetative parts, increased flower size, less fertile

Characteristics of Autoploids
•   Genetic ratios are simpler in allotetraploid as compared to autotetraploids. Eg. A and a
    alleles will result in 3 classes (AA, Aa, aa) in alloploid whereas it will result in 5 classes in
    case of Autoploid
•   AAAA = quadruplex, AAAa= triplex, AAaa=duplex, Aaaa=simplex, aaaa=nulliplex)
•   Consequently recessive combinations are less in autoploids forcing breeders to grow
    larger population

Autoploidy often result in
     •   Increased size of meristemetic and guard cells
     •   Decrease in total number of cells,
     •   Reduced growth rate
     •    consequently delayed flowering

Use of Autoploids
•   Due to more vegetative growth autoploids are more suitable in crops harvested for
    vegetative parts (Forages, root crops, vegetables, flowers)
•   Useful vigour is obtained by doubling chromosome contents of diploid with low
    chromosome numbers
•   Autoploids from cross pollinated crops are more successful than self pollinated crops
•   Bridging of ploidy levels in interspecific crosses
     – Diploid treated with colchicine to produce Autotetraploid X tetraploid species
Artificially Induced Alloploids
• Difficult to produce A, B first need to be hybridized and then doubled
  to form AABB

• After chromosome doubling chromosome from A genome pair with it’s
  A genome homolog and B with B genome, with no homoeolog pairing
  between A and B genome.

• Homoeolog pairing is restricted by certain genes in natural alloploids
  like, In wheat, Ph1 present at long arm of 5B chromosome inhibits
  pairing of homoeolog chromosomes from A and B genomes.

• Chromosomes originating from different but similar genomes (like A &
  B in wheat are different but similar being part of one species) are said
  to be homoeolog chromosomes (having genes and arrangement of
  genes in common)
                      Uses of Alloploidy
• Identifying genetic origin of polyploid plant
• Producing new plant genotypes and plant species
    – Production of Hexaploid (AABBRR) and Octoploid (AABBDDRR) triticale
      from rye (RR) and tetraploid (AABB) and Hexploid (AABBDD) wheat
• Facilitated transfer of genes from related species
    – Production of synthetic hybrids of wheat
    – Fibre strength in cotton
    – Arboreum(AA) X Thurberi (DD) chromosomes doubled to produce
      Allotetraploid which was further crossed to hirsutum (AADD)
    – Facilitating transfer or substitution of individual or pair of chromosomes
        • IB can tranlocate with 1R chromosome of rye (due to homoeolog)
      Variation in Chromosomal Number
1.2. Aneuploidy: Chromosome number of basic chromosome set is
  changed by addition or deletion of specific chromosomes


• Commonly results from nondisjunction during meiosis
   – Monosomy, trisomy, tetrasomy, etc.
   – Klinefelter and Turner syndromes are examples involving human sex
     chromosomes
       Variation in Chromosomal Number

• Chromosome deletion lines
   – Nullisomy - loss of one homologous chromosome pair; 2N – 2

   – Monosomy – loss of a single chromosome;     2N – 1


• Chromosome addition lines
   – Trisomy – single extra chromosome; 2N + 1

   – Tetrasomy – extra chromosome pair; 2N + 2
Substitution Line:
    – Exchange of chromosome between cultivars of same species
        • Wheat substitution lines
Alien Substitution Line:
    – Exchange of chromosome between cultivars of different species
        • Wheat X Rye resulting in triticale
                          HAPLOIDY
• HAPLOIDS: Plants having gamete number of chromosome
   – Occur in nature in very low frequency
   – In many species like corn, wheat, sorghum, barley, rye rice, flax,
     tobacco, cotton etc.
   – Can be differentiated from normal diploids (due to smaller size)
   – Haploidy can be efficiently confirmed by flow cytometery
   – Haploidy can be less efficiently confirmed by chromosome counting
   – Haploid plant can be made diploid by treating with colchicine
         Procedures for Haploid Production
1.       Identification and doubling of naturally occurring haploids
     •      In corn 1/1000 grains is a haploid that arise through development of
            an unfertilized egg into an embryo by parthenogenesis. Such haploid
            when doubled is far homozygous inbred line as compared to the one
            made through successive selfings
2.       Interspecific or intergeneric hybridization followed by elimination
         of the chromosomes of the wild or distantly related species
     •      In barley H. Vulgare, Rice, wheat through H.bulbosum
     •      In wheat through maize
     •      In wheat through pearl millet and others


3.       Anther or pollen culture
     •      In wheat
                     Use of Haploids
Doubling of chromosomes results in diploids that are completely
  homozygous
• This homozygosity achieved in one step is of higher level than
  normally achieved after 6 generations of selfing
• Recessive mutants can be observed at very early stage
• Selection of dominant alleles is facilitated
• Suitable for mapping populations development

				
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