Download Variation in Chromosome Number

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
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Diploid
Triploid
Tetraploid
Pentaploid
Hexaploid
Septaploid
Octoploid
2x
3x
4x
5x
6x
7x
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
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Easy to produce: AA doubled to AAAA
Characterized with thicker vegetative parts, increased flower size, less fertile
Characteristics of Autoploids
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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
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Increased size of meristemetic and guard cells
Decrease in total number of cells,
Reduced growth rate
consequently delayed flowering
Use of Autoploids
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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;
• Chromosome addition lines
– Trisomy – single extra chromosome; 2N + 1
– Tetrasomy – extra chromosome pair; 2N + 2
2N – 1
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
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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
Interspecific or intergeneric hybridization followed by elimination
of the chromosomes of the wild or distantly related species
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In barley H. Vulgare, Rice, wheat through H.bulbosum
In wheat through maize
In wheat through pearl millet and others
Anther or pollen culture
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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