Variation in Chromosome Number

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 Chromosomal variation

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 SpeciesCropBasic Chromosome Number (x) Haploid (Gametic) Number (n) Somatic (Diploid) Chromosome number (2n) Avena strigosaOats772n = 2x= 14 Avena barbataOats7142n = 4x= 28 Avena sativaOats7212n = 6x= 42 Gossypium arboreumCotton13 2n = 2x= 26 Gossypium hirsutumCotton13262n = 4x= 26 Triticum monococumWheat772n = 2x= 14 Triticum turgidumWheat7142n = 4x= 28 Triticum aestivumWheat7212n = 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