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PLANT SCIENCES II
BSC-201
Plant Domestication– Historical concepts
Search for Food
Hunter-gatherer society
Primary subsistence method involves the direct collection of edible plants and animals from the wild
Obtained most of the food (up to 80%) from gathering rather than hunting.
Main Source Wild wheat (from German Einkorn, literally "one grain" or "a grain")
– Wild einkorn: Triticum monococcum subsp. boeticum : 2n = 14; A genome
– Distribution : n. Syria, s. & w. Turkey, n. Iraq, Iran
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Harvest:
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Nutritional characteristics:
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abundant & dependable food production
conserves better than meat
Disadvantages:
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Acceptable/ good
Poor milling, baking quality
Advantages of einkorn wheat:
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hand-stripping + bag: average:2.05 kg/h
reconstructed sickle: average: 2.45 kg/h
46% by weight of actual grain (threshing with wooden mortar and pestle + wind winnowing)
quantity: > needs for one year
Fragile rachis
Hulled seed
Changes during domestication:
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tougher axis
threshing ratio: wild: 46% grain > cultivated: 73%
more adaptable
more productive
wider leaves
Hunter-gatherer societies tend to be relatively mobile:
•
to provide sufficient resources in order to sustain their population and the variable availability of
these resources owing to local climatic and seasonal conditions.
•
Their population densities tend to be small in number (10-30 individuals), but these may gather
together seasonally to temporarily form a larger group (100 or more) when resources are
abundant.
Domestication of Plants (food security) helped mankind to establish
– First farming societies
– develop implements
– and houses and towns
First Farming Societies (12,000-5,000 years ago)
Humans began to deliberately grow crops and domesticate a range of plants
Population density increased, 60–100 times greater than hunter-gatherer societies,
Because cultivated land is capable of sustaining higher population densities than land left
uncultivated.
The earliest place known to have lived mainly from the cultivation of crops is Jericho (Jordan River
in the West Bank of the Palestinian (Fertile Crescent).
. By around 8000 BC this community, occupying a naturally well-watered region, is growing
domesticated forms of wheat, soon to be followed by barley. Though no longer gatherers, these
people are still hunters.
Domestication of Plants and Growing them as Crops resulted in Abundant food and
consequently
Development of Jericho as the first known town, with a population of 2000 or more. A pioneering
agricultural community, surrounded by other tribes dependent on gathering food, offers easy
pickings which will need vigorous protection. Jerico has protective walls and a tower
The Fertile Crescent:
The light of civilization first dawned in the Middle East along what is known by historians as the fertile cresent - a cresentshaped region stretching from just south of modern-day Jerusalem then northward along the Mediterranean coast
to present-day Syria and eastward through present-day Iraq then southward along the Tigris and Euphrates rivers
to the Persian Gulf. Initially, the Fertile Crescent was only sparsely inhabited but around 8000 BC, it was in this fertile
valley that agriculture was first believed to have been developed. Wild wheat and barley grew in abundunce and tribes of
nomad hunters and herders began to settle down along the lush banks of the rivers and became the world's first farmers.
Agriculture was the spark which lit the flame of civilization. Farming gave rise to social planning on a larger scale as
groups of nomadic tribes settled down and joined co-operative forces. Irrigation developed as the need increased to feed
and support growing populations. Soon towns were built to afford comfort and protection for these early settlers. Towns
like Jericho, Jarmo, Ali Kosh, Catal Huyuk, Beidha and Hassuna were the basis of a new form of human social
organization and became the foundation for the first civilization.
Introduction to Plant Improvement
• What is plant Improvement?
– Selection/ development of plants with enhanced performance than the
existing genotype/ plants
• Early Plant improvement
– No scientific methodologies are involved, only on the basis of field
performance
• Land Races
– Varieties of crop plants whose genetic composition is shaped by
household agronomic practices and natural selection pressure over
generations of cultivation
• Systematic Plant Improvement
– Improvement in heredity of crops and Production of new crop
cultivars which are far better than the existing ones
This Systematic Plant Improvement is also referred to as
PLANT BREEDING
Plant Breeding
– is the art and science of improving the heredity of
plants for the benefit of the mankind
– The goal of Plant breeder is to change the heredity of
plant in ways that will improve plant performance
Why do we need plant breeding?
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Performance of CLCV
Susceptible Variety S12 Over The Years at
PSC, Farms
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(Tenant Area)
Yield (Mds/Ac)
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25
27
22
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Yield (Mds/Ac) (Direct Area)
10
1987
1988
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28
25
20
15
1987
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Plant Breeder Should Know
The person involved in Plant improvement
should have efficient knowledge of
• Needs of the growers and consumers
• Characteristics of the crop to be improved,
including its wild relatives
• Principles of Genetics & Cytogenetics Principles
• Special techniques adapted from related fields
for the solution of particular problems
• Principles of field plot techniques
• Principles involved in the design of experiments
and the statistical testing of data
Related Fields
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Botany
Genetics
Plant Physiology
Plant Pathology
Entomology
Agronomy
Plant Biochemistry
Statistics
Computer Sciences
– Wheat variety with high yield, disease resistance,
Aphid free, less moisture requirement, fertilizer
responsive, early maturing, high quality
History of Plant Breeding
Pre-History
– Improvement of wild wheat
Selection
– Improvement of wild Corn
– Improvement in wild potato
– Improvement of wild rice
Before Mendel
– Plant is correct unit for selection: Development of cultivars from
progeny of single plant
– Screening against wilt in cotton
Selection/
Introduction
– Reproductive systems in plants
Mendel
– Laws of inheritance
– New branch of science Genetics
After Mendel
Selection/
Introduction
– Hybrid Corn
Hybridization
– Dwarf Wheat
Tissue culture
– Dwarf rice
Biotechnology
– Strain building (synthetic) in forage
MAS
Basics of Plant Breeding Strategy
• Identify the morphological, physiological, pathological traits
in a cultivated plant species that contribute to its adaptation,
health productivity, and suitability for food, fibre or industrial
products
• Search out new genes that encode for desired traits in
different strains of the cultivated species and their close
relatives
• Combine genes for the desired traits into an improved
cultivar through traditional breeding or new biotechnological
procedures
• Performance assessment of the improved breeding lines in
the local environment in comparison with present cultivars
• Distribution of new cultivars that are superior to current
cultivars
Aims and Objectives of Plant Improvement
• Higher yield
• Better quality
• Shape, size, colour, nutrition, taste, Malting, milling, baking, (cereals)
sugar contents in scane, large, fine, strong fibre in cotton, flavour in
fruits
• Resistance to biotic and abiotic stresses
• Crop duration early or late maturity as desired
• Growth habit: height, type etc.
• Winter hardiness
• Lodging resistance
• High fertilizer responsiveness
• Easier thresh ability
• Wider adaptability
• Mechanised harvesting
Plant Genetic Resources
• Plant germplasm
– is the genetic source material used by plant breeders to develop new
cultivars
– Genotypes of particular species, collected from different sources and
geographical origins, for use in plant breeding.
• Sources of Plant germplasm
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Wild relatives
Land races & primitive cultivars
Obsolete cultivars
Advanced breeding lines & other products of plant breeding
program
– Current cultivars
• Thus the germplasm of a crop may be defined as
– The sum total of hereditary material i.e. all the alleles of various
genes, present in a crop species and its wild relatives
Center of Diversity
• Area where vast genetic diversity existed for a cultivated
crop species (NI Vavilov 1926)
Center of Origin
NI Vavilov proposed that center of diversity for a crop is
center of origin for that crop.
• primary centre of origins
– Areas where crop plants were domesticated
• Secondary Centre of origin
– areas where variation continued after domestication
Centers of diversity
• Chinese Centre
• Indian Centre
– Indomalayan centre
• Central Asiatic Centre
• Near Eastern Centre
• Mediterranean Centre
• Ethiopian Centre
• South Mexican and Central American Centre
• South American Centre
– Chiloe Centre
– Brazilian-Paraguayan Centre
Agencies engaged in plant breeding
• Asian vegetable Research and Development Center
(AVRDC) Taiwan
– Cabbage, Pepper, tomato, soybean & Mung bean
• International center for Agriculture Research in Dry
Areas (ICARDA) Syria
– Barley, Chick pea, faba bean, tropical forages, lentil &
wheat
• International center for wheat and maize Improvement
(CIMMYT) Mexico
– Maize, triticale and wheat
• International center for Tropical Agriculture (CIAT)
Colombia
– Dry beans, cassava, rice and tropical forages
Agencies engaged in plant breeding
• International Crop Research Institute for the Semi Arid
Tropics (ICRISAT) India
– Chickpea, millet, peanut, pigeon pea and sorghum
• International Institute of Tropical Agriculture (IITA)
Nigeria
– Cassava, cocoyam, cowpeas, lima bean, maize,
pigeon pea, rice, soybean, sweet potato, winged bean
and yam
• International Potato Center (CIP) Peru
– Potato and sweet potato
• International Rice Research Institute (IRRI) Phillipines
– Rice
3
Reproduction in crop plants
Flower Structure
Kinds of Flowers
• Complete: Having all four floral parts i.e. petal, sepal, stamens, pistils
e.g. cotton tobacco, potato, cowpeas, soybean, tomato etc.
• Incomplete: Lacking one or more floral organs e.g. corn, sorghum, millet,
wheat, oat, barley, rice, sugarcane etc.
• Perfect: Bisexual, having both stamen and pistil in same flower. Eg.
cotton tobacco, potato, cowpeas, soybean, tomato, wheat, oat, barley,
rice etc.
• Imperfect: Unisexual, having only stamen or pistil in flower
– Staminate: having stamen only
– Pistillate: having pistil only
• Monoecious: having pistillate and staminate on same plant e.g. corn,
cassava, castor
• Dioecious: Having pistillate and staminate flowers on different plants e.g.
hemp, papaya
• Cleistogamous: Pollination and fertilization occur before opening of
flower, promoting self pollination
Nuclear Cell Division
• Mitosis (Equational Division) : the daughter nuclei normally receive
an exact copy of each chromosome originally present in the nucleus
of the parent cell.
– is the method of cell division by which new cells are formed in the
normal growth and development of plant.
– The only form of cell division associated with asexual reproduction
• Meiosis Consists of two successive divisions, first reductional
(Meiosis I) and second equational (Meiosis II).
– Essentially reduces chromosome number from diploid (2n) in the
megaspore mother cell to haploid number (n) in gametes.
– Associated with sexual reproduction in higher plants
Length wise replication of chromosomes
to form two identical sister chromatids
Appearance of spindle fibres
Homologous chromosomes
• Chromosomes having genes
at corresponding loci
controlling a common
heredity characteristics
Crossing Over
• Exchange of chromatid
segments
Significance of Meiosis
• Maintenance of chromosome
numbers in plants
• Segregation of contrasting
alleles leads to their
subsequent recombination in
following generation
• Crossing over provides
mechanism for recombination
of linked genes and hence
new genetic variation.
Chromosome number in crop plants
Chapter 2
Reproduction in crop plants
Table 2.1
Types of Reproduction
1. Sexual Reproduction (Union of male & female gametes)
– Crops normally Self Pollinated (4 – 5% cross pollination)
• Self pollination is the transfer of pollen from an anther to stigma
of same flower or to a stigma of another flower on the same
plant, or within a clone.
• Fertilization resulting from union of male & female gametes
produced on the same plant is self –fertilization or autogamy
– Crops normally Cross Pollinated
• Cross pollination is the transfer of pollen from an anther to stigma
of same flower or to a stigma of another flower on a different
clone.
• Fertilization resulting from union of male & female gametes
produced on different clones is cross–fertilization or allogamy
– Crops both Self and Cross pollinated
• Usually the amount of cross-pollination exceeds that of the
normally self pollinated crops, yet does not reach that of the
normally cross-pollinated e.g. Cotton ( 5 -25%) pigeon pea (5-40%),
Sorghum, tobacco
Self Pollinated crops
• Barley
Tomato
• Bean
Triticale
• Chickpea
Vetch
• Cowpea
Wheat
• Flax
• Jute
• Lentil
• Millet
• Mungbean
• Oat
• Peanut
• Pea
• Potato
• Rice
• Sesame
• Soybean
• Tobacco
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Cross pollinated Crops
• Alfalfa
Sugar beet
• Cabbage
Sugarcane
• Carrot
Sunflower
• Castor
Sweet potato
• Cassava
• Clover
• Corn (maize)
• Cucumber
• Hemp
• Mustard
• Onion
• Pearlmillet
• Pepper
• Rye
• Safflower
• squash
Sporogenesis
• Production of microspores and megaspores is known as sporogenesis
• Microspores are produced in anthers (Microsporogenesis)
– Each anther has 4 pollen sacs with numerous pollen mother cells
• Megaspores are produced in ovules (Megasporogenesis)
– A single cell in each ovule differentiate into megaspore mother cell
Gametogenesis
• Production of male and female gametes in microspore and megaspores
is known as gametogenesis
Male gametogenesis
n
2n
PMC
Second Meiotic
Division
First Meiotic
Division
Meiosis
Pollen
Microsporogenesis
n
Nucleus
divides n
n
Microspore
n
Generative nucleus
Mitosis in the
Generative nucleus
Tube nucleus
Microgametogenesis
n
Male Gametes/
sperm
Tube nucleus
Female gametogenesis
n
n Degenerate
n
n
2n
MMC
Second Meiotic
Division
First Meiotic
Division
Meiosis
Megasporogenesis
Mitosis
Mitosis
n
n
Megaspore
Mitosis
Synergid
Egg cell
n
n
Megaspore
v
Polar
Nuclei
Antipodal
Cells
Megagametogenesis
v
v
Megagametophyte
(Embryo Sac)
• Pollen tube sends both male gametes/ sperms into embryo sac
• One male gamete/ sperm fuses with egg to form zygote. This
process is known as FERTILIZATION.
• Second male gamete/ sperm fuses with two polar nuclei. The
process is called triple fusion. These three nuclei together form
PRIMARY ENDOSPERM NUCLEUS.
• The fusion of one male gamete with egg and fusion of second with
polar nuclei is called DOUBLE FERTILIZATION.
• The fertilized egg develops into EMBRYO
• Primary endosperm nuclei divides to form many nuclei covered with
cell wall, collectively called ENDOSPERM
Asexual reproduction (No union of sexual gametes)
Plants may develop either from vegetative parts of plants
(Vegetative reproduction)
OR
Plants may arise from embryos that develop without fertilization
(APOMIXIS)
Vegetative Reproduction
– Propagation through vegetative plant parts (roots, tuber, stolons,
rhizomes, stems, leaf cutting or tissue culture)
Significance of Vegetative Reproduction
• Desirable plant may be used as a variety as there is no danger of
segregation
• Mutant buds/ branches or seedlings, if desirable can be multiplied
and used as varieties.
• However it does not allow transfer of genes from one variety to
another variety
Apomixis (without mixing)
In APOMIXIS, seeds are formed but the embryo develops without fertilization.
The resulting plants are identical to the parent plant.
APOMIXIS may be
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Facultative Producing strictly maternally similar plants,
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Obligate
Reproducing only through apomixis,
Obligate apomixis may be
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Vivipary: formation of plantlets instead of flowers, no seed is formed
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Agamospermy: Formation of seed without the union of egg and sperm
nuclei.
Agamospermy may be
1.
2.
Adventitious embyony: Embryo develops directly from vegetative cells of
ovule and it does not involve production of embryo sac e.g. mango, citrus etc.
Gametophytic Apomixis: Embryo develops without fertilization from egg cell.
Gametophytic Apomixis may be
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Apospory: Unreduced embryo sac is produced from vegetative cells of the ovule
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Diplospory: Unreduced embryo sac is produced from Megaspore Mother Cell
Diplospory may be
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Parthenogenesis: Embryo develops from the embryo sac without pollination
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Pseudogamy: Pollination is necessary for embryo development but
fertilization of egg cell does not take place. Fertilization of the secondary
nucleus takes place for endosperm development.
The endosperm can arise autonomously (Autonomous Apomixis) OR It can arise after
fertilization (Pseudogamy)
Significance of Apomixis
1.
2.
Apomixis allows plant breeder to fix heterosis
Apomixis allows for rapid multiplication and release of variety
Identification of Apomixis
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When progeny is identical to mother plant
Through flow cytometery (Flow cytometer is an instrument that
can accurately measure the DNA contents of thousands of nuclei
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
•
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
•
2.
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
•
•
•
3.
In barley H. Vulgare, Rice, wheat through H.bulbosum
In wheat through maize
In wheat through pearl millet and others
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
Breeding Self Pollinated Crops
What is a cultivar?
The cultivar (agricultural Variety) is a group of similar plants , which by structural features
and performance may be identified from other groups of genetically similar plants
within a species
Plant Kingdom is divided into - - - Family
Genus
cultivar
experimental strain/ strain / advance line.
•
species
Variety/
The distinction of being named and distributed commercially serves to set apart the
cultivar from strain
Essential Characteristics of Cultivar
1. Identity
feature of identification
2. Reproducibility
true to type or similar progeny, easy in self pollinated crops,
difficult in cross pollinated crops.
To learn breeding behavior of an individual plant
Progeny test: From the progeny test breeder learns that a superior looking plant is superior due to
its genetic make up or environmental influence: By growing progeny of selected superior plant,
approves or disapprove its genetic superiority
To identify the genotype of a plant
Test cross: Crossing of F1 plant to a homozygous recessive plant
To Accumulate desirable gene
Back cross: Cross of a hybrid to one of its parent, Succession of back crosses helps to add a gene
Significance of Pollination Method
1. Loci with identical genes (AA aa) will remain homozygous following self
pollination and
2. Loci with contrasting genes (Aa) will segregate, producing
homozygous and heterozygous progeny in equal proportions
Heterozygosity is reduced by 50% with each successive selfing thus
leading to homozygosity
S0
100%
Aa
S1
25% AA
50% Aa
25% aa
S2
25% AA
12.5% AA
25%Aa
12.5% aa
25% aa
S3
37.5%AA
6.25%AA
12.5%Aa
6.25%aa
37.5%aa
S4
43.75%AA
3.125%AA
6.25%Aa
3.125% aa 43.75%aa
S5
46.875%AA
1.562%AA
3.125%Aa 1.562% aa 46.875%aa
BREEDING METHODS
Crops can be improved by selecting
Mixture of plants, or a single plant from introduced germplasm
Mixture of plants, or a single plant from a local germplasm
Mixture of plants, or a single plant from a hybrid population
Three methods viz., introduction, selection and Hybridization are practiced
for crop improvement.
1. INTRODUCTION
Transfer of a collection of seed or planting material from one ecological
zone to other. This collection of germplasm resources may be from
–
–
–
–
wild relatives
Land races
Commercial cultivars from other similar locations (local or international)
Advanced lines from research institutes (local or international)
Acclimatization:
Adjustment of a species or a population to a changed environment over
a number of generations
2. Selection
Mixture of plants, or a single plant, selected from germplasm
– Population for selection may be Mixed population
Two methods for selection from mixed population in self pollinated crops
2.1. Mass selection
• Plants are selected and harvested on the basis of phenotypic expression (mostly
qualitative traits) and seed composite. Therefore cultivars developed by mass
selection are uniform for qualitative traits rather than quantitative traits further
they are not progeny tested.
– Objectives of mass selection:
• Purify a mixed cultivar or plant population by visually selecting plant with similar
phenotype
• Develop a new cultivar by improving the average performance of the population
2.2. Pure line selection:
• A pure line is a progeny descendent solely by self pollination from a single
homozygous plant. A pure line is genetically homozygous. Pure line selection
refers to the procedure of isolating pure lines from a mixed population followed by
progeny test
• Pure line cultivars were often developed from land races
• Pure line selection does not create a new genotype and improvement is limited to the best
genotype in the mixed population
How long a selection can remain pure? Depends on
• Seed mixture from other sources (mechanical Sources)
• Natural crossing with other cultivars or breeding lines (natural pollination etc)
• Mutations
– Constant rouging of off-type plants to maintain purity
3. Hybridization
•
Hybridization is a breeding method that utilizes cross-pollination
between genetically different parents to obtain gene recombination.
•
•
•
Segregation generations are grown
Selection of pure lines having desirable traits from both the parents
Selected lines are evaluated by progeny tests to verify presence of
desirable traits
Lines proven to be superior may be increased as anew cultivar
•
SELECTION PROCEDURES FOLLOWING HYBRIDIZATION
1.
2.
3.
4.
Pedigree Selection
Bulk Population
Single Seed Descent
Doubled Haploid
Pedigree Selection
• In pedigree selection, selection of desirable plants starts in F2
generation and continues in successive generation untill genetic
purity is reached
• Cross Cultivar A X Cultivar B
• F1 Generation: Grow 50-100 plants. Eliminate self plants
• F2 Generation: Grow 2000 – 3000 plants, space planted, select
superior plants, harvest superior plant separately
• F3 to F5: Grow progeny rows with seed harvested from superior
plants harvested in previous generation,
– Identify superior row, then select and harvest 3-5 plants within
these rows,
– normally 25 – 50 families are retained at the end of F5
generation,
– Identity of plants and rows is maintained and superior traits are
recorded
Pedigree Selection Cont---• F6 Generation: Grow families of plant rows, Uniform related families
may be harvested togather and seed bulked, the separate seed lots
are experimental lines/ advanced lines
• F7 Generation: Experimental lines are grown in preliminary yield
trials and compared with adapted cultivars
• F8 to F10: Continue yield testing at micro and national level.
Performance evaluation is recorded
• F11 and F12 generation: Increase seed of the best line and
distribute as new cultivar
–
–
–
–
–
–
–
Sometimes early Yield testing (F3 or F4) is started
Labour intensive
Requires detailed record keeping
Progeny lines with desirable genes are carried forward
Permits the collection of genetic information
Best suited to cereals, beans, peanut
12 years are required to develop new variety if one generation/year
PEDIGREE
METHOD
Bulk Population
• In bulk population, seeds harvested in F2 and successive
generation are bulked and selection of desirable plants is delayed till
F5 or F6 generation whenl genetic purity is reached
• Cross Cultivar A X Cultivar B
• F1 Generation: Grow 50-100 plants. Eliminate self plants
• F2 Generation: Grow 2000 – 3000 plants, harvest en masse and
bulk seed
• F3 to F4: Grow 1/50 to 1/100 of hectare plots with bulked seed
harvested from previous generation
• F5 Generation: Space plant 3000 – 5000 plants,
– Select 300 – 500 superior plants with desirable traits, harvest
separately
• F6 generation: Grow progeny rows of selected plants:
– Harvest 30 – 50 progenies having plants with desirable traits
Bulk Population Cont---• F7 Generation: Grow superior progeny as experimental lines in
preliminary yield trials and compared with adapted cultivars
• F8 to F10: Continue yield testing at micro and national level.
Performance evaluation is recorded
• F11 and F12 generation: Increase seed of the best line and
distribute as new cultivar
– Early Yield testing (F3 or F4) can be started
–
–
–
–
–
Simple, convenient, requires less labour
Requires no record keeping till selection generation
Easy to screen for biotic and abiotic stresses
Does not permits the collection of genetic information
Best suited to plant where it is difficult to harvest individual plants
– 12 years are required to develop new variety (one generation/year)
Bulk
Population
Method
Single seed descent
• The progenies of F2 plants are advanced rapidly through
succeeding generation from single seed
• Cross Cultivar A X Cultivar B
• F1 Generation: Grow 50-100 plants. Eliminate self plants
• F2 Generation: Grow 2000 – 3000 plants, harvest single seed from
each plant
• F3 to F4: Grow seeds harvested in previous generation, harvest
single seed from each plant
• F5 Generation: Space planting from seeds harvested in previous
generation. Select superior plants with desirable traits, harvest yto
collect seed from each plant separately
• F6 Generation: Grow progeny rows from plants harvested in
previous generation. Harvest rows superior for desirable traits. Each
row will have originated from a different F2 plant
Single Seed Descent Cont---• F7 Generation: Experimental lines are grown in preliminary yield
trials and compared with adapted cultivars
• F8 to F10: Continue yield testing at micro and national level.
Performance evaluation is recorded
• F11 and F12 generation: Increase seed of the best line and
distribute as new cultivar
– least optimum growing conditions and time requirement (only
one seed required) hence development period may also be
reduced
– Sometimes early Yield testing (F4 or F5) is started
– Maximum F2 plants are retained ensuring least genetic loss
– Does not requires detailed record keeping
– Does not permits the collection of genetic information
Single
SeedDescent
Method
Doubled Haploid
• Haploid plants are generated from anthers of F1 plants or by other
means and chromosomes are doubled to make doubled haploid
plants
• Cross Cultivar A X Cultivar B
• F1 Generation: Grow 2000 – 3000 haploid plants (from anthers)
• F2 Generation: Doubling of chromosomes and harvest seed from
doubled haploid plants
• F3 Generation: Grow seeds harvested from previous generation and
select superior plants with desirable traits
• F4 Generation: Grow progeny and select superior lines
• F5 Generation: Grow preliminary yield trials
• F6 to F8 Generation: Continue yield trials
• F9 & F10 Generation: Increase seed of the best line and distribute
as new cultivar
– Less time consuming, less labour, less record keeping, suitable
for molecular studies
Doubled
Haploid
Method
• Success in hybridization program can be increase by
– Selection of correct parents for hybridization
– Identifying superior plants from segregating population
– Selection of suitable selection procedure following hybridization
Backcross Breeding
• The backcross is a form of recurrent hybridization by which a
desirable allele for a character is substituted for the alternative allele
in an otherwise desirable cultivar.
• The adapted parent , to which the allele is being added, enters into
each cross and is called the recurrent parent
• The parent with superior character that enters into the initial cross
but does not enter into backcross, is termed as donor or nonrecurrent parent
• In case the desired trait is recessive, selfing is done after every
backcross to obtain homozygous recessive for next BC
– Donor parent is supposed to contribute only one trait, so enters only
once in the backcross
– In backcross, major gene share is expected from recurrent parent, so it
enters repeatedly into crosses as male parent.
• Why donor parent is always kept as female in backcross breeding?
Schematic Diagram of Backcross
Original Cross
Disease Resistant Cultivar
RR
X Adapted cultivar A
rr
First Backcross
F1
(Rr, 50% genes from A)
X Adapted cultivar A
rr
2nd Backcross
BC1
(Rr:rr, 75% genes from A)
X Adapted cultivar A
rr
3rd Backcross
BC2
(Rr:rr 87.5% genes from A)
X Adapted cultivar A
rr
4th Backcross
BC3
(Rr:rr, 93.75% genes from A)
X Adapted cultivar A
rr
BC4
(Rr:rr, 96.875% genes from A)
Self Rr plants from BC4 to
obtain plants homozygous for
RR i.e. 1RR:2Rr:1rr
DNA marker Assisted Backcross Breeding
• Marker-Assisted backcrossing is particularly useful when phenotype
of the gene(s) being transferred is not readily identifiable i.e.
– Transferring recessive genes
– The phenotype is visible only after pollination
– The gene is associated with a quantitative trait that would require
replicated testing
– Plant has to be destroyed to assay for the correct phenotype
Multiline Breeding
• Multiline cultivar is a composite of isolines.
• Isolines: Lines that are genetically identical except for one gene.
Donor
Donor
Genes
# of
Backcrosses
Recurrent
Cultivar
Isolines
Donor 1
(R1R1)
5
A1 (rr)
Donor 2
(R2R2)
5
A1 (rr)
Donor 3
(R3R3)
5
A1 (rr)
Donor 4
(R4R4)
5
A1 (rr)
A1(R4R4)
Donor 5
(R5R5)
5
A1 (rr)
A1(R5R5)
Multilines
A1(R1R1) First isolines are
developed and then
A1(R2R2) these Isolines are
composite to
synthesize
A1(R3R3)
Multiline cultivar
• Multiline cultivar has many disadvantages
–
–
–
–
There is no genetic improvement in yield or other traits
Limited utility only under regular high incidence of disease
More labor required to maintain isolines
More time required to produce sufficient seed of each isoline
• Mixture of genetically diverse lines combined in various
ways to buffer against environmental stresses are called
Composites.
• Variety Blend:
– A variety blend is a composite cultivar produced by mixing seed
of two or more cultivars.
– It is supposed that a blend of genotype will yield consistently
higher than the pure line due to better buffer against G X E
interaction over location and years
Other Breeding Procedures Practiced in self
pollinated crops
• Population Improvement through
– Recurrent Selection: is a population improvement procedure
designed to increase the frequency of desirable alleles for a
particular quantitative character by frequent intermatings among
superior genotypes within the population
– Multiple Crosses: or convergent cross is produced by crossing
pairs of parents and then crossing pairs of F1s until all parents
enter into a common progeny. Eight parents result in 4 F1s, 4F1s
result in 2F1s and 2F1s are crossed to one F1 having all
parents.
– Male-sterile Facilitated Hybridization: 1)Production of male
sterile isolines through backcrossing 2)Male-sterile isolines are
pollinated with a group of cultivars and resultant seed is bulked
to form a composite
• Early Testing
– Early generation testing allows for early identification of superior
genotypes, further that can also be used in breeding programme
as parent: saving time