Crop improvement –

Conventional Breeding
Introduction
Conventional crop improvement: involves the development or
improvement of cultivars using conservative tools for
manipulating plant genome
• Early work were largely based on empirical observations
• Plant breeding techniques have evolved since the start of agricutlure
• From crop domestication to crop improvement, the objectives were around
producing crop varieties suited to human needs (productivity, disease resistance,
stress resistance)
• Mendel’s work on genetics marked the start of the scientific age of plant breeding
• Over 500 out of over one million plant species have been domesticated or
cultivated as crop plants throughout history
PLANT BREEDING HISTORY
1.Crop Domestication: developed initial gene pools of all important crops
2.Classical Plant Breeding: selection and improvement of crop gene pools for benefit of mankind
3.Plant Breeding at present: Use of novel molecular tools and technology to continue the
improvement of crop plant germplasm

Early Selection Systematic Wide

Selection Hybridization

Plant Systematic Mutation Genetic

Domestication Crossing Induction Transformation

Figure 1. Historical development of plant breeding

Vavilov’s center of crop origin
Crop improvement - Timeline
700BC Babylonians and Assyrians pollinated date artificially
1717 Thomas Fairchild produced the first artificial hybrid of date palm

1760-1766 Joseph Kolreuter ( German ) made crosses in tobacco

1759- 1835 Knight was perhaps the first to use artificial hybridization to develop several
new fruit varieties.
1840 Le Courteur and Shireff used individual plant selection and progeny test to
develop cereal varieties.
1856 Vilmorin further developed progeny test and successfully improved
sugarbeets.
1900 Individual plant selection method was developed in detail by Nilsson et.al.in
Sweden.
1902 Walter and Sutton pointed out that the mutual inter- relationship between
cytology and Mendelism , closing the gap between cell morphology and
heredity.
1903 Johannsen proposed the pureline theory that provided the genetic basis for
individual plant selection. First inheritance studies on disease resistance.

1904 Hannig proposed the idea of embryo culture

Crop improvement - Timeline
1905 Inbreeding program for corn was initiated by Edward Murray East and George H.
Shull
1908 Nilsson- Ehle ( Sweden) . Multiple factor explanation of inheritance of wheat pericarp
color the beginnings of studies on quantitative inheritance. First to use the bulk
pollination method. His experiment provided a useful model for further analysis of
continuous variation of quatitative characters.
1909 EM East formulated the alleomorphs concept; George Harisson Shull. Extensive
crossing of inbreds made to form hybrids.Found that differences between selfing and
sib – crossing became progressively less as the number of generations of inbreeding
increased.
1910 AB Bruce , F Keeble , C. Pellew. That hybrid vigor in a character was due to the
operation of favourable dominance in 2 or more of its components.
1911 GNB Collins and Kemton demonstrated linkage in corn.
1912 HS Jennings. Selfing reduces heterozygosity by one each generation
1916 GH Shull introduced the term heterosis to replace heterozygosis to avoid implication
that all genetic differences which stimulate cell division , growth and other
physiological causes are Mendelian in their inheritance.
1917 First commercial hybrid corn ( double cross ) was developed.
1919 Donald Forsha Jones provided explanation of hybrid vigor as due to dominant linked
favourable genes which is up to now most accepted.
1920 East and Jones proved the existence of self – incompatibility factors in Nicotiana.
Crop improvement - Timeline
1926 Pioneer Hybrid Corn Co., the first commercial company in the US was organized.

1927 L J Stadler .X- ray work with plants or artificial transmutation of genes. In 1930 , he devised and
perfected methods of determining rates of different genes mutated at widely different rates.

19 30’s Use of colchicines in 1937, embryo culture; commercial use of hybrid corn in the US started.

1940’s- 1950’s Radiation genetics as a result of the coming in of the atomic age; biochemical and physiological
genetics getting started. Use of hybrid corn and improved varieties in developed countries getting
extensive.
1950’s to early 60’s Domination of interests in quantitative genetics , the bandwagon of the time. Use of cytoplasmic
and genetic male sterility in the commercial production of hybrid seeds is extensive. In 1953 , JD
Watson and FH Crick came out with the DNA configuration model which later earned for them
the Nobel prize award.
1960’s- 1970’s Knowledge explosion. Resurgence of interests in molecular biology, physiologic approaches in
plant breeding cell hybridization and culture . A network of international institutes was
established: green revolution brought about by new high yielding varieties sweeping Asia and the
Americas, a tangible proof of success of plant breeding as an applied science.

1980’s-1990’s Conventional plant breeding continues to make its impact around the world especially in
addressing specific problem areas through genetic intervention. The recombinant DNA
technology has found its application in crop improvement. Transgressive species of rice ,tobacco,
cotton , tomato, melons , squash, corn potato, peanuts and etc have been created with gene
constructs derived from plant disease virusis and Bacillus thuringienses.
Classical Plant Breeding
Outcome over the past 100 years:
• Increase crop productivity
• Developed and introduced hybrid cultivars
• Developed and introduced resistance to disease and insect pests
• Increased food quality and quantity
• Developed crops adaptable to mechanical harvest
Methods in Classical Plant Breeding (Crop
Improvement)
Unintentional selection
• Selection of variants may have been unintentional or deliberate during early
agriculture
 Simplest form of traditional grain agriculture involves planting the seeds into tilled
fields, harvesting grain-bearing structures, threshing out the seeds by mechanical
agitation
 This method creates a selection pressure that favors plants that do not shed their seeds
before the harvester is ready
 In the wild, most seeds would be shed from the reproductive structures
 In tilled fields however, such seeds would fall to the ground and hence not be saved
for re-planting
 Gradually wild-type seed shedding trait is lost from the population and dominated by
non-shedding seeds
 Another example is immediate and late germination: most wild-type varieties do
not germinate straightaway. They usually enter a phase of dormancy. In agricultural
systems however, seeds that usually germinate immediately are selected and hence the
late germinating seeds will be lost from the domestic crop gene pool
Pre-scientific empirical breeding
• Based on an informed trial-and-error process
• Conscious selection without proper scientific knowledge
• For example, instead of saving a random sample of harvested grain, a
farmer might select only the larger, better-looking seeds
• Selection of seeds from best performing fields was another technique
that resulted in better crops
• Farmers might have selected a few healthy plants and their seeds
from a diseased field – this led to selection of disease resistant
varieties
Traditional breeding
Hybrid crops
• Since the early nineteenth century
• F1 hybrid is created by using pollen from one parent to fertilise a genetically
dissimilar parent
• Such hybrids however, cannot be propagated – their progeny tend to be highly
variable
• Hence, F1 hybrids need to be recreated by a breeder every season (commercially
profitable for breeders)
• The first commercial “crossed corn” was produced in 1917
• To produce seeds on a commercial scale, it is necessary to cross two different F1
hybrids together – results in a more vigorous F2 or double-cross, hybrid variety
• Yield increase of upto 30% were observed in such varieties compared to conventional
varieties
• Most of such hybrids have been developed by intraspecific or interspecific crosses
Traditional breeding
Intergenic Hybrid crops
• Much complex process and time consuming
• Cross between plants of two different genera
• Triticale was the first commercially successful
manmade crop species developed from intergenic
crosses
• Triticale: hybrid of wheat genus (Triticum spp.) with Fig: Triticale crop
rye (Scale cereale)
• It combines multiple useful agronomic qualities of
wheat and rye
• Can be grown under a wider range of climatic and soil
conditions than wheat
• Nutritionally superior to wheat in protein content and
amino acid profile
• Confer multiple health benefits otherwise not present
in wheat
Traditional breeding
• Desired traits can also be introduced into a chosen best recipient line through backcrossing of the
selected progeny with the recipient line for several generations to reduce unwanted phenotype
combinations

Wide Cross:
• Crossing a crop cultivar with a distantly related plant outside of the immediate gene pool
(normally sexually incompatible)
• Normal purpose of plant breeding is to obtain a plant identical to original crop cultivar, except
for a few genes contributed by the distant relative
• Strategy made possible by advances in plant tissue culture techniques in the 1930s
• Embryo abortion occurs at a extremely early stage when wide crosses are made between very
distantly related species
• However, culture techniques allow these minute embryos to be rescued
• Mortality rates are often high, but enough embryos normally survive
Traditional breeding
Wide Cross:
• First-generation wide-hybrid plants are rarely
suited for cultivation (have only half of the
genes from crop parent)
• The other half consists of the desired gene from
P2 in addition to other undesired genes (needs to
be removed)
• They (F1) are hence re-crossed with the original
crop plant
• Another round of embryo rescue is required to
grow the new hybrids (F2 – 75%)
• Selection of progenies that contain small number
of desirable genes from the non-crop parent
• Backcrossing is then repeated for 6-7 generation
• The resulting plant is 99.9% identical to the
original crop cultivar
• Wide crossing programs take mutliple years
(around 10 years or more at times)
Traditional breeding
Wide Cross – Success story
• Success story of International Rice Research Institute (IRRI)
• Grassy stunt virus is a major pathogen of rice (little resistance)
• Pathogen is transmitted by a leaf-dwelling insect known as the brown planthopper,
Nilaparvata lugens
• By the 1960s and 70s, the grassy stun virus had become an endemic disease affecting
rice cultivation. One major outbreak was in Indonesia in the mid 1970s
• IRRI scientists came across a tiny population of a wild Indian plant (Oryza nirvara) in
India that was resistant to the virus
• Multiple years of backcrossing of Oryza sativa with Oryza nirvara led to the
development and release three new virus resistant varieties
• Other examples include, development of four new rice varieties by wide crossing with
Oryza officinalis that are resistant to the brown planthopper
Mutation breeding
• A key technique in conventional breeding
• Mutagenesis: phenomenon in which sudden heritable changes occur in the genetic material of
an organism
• Can occur spontaneously in nature
• Or artificially via exposure to chemical, physical or biological agents
• Mutation breeding classified based on three known types of mutagenesis:
1. Radiation-induced mutagenesis: gamma rays, X rays, ion beams
2. Chemically induced mutagenesis
3. Insertional mutagenesis: DNA insertions via genetic transformation and T-DNA insertion or
site directed mutagenesis
• Plant mutation dates back to 300 BC
• However, mutation as a terminology was first used in 1901 AD by Hugo De Vries (research
led to observations that new characteristics sometimes suddenly appear in plants and animals,
and these characters may sometimes be inherited by their offspring)
• Mutagens: artificial mutation-causing agents – physical and chemical mutagens
• Can induce mutation in almost any planting materials (in vitro cultured cells, seedlings,
whole plants)
Mutation breeding
Mutation breeding – Mutagens
Successful outcomes of conventional crop improvement
techniques