Transcript The Near East - University of Kentucky

History of Plant Breeding
David Van Sanford
University of Kentucky
Origins of Agriculture
Harlan (1992) outlined six regions in which agricultural
origins occurred
We will examine, briefly, the Near East and Meso-America.
The Near East serves as a general introduction to self-pollinated
species, focusing on common wheat (Triticum aestivum,
2n=6x=42, genomes ABD)
Meso-America serves as a general introduction to cross
-pollinated species focusing on corn
(Zea mays L., 2n = 2x = 20).
Ch. 1-3 Fehr
Topics 1-3 Murphy
The Near East
At the eastern end of the Mediterranean Sea,
across a broad arching zone of grasslands and
open oak-pistachio woodlands called the Fertile
Crescent the world’s first agricultural economies
emerged between 10,000 and 8,000 years ago
(the Neolithic revolution).
The Near East
Landraces of Self-Pollinated Species
The heterogeneous populations grown by Neolithic farmers are called
landraces—heterogeneous cultivated forms that evolved from natural
populations of plant species.
Landraces of Self-Pollinated Species
Wheat is a self-pollinated species with low levels of outcrossing.
Any given plant in a Neolithic farmer’s field was homozygous at most, or
all, loci.
In general, seeds harvested from any individual plant faithfully
reproduced the parental genotype the following generation.
This constancy was maintained through generations in the absence of
outcrossing to a different genotype or the occurrence of spontaneous
mutations.
If a second plant was considered in this Neolithic farmer’s field, the
same constancy of genotype from generation to generation occurred.
But there was a noticeable difference--this second plant likely had
different alleles at many loci when compared locus-by-locus with the
first plant.
Landraces of Self-Pollinated Species
At harvest, the seeds from individual plants were not kept separate,
but a conglomeration of seeds from all genotypes in the field was
bulked into a seed store.
The following season a random sample of this seed was planted.
This random sample of seed contained a random sample of the
genotypic array found in the field the previous season.
The Composition of an Autogamous (Self-Fertilizing) Landrace
Population
1. Contain many genetically distinct homozygous plants—e.g.,
AABBCC; AABBcc; aaBBcc. They have similar alleles at
corresponding loci on homologous chromosomes.
2. Although these plants exist side-by-side, they remain
more or less independent of each other in reproduction.
3. Plants in these populations are generally fully vigorous
homozygotes. Deleterious recessives are rare and are
eliminated rapidly.
4. Prolonged inbreeding is tolerated in the absence of
inbreeding depression.
5. Local adaptation of individuals is high. Reproductive
isolation shields adapted gene complexes from breakup
through recombination.
6. As a result, individuals have a low flexibility of response
to a changing environment.
Homozygous genotypes do not imply a homogeneous
population.
Landraces of self-pollinating species contain a
preponderance of homozygous plants, but they are not
homogeneous.
Homogeneity in a population exists only if all individuals
have the same genotype.
Individual loci can be homozygous or heterozygous in a
homogeneous population.
History of Plant Breeding
The Intellectual Landscape
• What was the mindset?
• How to regard phenotypic variation?
Essentialism in Biology
Before evolution, there existed an essentialist view of
biology that proposed all species to be unchanging
throughout time. Some religious opponents of evolution
continue to maintain this view of biology.
The bottom line was--essentialists could not handle
intraspecific variation; as a result, they could never
be plant breeders.
The Significance of Intraspecific Variation
Newton’s physical laws were exact and dealt in constancy of the
universe and its systems. Scientists assumed that biological laws
must have similarly rigid descriptions.
The Plant Breeders
1) Individual plant selections were made within landraces and seed
was used to form a) a pure line or b) an improved landrace
2) Intraspecific crosses were made between cultivars differing in one or
two characteristics.
Le Coteur, Isle of Jersey, 1830s
He observed that progeny from remarkable plants
tended to have the same remarkable characteristics.
This led to the development of inbred homozygous
homogeneous lines from landraces.
The Swedish Seed Association
Known for utilizing the Pure Line Method is the Swedish
Seed Association at Svalof in southwestern Sweden. Nilson
in the 1880s, directed a large-scale program of selecting
single heads from landraces followed by progeny tests.
Early Development
Hybridization
of
Cultivars
Through
Pringle released an oat cultivar in 1875 in
Vermont and Jones released a wheat cultivar in
New York in 1893 that resulted from artificial
hybridizations.
Farrer had notable success
during the same period in Australia. Following
the rediscovery of Mendel’s laws governing
particulate inheritance, the popularity of
hybridization to develop breeding populations
for cultivar development increased
The Influence of Darwin on Plant
Breeding
Darwin considered intraspecific variation to be fundamental
underpinning of evolution. Variation within a single species
was no longer considered a distortion of divine creation.
This viewpoint replaced the abstract or metaphysical view
of variation of the Essentialist. Darwin believed that
evolution was like the application of the plant and animal
breeder’s activities to nature as a whole.
Influence of Mendel
Mendel focused on the variation among the
offspring of his crosses rather than on an average
description of them. He divided progeny into
categories, counted offspring in each category, and
calculated ratios of individuals in each category.
Mendel’s approach was new because he
considered the whole population and sought out the
underlying mechanisms controlling the variation in
the populations.
The Confluence of Darwinism, Mendelism and
Plant Breeding
Genetics could explain how progeny resembled parents,
and yet why there was variation among progeny.
Organisms resemble parents (thesis)
Variation in progeny (antithesis)
Genetics (synthesis)
Seed Size in Common Bean - Johannsen
Landrace – seed size variation
Princess
Beans
Line 1
Mean
Range
Progeny Mean
64
Line 25
35
43-87
24-47
64
35
The Pure Line Theory-Johannsen,
1913
Conclusions:
1)selection for seed weight was effective;
2) the original landrace Princess consisted of a
mixture of homozygous plants
3) within-line phenotypic variation was
environmental in nature and further selection
within a pure line will not result in further
genetic change
Johannsen clarified the difference between
phenotype and genotype and gave selection a
firm scientific basis.
Meso-America--A General Introduction to CrossPollinated Species - Maize
Current evidence suggests that corn was domesticated from the
annual wild grass teosinte (Zea mexicana) in the Balsas River
drainage in south central Mexico
Monoecious:
Staminate (Tassel) and
Pistillate (Ear)
inflorescences
Landraces of Cross-Pollinated Species
Corn is a cross-pollinated species with high levels of outcrossing.
Any given plant in an Aztec farmer’s field contained both heterozygous
and homozygous loci. Heterozygosity occurred only at the loci for
which more than one allele was present in the landrace population.
The frequency of heterozygosity at a locus depended on the frequency
of the different alleles in the population.
Seed harvested from any individual plant in an open pollinated
landrace did not faithfully reproduce itself the following season
It was half-sib seed and the plants it produced the following generation
represented the female parent plus the array of male parents that
contributed pollen to the female’s silks.
The following season a random sample of these seeds was planted.
This random sample contained a random array of the genotypes
found in the field the previous season.
The Composition of an Allogamous (Cross-Pollinating)
Population
1. Comprised of a highly heterozygous and
heterogeneous plant population--e.g.,
AaBbcc:AABbCc:AaBBcc:etc.
2. A consequence of open-pollination is the retention
of recessive alleles in the population.
3. An open-pollinated landrace has the best potential for
evolutionary flexibility.
One way to quantify this flexibility is to estimate of Gene Diversity or
Polymorphic Index at a locus :
n
 Pi(1-Pi)
i=1
where n = number of alleles at the locus and Pi the frequency of the
ith allele.
A locus with two alleles present at frequencies of 0.9 and 0.1 has a
polymorphic index of 0.18. If the frequencies are more equitable at
values of 0.5 each, say, the diversity measure increases to 0.5.
4. Open-pollination in a heterozygous population
promotes effective recombination between linked alleles
.
Approximating linkage equilibrium
Early Corn Improvement in the United States
( see Troyer (1999). Crop Sci. 39: 601-625)
The early corn breeders all practiced a form of mass selection
through the selection of desirable open-pollinated ears from
superior plants
Some open-pollinated cultivars were developed by crossing
populations and conducting mass selection on the resulting
progenies, eg. Reid Yellow Dent
Blount was probably the first to conduct mass selection while
controlling the pollen source. He utilized this approach
during the 1870s in Tennessee.
It took approximately 15 years for the world’s population
to grow from 4 to 5.3 billion. The population is expected
to reach 9-10 billion in the 21st century. By the last
decade of your professional careers the world will be
consuming twice the calories per day that were
consumed when you were 10 years old.
How Will We Feed 9-10 Billion
People?
• Increase land area devoted to crop production?
• Increase yields?
• Need to double yields by the end of the 21st century
• Huge challenge for breeders
• Yield gains are diminishing
Plant Breeders – who employs them?
Frey (1996); 2205 science person years employed in plant breeding
Private sector 68% $340m $148-290,000 per SY
University
24% $156m $293,000 per SY
USDA
8% $153
$300,000 per SY
Agronomic crops (71%), Horticultural crops (29%)
Corn (~30%), soybeans (7%), wheat (6%), cotton (6%), temperate
fruits and nuts (5%), tomato(4%), forage legumes(4%), and
ornamentals(4%)
Cultivar development (65%); Germplasm development (18%)
Basic research (17%)
Consultative Group on International Agricultural Research
1000 scientists of 60 nationalities working in 40 developing
countries
Sir Roland Biffin