Plant Breeding as an Art and Science

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Plant Breeding as an Art and Science
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,
2n6x42, genomes ABD) Meso-America serves as
a general introduction to cross -pollinated
species focusing on corn (Zea mays L., 2n 2x
20).
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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 worlds first
agricultural economies emerged between 10,000 and
8,000 years ago (the Neolithic revolution).
Landraces of Self-Pollinated Species  The
heterogeneous populations grown by Neolithic
farmers are called landracesheterogeneous
cultivated forms that evolved from natural
populations of plant species.
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Landraces of Self-Pollinated Species
Wheat is a self-pollinated species with low
levels of outcrossing. Any given plant in a
Neolithic farmers 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 farmers 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.
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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.
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• The Composition of an Autogamous
  (Self-Fertilizing) Landrace Population
•  
• Contain many genetically distinct homozygous
  plantse.g., AABBCC AABBcc aaBBcc. They have
  similar alleles at corresponding loci on
  homologous chromosomes.

• Although these plants exist side-by-side, they
  remain more or less independent of each other in
  reproduction.
• 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.
• As a result, individuals have a low flexibility
  of response to a changing environment.
•  
•  

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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.
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Essentialism in BiologyBefore evolution was
developed as a viable scientific theory, there
existed an essentialist view of biology that
posited 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.
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Confusion Over the Significance of Intraspecific
Variation
Carl Von Linnaeus (1707-78) Species Hybridizers
Newtons physical laws were exact and dealt in
constancy of the universe and its systems, thus
scientists assumed that biological laws must have
similar rigid descriptions.
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The Plant Breeders

• 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.

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The Swedish Seed Association The organization
that has received the most notoriety for
utilizing the Pure Line Method on a grand scale
is the Swedish Seed Association at Svalof in
southwestern Sweden. Under the direction of
Nilson in the 1880s, a large-scale program of
selecting single heads from landraces followed by
progeny tests was initiated.
Early Development of Cultivars Through
Hybridization  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 Mendels laws
governing particulate inheritance, the popularity
of hybridization to develop breeding populations
for cultivar development increased
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The Influence of Darwin and Mendel on Plant
Breeding
Darwin began the age of population thinking. It
replaced the abstract or metaphysical view of
variation of the Essentialist with the
materialist view of the Darwinist by focusing on
the variation among organisms as a pivotal fact
of nature. Darwin considered intraspecific
variation to be the cornerstone of evolution.
The variation among members of a single species
was no longer considered an annoying distortion
of the ideal divine creation.
Darwin believed that evolution was simply the
application of the plant and animal breeders
activities to the mechanisms of nature as a whole.
Mendel put emphasis on the variation among the
offspring of his crosses rather than on an
average description of them. He divided the
progeny into categories, counted the number of
offspring in each category, and then calculated
ratios of numbers of individuals in each
category. Unlike his predecessors who seemed to
be satisfied to just obtain progeny from crosses,
Mendels approach considered the whole population
and sought out the underlying mechanisms
controlling the variation in the populations.
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At the turn of the century the stage was set for
the emergence of scientific plant breeding with
the joining of Darwinian theory on the
reproductive advantage of better adapted
individuals and Mendelian interpretation of the
inheritance of those adaptations.
One of the triumphs of genetics is that it can
explain both the constancy of inheritance and its
variation. It explains the apparent
contradiction--that individuals resemble their
parents and differ from their parents. It
contains elements for everyone--overall constancy
for the Essentialist, yet the constancy is
tempered by variation for the Darwinist
Organisms resemble their parents
(thesis) Variation in progeny
Genetics (antithesis)
(synthesis)
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The Pure Line Theory
His first conclusion was that selection for seed
weight was effective.
His second conclusion was that the original
landrace consisted of a mixture of homozygous
plants
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Thus, his third conclusion was that the
within-line phenotypic variation was
environmental in nature and further selection
within a pure line will not result in further
genetic change Johannsens results clarified the
difference between phenotype and genotype and
gave selection a firm scientific basis.
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Meso-America--A General Introduction to
Cross-Pollinated Species   We now shift focus to
corn and discuss a cross-pollinated breeding
system. Current evidence suggests that corn was
domesticated from the annual wild grass teosinte
(Zea mexicana) in the Balsas River drainage in
south central Mexico (Harlan, Fig. 11-2 Galinat,
Fig. 2). Mimicking the situation for
self-pollinated species, many distinct types or
races of corn evolved under human and
environmental selection as the species spread
through North and South America.
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Monoecious Staminate (Tassel) and Pistillate
(Ear) inflorescences
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Landraces of Cross-Pollinated Species
Corn is a cross-pollinated species with high
levels of outcrossing. Any given plant in an
Aztec farmers 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 females 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.
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The Composition of an Allogamous
(Cross-Pollinating) Population
1. Comprised of a highly heterozygous and
heterogeneous plant population--e.g.,
AaBbccAABbCcAaBBccetc.
2. A consequence of open-pollination is the
retention of recessive alleles in the population.
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3. An open-pollinated landrace has the best
potential for evolutionary flexibility.
This diversity is measured by the number and
frequencies of alleles at each locus. One
estimate of Gene Diversity or Polymorphic Index
at a locus is   n ? Pi(1-Pi) i1  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. How does
a population with three alleles each present at a
frequency of 0.33 rate relative to the former two?
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• Open-pollination in a heterozygous population
  promotes
• effective recombination between linked
  alleles .
• Approximating linkage equilibrium

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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.
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It took approximately 15 years for the worlds
population to grow from 4 to 5.3 billion. In 25
years the population will increase another 55 to
8.2 billion. 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.
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Scientific advance good for society was
unquestioned
Nuclear age watershed in public opinion
Criticism of Green Revolution
Southern Corn Leaf Blight epidemic. Genetic male
sterility system
GMOs, Terminator technology, control of plant
genetic resources, patenting life forms, etc.,
etc.
First the seed Kloppenburg (1988) Shattering
Fowler and Mooney (1990)
The Frankenfood Myth How Protest and Politics
Threaten The Biotech Revolution - Miller and
Conko (2004)
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Plant Breeders who employs them?
Plant Variety Protection Act (PVPA),
1970 Cultivar owner determines who may market
seed. Farmers could save their own seed, but
problems arose with Brown Bagging, or the sale
of excess seed Plant Variety Protection Act
(PVPA), 1994 Limits quantity of seed an
individual may save to the amount of seed needed
to plant own farm. If plans change, can sell seed
only with permission. Utility Patents Protection
of genetically engineered cultivars. May not
save, clean or sell seed
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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)
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Consultative Group on International Agricultural
Research
1000 scientists of 60 nationalities working in 40
developing countries