Transcript history_0f_pb 01_16_..

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,
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).
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).
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.
Essentialism in Biology
Before 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.
Confusion Over the Significance of Intraspecific Variation
Carl Von Linnaeus (1707-78)
Species Hybridizers
Newton’s 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.
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.
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 Mendel’s laws governing
particulate inheritance, the popularity of hybridization to
develop breeding populations for cultivar development
increased
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 breeder’s 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, Mendel’s approach considered the whole population and sought
out the underlying mechanisms controlling the variation in the populations.
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
(antithesis)
Genetics
(synthesis)
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
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
Johannsen’s results clarified the difference between
phenotype and genotype and gave selection a firm
scientific basis.
Meso-America--A General Introduction to CrossPollinated 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.
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.
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)
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. How
does a population with three alleles each present at a frequency of
0.33 rate relative to the former two?
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. 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.
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
GMO’s, 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)
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
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