Choice of Parental Germplasm and Population Formation
Is a Crucial First Step
It is essential that you build adaptation, agronomic performance,
pest resistance and end use quality into the populations in which
you will select . You will need to develop:
• Knowledge of the evolution of the genus, plus a classification
of the
cultivated species and its relatives.
• A comprehensive history of cultivar development in the
species.
• Knowledge of the biotic and abiotic stresses found in your
target environments and their relative levels of importance to crop
production.
• Knowledge of the germplasm resources available. Of particular
importance is their adaptation to your target environments.
N.I. Vavilov was the foremost plant
geographer of his time and took part
in over 100 collecting missions to
64 countries. He scoured five
continents in the 1920s and 1930s
for wild and cultivated corn, potato
tubers, grains, beans, fodder, fruits
and vegetable seeds. The Vavilov
Institute became the world's largest
crop research institute under
Vavilov's leadership. Vavilov even
organized an office in New York
City during the 1920s. Russian
immigrants in the United States
collected seeds and sent them to the
office, and the seeds were then
shipped back to Russia.
Vavilov’s Centers of Origin – First Indicated
By Vavilov to be a Center of Plant Domestication
Classification of the cultivated species and its relatives
Plant breeders may not be able to use strict taxonomists
classification schemes. They just don’t always make a lot of sense.
For example, broccoli, brussel sprouts, cabbage and cauliflower
are grossly different morphologically but they are in the same
species in a biological sense--i.e., they can be crossed and the
hybrids are fertile.
Many breeders have adopted Harlan and deWet’s more
informal and intuitive classifications as to what constitutes
useful groupings based on practical experience.
Harlan and deWet’s Gene Pool System
Informal genetic
perspective
Primary gene pool:
Biological species
Secondary gene pool :
Allele transfer a
struggle
Tertiary gene pool:
Outer limit of potential
genetic reach.
Sources of Information for the Plant Breeder
The in-depth compilations published in the United States Department
of Agriculture Yearbooks of 1936 and 1937 provide a valuable starting
point for many species.
Species monographs published by The American Society of
Agronomy and similar organizations – eg. Wheat and Wheat
Improvement
Pedigrees and methodologies utilized in cultivar development in a
diverse range of species can be found in Registration articles in
the Journal of Plant Registrations.
Biotic and Abiotic Stresses
in Your Target Area Are
Critical
• Know and understand the stresses
that are found in your target area
• Be aware of economic thresholds
and frequency of occurrence when
setting priorities
– Eg. BYDV resistance in wheat
Biotic and Abiotic Stresses
in Your Target Area
• Plant Breeder’s Mantra: Resistance,
or tolerance to all economically
important biotic and abiotic stresses
must be incorporated into breeding
populations constructed for cultivar
development.
BYDV – Aphid Transmitted
Genetic resistance
to BYDV is
difficult to work
with. An
application of
Warrior insecticide
is cheap
Knowledge of Diseases
• Life cycles - sexual or
asexual - Races or
Biotypes - virulence
changes (e.g. powdery
mildew in wheat) host resistance
quantitative or
qualitative - - average
life of qualitative allele
Cereal Disease Lab
USDA / U. Minn.
Track virulence
patterns of rust fungi
Isoline series?
Each isoline contains
a major gene in a
common genetic
background
Recessive
Knowledge of Available Germplasm Resources
The vast majority of all germplasm used in cultivar
breeding programs comes from cultivated species
adapted to your target environment
Note: ‘Exotic germplasm’ refers to not just wild or progenitor
species, but to cultivated types adapted to different target
environments. For example, we have used spring wheats in
crosses to get scab resistance.
Step 1: Adapted/Exotic Cross. An adapted, high yielding
cultivar with excellent baking quality, but poor fungal resistance, is
crossed to an exotic cultivar with excellent fungal resistance but poor
baking quality characteristics. The fungal resistance is controlled
qualitatively, so the breeder is only seeking a single resistance allele
. The inbred progeny from this cross will contain some excellent
fungal resistance, but none will contain the required baking quality,
which is quantitatively controlled
Step 2:
Adapted/Exotic/2/Adapted Backcross.
Next the
breeder backcrosses the F1 to an adapted parent. A step in the right
direction, but still one is unlikely to recover inbred progeny with
acceptable baking quality.
Step 3: Adapted/Exotic/2/Adapted/3/Adapted Backcross.
Thus an extended backcrossing program (prebreeding or parent
building) is required.
Using Exotic Germplasm
• Adapted/Exotic Cross - Adapted, susceptible high yielding
cultivar with excellent quality crossed to exotic, resistant
cultivar with poor quality. Resistance controlled by a single
allele; inheritance of quality is complex
• There will be resistant progeny from this cross but quality
will be poor
• Thus, another cross back to the adapted parent is required:
Adapted/Exotic//Adapted
When you must use exotic parents, the order of
preference will be:
1. Improved cultivars and breeding lines
2. Landraces or older cultivars
3. Closely related species
4.More distantly related species and genera
Most breeders will not get beyond step 1. If there is a
crisis (eg Russian wheat aphid) there will be a concerted
effort to find a deploy resistance .
A List of Parents for Cultivar Development.
1. The Old Standbys
These provide the breeding program with overall adaptation and
high yield potential. Typically there will be varieties that perform
solidly year after year, and year after year they stay in your
crossing block.
2. Elite Germplasm.
These are typically advanced breeding lines from your program
or a colleague’s program. Important because they may contain
new combinations of alleles that are superior to the standbys
described above.
3. Allele Sources for Specific Traits.
As a breeder, you can request germplasm from the National
Plant Germplasm System. The accessions may be cultivars,
landraces, breeding lines and wild relatives. (http://www.arsgrin.gov/)
4. Remnant F1 seed
Most breeders have remnant F1 seed that can be used for
several crossing cycles in a row.
Types of Crosses
•
•
•
•
Single cross
Three way cross
Four way cross
Complex cross
Single cross
•
•
•
•
Easy to make good x good
Pembroke/USG 3350
Choose parents to complement one another
Choose parents from different “heterotic
groups”
• Some crops have many breeding targets
• Two parents unlikely to have all traits
Three Way Cross
• True or modified backcross
• Widely used in wheat
• Third parent is critical (50%)
• KY97C-0519-04-07 // Pembroke/KY02C3006-19
Four Way Cross
• Cross two single cross F1’s
• Not as successful as 3 way crosses in wheat
• One modification: Use two F1’s which
have one parent in common - same genetic
composition as 3 way, but increased
recombination
Complex Crosses
• > 4 parents
• See Fehr chapter 12 for methods of
combining parents
• Will discuss polycrosses with Dr. Phillips
Assessing Parental Value
• Can make crosses using parents to evaluate
their “combining ability”
• Typically we would cross parents in all
possible combinations - referred to as a
diallel
• Requires considerable time; most breeders
will not do this
Assessing Parental Value – The
Diallel
• Consider 10 parents: (10x9)/2=45 crosses
• Produce enough crossed seed to yield test
(ok in tobacco or corn, not wheat or
soybean)
• Otherwise, increase F1’s to F2 generation
and yield test
• Compare parental performance and decide
which of the 10 you will use
The Murder of Nikolai Vavilov
• Vavilov recognized the importance of
Darwin, Mendel
• Became protégé of Bateson, father of
Genetics
• Knew Russians were far behind American,
European counterparts
• Wanted to improve Russian agriculture by
applying the science of genetics to plant
varieties
Timeline
• 1920 – went to Petrograd to take over
Bureau of Applied Botany
• 1921-22 – terrible famine in Russia
• 1923 – went to USA, Europe, met with
breeders, geneticists and acquired seed:
• 6224 packets, 2 tons native corn, 66 boxes
from Luther Burbank, 20,000 seeds from
Europe
Timeline
• 1924 -Began his collection trips:
– Afghanistan
– Ethiopia
– Northern Africa
• Tailed by spies working for dictator
• Incredibly persuasive and lucky
Trouble Brewing
• Trofim Lysenko: so called barefoot
scientist, rejected academic learning,
believed in acquired characteristics induced
by the environment
• Completely opposed to everything Vavilov
believed in
• When Stalin came to power in 1924,
Lysenko was “the man”
Timeline
• 1930-Lysenko put in charge of all soviet
agriculture
• 1940 – Vavilov imprisoned
• 1943 – Dies of malnutrition
• 1964 – USSR rejects Lysenkoism
• 1965 – USSR establishes Vavilov award
From the book The Murder of..
• The message was clear. Here was a shining example of
socialist agriculture..practical peasant unencumbered by
academic theory
• But no one could have told at this stage that this simple
peasant propagator of peas would become a monster,
willing to falsify scientific experiments to satisfy his
political masters, viciously attacking his colleagues,
watching them publicly harassed, imprisoned even
executed and playing the lead villain in the Soviet Union’s
attack on biological science.