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 IndicatedBy 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 LabUSDA / U. Minn.

Track virulence patterns of rust fungi

Isoline series?Each isoline containsa major gene in a common geneticbackground

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.ars-grin.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/KY02C-3006-19

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.