Developing New Varieties:Plant Breeding

Sherry Flint-Garcia

Research Geneticist

USDA-ARS

Department of Agronomy

Source: USDA

Source: USDA

Source: USDA

Source: USDASource: USDA

Plants

~ 350,000 species world-wide

< 300 used for food

~ 150 important in world commerce

15 major food sourceswheat, corn, rice, barley, sorghum

soybean, phaseolus (bean), peanut

potato, sweet potato, cassava

coconut, banana

sugar cane, sugar beet

Plant Breeding

Private Sector, Universities,

USDA, International Organizations

Biochemistry

Yield

ProtectionQualityEntomology

Physiology

PlantPathologyNutrition

Genetics

Molecular Biology

Statistics

Source: USDA

Plant Breeding

Identify/generate variationGermplasm collections

Induced variation

Hybridization/recombination

Select for desired characteristicsDepends on the crop and its end use

Evaluate selections

Sou

rce:

US

DA

Source: USDA

Variation – Germplasm Collections

Primary gene pool (same species) Elite cultivars Landraces (primitive cultivars) Wild plants of the same species

Secondary gene poolCultivars, landraces, or wild

plants of different species“Wide crosses”

Potato Germplasm Introduction Station (NR6)

North Central Regional PlantIntroduction Station (NC7)

Sou

rce:

US

DA

Source: USDA

Variation – Induced Variation

PolyploidyTreat plants with chemical to induce

polyploidy

MutationNatural rates are ~1 per million cell divisions

Increase rate by using radiation or chemicals

Biotechnology/Genetic Engineering Ability to cross the species/kingdom barriers

Variation – Hybridization and Recombination

Parent 1 Parent 2

F1

F2

(self pollinate) Recombination

Hybridization

Self- vs. Cross-pollinated Crops

Self-pollinated – wheat and soybeansUses pollen and egg from the same plant to

produce seed

Few seeds per hand pollination (3-15)

Cross-pollinated – Corn and squashUses pollen from one plant to fertilize an egg

from another plant

Many seeds per hand pollination (300-400)

Selection

Figure courtesy of Larry Darrah

SelectionSelf-pollinated crops

Mass selection – bulking of selections

Pure line – test each selection separately

Cross-pollinated cropsMass selection

Half- and Full-sib selection – hybrids

Recurrent selection – intermate selected lines

Backcross

Marker-assisted selection

Recurrent Selection

Used for parent building/population developmentGenerate families

Test family performance

Recombine selected families to complete a cycle of selection

random mate

Cycle 0

Cycle 1

Stalk Lodging – An Example of Recurrent Selection

Divergent Selectionfor Stalk Strength

0 1 2 3 4 5 6 7 8 9 10 11

Rind penetrometer resistance bins (each 0.25 kg)

0

100

200

300

400

500

Fre

quen

cy

2530 data points for Cycle 0Mean = 4.37 for B73 x Mo17Mean = 4.40 for Cycle 0

2699 data points for B73 x Mo17

B73 x Mo17

Cycle 0

0 1 2 3 4 5 6 7 8 9 10 11

Rind penetrometer resistance bins (each 0.25 kg)

0

100

200

300

400

500

Fre

quen

cy

2446 data points for high2508 data points for low

High sel.

Low sel.

Hig

h m

ea

n =

6.5

4

Lo

w m

ea

n =

3.0

9

Cycle 0 and B73 x Mo17 Cycle 6 low and high

Divergent Selection Results

Figure courtesy of Larry Darrah

Evaluation

Phenotypic (trait) variation can be caused by:Environment = soil fertility, weather, biotic and

abiotic stresses

Genotype** = genes responsible for trait

** This is what plant breeders want to exploit

Replicated trials to find stability across environments

Look at many – save a few!300 Lines

5-10 advance to the next generation

GOAL:

Rapid Reliable

Inexpensive

Corn Anatomy

Tassel - ♂♂ Sheds pollen at maturity

Ear - ♀♀Each silk is attached to one ovule.

Pollen tube grows down silk and fertilizes ovule.

Photo courtesy of Maize Mapping Project

Shoot Bagging

Controlled Pollinations of Maize http://www.maizegdb.org

Cutting Back the Ear

Controlled Pollinations of Maize http://www.maizegdb.org

Tassel Bagging

Controlled Pollinations of Maize http://www.maizegdb.org

Corn Pollination

Controlled Pollinations of Maize http://www.maizegdb.org

Soybean Anatomy

Soybean Anatomy

Soybean Anatomy

Stamen - ♂♂

Pistil - ♀♀

Soybean Pollination

Photo courtesy of Duane Dailey

Photo courtesy of Duane Dailey

First Plant Breeders

Thousands of years ago:Likely women selected plants that they liked

Hundreds of years ago:Farmers had their favorite “family” variety

1926 Pioneer Hi-Bred was foundedFirst commercial hybrid seed company

First Traits Selected

Non-shattering Altered photoperiod and vernalizationLoss of dormancy/rapid germinationSeed size and abundanceAnnual habitLoss of defensive structuresUniformity (germination and maturation)

Today’s Important Traits

Altered Seed CompositionAmino acid, fatty acid, starch

Grain Quality:Baking quality, brewing quality

Resistance to biotic and abiotic stressesTolerance/resistance to diseases and insects

Tolerance of poor soils – salinity, acidity

Yield – the bottom line!

Bu

shel

s P

er A

cre

Year

Open Pollinated Varieties

Double Cross Hybrids

Single Cross Hybrids

Does plant breeding work?

Corn Yield Trends: 1870 to Today

Heterosis in Maize

Bu

sh

els

Pe

r A

cre

Year

Single CrossHybrid

InbredA

InbredB

Double Cross Hybrid

InbredA

InbredB

InbredC

InbredD

Hybrid1

Hybrid2

Green Revolution in Wheat

Funded by the Rockefeller Foundation in 1940s and 1950s

Led by Dr. Norman Borlaug Wheat pathologist and breeder

Semi-dwarf varieties with resistance to stem rust

Wheat yields in Mexico From 11 bu/a (1943) to 30 bu/a (1963)

Examples of

New Varieties

Wheat variety ‘Ernie’

Derived from pedigree selection at the University of Missouri

First soft red winter wheat with Scab resistance (fungal disease)

Very early maturity that permits its use in double cropping systems

Photo courtesy of Anne McKendry

Photo courtesy of Anne McKendry

“Calrose 76”

Semi-dwarf rice variety

Developed in California in 1976

Product of mutation-breeding program

Continues to be a parent in breeding programs today

Triticale: A “new” crop

Triticale is a cross of wheat (♀♀) and rye (♂♂) followed by induced polyploidization

High yield and baking qualityConfers traits of tolerance to acid soils and

salinity, drought tolerance, winter hardiness, rust and mildew resistance, and higher lysine

Grown on 7.5 million acres (acid and marginal soils) in the world – primarily Australia, Brazil, France, Germany, Poland, and South Africa

Triticale on Acidic Soil

Photo courtesy of Perry GustafsonPhoto courtesy of Perry Gustafson

New Birdsfoot Trefoil with Rhizomes

U.S. trefoil has persistence problems because of root and crown rot

U.S. varieties lack rhizomesPaul Beuselinck, USDA-ARS, Columbia,

collected a rhizomatous birdsfoot trefoil in Morocco and has bred it into U.S. germplasm resulting in release of ARS-2620

Grazing studies show increased persistence in pastures

No rhizomes Rhizomes

Photo courtesy of Paul Beuselinck

Where Do Most New Varieties Come From?

Selfing out of existing varieties and testingCrosses among existing lines and

varieties followed by selfing and testing

Yes, this approach seems to be self-limiting (funnel), but it has worked well thus far in many crops

Bu

shel

s P

er A

cre

Year

Can We Continue Indefinitely?

Glossary

Phenotype: The observable physical or biochemical characteristics of an organism

Genotype: The genetic constitution of an organism

Inbred: A plant that is produced through self-pollination over many generations; “true breeding”

Hybrid: A plant that is produced by cross-pollinating two inbreds

Hybridization: The act of mixing different species or varieties of animals or plants

Recombination: A combining of genes or characters different from what they were in the parents