Plant Breeding & Center of Domestication

Introduction to Plant Breeding

What is Plant Breeding?

Plant breeding is:

• Accelerated evolution guided by humans rather than nature.
• A scientific process where human selection replaces natural selection.
• Aimed at changing plant heredity to suit human needs.
• Focused on developing cultivars with:
  • Higher yield
  • Better quality
  • Increased resistance to pests and diseases
  • Improved tolerance to environmental stresses (drought, heat, flood)
  • Better nutritional value

Primary Goal

• Improved yield

Yield is influenced by production methods, fertilizers, pest management, and especially improved cultivars.

Contribution of Breeding to Yield

• 30–50% of yield improvement (1920–1980) came from breeding.
• ~1.5% yield increase per year due to plant breeding advancements.

Plant Diversity and Global Food Base

• Earth has ~320,000 seed-bearing plant species.
• ~7,000 species are used as food plants.
• Six crops provide 60–80% of global human calories:
  1. Wheat
  2. Rice
  3. Maize (corn)
  4. Potato
  5. Sweet potato
  6. Cassava

These crops form the backbone of global food security.

History of Plant Breeding

Early Domestication

• Began with early farmers saving seeds of their best plants.
• Plant domestication started ~10,000 years ago in the Near East.
• Root crops and legumes: domesticated 2,000–3,000 years ago.
• Forage, drug, and ornamentals: domesticated ~2,000 years ago.

Scientific Breeding

• Major progress occurred after:
  • Industrialization
  • Rediscovery of Mendel’s laws (1900s)
• Systematic scientific breeding has been practiced for only ~200 years.

Genotype × Environment Interaction (G × E)

Plant performance depends on:

• Genetics (Genotype)
• Climate & soil (Environment)
• Their combined interaction (G × E)

Example: Wheat varieties respond differently to fertilizers depending on their genotype.

Case Study: Breeding of Sugar Beet (Beta vulgaris)

• 4000 BC: Sugar cane cultivation in India.
• 800 AD: Sugar cane grown in Italy and Spain.
• 18th century: Shortage of sugar stimulated alternative sources.
• 1747: Andreas Marggraf discovered sugar in fodder beet.
• 1786: Archard began selecting beets with high sugar content.
• 1802: First sugar beet refinery established in Germany.
• 1952: Discovery of the monogerm gene (single-seeded fruits).

Centers of Origin & Domestication (Vavilov’s Concept)

Definition

Regions where early humans first domesticated plants and where:

• Genetic diversity is highest,
• Wild relatives of crops are abundant,
• Agriculture has ancient roots.

Importance of Centers of Origin

These regions are vital for:

• Crop improvement
• Breeding for resistance
• Climate resilience
• Germplasm conservation (gene banks, field banks, in-situ conservation)

Major Centers of Domestication & Their Crops

North American Region

Crops:

• Sunflower
• Maize
• Cotton
• Tomato
• Zinnia
• Marigold

Notes:

• Maize domesticated from teosinte in Mexico.
• Sunflower—major global oilseed.
• Cotton (G. hirsutum) originated here.

Mesoamerican / Central American Region

Crops

• Maize
• Beans
• Chili peppers
• Squash
• Tomato
• Cotton

Notes

• Richest center for vegetable crops.
• Valuable genes for disease resistance.

South American (Andes–Amazon) Region

Crops

• Potato
• Cassava
• Peanut
• Sweet potato
• Coca
• Minor tubers (oca, ulluco)

Notes

• Potato became a global staple.
• Cassava critical for tropical food security.

European / Mediterranean Region

Crops

• Wheat
• Barley
• Oats
• Apple
• Grapes
• Olives
• Fodder crops

Notes

• Early cereal domestication.
• Major fruit and oil crops.

Near Eastern (Fertile Crescent) Region

Crops

• Wheat (emmer, einkorn)
• Barley
• Lentil
• Pea
• Chickpea
• Flax

Notes:

• First settled agriculture ~10,000 years ago.
• Birthplace of cereal and pulse domestication.

Central Asian Region

Crops

• Wild apples
• Grapes
• Onion
• Carrot

Notes

• Extremely rich fruit diversity.
• Important for improving apple cultivars.

Chinese Region

Crops

• Rice (Oryza sativa)
• Soybean
• Peach
• Apricot
• Citrus
• Tea

Notes

• One of world’s oldest agricultural centers.
• Rice domesticated ~10,000 years ago.

Indo-Malayan / Southeast Asian Region

Crops

• Banana
• Sugarcane
• Coconut
• Taro
• Black pepper
• Turmeric
• Citrus spp.

Notes

• Center of tropical fruits and spices.
• Sugarcane and banana originated here.

African Centers

A. Ethiopian Highlands

Crops

• Coffee
• Finger millet
• Teff
• Castor
• Chickpea (secondary center)

B. West African Region

Crops

• Sorghum
• Oil palm
• Yam
• Cowpea
• Watermelon (wild type)

Notes:

• Known for drought-tolerant crops.

Summary Table of Crop Domestication

Importance of Domestication Centers in Modern Breeding

A. Genetic Diversity

Provides raw material for:

• Disease resistance
• Drought and heat tolerance
• Pest resistance
• Nutritional enhancement

B. Germplasm Conservation

Preserved through:

• Gene banks
• Field gene banks
• In situ conservation

C. Crop Improvement

Used for:

• Hybrid breeding
• Backcrossing
• Mutation breeding
• Genome editing (CRISPR)

D. Climate Change Adaptation

Wild genes help breed:

• Drought-resistant wheat
• Heat-tolerant maize
• Flood-tolerant rice (e.g., SUB1 gene)
• Insect- and disease-resistant cultivars

Strategy of Plant Breeding

Plant breeding is a systematic, scientific process aimed at developing superior crop varieties with improved yield, stress tolerance, and adaptability. The strategy typically follows four key steps:

Identification of Useful Traits

The first step in plant breeding is to identify traits that can improve the performance or usefulness of a crop. These may be:

I. Morphological Traits

These are visible and structural features of the plant.
Examples:

• Plant height
• Leaf size and shape
• Seed size and color
• Number of tillers
• Root architecture
• Days to maturity

Importance
Morphological traits help in improving crop architecture, harvesting efficiency, and consumer preference.

II. Physiological Traits

Traits related to internal biological processes.
Examples:

• Photosynthetic efficiency
• Water-use efficiency
• Nitrogen-use efficiency
• Drought tolerance mechanisms
• Salt and heat tolerance

Importance
Physiological traits contribute to stress resistance and productivity under variable climate conditions.

III. Pathological Traits

Traits related to plant resistance or susceptibility to diseases and pests.
Examples:

• Resistance to fungal diseases (rust, blight, mildew)
• Resistance to viral diseases (mosaic virus, leaf curl)
• Resistance to insects (stem borer, aphids)

Importance
These traits protect yield and reduce reliance on chemical pesticides.

IV. Sources of Useful Traits

Breeders explore traits using:

• Wild relatives (rich in resistance genes)
• Landraces (locally adapted accessions)
• Mutants
• Existing improved varieties
• Germplasm banks

Identification of Genes/Loci Responsible

After identifying a useful trait, the next step is to determine the genetic basis behind it.

I. Gene Mapping

Using tools like:

• Linkage mapping
• QTL (Quantitative Trait Loci) analysis
• GWAS (Genome-Wide Association Studies)

II. Molecular Marker Techniques

• RFLP
• RAPD
• SSR
• SNP markers

Uses
Markers linked to desirable traits help in faster selection through marker-assisted selection (MAS).

III. Identifying Gene Function

Through:

• Gene expression studies
• Knock-out mutants
• Functional genomics
• CRISPR gene editing

Importance:
Knowledge of gene location and function allows more precise breeding and biotechnology-based improvements.

Combining Genes into an Improved Cultivar

Once genes are identified, breeders combine them into a single genotype through:

I. Hybridization

Crossing genetically diverse parents to combine desirable traits.
Example:

• Disease-resistant parent × High-yielding parent

Outcome: Hybrid population with combined traits.

II. Selection

Choosing individuals superior for desired traits.
Methods include:

• Mass selection
• Pedigree selection
• Bulk selection
• Recurrent selection
• Marker-assisted selection (MAS)

III. Backcross Breeding

Used to transfer a specific gene (e.g., disease resistance) into an elite variety.

IV. Mutation Breeding

Using radiation or chemicals to create beneficial mutations.
Example: Improved rice varieties (e.g., Sharbati Sonora).

V. Biotechnology-Based Approaches

• Genetic engineering (GM crops)
• CRISPR/Cas genome editing
• Tissue culture and micropropagation
• Doubled haploids (to fix traits quickly)

Goal
To assemble all desirable genes in a stable, uniform, high-performing cultivar.

Evaluation of Performance Across Multiple Environments

After developing potential new varieties, breeders test them under diverse conditions.

I. Multi-location Trials

Conducted across:

• Different agro-climatic zones
• Varied soil types
• Different seasons

II. Parameters Evaluated

• Yield
• Disease/pest resistance
• Stress tolerance (drought, salinity, heat)
• Quality traits (protein, sugar content, fiber quality)
• Maturity duration

III. Genotype × Environment (G × E) Interaction

Varieties may perform differently in different environments.
Example:
A variety performing well under high soil fertility may not yield well in low-input conditions.

IV. Stability Analysis

Breeders use statistical models to identify stable and high-performing genotypes across environments.

V. Official Testing

Before release, varieties undergo:

• Pre-release trials
• National performance trials
• Distinctness, Uniformity, Stability (DUS) testing

Outcome
Only the best-performing, stable, and superior lines are released as new varieties.

Multiple Choice Questions (MCQs)

Q1. Which of the following crops is NOT one of the six crops providing 60–80% of global calories?

A. Wheat
B. Potato
C. Cassava
D. Sorghum

Answer: D. Sorghum

Q2. Plant breeding is best defined as:

A. Natural evolution of crops
B. Evolution guided by humans through selection
C. Mutation of wild plants
D. Hybridization of animals

Answer: B. Evolution guided by humans through selection

Q3. Which scientist first proposed Centers of Origin of cultivated plants?

A. Charles Darwin
B. Gregor Mendel
C. Nikolai Vavilov
D. Norman Borlaug

Answer: C. Nikolai Vavilov

Q4. The earliest region where settled agriculture began (~10,000 years ago) is:

A. Ethiopian Highlands
B. Fertile Crescent (Near East)
C. Andes Mountains
D. East Asia

Answer: B. Fertile Crescent (Near East)

Q5. Maize was domesticated from which wild grass?

A. Triticum
B. Teosinte
C. Hordeum
D. Oryza nivara

Answer: B. Teosinte

Q6. Which center is known for the domestication of rice, soybean, and tea?

A. Central Asia
B. China
C. Indo-Malayan region
D. Mediterranean

Answer: B. China

Q7. Which crop originated in the Ethiopian Highlands?

A. Cassava
B. Coffee
C. Banana
D. Sorghum

Answer: B. Coffee

Q8. Which of the following crops originated in South America (Andes region)?

A. Sorghum
B. Wheat
C. Potato
D. Finger millet

Answer: C. Potato

Q9. Which factor contributed 30–50% to yield improvement between 1920–1980?

A. Irrigation
B. Chemical fertilizers
C. Plant breeding
D. Mechanization

Answer: C. Plant breeding

Q10. Sugar beet improvement began after the discovery of sugar in fodder beet by:

A. Archard
B. Vavilov
C. Marggraf
D. Darwin

Answer: C. Marggraf

Q11. Monogerm sugar beet seeds are beneficial because:

A. They contain more sugar
B. They allow easier mechanical sowing
C. They resist pests better
D. They have deeper roots

Answer: B. Easier mechanical sowing

Q12. The Indo-Malayan region is known as the center for which major crops?

A. Wheat and barley
B. Banana and sugarcane
C. Grapes and apple
D. Soybean and tea

Answer: B. Banana and sugarcane

Q13. The concept of Genotype × Environment interaction means:

A. Environment has no effect on yield
B. All varieties give same yield everywhere
C. A genotype performs differently across environments
D. Genes remain stable in all environments

Answer: C. A genotype performs differently across environments

Q14. The primary goal of plant-breeding programs is:

A. Bigger leaves
B. Higher yield
C. Taller plants
D. Faster flowering

Answer: B. Higher yield

Q15. Which region contributed the highest diversity of fruit crops like apple and grapes?

A. Central Asia
B. Mediterranean
C. China
D. North America

Answer: A. Central Asia

Q16. Cassava originated in which domestication center?

A. Africa
B. South America
C. Mesoamerica
D. Near East

Answer: B. South America

Q17. Which crop originated in West Africa?

A. Yam
B. Wheat
C. Potato
D. Soybean

Answer: A. Yam

Q18. Vavilov’s centers are important for plant breeding because they provide:

A. Uniform crop varieties
B. Maximum genetic diversity
C. High-yielding hybrids only
D. Modern varieties

Answer: B. Maximum genetic diversity

Q19. Which of the following is NOT a major crop from the Near East?

A. Lentil
B. Wheat
C. Barley
D. Peanut

Answer: D. Peanut

Q20. Which crop originated in North America?

A. Sunflower
B. Coffee
C. Tea
D. Sorghum

Answer: A. Sunflower

References

• Brooker R.J. (2005). Genetics: Analysis and Principles.
• Griffiths et al. (2015). Genetic Analysis.
• Pierce B. (2014). Genetics: A Conceptual Approach.
• Russell P.J. (2010). iGenetics: A Molecular Approach.