Genetic Basis and Methods of Breeding Cross-Pollinated Crops

1. Introduction

Cross-pollinated crops primarily undergo pollination between flowers of different plants. This ensures genetic recombination, resulting in high heterozygosity and diversity. Examples include maize, sunflower, pearl millet, sorghum, onion, and carrot. Breeding these crops aims to exploit heterosis, improve yield, and enhance resistance to stresses. Unlike self-pollinated crops, breeding cross-pollinated crops focuses on maintaining and utilizing genetic variability.

2. Genetic Basis of Cross-Pollinated Crops

2.1 Genetic Variability

Cross-pollinated crops show high genetic variability due to:

• Outcrossing between genetically different individuals.
• Heterozygosity in gene loci.
• Segregation in successive generations.

This variability enables selection of superior genotypes, hybrid development, and adaptation to different environments.

2.2 Heterosis (Hybrid Vigor)

Heterosis is the superior performance of F₁ hybrids over their parents, common in cross-pollinated crops. Caused by:

• Dominance hypothesis: masking of deleterious recessive alleles.
• Overdominance hypothesis: heterozygote advantage at certain loci.

Exploiting heterosis is a major objective in crops like maize, sunflower, and sorghum.

2.3 Inbreeding Depression

Inbreeding reduces vigor, fertility, and yield due to homozygosity of deleterious alleles. This emphasizes the importance of maintaining heterozygosity in cross-pollinated crops.

2.4 Genetic Makeup

Cross-pollinated crops are predominantly heterozygous. Large populations are required to capture full genetic variability. Selection is based on population performance rather than individual performance.

3. Objectives of Breeding Cross-Pollinated Crops

• Improve yield potential.
• Enhance resistance to pests and diseases.
• Improve quality traits (nutritional value, oil/sugar content).
• Increase adaptability to environmental stresses.
• Develop hybrids to exploit heterosis.

4. Methods of Breeding Cross-Pollinated Crops

Breeding methods are divided into population improvement methods and hybrid breeding methods.

4.1 Population Improvement Methods

These methods increase the frequency of desirable alleles in populations.

4.1.1 Mass Selection

Selection of superior plants based on observable traits. Seeds of selected plants are bulked for the next generation.

 [Read Here]

• Advantages: Simple and effective for high heritability traits.
• Limitations: Less effective for low heritability traits; environmental effects may influence selection.

4.1.2 Recurrent Selection

Repeated cycles of selection and intercrossing to increase favorable alleles. Types:

[Read Here]

• Phenotypic Recurrent Selection: Based on observable traits.
• Half-Sib Recurrent Selection: Based on progeny performance of maternal half-sib families.
• Full-Sib Recurrent Selection: Based on progeny performance of full-sib families.

Advantages: Enhances additive and non-additive gene effects; suitable for polygenic traits.

4.1.3 Mass Pedigree Method

Superior plants are selected individually and seeds are kept separately for pedigree record. Selection continues over generations.

4.1.4 Synthetic Varieties

Developed by intercrossing selected lines and maintaining them through open-pollination. Suitable for highly cross-pollinated crops like pearl millet and sunflower.

[Read Here]

4.1.5 Composite Varieties

Formed by intercrossing several highly variable superior genotypes. Maintains heterozygosity and adaptability; commonly used in forage crops.

[Read Here]

4.2 Hybrid Breeding Methods

Hybrid breeding exploits heterosis in cross-pollinated crops.

4.2.1 Two-Line Method (EGMS-based)

Uses a male sterile line and a fertile line to produce F₁ hybrids showing heterosis. Applied in rice and sunflower.

4.2.2 Three-Line Method (CMS-based)

Components:

• A line: Cytoplasmic male sterile line
• B line: Maintainer line (fertile, maintains sterility in A line)
• R line: Restorer line (restores fertility in F₁)

Common in maize, sorghum, and sunflower for producing high-yielding hybrids.

4.2.3 Single and Double Cross Hybrids

• Single Cross: Cross between two inbred lines (A × B)
• Double Cross: Cross between two single-cross hybrids ((A × B) × (C × D))

Double crosses are more adaptable but less uniform than single crosses.

4.2.4 Synthetic Hybrids

Developed by intercrossing selected lines and harvesting bulked seeds. Less heterotic than single-cross hybrids but stable across generations.

5. Selection Criteria in Cross-Pollinated Crops

• Traits with high heritability and economic importance.
• Resistance to diseases and pests.
• Adaptability to target environments.
• Stability of yield across seasons.
• Quality traits like oil, protein, and sugar content.

6. Challenges in Breeding Cross-Pollinated Crops

• Maintaining genetic diversity while improving populations.
• Managing inbreeding depression in selected lines.
• Complex hybrid seed production.
• High cost and labor requirements for hybridization.
• Environmental influence due to strong genotype × environment interaction.

7. Conclusion

Breeding cross-pollinated crops is complex due to their heterozygous and outcrossing nature. Understanding heterozygosity, heterosis, and inbreeding depression is essential. Methods like mass selection, recurrent selection, synthetics, composites, and hybrid breeding improve yield, quality, and adaptability while maintaining genetic diversity.

8. References

• Allard, R.W. (1999). Principles of Plant Breeding. 2nd Edition. Wiley, New York.
• Singh, B.D. (2015). Plant Breeding: Principles and Methods. 11th Edition. Kalyani Publishers, India.
• Poehlman, J.M., & Sleper, D.A. (1995). Breeding Field Crops. 4th Edition. Iowa State University Press.
• Acquaah, G. (2012). Principles of Plant Genetics and Breeding. 2nd Edition. Wiley-Blackwell.