🧬 Wheat — Origin, Cytogenetics, Breeding Objectives

This lesson builds a complete exam-ready base for wheat improvement by linking origin, ploidy evolution, and practical breeding targets used in Indian programmes.

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Origin and Distribution

Wheat (Triticum aestivum L.) is the most widely cultivated cereal crop in the world and a staple food for over one-third of the global population. The primary centre of origin is the Fertile Crescent region of South-West Asia, encompassing present-day Iraq, Syria, Turkey, and Iran. N.I. Vavilov identified the Near East gene centre as the primary centre of diversity. Secondary centres include Ethiopia and the Mediterranean region. Wheat was introduced to India around 6000 BC and is now the second most important food grain after rice.

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Cytogenetics

Wheat species are classified into three ploidy groups based on their chromosome number. Diploid wheats (2n = 2x = 14) include T. monococcum (einkorn wheat) with genome AA. Tetraploid wheats (2n = 4x = 28) include T. turgidum (emmer wheat) and T. durum (macaroni wheat) with genome AABB. Hexaploid wheat (2n = 6x = 42) includes T. aestivum (bread wheat) with genome AABBDD. The A genome was contributed by T. urartu, the B genome by Aegilops speltoides (or a related species from the Sitopsis section), and the D genome by Aegilops tauschii (syn. Ae. squarrosa). The polyploid nature of wheat provides buffering capacity and genetic redundancy, enabling greater adaptability to diverse environments.

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Breeding Objectives

The primary breeding objectives in wheat include yield enhancement through development of semi-dwarf high-yielding varieties using dwarfing genes Rht1 and Rht2 derived from the Japanese variety Norin-10. Rust resistance is critical, targeting stem rust (Puccinia graminis tritici), leaf rust (P. triticina), and stripe rust (P. striiformis). Breeders aim to incorporate multiple resistance genes (Sr, Lr, Yr series) for durable resistance. Quality improvement focuses on high protein content, superior gluten strength, grain hardness, and milling and baking quality. Additional objectives include tolerance to heat and drought stress, resistance to Karnal bunt (Tilletia indica), lodging resistance, early maturity for fitting into crop rotations, and biofortification for enhanced iron and zinc content. Indian wheat breeding is coordinated through the ICAR-Indian Institute of Wheat and Barley Research (IIWBR), Karnal.

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Summary Cheat Sheet

Quick Recall Points

• Bread wheat is hexaploid (2n = 42) with genome AABBDD.
• A, B, and D genomes are linked to T. urartu, Aegilops speltoides group, and Ae. tauschii.
• Semi-dwarf wheat revolution used Rht1/Rht2 genes from Norin-10.

Exam Traps

• Do not confuse durum wheat (AABB, 2n=28) with bread wheat (AABBDD, 2n=42).
• Rust breeding means combining multiple Sr/Lr/Yr genes, not a single resistance source.

References

2 sources • [1] [2]

[1]

Principles of Plant Breeding — R.W. Allard

Book

[2]

ICAR-IIWBR, Karnal

Official