🧬 Cotton — Genetics, Breeding, Bt Cotton

Cotton breeding combines cytogenetics, fibre technology, and pest management at commercial scale. This lesson connects species-genome diversity, quality metrics, hybrid strategy, and Bt deployment outcomes.

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Origin and Species

Cotton is the world's most important fibre crop, belonging to the genus Gossypium (family Malvaceae). Four species are cultivated:

Species	Genome	Ploidy	Fibre Quality
G. arboreum	A2	Diploid (2n=26)	Coarse, short
G. herbaceum	A1	Diploid (2n=26)	Coarse, short
G. hirsutum	(AD)1	Tetraploid (2n=52)	Medium, upland
G. barbadense	(AD)2	Tetraploid (2n=52)	Fine, extra-long staple

The tetraploid species (G. hirsutum and G. barbadense) are allopolyploids arising from natural hybridization between an A-genome African diploid and a D-genome American diploid ancestor, followed by chromosome doubling. G. hirsutum (American upland cotton) accounts for about 90% of world cotton production.

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Fibre Quality Traits

Cotton fibre is a single-celled trichome that elongates from the seed coat. Key quality parameters include:

• Staple length — short (< 21 mm), medium (21-27 mm), long (27-32 mm), extra-long (> 32 mm).
• Fibre fineness (micronaire) — 3.5-4.9 is ideal for spinning.
• Fibre strength — measured in g/tex; higher values give stronger yarn.
• Uniformity ratio and elongation percentage — affect processing efficiency.

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Breeding Objectives

1. High lint yield — through high ginning outturn (lint percentage), boll weight, and number of bolls per plant.
2. Superior fibre quality — long staple, high strength, and fine micronaire for textile industry requirements.
3. Disease resistance — bacterial blight (Xanthomonas citri pv. malvacearum), Fusarium wilt, Verticillium wilt, and cotton leaf curl virus (CLCuV).
4. Insect resistance — bollworms (Helicoverpa armigera, pink bollworm, spotted bollworm) are the most destructive pests.
5. Drought and salinity tolerance — for rainfed cotton in central and peninsular India.

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Hybrid Cotton

India is the only country with large-scale commercial hybrid cotton production. Both intraspecific (hirsutum x hirsutum) and interspecific (hirsutum x barbadense) hybrids are grown. The first hybrid cotton released was H4 (1970) from Gujarat. Hybrid seed production relies on hand emasculation and pollination, which is labour-intensive.

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Bt Cotton

Bt cotton expresses insecticidal crystal proteins (Cry1Ac, Cry2Ab) from Bacillus thuringiensis that are toxic to bollworm larvae. Bt cotton was approved for commercial cultivation in India in 2002 (Bollgard I with Cry1Ac) and 2006 (Bollgard II with stacked Cry1Ac + Cry2Ab). Bt adoption reached over 95% of cotton area in India, dramatically reducing insecticide usage and increasing yields. However, pink bollworm has developed field resistance to Cry proteins in some regions, necessitating refuge-in-bag (RIB) strategies and new resistance management approaches.

Summary Cheat Sheet

Key Facts

Topic	Value
Dominant cultivated species	G. hirsutum
Tetraploid chromosome number	2n = 52
Bt approval in India	2002 (BG-I), 2006 (BG-II)

Exam Traps

• Bt controls bollworms, not all cotton pests.
• Fibre quality is multidimensional: staple length, micronaire, strength, and uniformity all matter.

References

2 sources • [1] [2]

[1]

ICAR eCourse: GPBR 213 Crop Improvement-I (Kharif Crops)

Book

[2]

ICAR Crop-specific research bulletins (IIRR, IIMR, IIMR Sorghum, ICRISAT, CICR, SBI, CRIJAF)

Website