PB Ch 23. Heterosis and Hybrid Vigour

• When plant breeders force cross-pollinated crops to self-pollinate, the plants suffer from inbreeding depression—they become stunted, weak, and yield poorly.
• But an incredible biological phenomenon occurs when you take two of these weak, completely different inbred lines and cross them: their offspring don't just return to normal; they explode with unprecedented size, speed, and yield.
• This dramatic superiority of an offspring over its parents is called Heterosis, or Hybrid Vigor.
• The term "heterosis" was coined by G.H. Shull in 1914 (though he and E.M. East independently described the mechanics in 1908). However, the concept is much older.
• Charles Darwin noted in 1876 that crossing unrelated plants created highly vigorous offspring, and by the 1920s, the first commercial hybrid maize was transforming American agriculture.
• Decades later, India launched its own massive hybrid programs, releasing its first commercial sorghum hybrid (CSH-1) in 1964 and pearl millet hybrid (HB-1) in 1965.

Historical milestones

• 1673 — Koelreuter: first reported hybrid vigour in Nicotiana tobacco hybrids
• 1876 — Darwin: concluded that hybrids from unrelated plant types were highly vigorous
• 1877-1882 — Beal: intervarietal maize hybrids yielded up to 40% more than parental varieties
• 1908 — Davenport: proposed Dominance hypothesis; East and Shull: proposed Overdominance hypothesis
• 1912 — East and Hays: advocated heterosis breeding as an alternative strategy
• 1917 — Jones: proposed concept of double cross hybrids; 1918 — first double cross hybrid Burr Leaming Dent
• 1922 — USA: First commercial double cross maize hybrid (Burr Leaming Dent)
• 1952 — India: hybrid maize programme initiated under AICRP with Rockefeller Foundation
• 1964 — India: CSH-1 (first commercial sorghum hybrid); 1965: HB-1 (first commercial pearl millet hybrid)

Manifestations of Heterosis 

• Increased yield: Most economically important. Commercial exploitation in maize, sorghum, pearl millet, rice, sunflower, cotton. US maize: open-pollinated 20-32 bu/acre (1870-1930) → double cross hybrids 55 bu/acre (1960s) → single cross hybrids 120 bu/acre (1990s).
• Increased reproductive ability: Higher seed or fruit production. More seeds per head in cereals; more tubers in potato; more tillers in bajra hybrids.
• Increase in size and general vigour: Larger and more vigorous plants — increase in number and size of cells. Larger fruit in tomato, larger cob in maize, larger head in jowar.
• Better quality: Improved oil content, protein content, vitamin content. Onion hybrids — better keeping quality. Tomato hybrids — higher vitamin C content.
• Earlier flowering and maturity: Many hybrids flower and mature earlier than their parents — valuable for multiple cropping and escaping terminal drought or frost. Tomato F1 hybrids are regularly earlier than parents.
• Greater resistance to diseases and pests: Some F1 hybrids are more resistant than either parent — due to dominant resistance alleles from both parents combining.
• Greater adaptability: Hybrids show smaller variance across environments than inbreds — more stable performers across varying conditions.
• Faster growth rate: Hybrids may show faster seedling emergence and early growth — giving competitive advantage over weeds.
• Increase in number of plant parts: More nodes, leaves, tillers in some hybrids (e.g., beans).

Types of Heterosis 

This classification is directly tested in IFoS:

Genetic Hypotheses for Heterosis — Three Theories

Three main genetic hypotheses have been proposed to explain heterosis. 

A. Dominance Hypothesis (Davenport, 1908; elaborated by Keeble & Pellew, 1910)

• At each locus, the dominant allele has a favourable effect while the recessive allele is deleterious. In the heterozygous F1, the deleterious effects of recessive alleles from both parents are masked by dominant alleles from the other parent.
• Example: If Parent 1 = AAbbCCdd and Parent 2 = aaBBccDD, 

• then F1 = AaBbCcDd — all four loci now have at least one dominant allele. 
• F1 combines favourable dominants from both parents.

• Objection 1: According to this hypothesis, it should be possible to isolate inbreds with all dominant alleles (AABBCCDD) — as vigorous as the F1. 

• Such inbreds have NOT been isolated in most studies. 
• However, In some studies, it has been possible to recombine genes so that inbred lines as good as or superior to the heterotic hybrids were isolated.'

• Objection 2 and explanation (Jones 1917): 

• F2 populations should NOT show symmetrical distributions if dominance alone caused heterosis. 
• Jones explained this by 'dominance of linked genes' — dominant and recessive genes are linked in repulsion, so isolating all-dominant inbreds requires multiple precise crossovers, which is extremely rare.

• Collins (1921): 

• If many genes govern a quantitative character, symmetrical F2 distribution is expected even without linkage. Environmental effects also push the distribution towards symmetry.

B. Overdominance Hypothesis (East and Shull, 1908; also called Superdominance or Single Gene Heterosis)

• Heterozygote Aa is SUPERIOR to both homozygotes AA and aa. Heterozygosity per se is the cause of heterosis.
• Mechanism: Different alleles perform somewhat different molecular functions. 

• The hybrid, having both alleles, can perform the functions of both. 
• East (1936) proposed that alleles with more divergent functions produce more heterosis.

• Evidence: 

• Some oligogenes show clear heterozygote superiority. 
• Maize gene ma (affecting maturity): Ma ma is more vigorous and later than either homozygote. 
• Two barley chlorophyll mutants: heterozygotes show larger and more seeds than normal homozygotes (Gustafsson). 
• Sickle cell Ss heterozygotes in Africa: superior to SS (more malaria-resistant).

• Main objection: 

• Overdominance has NOT been demonstrated unequivocally for any polygenic trait. 
• Most QTL studies in maize suggest dominance is the primary cause, not overdominance.
• Many apparent overdominance cases may be due to linkage in repulsion phase (two closely linked dominant genes in repulsion mimic overdominance).

C. Epistasis Hypothesis (Gowen, 1952)

• Non-allelic gene interactions (epistasis) between genes from different parents contribute to heterosis.
• Mechanism: 

• Complementary epistasis between parents with different allelic combinations. 
• If interacting gene pairs are dispersed between the two parents (each parent has dominant allele of one gene but recessive of the other), the F1 will show enhanced expression due to complementary interaction.

• Evidence: 

• Majority of heterotic crosses show significant epistasis. 
• In rice hybrid Zhenshan 97 x Minghin 63: overdominance was observed for most QTLs for yield, and digenic epistasis was frequent and widespread.

Conclusion: 'Heterosis, to a large extent, is due to dominance gene action, but epistasis and overdominance are also involved. The relative importance of these phenomena is not clearly understood.' Modern view: heterosis results from a combination of all three mechanisms — dominance + overdominance (real or apparent) + epistasis.

Physiological Basis of Heterosis

• Metabolic concept: Inbreds have unbalanced metabolic systems with certain enzymes in rate-limiting concentrations. Different enzymes are rate-limiting in different inbreds. When two complementary inbreds cross, the F1 has no rate-limiting enzymes — resulting in vigorous metabolism.
• Net assimilation rate / Leaf Area Index (LAI): Heterotic hybrids show higher LAI, especially in early growth phases — leading to more biomass and yield. Studies in cotton and rice demonstrate clear LAI advantage in seedling stage.
• Root growth: Many hybrids show heterosis for root growth — longer root systems; access more nutrients and water.
• Hormone balance: Hybrids may have superior hormonal balance. Maize inbreds contain lower GA3 levels and respond to exogenous GA3; hybrids have relatively higher endogenous GA3. However, evidence is limited to seedling stage.
• Mitochondrial complementation (Sarkissian 1972):

  • Mitochondria from heterotic hybrids may show higher oxidative phosphorylation efficiency. But the correlation with grain yield is inconsistent.

  • Overall conclusion:  'The understanding regarding the exact role of the many physiological contributions for the expression of heterosis still remains unclear.'