Heterosis, male sterility and hybrid seed
Why hybrids out-yield their parents, how heterosis is measured, and how male sterility and restorer genes make hybrid seed production possible.
On this page
Heterosis, or hybrid vigour, is the superiority of a hybrid over its parents in yield, growth, uniformity or hardiness. It is why most maize, sorghum, sunflower and many vegetables are now grown from hybrid seed, and why hybrid rice covers a large share of the rice area in China and a growing area elsewhere.
Exploiting heterosis depends on making cross-pollinated seed cheaply and on a large scale, and that depends on male sterility. Both ideas are core plant breeding exam topics, often with a calculation of heterosis.
What heterosis is
When two genetically different, usually inbred, parents are crossed, the F1 often outperforms both. The term was coined by George Shull in 1914; he and Edward East had shown, around 1908–1909, that inbred maize lines lost vigour but that crosses between them recovered it and more.
The opposite of heterosis is inbreeding depression: the loss of vigour and fertility when naturally cross-pollinated species are self-pollinated or mated with close relatives. Cross-pollinated crops such as maize, sunflower and many brassicas show strong inbreeding depression and strong heterosis; naturally self-pollinated crops such as wheat and rice show both less, though rice hybrids still yield usefully more.
Measuring heterosis
Three measures are used, and exams ask for all of them.
Only standard heterosis, against the best variety or hybrid farmers already grow, decides whether a new hybrid is worth releasing.
Calculating heterosis
Two inbred lines yield 4.0 and 3.0 t/ha. Their F1 yields 6.3 t/ha. The best commercial variety yields 5.5 t/ha.
- Mid-parent value: (4.0 + 3.0) ⁄ 2 = 3.5 t/ha. Mid-parent heterosis = (6.3 − 3.5) ⁄ 3.5 × 100 = 80 per cent.
- Better parent: 4.0 t/ha. Heterobeltiosis = (6.3 − 4.0) ⁄ 4.0 × 100 = 57.5 per cent.
- Standard heterosis = (6.3 − 5.5) ⁄ 5.5 × 100 = 14.5 per cent.
If a farmer saves and sows F2 seed, then under the simplest genetic model (no epistasis) half of the heterosis is lost: expected F2 = F1 − (F1 − MP) ⁄ 2 = 6.3 − 1.4 = 4.9 t/ha, and the crop is no longer uniform. This is why hybrid seed must be bought fresh each season.
Why it happens
There is no single accepted explanation; the main hypotheses are complementary.
- Dominance hypothesis (Davenport, Bruce, Jones, 1908–1917): each inbred carries different harmful recessive alleles; in the hybrid, the dominant favourable allele from one parent masks the recessive from the other at many loci.
- Overdominance hypothesis (Shull, East): at some loci the heterozygote itself (A1A2) is superior to either homozygote.
- Epistasis: favourable interactions between alleles at different loci.
Modern genomics finds evidence for all three, plus effects on gene expression. For breeding, the practical rule is that heterosis is greatest between parents from different, complementary genetic groups (heterotic groups), each improved separately.
Why hybrid seed is hard to make
To make hybrid seed, every seed harvested from the female parent must come from the male parent's pollen. In crops with separate male and female flowers on the same plant, such as maize, this can be done by detasselling: removing the male tassels from the female rows by hand or machine. In crops with perfect flowers (both sexes in one flower), such as rice, sorghum and sunflower, removing anthers by hand is impossibly slow for commercial seed. The female parent must be made unable to produce pollen. That is male sterility.
Male sterility
Male sterility is the failure of a plant to produce functional pollen while its female parts work normally. It occurs naturally and is used in several forms.
| Type | Controlled by | How it is maintained | Main use |
|---|---|---|---|
| Genetic (genic) male sterility (GMS) | recessive nuclear genes (ms ms) | crossing with heterozygotes; half the progeny fertile and must be removed | some vegetables; limited use |
| Cytoplasmic male sterility (CMS) | mitochondrial genes in the cytoplasm | crossing with a fertile line of the same nucleus; all progeny sterile | crops where the harvest is vegetative (onion, carrot, cabbage for leaves) |
| Cytoplasmic-genetic male sterility (CGMS) | sterile cytoplasm plus nuclear restorer genes (Rf) | three-line system | grain and seed crops: rice, sorghum, pearl millet, sunflower, maize |
| Environment-sensitive genic male sterility (TGMS, PGMS) | nuclear genes switched by temperature or day length | sterile in one environment, fertile in another | two-line hybrid rice |
| Chemically induced | gametocides that kill pollen | applied each season | some wheat and other crops |
Cytoplasm is inherited only through the egg, so cytoplasmic sterility passes from the female parent to all offspring. For crops harvested as grain or seed, the hybrid must be fertile, so a restorer gene has to be brought in by the male parent. That is the logic of the three-line system.
The three-line system
- A line (male-sterile female): sterile cytoplasm (S) and no restorer genes (rf rf). It produces no viable pollen.
- B line (maintainer): normal cytoplasm (N) but the same nucleus as A (rf rf), so it is fertile. Pollinating A with B gives more A-line seed, still sterile, because the cytoplasm comes from A.
- R line (restorer): carries the dominant restorer gene, Rf Rf. Pollinating A with R gives the commercial F1 hybrid, S Rf rf: fertile, because one Rf allele restores pollen production.
The two-line system, used widely for hybrid rice in China since the 1990s, avoids the maintainer: a single line is sterile when grown under one temperature or day length (to produce hybrid seed) and fertile under another (to multiply itself). It gives more freedom in choosing parents but depends on predictable weather.
Notable cases. The wild-abortive CMS found in a wild rice plant in China in 1970 opened the way to the first commercial hybrid rice, associated with Yuan Longping. Sunflower hybrids worldwide rest largely on one CMS source found in 1969. The danger of depending on one cytoplasm was shown in 1970, when most hybrid maize in the United States used Texas (T) cytoplasm, which proved highly susceptible to race T of southern corn leaf blight; the epidemic destroyed a large part of the crop. Cytoplasmic diversity is now valued deliberately.
Producing hybrid seed in the field
- Isolation from other varieties of the same crop, by distance or by time, so stray pollen cannot contaminate the female rows.
- Row ratios: several female rows to each male row (for example 2 male to 6–10 female in rice, depending on the pollen supply), with only female rows harvested for seed.
- Synchronising flowering of the two parents by staggered sowing, so pollen is shed when female stigmas are receptive.
- Rogueing: removing off-types and any fertile plants in the female rows.
- Help with pollination: in rice, shaking the male rows with ropes or poles at flowering; plant growth regulators to improve panicle exsertion in some systems.
- Certification: hybrid seed is inspected and tested like other certified seed.
Key points
- Heterosis is the superiority of an F1 over its parents; inbreeding depression is its reverse.
- Measures: mid-parent heterosis, better-parent heterosis (heterobeltiosis), standard heterosis against a check variety.
- Explanations: dominance, overdominance and epistasis; heterosis is greatest between distinct heterotic groups.
- F2 seed loses about half the heterosis and its uniformity, so hybrid seed is bought every season.
- Male sterility makes cross-pollination practical in perfect-flowered crops: genetic, cytoplasmic, cytoplasmic-genetic, environment-sensitive and chemical.
- Three-line system: A (S, rf rf, sterile) × B (N, rf rf) maintains A; A × R (Rf Rf) gives the fertile hybrid. Two-line rice uses temperature- or day-length-sensitive sterility.
What the exams ask
- School-leaving and entrance papers: definitions of heterosis, inbreeding depression and male sterility, with crop examples.
- Degree and research-fellowship entrances: calculating mid-parent, better-parent and standard heterosis, genetic theories, CMS versus CGMS versus GMS, the A, B and R lines, two-line hybrids, and the T-cytoplasm epidemic.
- Seed technology and professional examinations: isolation distances, row ratios, synchronisation, rogueing and certification standards for hybrid seed production.
Described in general terms: the kind of examination, not a past paper. Laisha never reproduces exam questions.
Questions people ask
Can farmers save seed from hybrids?
They can sow it, but the next generation segregates: plants differ from each other and on average yield noticeably less. That is why hybrids are usually bought each season.
Why are there few wheat hybrids?
Wheat is self-pollinating with little natural outcrossing, its pollen is shed briefly and travels poorly, and heterosis is modest, so producing hybrid seed is costly compared with the gain. Chemical and genetic systems for hybrid wheat exist, and the area is growing slowly.
Is a hybrid the same as a genetically modified crop?
No. A hybrid is simply the first generation from crossing two parent lines by pollination. Genetic modification inserts or edits specific genes in a laboratory; a hybrid may or may not also carry such traits.
Sources
- Allard, R. W. Principles of Plant Breeding, 2nd edition. Wiley, 1999.
- Acquaah, G. Principles of Plant Genetics and Breeding, 3rd edition. Wiley-Blackwell, 2020. Chapters on heterosis and hybrid cultivars.
- Shull, G. H. "The composition of a field of maize." American Breeders' Association Report 4, 296–301 (1908).
- Virmani, S. S. et al. Two-line Hybrid Rice Breeding Manual. International Rice Research Institute, 2003.
- Ullstrup, A. J. "The impacts of the southern corn leaf blight epidemics of 1970–1971." Annual Review of Phytopathology 10, 37–50 (1972).
- Singh, B. D. Plant Breeding: Principles and Methods. Kalyani Publishers, latest edition.
More in genetics and plant breeding
Discussion
Questions, corrections and notes from readers on this article. Every comment is checked before it appears: keep it on the topic, and do not share phone numbers, links or contact details.