Breeding cross-pollinated crops
How cross-pollinated crops are bred: population genetics, mass and recurrent selection (GCA, SCA, reciprocal), synthetics, composites and hybrids.
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Maize, pearl millet, rye, sunflower, castor, cucurbits, onion, carrot, lucerne and most forage grasses are cross-pollinated. Pollen moves between plants by wind or insects. Some species enforce this through self-incompatibility, separate male and female flowers, or flowers in which the male and female parts mature at different times.
The result is a population that is very different from a pure line. Plants are heterozygous at many genes, and each plant is genetically unique. The population behaves as a gene pool whose allele frequencies stay stable under random mating, as Hardy–Weinberg describes. Forcing these plants to self causes inbreeding depression: loss of vigour and yield as harmful recessive alleles become homozygous.
Breeders of cross-pollinated crops therefore aim either to improve the whole population while keeping its heterozygosity, or to exploit heterosis through hybrids.
Population improvement
Mass selection
The simplest method is to choose the best-looking plants, harvest them, and sow the bulked seed. Mass selection works for traits with high heritability, such as plant height, maturity and ear shape. It works poorly for yield, for two reasons: single-plant phenotypes are noisy, and selection usually acts only on the female parent, since pollen comes from all plants, good and bad.
The famous Illinois long-term selection experiment, started by Hopkins in 1896, shows how far steady selection can go. More than a century of selection for high and low oil and protein in maize kernels produced lines with many times their original differences, and the response has still not run out.
Improved forms of mass selection include grid (stratified) selection, which compares plants only with their neighbours, and ear-to-row selection, which tests progeny before selecting.
Recurrent selection
Recurrent selection repeats a cycle: select superior plants, test them or their progeny, and intermate the best to form the next cycle's population. Each cycle increases the frequency of favourable alleles while recombination keeps creating new combinations.
| Type | What is tested | Improves |
|---|---|---|
| Simple recurrent selection | Plants themselves (phenotype) | Traits with high heritability |
| Half-sib and full-sib selection | Progeny from open or paired crosses | Additive (general) effects |
| S1 and S2 progeny selection | Selfed progeny | Additive effects; removes harmful recessives |
| For general combining ability (GCA) (Jenkins, 1940) | Testcross to a broad-based tester | Average performance in crosses (additive effects) |
| For specific combining ability (SCA) (Hull, 1945) | Testcross to an inbred tester | Performance with one specific partner (dominance and overdominance) |
| Reciprocal recurrent selection (Comstock, Robinson and Harvey, 1949) | Two populations, each tested against the other | Both GCA and SCA; ideal for hybrid breeding |
Reciprocal recurrent selection underlies modern heterotic groups, such as the Stiff Stalk and non-Stiff Stalk groups of US maize. Two populations are improved in parallel so that crosses between them perform especially well.
Gain per cycle follows the breeder's equation: R = i h2 σP, divided by the years each cycle takes. Shorter cycles and accurate testing drive progress.
Synthetic and composite varieties
A synthetic variety is made by intercrossing several inbred lines or clones that have been tested and found to combine well, usually 5 to 10. The resulting population is maintained by open pollination. Farmers can save seed for a few seasons with only a modest fall in yield. This suits forage crops, pearl millet and maize in areas where hybrid seed is costly or hard to obtain.
Sewall Wright showed how much yield is lost when the synthetic advances to the next generation.
Choosing the number of parents
Six inbred lines average 3.0 t/ha. All 15 single crosses among them average 6.0 t/ha. The expected Syn2 yield is 6.0 − (6.0 − 3.0) ⁄ 6 = 5.5 t/ha. With three parents it would be 6.0 − 1.0 = 5.0 t/ha. Using more parents reduces the loss, but only if extra lines can be added without lowering the F1 mean.
A composite variety is made by mixing seed of several open-pollinated varieties and allowing them to intermate. It needs no inbred lines and is cheaper to make than a synthetic, though less precise. Several Indian maize composites of the 1960s and 1970s were made this way.
Hybrid varieties
Hybrids exploit heterosis directly. Inbred lines are developed by selfing for five to seven generations, then tested in crosses, and the best combination becomes the hybrid.
- A single cross (A × B) is the most uniform and highest-yielding hybrid, and most maize hybrids today are single crosses.
- A three-way cross ((A × B) × C) and a double cross ((A × B) × (C × D)) were widely used when inbreds yielded poorly and seed was expensive to produce.
- A top cross is an inbred crossed with an open-pollinated variety.
Seed production uses detasseling (maize), cytoplasmic male sterility (pearl millet, sorghum, sunflower) or self-incompatibility (brassicas). Hybrid seed must be bought each season.
Clonally propagated crops
Many outcrossing crops, such as potato, sugarcane, banana, cassava and most fruit trees, are propagated vegetatively. Breeders cross diverse parents, select the best seedlings, and multiply them as clones. Any good genotype, however heterozygous, is fixed immediately, which is a major advantage.
Key points
- Cross-pollinated crops are heterozygous populations; selfing causes inbreeding depression.
- Mass selection works for traits with high heritability but is weak for yield.
- Recurrent selection cycles through selection, testing and intermating. The main types are for GCA (broad tester), SCA (inbred tester) and reciprocal (two populations).
- Synthetic varieties come from intercrossing tested inbreds. Wright's formula: Syn2 = F1 − (F1 − P) ⁄ n.
- Composite varieties come from intermating several open-pollinated varieties.
- Hybrids use inbred lines; single crosses are the most uniform and productive.
What the exams ask
- Distinguish recurrent selection for GCA and SCA, and explain reciprocal recurrent selection.
- Compare synthetic and composite varieties.
- Apply Wright's formula.
- Explain why mass selection is ineffective for yield in maize.
- Name types of hybrids and explain why hybrid seed cannot be saved.
Described in general terms: the kind of examination, not a past paper. Laisha never reproduces exam questions.
Questions people ask
Why does maize suffer from inbreeding but wheat does not?
Wheat has been self-pollinating for thousands of years, so harmful recessive alleles were exposed and removed long ago. Maize has always outcrossed, so many harmful recessives are hidden in heterozygotes. Selfing exposes them, which causes inbreeding depression.
Is an open-pollinated variety the same as a landrace?
Not quite. A landrace is a farmer-maintained local population shaped mainly by natural and farmer selection. An open-pollinated variety is a population developed and released by breeders, often through recurrent selection, and maintained to a recognisable standard. Both can be saved and resown.
Why do breeders use testers?
The value of an inbred line lies in how its offspring perform. Crossing every candidate with a common tester gives a fair comparison of combining ability without making every possible cross. A broad-based tester measures general combining ability, and an elite inbred tester measures specific combining ability.
Sources
- Hallauer, A. R., Carena, M. J. and Miranda Filho, J. B. Quantitative Genetics in Maize Breeding, 3rd edition. Springer, 2010.
- Allard, R. W. Principles of Plant Breeding, 2nd edition. Wiley, 1999.
- Singh, B. D. Plant Breeding: Principles and Methods, 12th edition. Kalyani Publishers, 2021.
- Dudley, J. W. and Lambert, R. J. "100 generations of selection for oil and protein in corn." Plant Breeding Reviews 24 (part 1), 79–110 (2004).
- Comstock, R. E., Robinson, H. F. and Harvey, P. H. "A breeding procedure designed to make maximum use of both general and specific combining ability." Agronomy Journal 41, 360–367 (1949).
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