Breeding self-pollinated crops
Breeding methods for self-pollinated crops: pure-line and mass selection, pedigree, bulk, single seed descent, backcross, multilines and doubled haploids.
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Rice, wheat, barley, oats, most pulses, groundnut, soybean, tomato and many other crops are self-pollinated: each flower is usually fertilised by its own pollen. Typically less than 5% of seeds come from cross-pollination. Breeding these crops follows from one genetic fact: selfing drives a population toward homozygosity. A variety of a self-pollinated crop is therefore a pure line, or a mixture of pure lines, and it breeds true from saved seed.
The breeder's job is to create variation, usually by crossing two parents, and then select among the homozygous lines that appear as the population is selfed. The methods differ mainly in when selection starts and how records are kept.
Genetics of self-pollination
Each generation of selfing halves the proportion of heterozygotes. Starting from an F1 heterozygous at one gene, after m generations of selfing:
In F2 (m = 1), 50% of plants are homozygous at a single gene. By F6 (m = 5), 96.9% are. This is why breeders consider lines from F6 to F8 "fixed" enough to test.
Self-pollinated crops do not lose vigour on selfing the way maize does, because natural selection over many generations has already removed most harmful recessive alleles.
Methods without crossing
Plant introduction
Plant introduction brings in varieties from other regions or countries. Some can be released directly; Sonalika and Kalyan Sona wheats came from CIMMYT material. Introductions pass through quarantine (NBPGR in India) and are tested under local conditions.
Pure-line selection
Pure-line selection picks single plants from a mixed landrace and multiplies the progeny of each separately. Johannsen's 1903 experiments with beans showed that selection within a pure line is useless, because all its variation is environmental. Many early Indian varieties came from pure-line selection of local material.
Mass selection
Mass selection picks many plants with the desired type and bulks their seed together. It is quick and keeps the variety adaptable, but it gains less per cycle. It is often used to purify existing varieties.
Methods after hybridisation
Pedigree method
The pedigree method selects individual plants from F2 onward. Their progeny are grown in separate rows or families, with records of every plant's ancestry. Selection continues within and between families until about F6 to F8, when the best uniform lines enter yield trials.
- Strengths: the breeder controls selection closely, so it is effective for traits that are easy to see and highly heritable, such as disease resistance, height and maturity.
- Weaknesses: it needs a lot of labour and record-keeping, and it relies on selection for yield in early generations, which is unreliable because of heterozygosity and small plots.
Most Green Revolution wheat and rice varieties were bred by the pedigree method.
Bulk method
In the bulk method, the F2 and later generations are grown as one population and harvested together. Selection starts only around F5 or F6, when most plants are homozygous. Nilsson-Ehle introduced the idea in Sweden.
- Strengths: it is simple and cheap, and natural selection within the bulk can favour adapted types, for example for winter hardiness.
- Weaknesses: natural selection may favour competitive tall plants rather than high-yielding ones, and there are no records of ancestry.
Single seed descent (SSD)
Single seed descent advances one seed from each F2 plant through each generation, often in a greenhouse or off-season nursery that gives two or three generations a year. Each F2 plant gives rise to one homozygous line by about F6. SSD keeps the genetic variation of the F2 and reaches homozygosity fast. Lines are selected at the end of the process.
Backcross method
The backcross method transfers one or a few genes, usually for disease resistance, from a donor into an otherwise good variety, the recurrent parent. Harlan and Pope proposed it in 1922. The F1 and each later generation are crossed back to the recurrent parent while selecting for the transferred gene.
How many backcrosses?
After the first cross (F1), the recurrent parent contributes 50%. After BC1 it is 75%, after BC2 87.5%, after BC3 93.75%, and after BC5 98.4%. Breeders usually make five or six backcrosses and then self to recover a homozygous line that carries the new gene. With DNA markers for the background genome, the same recovery can be reached in two or three backcrosses.
The result is a variety almost identical to the original, plus the new trait. Because farmers already know the variety, it is easy to release. Swarna-Sub1, a submergence-tolerant version of the popular rice Swarna, was made by marker-assisted backcrossing.
Other methods
- Multiline varieties are mixtures of near-isogenic lines that carry different rust-resistance genes in the same background (Jensen, 1952; developed by Borlaug). When a new rust race appears, only some lines are susceptible, so the epidemic slows.
- Doubled haploids come from haploid plants, produced by anther culture or wide crosses such as wheat × maize, whose chromosomes are doubled with colchicine. They give completely homozygous lines in one generation.
- Hybrid varieties are possible in self-pollinated crops when a male-sterility system makes seed production practical. Hybrid rice in China and India is the main example.
- Mutation breeding and speed breeding (extended day length to fit up to six generations a year) are increasingly combined with these methods.
Key points
- Selfing halves heterozygosity every generation; homozygosity after m generations = 1 − (1⁄2)m.
- Self-pollinated varieties are pure lines and breed true.
- Pure-line selection picks single plants from landraces; mass selection bulks many selected plants.
- Pedigree selects individuals from F2 with records; bulk delays selection to F5–F6; SSD advances one seed per plant.
- Backcrossing adds a gene to a good variety; recurrent parent recovery = 1 − (1⁄2)t+1, about 98% after five backcrosses.
- Multilines, doubled haploids and hybrid rice add further options.
What the exams ask
- Compare the pedigree and bulk methods, with merits and demerits.
- Calculate homozygosity after a given number of selfings.
- Calculate recurrent parent recovery after a given number of backcrosses.
- Explain Johannsen's pure-line theory.
- Define multiline varieties and doubled haploids.
Described in general terms: the kind of examination, not a past paper. Laisha never reproduces exam questions.
Questions people ask
Why can farmers save seed of wheat varieties but not of hybrids?
A wheat variety is a pure line, so its seed produces plants just like the parent. A hybrid is heterozygous, so its F2 seed segregates into variable plants and loses much of the hybrid vigour. Farmers must buy fresh hybrid seed each season.
Why not select for yield in F2?
F2 plants are mostly heterozygous and are grown as single plants, which are strongly affected by soil and spacing. Yield has low heritability on a single-plant basis, so early selection for yield gains little. Breeders select simple, visible traits early and test yield on homozygous lines in replicated plots.
What is a near-isogenic line?
A near-isogenic line (NIL) is genetically almost identical to another line except at one region carrying a specific gene. Backcrossing produces NILs. They are used to study the effect of single genes and to build multiline varieties.
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.
- Singh, B. D. Plant Breeding: Principles and Methods, 12th edition. Kalyani Publishers, 2021.
- Poehlman, J. M. and Sleper, D. A. Breeding Field Crops, 5th edition. Iowa State University Press, 2006.
- Septiningsih, E. M. et al. "Development of submergence-tolerant rice cultivars: the Sub1 locus and beyond." Annals of Botany 103, 151–160 (2009).
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