Centres of origin of crop plants
Vavilov's eight centres of origin, primary and secondary centres, Harlan's centres and non-centres, where major crops come from and why it matters.
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Every crop was domesticated from a wild plant somewhere. Wheat comes from wild grasses of the Fertile Crescent, maize from teosinte in Mexico, and potato from wild tuber-bearing Solanum in the Andes. Knowing where crops came from matters for more than history. The regions where a crop and its wild relatives evolved still hold most of its genetic diversity: resistance genes, stress tolerance and quality traits that breeders need. That knowledge guides germplasm collection, conservation and quarantine.
The Russian geneticist Nikolai Vavilov (1887–1943) made the first systematic attempt to map these regions. He led more than a hundred collecting expeditions across five continents, and his collections laid the foundation of the world's gene banks.
Vavilov's theory
Vavilov reasoned that the region where a crop shows the greatest genetic diversity, with many varieties, forms and wild relatives, is likely to be where it originated. He called these regions centres of origin (or gene centres). He set out his ideas in 1926 and refined them in 1935, identifying eight main centres and three sub-centres:
| No. | Centre | Example crops |
|---|---|---|
| 1 | Chinese | Soybean, foxtail and proso millet, tea, peach, Chinese cabbage, adzuki bean |
| 2 | Indian (Hindustan) | Rice, sugarcane, pigeonpea, chickpea (secondary), brinjal, mango, cucumber, black pepper, jute, Asiatic cotton |
| 2a | Indo-Malayan | Banana, coconut, nutmeg, breadfruit |
| 3 | Central Asian | Pea, onion, garlic, spinach, carrot, apple, bread wheat (diversity) |
| 4 | Near Eastern (Asia Minor) | Einkorn and emmer wheat, barley, rye, lentil, chickpea, flax, grape, fig |
| 5 | Mediterranean | Olive, oats, durum wheat (diversity), cabbage, lettuce, sugar beet |
| 6 | Abyssinian (Ethiopian) | Coffee, sorghum, finger millet, teff, durum and emmer wheat (diversity), castor |
| 7 | South Mexican and Central American | Maize, common bean, upland cotton, chilli, sweet potato, papaya, cocoa |
| 8 | South American (Peru, Ecuador, Bolivia) | Potato, tomato, Sea Island cotton, tobacco |
| 8a | Chiloé (Chile) | Common potato (long-day forms) |
| 8b | Brazilian–Paraguayan | Groundnut, cassava, pineapple, rubber |
Vavilov placed more than a hundred cultivated species in the Indian centre. This underlines the importance of India's own genetic resources.
He also proposed the law of homologous series in variation (1920). Related species and genera show parallel series of variation, so a trait found in one crop, such as hull-less grain, can be expected in its relatives.
Primary and secondary centres
Vavilov distinguished two kinds of centre:
- A primary centre is where a crop was first domesticated. It usually has many wild relatives and many dominant alleles.
- A secondary centre is a region where a crop developed great diversity after it was introduced. This happens through isolation, new environments, crossing with local wild relatives and farmer selection. Secondary centres are often rich in recessive variants.
Ethiopia, for example, is a secondary centre of great diversity for barley and durum wheat, though both were domesticated in the Near East. India is a secondary centre for crops such as chilli, which came from the Americas only about 500 years ago and now has hundreds of Indian landraces.
Later revisions
Research since Vavilov has shown that the centre of diversity is not always the centre of origin. Diversity can build up in a secondary area, and some crops were domesticated across broad regions rather than at one point.
Jack Harlan (1971) proposed three centres where agriculture clearly began, each paired with a non-centre, a wide area where domestication was dispersed:
- the Near East centre (A1) and the African non-centre (A2);
- the North Chinese centre (B1) and the Southeast Asian and South Pacific non-centre (B2);
- the Mesoamerican centre (C1) and the South American non-centre (C2).
Zhukovsky (1968–1975) enlarged the scheme to 12 megacentres, adding regions such as Australia and North America.
Archaeology and DNA have refined the origins of many crops:
- Maize was domesticated from Balsas teosinte in south-west Mexico about 9,000 years ago.
- Asian rice (Oryza sativa) was domesticated in the Yangtze valley of China, with indica rice probably arising after japonica spread into India and hybridised with local wild rice.
- Sugarcane (Saccharum officinarum) is now thought to have been domesticated in New Guinea. India was nevertheless an early centre of sugar making, and the source of S. barberi.
- Pearl millet comes from the West African Sahel and finger millet from the East African highlands, before both spread to India.
Why breeders go back to the centre
In the 1840s, potato late blight caused the Irish famine. Later, breeders found strong resistance genes (R genes) in the wild species Solanum demissum and other Mexican potatoes. Mexico, home of the late blight pathogen, is also where wild potatoes evolved alongside it. Pathogens and their hosts often co-evolve in the same centre, so that is where resistance is easiest to find. The same logic led breeders to search wild relatives in the Near East for wheat rust resistance.
Why centres matter today
- Germplasm collection: centres of diversity are priority areas for collecting landraces and wild relatives.
- In situ conservation: protecting wild relatives in their natural habitat, for example wild rice in India and teosinte in Mexico.
- Plant quarantine: introducing seed from a crop's centre of origin also risks introducing its co-evolved pests and diseases.
- Benefit sharing: the International Treaty on Plant Genetic Resources and the Nagoya Protocol give countries rights over genetic resources, which affects access to them.
Key points
- Vavilov's theory: the centre of greatest diversity marks the centre of origin.
- The eight centres are Chinese, Indian (with Indo-Malayan), Central Asian, Near Eastern, Mediterranean, Abyssinian, Mexican–Central American and South American (with Chiloé and Brazil–Paraguay).
- The Indian centre includes rice, sugarcane, pigeonpea, brinjal, mango, cucumber and black pepper.
- A primary centre is where a crop was domesticated; a secondary centre is where it diversified after introduction (for example barley in Ethiopia).
- Harlan proposed three centres and three non-centres; Zhukovsky proposed 12 megacentres.
- The law of homologous series predicts parallel variation in related species.
What the exams ask
- List Vavilov's eight centres with crops, especially the Indian centre.
- Distinguish primary and secondary centres of origin.
- State the law of homologous series in variation.
- Describe Harlan's centres and non-centres.
- Give the centre of origin of a named crop.
Described in general terms: the kind of examination, not a past paper. Laisha never reproduces exam questions.
Questions people ask
Is India the origin of rice?
Partly. Genetic evidence suggests japonica rice was first domesticated in China, while indica rice formed when domesticated rice reached India and crossed with local wild rice (Oryza nivara and O. rufipogon). India is certainly a major centre of rice diversity, with tens of thousands of landraces.
What happened to Vavilov?
He fell out of favour under Stalin because of his opposition to Lysenko's pseudoscience. He was arrested in 1940 and died in prison in 1943. During the siege of Leningrad, staff of his institute protected the seed collection, and several of them starved to death rather than eat the seeds.
Do centres of origin still have the most diversity?
Often, but not always. Modern varieties have replaced many landraces in their home regions, which is called genetic erosion. Some of the diversity now survives mainly in gene banks, and some crops have greater variation in secondary centres than in their primary one.
Sources
- Vavilov, N. I. Origin and Geography of Cultivated Plants. Translated by D. Löve. Cambridge University Press, 1992.
- Harlan, J. R. "Agricultural origins: centers and noncenters." Science 174, 468–474 (1971).
- Harlan, J. R. Crops and Man, 2nd edition. American Society of Agronomy, 1992.
- Meyer, R. S. and Purugganan, M. D. "Evolution of crop species: genetics of domestication and diversification." Nature Reviews Genetics 14, 840–852 (2013).
- Matsuoka, Y. et al. "A single domestication for maize shown by multilocus microsatellite genotyping." PNAS 99, 6080–6084 (2002).
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