Centers of origin and diversity of crop plants




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Centers of origin and diversity of crop plants
Crop genetic diversity refers to the variety of genes and genotypes found in a particular crop species. Genetic diversity is essential to develop improved cultivars with broad genetic base and wide adaptability. Moreover, existence of genetic diversity is also essential to meet current and future breeding requirements. If a crop species has large number of genetic variants it is said to be genetically diverse. Genetic diversity provides broad genetic base to the population.

N.I Vavilov (1926, 1951) a Russian geneticist and plant breeder, was the pioneer man who realized the significance of genetic diversity for crop improvement. Vavilov and his colleagues visited several countries and collected cultivated plants and their wild relatives for use in the Russian breeding programme to develop varieties for various agroclimatic conditions of USSR. Based on his studies of global exploration and collection, Vavilov proposed eight main centres of diversity and three subsidiary centres of diversity.

Centre of diversity refers to a place, region or area where maximum variability and diversity of crop plants is observed. The centres of diversity were also considered as centres of origin of crop plants. The various centres of diversity as proposed by N.I Vavilov include:

(1) Chinese Centre,

(2) India Centre,

(2a) Indo-Malayan Centre,

(3) Central Asiatic Centre,

(4) The Asia Minor Centre

(5) Mediterranean Centre,

(6) Abyssinian Centre

(7) South Mexican and Central American Centre,

(8) South American Centre

(8a) Chilean Centre, and

(8b) Brazilian- Paraguayan centre



All these centres are also known as Vavilovian centers of diversity

Table .7 Vavilovian centres of diversity of crop plants (after Vavilov, 1951)

Name of centre

Main crops for which genetic diversity is found

  1. Chinese Centre:

It consists of the mountainous regions of central and western China and the neighbouring low lands. It is the largest and oldest independent center.

Naked oat (SC), Soybean, Adzuki bean, Common bean (SC) Small Bamboo, Leaf Mustard (SC), Apricot, Peas, Orange, Sesame (SC), China tea, etc.

2. Indian Centre: This includes Burma, Assam, Malaya, Java Borneo,Sumatra and Philippines, but excludes North West India, Punjab and North Western Frontier Provinces.

Rice, Chick pea, Moth Bean, Rice bean, Horse gram, Brinjal, Cucumber, Tree Cotton, Jute, Pepper, African Millet, Indigo, etc.

2(a) Indo-Malayan Centre

Banana, Coconut, Yam and Pomelo

3. Central Asiatic Centre: It includes North West India, all of Afghanistan, the Soviet Republics of Tadjikistan and Tian Shan. It is also known as the Afghanistan center of origin.

Bread wheat, Club wheat, Shot wheat, Rye (SC) Lentil, Chickpea, Sesame, Flax, Safflower, Carrot, Radish, Apple, Pear and Walnut

4. Asia Minor Centre: This is also known as the Near East or the Persian Centre of Origin. It includes the interior of Asia Minor, the whole of Transacaucasia, Iran and Highlands of Turkmenistan.

Einkorn wheat, Durum wheat, Poulard wheat, Bread wheat, Two Rowed barley, Rye, Red oat, Chickpea (SC), lentil, pea (SC), Flex, Almond, Pomegranate, Pista, Apricot and Grape

5. Mediterranean Centre

Durum wheat, Husked oats, Cabbage, Olive, Broad beans and Lettuce.

6. Abyssinian Centre: It includes Ethiopia and hill country of Eritrea.

Durm wheat, Poulard wheat, Emmer wheat, Barley, Chickpea, Lentil, Pea, Flax, Sesame, Castor bean, African millet and Coffee

7. South Mexican and Central American Centre : This includes South Mexico and Central America. It is also referred to as the Mexican Centre of Origin.

Maize, Common bean, Upland cotton, Pumpkin, Gourd, Squash, Sisal hemp and Pepper

8. South American Centre: This center includes the high mountainous regions of Peru, Bolivia, Ecuador, Colombia, parts of Chile, and Brazil and whole of Peraguay.

Potao, Sweet potato, Lima bean, Tomato, Papaya, Tobacco and Sea Island cotton.

8(a) Chilean Centre

Potato

8(b) Brazilian Paraguan Centre

Peanut, Rubber Tree, Cocoa (SC), Pineapple, etc.

SC = Secondary centre


Vavilov could not adequately cover Africa, Moreover, Australia was not at all covered. These two continents have tremendous wealth of crop genetic diversity of several crop plants. The main difference between centre of origin and centre of diversity is that centre of origin of a crop species is generally confined to one place, whereas the diversity of a crop may be found at more than one place.

It is clear from the various centres of diversity that some crop plants have more than one centre of diversity. Moreover, centres of diversity are of two types, viz., (1) primary centres of diversity, and (2) secondary centres of diversity. They are briefly discussed below.



Primary centres of diversity

Primary centres are regions of vast genetic diversity of crop plants. These are original homes of the crop plants which are generally uncultivated areas like, mountains, hills, river valleys, forests, etc. Main features of these centres are given below.



  1. They have wide genetic diversity

  2. Have large number of dominant genes

  3. Mostly have wild characters

  4. Exhibit less crossing over

  5. Natural selection operates

Secondary centres of diversity

Vavilov suggested that valuable forms of crop plants are found far away from their primary area of origin which he called secondary centres of origin or diversity. These are generally the cultivated areas and have following main features.




  1. Have lesser genetic diversity than primary centres

  2. Have large number of recessive genes

  3. Mostly have desirable characters

  4. Exhibit more crossing over

  5. Both natural and artificial selections operate

Microcentres

In some cases, small areas within the centres of diversity exhibit tremendous genetic diversity of some crop plants. These areas are referred to as microcentres. Microcentres are important sources for collecting valuable plant forms and also for the study of evolution of cultivated species. The main features of micro centres are given below.



  1. They represent small areas within the centres of diversity

  2. Exhibit tremendous genetic diversity

  3. The rate of natural evolution is faster than larger areas

  4. They are important sites for the study of crop evolution

Vavilov also developed the concept of parallel series of variation or Law of homologous series of variation. This concept states that a particular variation observed in a crop species is also expected to be available in its another related species. For instance, if we get dwarf collections in one crop species the same may be found in another related species also.


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