Primary, secondary and microcenters.
Introduction
The availability and utilization of genetic variability is the backbone of plant breeding. The concept of centers of origin and diversity of crop plants was proposed by the eminent Russian scientist N. I. Vavilov (1926). According to him, crop plants show maximum genetic diversity in specific geographical regions, known as centers of origin. These centers are classified into primary centers, secondary centers, and microcenters based on the extent and location of genetic diversity. Understanding these centers is crucial for germplasm collection, conservation, and crop improvement.
1. Primary Centers of Origin
Definition
Primary centers of origin are geographical regions where a crop plant originated and was first domesticated, and where it shows maximum genetic diversity along with the presence of wild relatives and primitive cultivars.
Characteristics
Highest degree of genetic variability
Presence of wild ancestors, landraces, and weedy forms
Long evolutionary and domestication history
Natural and human selection pressure
Continuous cultivation over centuries
Examples of Major Primary Centers (Vavilov)
Chinese Center – Rice, soybean, millets
Indian (Indo-Burma) Center – Rice, sugarcane, banana, brinjal
Central Asiatic Center – Carrot, onion, garlic
Near Eastern Center (Fertile Crescent) – Wheat, barley, rye
Mediterranean Center – Cabbage, beet, olive
Abyssinian (Ethiopian) Center – Sorghum, coffee, barley
Central American Center – Maize, cotton
Andean Center – Potato, tomato
Importance in Plant Breeding
Source of genes for resistance to diseases and pests
Provides tolerance to drought, salinity, heat, and cold
Helps broaden the genetic base of cultivated crops
Essential for long-term breeding and sustainability
2. Secondary Centers of Origin
Definition
Secondary centers are regions where a crop plant was introduced from its primary center and later developed considerable genetic variability due to mutation, recombination, natural selection, and human selection.
Characteristics
Moderate level of genetic diversity
Wild progenitors generally absent
Evolution occurs under new agro-climatic conditions
Development of locally adapted varieties
Causes of Formation
Spread of crops through migration and trade
Hybridization among introduced varieties
Selection by farmers for desirable traits
Adaptation to new environments
Examples of Secondary Centers
China and Japan – Rice
Ethiopia – Wheat
Europe – Potato
South America – Maize
North America – Sunflower
Role in Plant Breeding
Source of genes for adaptability and yield
Provides quality traits like grain size and taste
Useful in regional breeding programs
3. Microcenters of Diversity
Definition
Microcenters are small and localized geographical areas within primary or secondary centers that exhibit exceptionally high genetic diversity of a particular crop species.
Characteristics
Very limited geographical extent
High concentration of rare and valuable genes
Closely associated with traditional farming systems
Often located in isolated regions (hills, valleys)
Examples of Microcenters
North-Eastern India – Rice diversity
Nilgiri Hills (India) – Banana, pepper
Andean Valleys – Potato
Ethiopian Highlands – Barley
Western Ghats – Black pepper
Importance in Plant Breeding
Source of unique genes for resistance and quality
Important for in situ conservation
Helps in identifying novel germplasm
Significance of Centers of Origin in Plant Breeding
Guides germplasm exploration and collection
Helps in conservation of plant genetic resources
Supports resistance breeding and climate-resilient crops
Aids in crop diversification and food security
Conclusion
Primary, secondary, and microcenters of origin provide the genetic foundation for plant breeding. The identification and conservation of these centers are essential for sustaining agricultural productivity and meeting future challenges such as climate change, pests, and population growth. Vavilov’s concept continues to guide modern plant breeding and genetic resource management.
In plant breeding, the concept of "centers of diversity" was pioneered by N.I. Vavilov. These are geographical areas where a high degree of genetic variation for a particular crop and its wild relatives is found. Understanding these centers is crucial for breeders to locate new genes for disease resistance, yield, and environmental adaptation.
The classification includes Primary Centers, Secondary Centers, and Microcenters.
1. Primary Centers of Diversity
The primary center is the original home of a crop plant. It is the region where the crop first evolved from its wild ancestors.
-
Key Characteristics:
- Wild Relatives: These areas contain the wild species from which the crop was domesticated.
- Genetic Dominance: Typically characterized by a high frequency of dominant alleles.
- Environment: Mostly located in mountainous regions, hills, or river valleys where natural selection is the primary force.
- Examples: Central America is the primary center for Maize; Ethiopia (Abyssinian center) for Coffee.
.
2. Secondary Centers of Diversity
A secondary center is a region where a crop shows a high degree of variation, even though it did not originate there. This happens when a crop is introduced to a new area and undergoes extensive natural and artificial selection.
-
Key Characteristics:
- Lack of Wild Relatives: Wild ancestors of the crop are generally absent in these regions.
- Recessive Traits: These centers often show a higher frequency of recessive alleles and desirable cultivated traits.
- Evolutionary Force: Human-driven artificial selection and crossing between different introduced varieties play a major role.
- Example: While Maize originated in Mexico (Primary), it shows significant secondary diversity in parts of Asia and Africa.
ca.
3. Microcenters
Microcenters are small, localized areas within a larger center of diversity (primary or secondary) that exhibit an exceptionally high concentration of genetic variation.
-
Significance:
- They are "hotspots" for evolution where rapid diversification occurs.
- They are the most valuable sites for germplasm collection because breeders can find a vast array of forms within a very small geographic footprint.
- They often occur in regions with varied topography (e.g., small mountain valleys) which creates different micro-climates, forcing the plants to adapt in diverse ways.
.
Comparison Table primary and Secondary centre
1. The concept of centers of origin of cultivated plants was proposed by
a) Charles Darwin
b) Gregor Mendel
c) N. I. Vavilov
d) Linnaeus
✅ Answer: c) N. I. Vavilov
Origin of Cultivated Plants” (1882) : Alphonse de Candolle
2. Primary centers of origin are characterized by
a) Minimum genetic diversity
b) Absence of wild relatives
c) Maximum genetic diversity
d) Artificial selection only
✅ Answer: c) Maximum genetic diversity
3. Wild progenitors of crops are usually found in
a) Secondary centers
b) Microcenters only
c) Primary centers
d) All centers
✅ Answer: c) Primary centers
4. The Fertile Crescent is the primary center for
a) Rice
b) Maize
c) Wheat
d) Potato
✅ Answer: c) Wheat
5. Potato originated in the
a) Mediterranean region
b) Andean region
c) Ethiopian region
d) Central Asian region
✅ Answer: b) Andean region
6. Secondary centers of origin develop mainly due to
a) Domestication of wild plants
b) Introduction and selection
c) Natural mutation only
d) Polyploidy
✅ Answer: b) Introduction and selection
7. Which of the following is NOT a feature of secondary centers?
a) Moderate genetic diversity
b) Absence of wild ancestors
c) Long history of cultivation
d) Presence of wild progenitors
✅ Answer: d) Presence of wild progenitors
8. Europe is considered a secondary center for
a) Rice
b) Sorghum
c) Potato
d) Sugarcane
✅ Answer: c) Potato
9. Microcenters are best described as
a) Large geographical regions
b) Areas with no variability
c) Small areas with very high genetic diversity
d) Regions of crop extinction
✅ Answer: c) Small areas with very high genetic diversity
10. North-Eastern India is a microcenter for
a) Wheat
b) Rice
c) Barley
d) Maize
✅ Answer: b) Rice
11. Ethiopian highlands are a microcenter for
a) Rice
b) Barley
c) Sugarcane
d) Cotton
✅ Answer: b) Barley
12. Which center shows maximum overall genetic variability of a crop?
a) Secondary center
b) Microcenter
c) Primary center
d) All centers equally
✅ Answer: c) Primary center
13. The main importance of centers of origin in plant breeding is
a) Mechanization of farming
b) Germplasm collection and utilization
c) Fertilizer recommendation
d) Weed management
✅ Answer: b) Germplasm collection and utilization
14. Maize originated in
a) China
b) Ethiopia
c) Central America
d) Mediterranean region
✅ Answer: c) Central America
15. Microcenters are commonly associated with
a) Modern intensive agriculture
b) Traditional farming systems
c) Industrial farming
d) Monoculture cropping
✅ Answer: b) Traditional farming systems.
Comments