WHAT IS GENETIC DIVERSITY?
Genetic diversity of wheat species illustrates the potential of plant breeding (picture from CIMMYT genebank).
Genetic diversity within a population refers to the number of different alleles (the alternate forms of genes) of all genes and the frequency with which they appear.
Variation is high when there are many different alleles of all genes and many different combinations of those alleles.
A gene pool is the collective set of alleles found in a population. Usually gene pools refer to a particular species (individuals can interbreed freely). Plant breeders can also make use of the gene pool of closely related species that can cross and produce fertile hybrids (called interspecific hybrids).
On the left picture, one of the most famous example of genetic diversity : maize ears from CIMMYT’s collection, showing a wide variety of colors and shapes. CIMMYT’s germplasm bank contains about 28,000 unique samples of cultivated maize and its wild relatives.
The level of genetic variation in a population is constantly changing: different alleles of a single gene can appear and disappear from time to time within a population. This means that the gene pool of a population is always dynamic.
For some characteristics, such as maize grain colour, there is a great level of genetic variation in nature.
WHAT IS THE MAIN REQUIREMENT FOR BASIC PLANT BREEDING?
The most important factor for basic selective breeding is genetic variation in the characteristic of interest.
For example, a farmer who wants to select a plant with resistance to an insect pest will watch for the plants that survive an insect attack. Another farmer wanting larger fruits will save seeds from plants yielding the biggest fruits in the field.
For some traits, such as the colour of maize kernels, or the ear size and shape of barley (picture below from ICARDA, diversity of 6 rows barley) or wheat, there is a great level of genetic variation in nature.
WHERE DO PLANT BREEDERS FIND GENETIC DIVERSITY?
The source of genetic variation for resistance to a pest or disease, for example, can be found in cultivated varieties of plants, or among landraces – local varieties of domesticated plant species adapted to the natural and cultural environment in which they are grown. If the required variation cannot be found among cultivars, the plant breeder may look into collections of seed of the wild ancestor of the crop. The main collections and genebank resources for barley and wheat are at ICARDA and CIMMYT (see the Crop Genebank Knowledge Base Website from CGIAR for more information : http://cropgenebank.sgrp.cgiar.org/ )
If there is no expected variation within the cultivar and willd ancestors, then the breeder will look into the gene pool of related species which can be crossed with the crop.
If crossing plants is not possible, the genetic engineering route can be examined. But this is another story…
Sources for text elements and photos :
CIMMYT, ICARDA, CGIAR, Biosciences for farming in Africa.