Part 3: Preserving and Improving Genetic Diversity
Options to Improve Diversity
What options then do plant breeders have to incorporate diversity into their program’s germplasm? What work is being done internationally to slow the loss of genetic diversity? To conserve and preserve genetic diversity, most crops have many accessions stored in genebanks, or germplasm banks, that are available free of charge or with a shipping and handling fees. A few examples:
- The USDA-ARS National Plant Germplasm System is a government organization that acquires and stores germplasm.
- The CGIAR system has a number of genebanks around the world, storing crop germplasm such as rice, maize and many others. An example is the International Rice Research Institute (IRRI) genebank in Los Banos, Philippines, stores over 80,000 accessions of rice, shown in Figure 7.
Figure 7: Genebanks are reserves for wild relatives of domesticated crops. The International Rice Research Institute (IRRI) genebank in Los Banos, Philippines, stores over 80,000 accessions of rice.
In addition to crop germplasm, many wild relatives of our crops have also been saved in genebanks around the world (Figure 7 and 8). The CGIAR centers (Consultative Group on International Agricultural Research) together curate more than 700,000 accessions of crop, forage and agroforestry species.
Figure 8: CGIAR centers curate thousands of accessions of economic crops as well as their wild relatives. Rice is just one example. Photo: CGIAR-IRRI
Alleles which can be transferred from a wild relative to a crop plant not only increase the crop's genetic diversity, but also can lead to an improvement in a valuable trait, even when this could never be predicted from the wild relative's phenotype. Although a wild relative may appear not to have many desirable characteristics, we now know that there may be many hidden alleles that could affect the trait in the direction that we want. For example, genes for increasing yield can be found in a low-yielding plant (Eshed and Zamir 1995, Mallikarjuna Swamya and Sarla 2008). This is especially true for quantitative traits, which we are discussed in a later module. In the example diagrammed in Figure 9, in a genome with 6 chromosomes there are 2 genes (or QTL) (red) that are associated with high yield, but 4 genes (blue) that are associated with lower yield, so overall the individual may be a poor yielder and yet have some useful genes. MAB techniques, such as those in the upcoming modules, can help identify those genes with the positive effects (ie the red ones in this example).
Figure 9: The phenotype may not totally represent the genotype. Image by Theresa Fulton.