Molecular Breeding and Marker-Assisted Selection

The process of developing new crop varieties requires many steps and can take almost 25 years. Now, however, applications of biotechnology have considerably shortened the time it takes to bring them to market. It currently takes 7-10 years for new crop varieties to be developed. One of the tools, which makes it easier and faster for scientists to select plant traits is called marker-assisted selection (MAS).

Molecular shortcut

 

The differences which distinguish one plant from another are encoded in the plant’s genetic material, the DNA. The DNA occurs in pairs of chromosomes (strands of genetic material), one coming from each parent. The genes, which control the plant’s characteristics, are specific segments of each chromosome. All of the plant’s genes together make up its genome.

 

Some traits, like flower color, may be controlled by only one gene. Other more complex characteristics, however, like crop yield or starch content, may be influenced by many genes. Traditionally, plant breeders have selected plants based on their visible or measurable traits, called the phenotype. But, this process can be difficult, slow, influenced by the environment, and costly - not only in the development itself, but also for the economy, as farmers suffer crop losses.

 

As a shortcut, plant breeders now use marker-assisted selection. To help identify specific genes, scientists use what are called molecular markers. The markers are a string or sequence of nucleic acid which makes up a segment of DNA. The markers are located near the DNA sequence of the desired gene. Since the markers and the genes are close together on the same chromosome, they tend to stay together as each generation of plants is produced. This is called genetic linkage. This linkage helps scientists to predict whether a plant will have a desired gene. If researchers can find the marker for the gene, it means the gene itself is present

 

As scientists learn where each of the markers occurs on a chromosome, and how close it is to a specific gene, they can create a map of the markers and genes on specific chromosomes. These genetic linkage maps show the location of markers and genes, and show their distance from other known genes. Scientists can produce detailed maps in only one generation of plant breeding.

 

Previously, scientists produced very simple genetic maps using conventional techniques. It was observed long ago that as generations of plants were crossed, some traits consistently appeared together in the new generations (genetic linkage). However, since researchers could concentrate on only a few traits in each attempt at cross-breeding, it took many crosses to obtain even a very simple genetic map. Using very detailed genetic maps and better knowledge of the molecular structure of a plant’s DNA, researchers can analyze a tiny bit of tissue from a newly germinated seedling. They don’t have to wait for the seedling to grow into a mature plant so that they can test for a specific characteristic. Once the tissue is analyzed, scientists know whether that seedling contains the appropriate gene. If it doesn’t, they can quickly move on and concentrate analysis on another seedling, eventually working only with the plants which contain a specific trait.

 

It should be noted, however, that molecular breeding through marker assisted selection is somewhat limited in scope compared to genetic engineering or modification because:

 

  1. it only works for traits already present in a crop;

  2. it cannot be used effectively to breed crops which have long generation time (e.g. citrus); and

  3. it cannot be used effectively with crops which are clonally propagated because they are sterile or do not breed true (this includes many staples such as yams, bananas, plantain, sweet potato, and cassava).

 

Sources:

Ag-West Biotech Inc. 1998. Marker assisted selection: Fast track to new crop varieties. Agbiotech Infosource. Canada. (http://www.agwest.sk.ca/sabic_bioinfo.shtml)

FAO 2002 Crop Biotechnology: A working paper for administrators and policy makers in sub-Saharan Africa. Kitch, L., Koch, M., and Sithole-Nang, I.