Marker Assisted Breeding

Biotechnology is often discussed as an alternative to conventional, but it can also provide invaluable assistance to conventional breeding. Marker-Assisted Breeding (or Marker-Assisted Selection) is a form of biotechnology that does not involve genetic modification, and that is already benefitting conventional breeding programs in developing countries.

MAB allows the breeder to see more clearly and more quickly what has happened each time plants are crossed. Consider that a typical breeding project may unfold as follows. An agricultural plant is being attacked by a pest, and a wild relative of the crop is found to have resistance to the pest. The breeder wishes to transfer the resistance from the wild relative, but without disrupting the crop's other traits. The crop and the wild relative are crossed, and the progeny are exposed to the pest to isolate the plants that have inherited the resistance. These plants will also have inherited many other unwanted traits from the wild parent as well; therefore the breeder crosses the offspring with the crop parent again (this is called backcrossing). The pest-resistant offspring are again isolated, and again backcrossed. The strategy is to continue isolating resistant offspring and backcrossing until a plant is achieved that has none of the wild plant's traits except for the desired pest resistance. The process can be extremely time-consuming, especially since it is often necessary to raise each generation to maturity in order to test for inheritance of the desired trait.

Fig. 1. The marker acts as a red flag to indiucate the presence of the gene of interest (represented by the green stretch of DNA).
Such breeding can be greatly expedited by the use of genetic markers. A marker is usually a distinctive stretch of DNA located close to the gene of interest. It may occur within the gene itself, in another nearby gene, or in the "non-coding" DNA between genes (markers may also be in proteins instead of the DNA, although protein markers are used less commonly). Since the marker is located very close to the gene, it will almost always be inherited along with the gene, and since it is highly recognizable it acts as a red flag indicating the presence of the gene of interest (Figure 1). Rather than raising a plant to maturity and performing an experiment on the plant (such as exposing it to an agricultural pest), the breeder can check for the marker in the plant's DNA. MAB is the strategy of alternating between biotechnology (to select plants with desired traits) and conventional breeding (to produce each successive generation of plants).

What makes this strategy particularly valuable is that markers can be checked quickly and inexpensively, using immature plants. Marker-assisted breeding is even applauded by staunch opponents of crop genetic modification like Britain's Soil Association, and it is already playing a significant role in public breeding. Indeed, public researchers recently wrote, "because of their relative simplicity, easy integration into conventional breeding, and minimal background intellectual property, DNA marker technology and marker-assisted selection are expected to be strong driving forces in crop improvement in the future" (Fischer, Leung and Khush 2000).

FISCHER, KEN, HEI LEUNG AND GURDEV KHUSH. 2000. Molecular breeding: biotechnology at work for rice. In Serageldin, Ismail, and G.J. Persley. 2000. Promethean Science: Agricultural Biotechnology, the Environment, and the Poor. Consultative Group on International Agricultural Research, Washington, D.C.