Tuesday, February 16, 2010

Marker Assisted Selection (MAS)

Some of farmers’ favorite rice varieties have a flaw or two. African rice farmers often want to keep a variety they have grown for years, but it may be susceptible to a certain disease, or easily damaged by drought or salinity, which cause large losses of rice in Africa.

Geneticists at AfricaRice are using a technique called “marker assisted selection” (MAS) to improve the resistance of rice varieties to environmental stresses, using conventional plant breeding combined with molecular markers.

Dr. Marie-Noëlle Ndjiondjop prepares a DNA sample. MAS is not genetic modification. But by mapping useful genes in the lab, geneticists can save conventional plant breeders years of work
MAS is not genetic engineering. Molecular markers are naturally occurring and there are several types, but for rice the preferred ones are simple sequence repeats (SSRs) which are simply that, short sections of DNA that repeat themselves over and over. SSRs are abundant in rice so geneticists can use them as landmarks to pinpoint the sections of DNA that affect the desired traits.  SSR markers may occur in a gene, or near it. The geneticists plot the interesting genes onto a core map of the rice genome. Often a desired trait is coded by several genes, each one contributing a small part of the trait. MAS helps to find these genes and guide them into a popular variety.

How MAS works. Geneticists cross a popular rice variety with another one with a desired trait (but perhaps few other traits of interest). Then researchers test the offspring of the two varieties in the field to confirm that they have the trait of interest. As they begin backcrossing the offspring with the favorite variety, the trick is to keep as much genetic material as possible from the favorite variety, including only the desired genes from the other variety.

Finding markers that are genetically linked to a trait can help identify superior plants faster. DNA can be extracted from very young rice plants and the marker assay carried out long before the plant expresses the actual traits. MAS helps to develop promising new varieties in fewer generations, saving years of plant breeding.

Tougher rice. AfricaRice geneticist Dr. Marie-Noëlle Ndjiondjop is now using MAS to breed a new generation of improved inter-specific varieties which will be more like their African parents in ways that will help them adapt to local constraints. She has also used MAS to introduce a gene for resistance to rice yellow mottle virus (RYMV) into four popular varieties from Burkina Faso, The Gambia, Guinea and Mali. The new lines were tested in farmers’ fields and selected lines are now ready to be field evaluated with farmers using participatory varietal selection (PVS) in national programs in African countries.

Stress tolerant rice. As part of the STRASA project (Stress Tolerant Rice for Africa and South Asia) working in 14 African countries and three in South Asia, funded by the Bill & Melinda Gates Foundation through the International Rice Research Institute (IRRI), AfricaRice now has some interesting lines of rice which are resistant to cold, drought, salinity and iron toxicity. AfricaRice post-doc Dr. Kofi Bimpong will soon do the lab work on cold and salinity tolerant lines at IRRI, in the Philippines.

Dr. Khady Nani Drame is working with STRASA to develop solutions to iron toxicity, a major problem in the lowlands. Dr. Drame hopes to breed varieties which have acceptable yield, and which also tolerate iron toxicity, but there has been little previous work on the genetics of iron tolerance. She is now using MAS to introduce the genes that confer tolerance to iron toxicity in rice to popular varieties of Guinea, Ghana, Burkina Faso, and Nigeria.

STRASA coordinator for Africa, Dr. Baboucarr Manneh, explains that within another year or so, some of these lines will be ready to test with national programs in Africa using participatory varietal selection (PVS) with farmers.