Wednesday, June 5, 2013

Genetic bridges between Africa and Asia

Achieving food security in Africa largely depends on growing high-yielding crops, such as rice, that are also well adapted to the continent’s harsh environment. GRiSP, through the Institut de Recherche pour leDéveloppement (IRD), International Center for Tropical Agriculture (CIAT), andAfrica Rice Center (AfricaRice), is building genetic bridges between African and Asian rice to accelerate the development of new robust and high-yielding rice varieties suitable for Africa.

Cross-breeding between different species of rice can produce new plants that stand up better to drought, salty soil, diseases, and insect pests. Since the 1990s, AfricaRice has been crossing African rice (Oryza glaberrima), which is particularly resistant to drought and diseases, with O. sativa, the high-yielding Asian rice species most widely grown in the world, to create the New Rice for Africa (NERICA) varieties.

NERICA rice has been hailed as a major advancement for Africa and could help Africa achieve much higher rice productivity. Currently, 78 NERICA varieties are available to rice farmers in sub-Saharan Africa, and, by 2011, adoption had reached more than 700,000 hectares.

The right gene mix

The initial problem in developing NERICA was that the two different rice species produced sterile offspring that could not be used to grow crops. Scientists at AfricaRice overcame this by repeatedly backcrossing into Asian rice to restore fertility. Although the early NERICA crosses exhibited low susceptibility to disease and insect attacks, the backcrossing process reduced the proportion of the African rice genome in the final NERICA varieties, consequently reducing the desired tolerance of pests.

“What if we were to increase the proportion of O. glaberrima genes—would we get better varieties?” Dr. Mandé Semon, an upland rice breeder at AfricaRice, wondered. He proposed reversing the gene route and bringing the desirable genes from Asian rice into African rice instead. However, sterility was still going to be a problem.

The bridge from sterility to fertility

To solve this problem, the iBridges (interspecific Bridges) project was launched in 2005 as a close cooperation among IRD, CIAT, AfricaRice, the Philippine Rice Research Institute, and African national agronomic research bodies such as the Institute of Rural Economy in Mali and the Institute of Environment and Agricultural Research in Burkina Faso. Originally supported by the CGIAR Generation Challenge Program, the second phase of iBridges is now being advanced under GRiSP.

Cutting-edge research under Mathias Lorieux, IRD director of research and plant geneticist at CIAT, led to the fine mapping of the S1 gene, previously identified as a key factor in interspecific sterility. Using the rice genome maps developed by the Oryza Map Alignment Project, the researchers identified the portion of the chromosome responsible for sterility.

“This resulted in the design of a genetic model that explains the sterility of interbred descendants, and opens the way to further research into the genetic control of sterility,” said Dr. Lorieux, who is also the iBridges co-project leader along with Dr. Alain Ghesquière of IRD. “Being able to identify the few and rare fertile individuals from interspecific crosses early in the breeding process eliminates many stages previously required for interspecific breeding and accelerates the development of new and improved African-Asian varieties that fully embrace the rich diversity of African rice alongside that of Asian rice.“

Open pool

Thanks to the partnerships that are being fostered by GRiSP, plant breeders now have access to the complete genetic diversity available in African rice. Under GRiSP, the iBridges project aims to develop new and fertile rice varieties derived from crosses between elite O. sativa and O. glaberrima rice varieties.

“We hope that the iBridges will make a significant breakthrough in improving African rice varieties,” said Dr. Lorieux. “It is the African rice farmers who will directly benefit from this technology, and this will certainly contribute to Africa’s food security.”

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