‘NIL 130’ was named “best rice variety” among seven being evaluated by a farmers’ cooperative in Gagnoa, Côte d’Ivoire. NIL 130 is a ‘near-isogenic line’ derived from ‘elite’ rice variety IR64 by the introduction of a gene for resistance to (RYMV) through a process known as ‘marker-assisted breeding’ (MAB).
“Rice yellow mottle virus (RYMV) is probably the stress that benefits the most from molecular biology at the Africa Rice Center (AfricaRice),” says AfricaRice molecular biologist Dr Marie-NoëlleNdjiondjop. RYMV is colloquially known as the “AIDS of rice”!
The AfricaRice MAB work targeting RYMV is carried out in collaboration with the Institut de recherchepour le développement (IRD), Montpellier, France, and various national (NARS) partners. AfricaRice has had fruitful collaboration on RYMV with IRD since 1994, shortly after the first devastating outbreaks of the disease occurred in various parts of West Africa.
RYMV is a disease of intensified irrigated rice production where high-yielding varieties have been introduced, and the vast majority of irrigated varieties are extremely susceptible to it. It only occurs in Africa. In 1995, AfricaRice discovered that a variety from Mozambique (Gigante) was virtually immune to the disease.
Gigante’s resistance was confirmed by Dr Ndjiondjop against a whole spectrum of RYMV isolates from diverse locations. Determining the genetic basis of Gigante’s resistance was given top priority. The resistance gene, rymv1-2, was identified and mapped in 1999 by Dr Ndjiondjop as part of her PhD studies at IRD.
“We are now using molecular breeding to improve the RYMV resistance of West African elite rice cultivars,” says Dr Ndjiondjop.
In 2005, a USAID-funded project enabled AfricaRice to carry out MAB to introgress (i.e. incorporate) rymv1-2 into elite rice cultivars of four West African countries (Burkina Faso, Gambia, Guinea and Mali) and to introduce molecular-marker work into the breeding programs of the NARS.
At least two elite varieties were proposed by each NARS and backcrossed three times to the donor Gigante. Resistant lines were identified by a combination of ‘foreground’ and ‘background’ markers.
Foreground markers occur in the DNA close to the RYMV-resistance gene in Gigante and therefore show that any particular plant has the gene, while background markers are from the elite variety and show how similar the plant is to the elite variety.
The resulting lines are near-isogenic lines, or NILs — that is, they are very similar to the elite variety, except that they carry the RYMV-resistance gene from Gigante.
The promising resistant NILs were then further screened under controlled conditions using a purified virus isolate. Trials were conducted at multiple locations in the target countries to confirm their resistance to diverse natural populations of RYMV.
Fixed (pure-breeding) RYMV-resistant NILs were then sent to the NARS for complete evaluation and incorporation into resistance breeding programs. The best NIL from each elite parent was selected for further trials in the four project countries plus six more countries (Côte d’Ivoire, Ghana, Liberia, Niger, Nigeria and Sierra Leone).
This activity was funded by USAID through the West and Central African Council for Research and Development(CORAF/WECARD). A number of these lines – like NIL 130 – are expected to be released in some of the countries in the near future.
In 2010, AfricaRice and IRD discovered a second resistance gene, rymv2, and a new allelic form of the first gene in African rice (Oryza glaberrima) varieties. As an insurance policy against RYMV overcoming single-gene resistance, the AfricaRice breeding strategy is to ‘pyramid’ two resistance genes in varieties for hot-spot areas.