‘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.
