|
Utilization of molecular markers to characterize and map useful genes of the cultivated African rice Oryza glaberrima
M.N.
Ndjiondjop,
A. Alonge, S. Mc Couch
& M. P. Jones
This
paper summarizes the progress made by the West African Rice Development
Association (WARDA) in developing the genetic tools and plant material
required for the identification and characterization of valuable genes
from Oryza glaberrima, and to assist the breeders by using
molecular-markers in the selection of genotypes that combine the high yield potential of O.
sativa with the stress tolerance of
O. glaberrima.
WARDA used anther culture and backcrossing to produce
several hundred fixed lines from interspecific crosses between varieties
CG14 (O. glaberrima) and
WAB56-104 (O. sativa, subsp. japonica).
We selected 50 doubled haploid and 50 backcross lines with superior
adaptation in farmer field. A set of 131 polymorphic microsatellites
dispersed over the 12 chromosomes of rice was used to assess the proportion of the genome from each parent in the interspecific lines. From
the graphical genotyping of all lines, we observed the
introgression of O. glaberrima alleles at the end of the long
arm of chromosome 2, the short arm of chromosome 6 and in the middle of
chromosomes 8, 9 and 10. The
introgressed fragment on chromosome 6 is probably linked to the
gamete-killer gene reported on this chromosome; however, on chromosomes
2, 8, 9 and 10 mechanisms other than the sterility gene may be involved.
On average, the introgressed segments from O. glaberrima
were small (average size 5 cM) and distributed on almost all
chromosomes, indicating that recombination occurs frequently in this
interspecific combination and that there is no genome-wide barrier to
recombination during meiosis. The mean of O. glaberrima alleles
across lines was 12% and the
mean of O. sativa alleles 86%. Some non-parental alleles were
identified. We believe that these alleles, which are not linked to the
high mutation rate due to microsatellites, are due to out-crossing and
have played an important role in increasing the genetic diversity of the
lines. In addition, the lines
were fixed in all parts of the genome. The phenotyping and
graphical genotyping of these lines has enabled the selection of three
interspecific lines. They are used to develop the near isogenic material
necessary to identify genes/QTLs controlling characters, such as wide
droopy leaves, number of grains per panicle, panicle size, tolerance to
blast and drought. These lines have also provided the opportunity to
characterize and to assess the genetic basis of male sterility.
To establish the marker-assisted
selection system at WARDA, we developed a genetic mapping program. Advanced
backcross and dihaploidization strategies from BC2F1
lines were used to produce segregating materials.
A good source of single gene resistance
to RYMV, a major constraint to rice production in several countries in
West Africa, has been identified in the variety Gigante (O.
sativa, subsp
indica) and mapped on chromosome 4. Fine
mapping allowed the determination of three microsatellite markers close
to the RYMV-resistance gene (2 cM), providing the opportunity to easily
transfer the resistance gene from Gigante into other genetic backgrounds
using marker-assisted selection. Three elite rice varieties, namely
BG90-2, Bouaké189 and Jaya have been selected by WARDA, since
they are well adapted to African lowland conditions, but these are
highly susceptible to RYMV. Two backcross progenies have been produced
and evaluated for resistance to RYMV. The BC2F1
resistant materials are now available and are being used to continue the
backcrossing process.
|
