Résumé
Drought is one of the most important abiotic constraints that negatively impacts rice productivity, especially at reproductive stage. Developing drought-tolerant cultivars possessing high yield under a favorable water regime and fairly good yield under drought stress is crucial to sustain farmers' efforts in intensifying rice cropping systems. To develop such cultivars, we launched a marker-assisted recurrent selection (MARS) program, a new approach in rice breeding, where quantitative trait loci (QTLs) for the target traits are mapped within the progeny of elite materials with complementary traits, and favorable alleles of those QTLs are accumulated through successive cycles of intercrossing and genotyping-based selection. A population of 230 F3:5-derived lines from a cross between IR64 and B6144F, and 10 check varieties, have undergone 14 field evaluations for yield under favorable conditions (2 years) during the wet season, and five field evaluations for yield under cyclical drought during the dry season (one year) in five locations (Banfora, Burkina Faso; Longorola, Mali; Ibadan and Badeggi, Nigeria; Los Baños, Philippines and Santa Rosa, Colombia). The population was also evaluated for drought tolerance in a rain-out shelter (ROS), and also genotyped with 484 single nucleotide polymorphism (SNP) markers polymorphic between the two parents. A wide level of variability for target traits, irrespective of the water regime, was observed among the progenies in all locations. Under favorable conditions, the population average yield was 5.06 t/ha in Banfora, 4.19 t/ha in Badeggi, 4.08 t/ha in Ibadan, and 3.9 t/ha under the ROS. Three progenies out-yielded all the checks in Badeggi, and several yielded more than both parents. Similar variability was observed during the trials for drought tolerance. Yield reduction due to drought averaged 72.6%, 72%, 33% and 47%, under the ROS, in Ibadan, Longorola and Banfora, respectively. Stability estimated from environmental variance revealed that some lines (16 in Ibadan, 24 in Burkina Faso, 9 in Mali, 5 under ROS) had more stable yield than the best check. Seven QTLs related to yield under favorable conditions were detected on chromosomes 1, 3, 4 and 9 in Badeggi, Banfora and Longorola. The QTL on chromosome 3 (LOD = 4) explained 20.1 and 19.8% of the yield phenotypic variation in Banfora and Badeggi, respectively. This QTL is also related to plant height and days to flowering. In Ibadan and under the ROS, two QTLs for grain yield under drought stress, explaining 25.4% and 54.48% of the phenotypic variation, were detected. These results underline the successful development of a reliable phenotyping network. The next step will be to combine favorable alleles of those QTLs into a single line. A three-step optimization process is undertaken: first, the genetic value of F3 lines is estimated using genotypic data; second, a small set of candidate F3 lines is selected; third, the expected genetic value of their progeny is computed and crosses made. Improved material identified at each selection cycle will join the AfricaRice and NARS partners' varietal development and release pipelines. This is already the case for F3 lines with improved yield stability. (Résumé d'auteur)