IDENTIFICATION OF QUANTITATIVE TRAIT LOCI UNDER DROUGHT CONDITIONS INTROPICAL MAIZE .2. YIELD COMPONENTS AND MARKER-ASSISTED SELECTION-STRATEGIES

In most maize-growing areas yield reductions due to drought have been observed. Drought at flowering time is, in some cases, the most damagi ng. In the experiment reported here, trials with F-3 families, derived from a segregating F-2 population, were conducted in the field under well-watered conditions (WW) and two other water-stress regimes affect ing flowering (intermediate stress, IS, and severe stress, SS). Severa l yield components were measured on equal numbers of plants per family . grain yield (GY), ear number (ENO), kernel number (KNO), and 100-ker nel weight (HKWT). Correlation analysis of these traits showed that th ey were not independent of each other. Drought resulted in a 60% decre ase of GY under SS conditions. By comparing yield under WW and SS cond itions, the families that performed best under WW conditions were foun d to be proportionately more affected by stress, and the yield reducti ons due to SS conditions were inversely proportional to the performanc e under drought. Moreover, no positive correlation was observed betwee n a drought-tolerance index (DTI) and yield under WW conditions. The c orrelation between GY under WW and SS conditions was 0.31. Therefore, in this experiment, selection for yield improvement under WW condition s only, would not be very effective for yield improvement under drough t. Quantitative trait loci (QTLs) were identified for GY, ENO and KNO using composite interval mapping (CIM). No major QTLs, expressing more then 13% of the phenotypic variance, were detected for any of these t raits, and there were inconsistencies in their genomic positions acros s water regimes. The use of CIM allowed the evaluation of QTL-by-envir onment interactions (Q x E) and could thus identify ''stable'' QTLs ac ross drought environments. Two such QTLs for GY, on chromosomes 1 and 10, coincided with two stable QTLs for KNO. Moreover, four genomic reg ions were identified for the expression of both GY and the anthesis-si lking interval (ASI). In three of these, the allelic contributions wer e for Short ASI and GY increase, while for that on chromosome 10 the a llelic contribution for short ASI corresponded to a yield reduction. F rom these results, we hypothesize that to improve yield under drought, marker-assisted selection (MAS) using only the QTLs involved in the e xpression of yield components appears not to be the best strategy, and neither does MAS using only QTLs involved in the expression of ASI. W e would therefore favour a MAS strategy that takes into account a comb ination of the ''best QTLs'' for different traits. These QTLs should b e stable across target environments, represent the largest percentage possible of the phenotypic variance, and, though not involved directly in the expression of yield, should be involved in the expression of t raits significantly correlated with yield, such as ASI.