Molecular Breeding 4: 419–426, 1998. © 1998 Kluwer Academic Publishers. Printed in the Netherlands. 419 Genetic dissection of the source-sink relationship affecting fecundity and yield in rice (Oryza sativa L.) Zhikang Li 1,2,4,* , Shannon R.M. Pinson 3 , James W. Stansel 2 and Andrew H. Paterson 1 1 Department of Soil and Crop Sciences, Texas A&M University, College Station, TX 77843, USA; 2 Texas A&M University Agricultural System Research and Extension Center, Beaumont, TX 77713, USA; 3 USDA-ARS, Beau- mont, TX 77713, USA; 4 Current address: International Rice Research Institute, P.O. Box 933, 1099 Manila, Philippines ( * author for correspondence) Received 7 november 1997; accepted in revised form 22 May 1998 Key words: plant breeding, morphology, QTL, epistasis, productivity Abstract The genetic basis underlying the relationship between the source leaves (the top two leaves) and the sink capacity in rice was investigated in a replicated trial of 2418 F 2 derived F 4 progeny from an inter-subspecific cross between cv. Lemont (japonica) and cv. Teqing (indica) and a complete linkage map with 115 well distributed RFLP markers. Path analysis indicated that 50% of the phenotypic variation in the primary sink capacity-grain weight per panicle was attributable to variation of the flag leaf area. Thirteen QTL and 30 pairs of epistatic loci were identified, which influence the length, width and area of the source leaves and the size of the primary sink (panicles) panicle length, floret density and floret number per panicle. Two QTL (QLl3b and QLw4) and 7 pairs of epistatic loci are largely responsible for the observed relationship between the source leaves and the sink capacity. The others appear to primarily influence the shape of the source leaves or panicle length/branching, and contribute little to the observed source-sink relationship and partially explain the yield component compensation. Our results suggest that important QTL affecting the source leaves can be manipulated through marker-assisted selection to increase sink capacity, which might result in improved yield potential in rice. Introduction Grain yield of major cereals including rice, wheat, barley and sorghum is largely determined by the source-sink relationship in which florets are the pri- mary photosynthate sink while the top three leaves on a stem (the flag leaf, flag-1 and flag-2), particu- larly the flag leaf, are the primary source [1–5]. In rice, over 80% of the carbohydrates accumulated in grains is produced by the top two leaves [3, 5–7]. The source and sink capacities in cereals are pheno- typically associated with some morphological traits such as size and shape (or type) of the source leaves, panicles and kernels [2, 6]. In cereal breeding efforts for high yield potential, selection for yield per se or on yield components in early segregating generations has not been effective because of low heritability of yield in the former case, and the negative correla- tion between yield components of latter, which is also known as the concept ‘yield component compensa- tion’ arising either from the developmental allometry or physiological competition [8, 9]. We report here an effort to genetically dissect the source-sink relationship based on mapped QTL and epistatic loci influencing several morphological traits associated with the source and sink capacities in rice. Our results revealed genetic aspects of the relationship between source and sink traits, which also provided a genetic basis for the ‘yield component compensa- tion’ and useful information for genetic improvement of yield potential in rice and other cereal crops.