Preserving Rice Quality: Fine-Mapping and Introgressing a Fissure Resistance Locus.

Rice (Oryza sativa L.) kernel fissuring is a major concern of both rice producers and millers. Fissures are small cracks in rice kernels that increase breakage among kernels when transported and milled, which decreases the value of processed rice. This study employed molecular gene tagging methods to fine-map a fissure resistance (FisR) locus found in ‘Cybonnet’, a semidwarf tropical japonica cultivar, as well as transfer this trait to rice genotypes of standard plant height.  This recombination of the FisR trait is particularly significant in that one of the three previously-identified FisR loci resides near the semidwarf sd1 locus on the long arm of chromosome 1.  This study began with F₂ progeny from a cross between a standard plant height (Sd1Sd1), U.S. inbred breeding line with high kernel breakage upon milling, and Cybonnet, which is semidwarf (sd1sd1) and noted for having improved milling quality due to increased FisR.  Molecular markers (SSRs) within the three genomic regions previously determined to contain QTLs affecting kernel FisR were used to identify F₂ progeny plants containing recombination within region of chromosome 1 previously found to contain the FisR locus qFIS1-2, as well as sd1 so that the qFIS1-2 locus could be mapped more precisely.  Ten individual plants were selected and advanced to F_2:3 based on the following criteria: 1) the individual possessed recombination in the chromosomal region found to contain both Sd1 and the qFIS1-2 locus, 2) the individual maintained at least one standard height allele (i.e. was either Sd1Sd1 or Sd1sd1) and 3) when possible, the individual was homozygous for the other two known FisR QTLs that are not closely linked to sd1.  To reduce the time and labor required for phenotyping, the F_2:3 progeny were genotyped prior to being phenotyped, such that only individuals found to be homozygous for the new recombination are to be analyzed for FisR. To date, progeny from two of ten populations have been phenotyped. Results from both populations indicate that the qFIS1-2 locus is approximately 4 cM distal to sd1 on chromosome 1.  This study provides further evidence that the previously identified QTL qFIS1-2 for FisR is linked to but not pleiotropic with the sd1 allele and can be introgressed into rice cultivars of standard height.