Validation of Quantitative Trait Loci for Spikelets per Panicle and Grain Weight in Rice

High grain yield is one of the most important traits for improvement in rice breeding program. Much attention has been given to the genetic basis of the number of spikelets per panicle (SPP) and grain weight (GW) because of their importance in rice yield. In this study, IL28, a near isogenic line (NIL) developed by introgressing “Moroberekan” chromosomal segments from into “Ilpumbyeo”, showed significant increase in the number of spikelets per panicle and 1,000 grain weight compared to the recurrent parent Ilpumbyeo. Quantitative trait locus (QTL) analysis using 1150 F2 plants derived from a cross between IL28 and Ilpumbyeo, indicated that both qspp6 and qgw6 were located in the interval RM3430 - RM20580. To narrow down the QTL region, substitution mapping of qspp6 and qgw6 was conducted using F4 lines. qspp6 was mapped to a 429-kb interval between RM20521 and RM20562 while qgw6 was mapped to a 267-kb interval between RM20562 and RM20572 based on the japonica genome sequence. This result seems to indicate that qspp6 and qgw6 are different genes. It is notable that these QTL, qspp6 and qgw6 are independent from undesirable height and flowering time. Moreover, there was no negative correlation between qspp6 and qgw6 when two genes were introgressed in the Ilpumbyeo genetic background. SSR markers tightly linked to the qspp6 and qgw6 will facilitate cloning of the gene underlying these QTLs as well as marker assisted selection for variation in SPP and TGW in rice breeding program.