IL-34 (NIL) developed by introgressing chromosomal segment substitution from an accession of Oryza minuta (2n=48, BBCC, Acc. No. 101141) into the O. sativa subsp. japonica cv. Hwaseongbyeo, showed significantly higher number of spikelets per panicle (SSP) than the recurrent parent Hwaseongbyeo. QTL analysis in F2 generation derived from the cross between IL-34 and Hwaseongbyeo revealed that ssp7, a QTL was located in the pericentromeric region of chromosome 7. The frequency distribution of spikelets per panicle followed 3:1 ratio for single locus segregation. The additive effect of the O. minuta allele at the QTL was 23 spikelets per panicle, and 43.6% of the phenotypic variance could be explained by the segregation of marker RM21596. To clarify whether ssp7 could be dissected genetically, we carried out fine-scale mapping with 3,700 F2 plants derived from the cross between IL-34 and Hwaseongbyeo using markers flanking spp7. 186 F2 plants having informative recombination breakpoints within the region flanked by two SSR markers RM500 and RM21615 were identified and used for fine mapping of ssp7. ssp7 was mapped between the SSR markers RM21596 and RM418 which was approximately 441kb in length based on the physical map of the region. Of great interests, the QTL region also had effects on primary branch number (PB), grains per panicle (SP) and grain yield (YD). These results are very useful for transferring or pyramiding ssp7 by molecular marker assistant selection in rice breeding programs.

Heading date in rice is a complex trait that is governed by multiple genes and environmental factors, such as day-length, temperature, and soil conditions. The genetic studies using DNA markers have facilitated the genetic dissection of heading date and many quantitative trait loci (QTLs) for heading date have been identified using several mapping population. In a previous study, a new quantitative trait loci (QTLs) for heading date have been identified using several mapping population. In a previous study, a new for heading date was detected near SSR marker RM215 on chromosome 9 using an advanced backcross line, WH29001, developed by introgressing chromosomal segments from an accession of Oryzaminuta (2n=48, BBCC, Acc. No.101141)into the O. sativa subsp. japonica cv. Hwaseongbyeo. The O. minuta allele of QTL contributed to an increase in heading date. To clarify whether dth9 could be dissected genetically, a high-resolution linkage mapping of dth9 was performed using alarge F2 population derived form the cross between one F4 plant which was homozygous for O.minuta in the target region RM5661-RM215 on chromosome9 and Hwaseongbyeo. Days to heading in the F2 population showed continuous variation rang form 102 to 113 days. The dth9 QTL further narrowed down at the interval between the SSR marker RM1553 and RM215 which was approximately 403kb in length based on the physical map of the region. The QTL for heading date(dth9) had not been detected in previous QTL studies between Oryza cultivars, indicating the existence of potentially novel alleles from O. minuta.