Heterosis describes the increased performance of F1 hybrid plants in terms of increased biomass, yield, vegetative growth rate, and tolerance against biotic and abiotic stresses as compared with their inbred parents. Two sets of rice materials, 166 RILs derived from a cross between Milyang 23 (Korean indica-type rice) and Tong 88-7 (japonica Rice), and BC1F1 hybrids derived from crosses between the RILs and the female parent, Milyang 23, were produced to identify QTLs for heterosis of yield and yield-related traits. The QTLs were detected from three different phenotype data sets including the RILs, BC1F1 hybrids, and mid-parental heterosis data set. A total of 57 QTLs were identified for nine traits. Of eight QTLs detected for yield heterosis, five overlapped with other heterosis QTLs for yield-related traits such as spikelet number per panicle, days to heading, and spikelet fertility. Four QTLs for yield heterosis, gy1.1, py6, gy10, and py11, were newly identified in this study. We identified a total of 17 EpQTLs for yield heterosis that explain 21.4 ~ 59.0 % of total phenotypic variation, indicating that epistatic interactions may play an important role in heterosis.

The rice sucrose synthase 3 (RSUS3) localized predominantly inrice seed endosperm may play an important role in the starch filling in the milky stage of rice seed. As the genetic diversity at this locus is not known yet, forty three rice varieties/accessions were objected to amplify full sequence of the RSUS3 to examine the distribution of DNA polymorphisms. A total of 254 sequence variants, including 82, 150 and 22SNP sand indels, were successfully identified in whole length of 7,733bp sequence comprising promoter, exon and intron, and 3’ down stream non transcribed region(NTR). Eleven haplotypes were distinguishable among 43 rice varieties based on the nucleotide variation on the three defined regions (5’NTR, transcript and 3’NTR). The promoter region showed the occurrence of a base change on a cis-element which might involve a functional role of the motif in seed-specific expression. Non random process seemed to be acted in the genetic diversity of RSUS3geneamongricegermplasmusedinthisstudy. The analysis of polymorphism sites indicated a history of eleven minimum recombination mostly occurred in the transcribed region. This result might provide an insight for a clasditic approach for establishing future genetic association studies of RSUS3locus.

Sucrose synthase 3 which is a third active gene present in rice, is localized predominantly in rice endosperm. This sucrose synthase 3 may play an important role in the starch filling in the milky stage rice seed, probably involving in the starch physicochemical properties. As the genetic diversity at this locus is little informed, forty three rice consisting of japonica, indica and Oryza rufipogon were targeted to amplify full sequence of sucrose synthase 3 to examine the frequency and distribution of nucleotide polymorphism. Total of 755 all sequence variants detected, 491 single nucleotide polymorphisms (SNPs) and 264 indels were successfully identified in 7618 bp of sequence containing the sucrose synthase 3 transcript, promoter and 3' non-transcribed region. The frequency of nucleotide changes and indels were high, on average one polymorphism per 15.5 bp and one indel per 28.9 bp with 11 sequence-based haplotypes distinguishable among the varieties and lines. Both the frequency of nucleotide changes and indels were frequent in non-coding region, but rare in coding region. Sequencing a polymorphism region in the promoter showed one base change on one of cis-element from T (CATGCATA to A (CATGCACA) that might implicate in seed specificity. The presence of a high number of haplotype shared by a few varieties indicated a little information on linkage disequilibrium.