Identification of QTLs conferring genetic variation for heading date among rice varieties at the northern-limit of rice cultivation

Appropriate variation of heading date within the optimum range in a particular region is an important objective in rice breeding programs. Quantitative trait loci (QTLs) conferring genetic variation for heading date were identified among the rice varieties in the northern-limits of rice cultivation, Hokkaido (Japan), Portugal, Hungary and Italy. Based on segregation analyses of heading date, these varieties were categorized into two groups: one included the Hokkaido varieties and the other included the European varieties. Two QTLs controlling heading date were detected on chromosomes 6 and 7 using backcross inbred lines (BILs) derived from a cross between Hayamasari (Hokkaido) and Italica Livorno (Italy). The most effective QTL, qDTH-7 on chromosome 7, accounted for 64.5% of total phenotypic variation. Also, clear relationships between days to heading and the marker genotypes that were located in the region of qDTH-7 were observed in the two BIL populations from the crosses of Hoshinoyume (Hokkaido) with Arroz Da Terra (Portugal) or Dunghung Shali (Hungary). Estimation of the photoperiod sensitivity for these varieties indicated that qDTH-7 is involved in photoperiod sensitivity. These results indicate that qDTH-7 was responsible for distinguishing the varieties in the northern-limits of rice cultivation into the two groups.