The inheritance of early heading in the rice variety USSR5.

USSR5, a japonica rice variety from the former Soviet Union, is an extremely early maturing rice variety. To elucidate the genetic basis for its early heading, genetic analysis was carried out by crossing it with a set of major gene nearly isogenic lines (NIL) and QTL-isogenic lines. The early heading of USSR5 was attributed to the presence of photoperiod-insensitive alleles at E(1) and Se-1 gene, the photoperiod-sensitive inhibitor gene i-Se-1, and the dominant earliness gene Ef-1. Analysis of a backcrossed population (BC(1)F(1)) derived from the cross USSR5 x N22 indicated that two quantitative trait loci (QTL) for early heading were located on chromosomes 7 and 8, accounting for 27.4% and 11.2% of the phenotypic variance, respectively, with both early alleles originating from USSR5. From an F(2) population of the same cross, early heading QTLs were detected on chromosomes 1, 2, 7, 9, and 10, with individual QTL accounting for between 4.1% and 15.4% of the phenotypic variance. Early heading alleles at four of these five QTLs originated from USSR5. A comparison of chromosomal locations suggests that one of these QTLs may be identical with the known gene Hd4 (E(1)). The relationship between the other QTLs and known genes for heading date are not clear. USSR5 is a promising source for propagating earliness for the development of improved early heading rice varieties.