PPD-B1 of Triticum dicoccoides, the Candidate Gene of a Major QTL Responsible for the Later Heading of Common Wheat-Wild Emmer Chromosome Arm Substitution Line 2BS

Wheat heading date is an important agronomic trait regulated by several genetic factors. In this study, a common wheat (Chinese Spring, CS)-wild emmer (Acc. TTD140) chromosome arm substitution line (CASL2BS) was found about fifteen days later in heading date compared to CS. To map the genes affecting the heading date on chromosome arm 2BS, three cross combinations involving CASL2BS as one parent were developed, including CASL2BS x CS (later heading x Parent), CASL3AL x CASL2BS (earlier heading x later heading), CASL4AL x CASL2BS (later heading x later heading), and planted in three different times forming six F₂ mapping populations. The heading date varied considerably in different populations. Average heading date of the populations derived from the cross with an early heading parent was earlier than those of the populations involving later parent. Using these populations, a genetic map of chromosome arm 2BS consisting of 13 SSR markers and one SSCP marker was constructed spanning 88.0 cM. A clear QTL accounting for a higher ratio of heading date variation was detected in the same interval (WMC154-WMS429) of the maps from five mapping populations expect for CASL4AL x CASL2BS, which did not show any QTL related to heading date. The SSCP marker, developed according to the sequence of photoperiod gene Ppd-B1, was mapped in above interval corresponding to the QTL and was the closest one to the highest peak, explaining 52.47%, 76.72%, 16.19%, 78.41%, 75.42% of phenotypic variation for heading date, with LOD score of 4.00, 19.34, 3.33, 10.57, 16.34 respectively in these five mapping populations. Based on these results, we concluded that the Ppd-B1 on chromosome arm 2BS is the candidate gene responsible for the major quantitative trait locus of heading date.