HL2 on chromosome 7D of wheat (Triticum aestivum L.) regulates both head length and spikelet number.

KEY MESSAGE: A major QTL QSpl.nau-7D, named HL2, was validated for its effects on head length and kernel number per spike using NIL, and mapped to a 0.2 cM interval using recombinants. Improvement in wheat inflorescence traits such as spike or head length and spikelet number provides an important avenue to increase grain yield potential. In a previous study, QSpl.nau-7D, the major QTL for head length on chromosome 7D, was identified in the recombinant inbred lines derived from Nanda2419 and Wangshuibai. To validate and precisely map this QTL, the Wangshuibai allele was transferred to elite cultivar Yangmai15 through marker-assisted selection. Compared with the recurrent parent, the resultant near-isogenic line (NIL) yielded not only 28% longer spikes on the average but also more spikelets and kernels per spike. Moreover, the NIL had a lower spikelet density and did not show significant kernel weight change. In the F2 population derived from the NIL, QSpl.nau-7D acted like a single semi-dominant gene controlling head length and was therefore designated as Head Length 2 (HL2). With this population, a high-density genetic map was constructed mainly using newly developed markers, and 100 homozygous recombinants including 17 genotypes were obtained. Field experiments showed that the recombinants carrying the 0.2-cM interval flanked by Xwgrb1588 and Xwgrb1902 from Wangshuibai produced longer spikes than those without this Wangshuibai allele. Comparative mapping of this interval revealed a conserved synteny among cereal grasses. HL2 is beneficial to wheat breeding for more kernels per spike at a lower spikelet density, which is a favored morphological trait for Fusarium head blight resistance.