Copy number variation of B1 controls awn length in wheat

Wheat awns contribute to photosynthesis and grain production. In this study, an F2 population and F2:3 families from a cross between the awned line 7D12 and the Chinese awnless variety Shiyou 20 (SY20) were used to identify loci associated with awn length. Bulked-segregant RNA sequencing and linkage mapping identified a single dominant locus in a 0.3 cM interval on chromosome 5AL. Five genes were in the interval, including the recently cloned awn inhibitor B1. Although a single copy of the B1 gene was detected in 7D12, SY20 carried five copies of the gene. Increased copy number of B1 in SY20 enhanced gene expression. Based on sequence variation among the promoter regions of five B1 gene copies in SY20, two dominant markers were developed and found to cosegregate with B1 in a population of 931 wheat accessions. All 77 awnless accessions harbored sequence variations in the B1 promoter regions similar to those of SY20 and thus carried multiple copies of the gene, whereas 15 randomly selected awned wheats carried only one copy. These results suggest that an increase in copy number of the B1 gene is associated with inhibition of awn length.