Structural variation in the Waxy gene and differentiation in foxtail millet [Setaria italica (L.) P. Beauv.]: implications for multiple origins of the waxy phenotype

The origin and evolution of the waxy type of foxtail millet Setaria italica (L.) P. Beauv] were studied by analyzing structural variation in the Waxy gene. Initially, the Waxy gene was amplified by RT-PCR, RACE and genomic PCR from a non-waxy strain to determine the structure of the wild-type gene. Secondly, we screened by PCR for polymorphisms at the Waxy locus in 79 strains with various waxy phenotypes. We then carried out genomic Southern analysis on 67 strains and identified seven RFLP classes which were designated as types I-VII. RFLP type was correlated with phenotype, such that types I and II corresponded to non-waxy, types III and VI to low-amylose, and types IV, V and VII to waxy phenotypes. The differences between RFLP types could be attributed to insertions in the Waxy gene. Types II and VI were caused by the insertion of a Tourist element into intron 1 and a SINE-like sequence into intron 12, respectively. Types III, IV, V and VII were characterized by the insertion of large sequences into the Waxy gene that may alter the expression of the gene. Thus, multiple, independent insertions in the Waxy gene appear to have caused the loss-of-function waxy phenotypes. Furthermore, the geographical distributions of the three RFLP types associated with the waxy phenotype (types IV, V and VII) were distinct, with type IV being found mainly in Taiwan and Japan, type V in Korea, and type VII in Myanmar. These results indicate a polyphyletic origin for the waxy phenotype in landraces of foxtail millet.