Molecular characterization of the waxy locus in sorghum

A comparison of approximately 4.5 kb of nucleotide sequence from the waxy locus (the granule-bound starch synthase I [GBSS I] locus) from a waxy line, BTxARG1, and a non-waxy line, QL39, revealed an extremely high level of sequence conservation. Among a total of 24 nucleotide differences and 9 indels, only 2 nucleotide changes resulted in altered amino acid residues. Protein folding prediction software suggested that one of the amino acid changes (Glu to His) may result in an altered protein structure, which may explain the apparently inactive GBSS I present in BTxARG1. This SNP was not found in the second waxy line, RTx2907, which does not produce GBSS I, and no other SNPs or indels were found in the approximately 4 kb of sequence obtained from RTx2907. Using one indel, the waxy locus was mapped to sorghum chromosome SBI-10, which is syntenous to maize chromosome 9; the waxy locus has been mapped to this maize chromosome. The distribution of indels in a diverse set of sorghum germplasm suggested that there are two broad types of non-waxy GBSS I alleles, each type comprising several alleles, and that the two waxy alleles in BTxARG1 and RTx2907 have evolved from one of the non-waxy allele types. The Glu/His polymorphism was found only in BTxARG1 and derived lines and has potential as a perfect marker for the BTxARG1 source of the waxy allele at the GBSS I locus. The indels correctly predicted the non-waxy phenotype in approximately 65% of diverse sorghum germplasm. The indels co-segregated perfectly with phenotype in two sorghum populations derived from crosses between a waxy and a non-waxy sorghum line, correctly identifying heterozygous lines. Thus, these indel markers or sequence-based SNP markers can be used to follow waxy alleles in sorghum breeding programs in selected pedigrees.

Characterization of waxy proteins and waxy genes of Triticum timopheevii and T. zhukovskyi and implications for evolution of wheat

The granule-bound starch (GBSS I, waxy protein) in Triticum timopheevii (AtAtGG) and T. zhukovskyi (AtAtAzAzGG) and a diagnostic section of the genes encoding GBSS-I from the Wx-TtA and Wx-G loci of T. timopheevii and the Wx-TtA, Wx-G, and Wx-TzA loci of T. zhukovskyi were investigated in this study. The waxy proteins in these two polyploid wheats could not be separated into distinct bands, in contrast to those in the T. turgidum (AABB) – T. aestivum (AABBDD) lineage. Alignment of sequences of the section covering exon4–intron4–exon5 of the various waxy genes led to the identification of gene-specific sequences in intron 4. The sequences specific to the Wx-TtA and Wx-G genes of T. timopheevii were different from those of the Wx-A1 gene and Wx-B1 genes of T. turgidum and T. aestivum. A surprising observation was that the Wx-TzA of T. zhukovskyi did not match with the Wx-TmA of T. monococcum, a putative donor of the Az genome, but matched unexpectedly and perfectly with the Wx-B1 gene on chromosome 4A, which is proposed to have translocated from the chromosome 7B of T. aestivum. The possible genetic mechanism explaining these observations is discussed.Key words: waxy proteins, waxy genes, T. timopheevii, T. zhukovskyi, wheat evolution.