Francaise You

To better understand the molecular evolution of the large α-gliadin gene family, a half-million bacterial artificial chromosome library clones from tetraploid durum wheat, Triticum turgidum ssp. durum, were screened for large genomic segments carrying the α-gliadin genes of the Gli-2 loci on the group 6 homoeologous chromosomes. The resulting 220 positive BAC clones - each containing between one and four copies of α-gliadin sequences - were fingerprinted for contig assembly to produce contiguous chromosomal regions covering the Gli-2 loci. While contigs consisting of as many as 21 BAC clones and containing up to 17 α-gliadin genes were formed, many BAC clones remained as singletons. The accuracy of the order of BAC clones in the contigs was verified by Southern hybridization analysis of the BAC fingerprints using an α-gliadin probe. These results indicate that α-gliadin genes are not evenly dispersed in the Gli-2 locus regions. Hybridization of these BACs with probes for long terminal repeat retrotransposons was used to determine the abundance and distribution of repetitive DNA in this region. Sequencing of BAC ends indicated that 70% of the sequences were significantly similar to different classes of retrotransposons, suggesting that these elements are abundant in this region. Several mechanisms underlying the dynamic evolution of the Gli-2 loci are discussed. © Springer-Verlag 2004.

Two overlapping bacterial artificial chromosome clones from the B genome of the tetraploid wheat Triticum turgidum were identified, each of which contains one of the two high-molecular-weight glutenin genes, comprising the complex Glu-B1 locus. The complete sequence of this chromosomal region was determined. The two paralogous x-type and y-type HMW-glutenin genes of the complex Glu-B1 locus were found to be separated by ca. 168 000 bp instead of the 51 000 bp separation previously reported for the orthologous Glu-D1 locus of Aegilops tauschii, the D-genome donor of hexaploid wheat. This difference in intergene spacing is due almost entirely to be the insertion of clusters of nested retrotransposons. Otherwise, the orientation and order of the HMW glutenins and adjacent genes were identical in the two genomes. A comparison of these orthologous regions indicates modes and patterns of sequence divergence, with implications for the overall Triticeae genome structure and evolution. A duplicate globulin gene, found 5′ of each HMW-glutenin gene, assists to tentatively define the original duplication event leading to the paralogous x- and y-type HMW-glutenin genes. The intergenic regions of the two loci are composed of different patterns and classes of retrotransposons, indicating that insertion times of these retroelements were after the divergence of the two wheat genomes. In addition, a putative receptor kinase gene near the y-type HMW-glutenin gene at the Glu-B1 locus is likely active as it matches recently reported ESTs from germinating barley endosperm. The presence of four genes represented only in the Triticeae endosperm ESTs suggests an endosperm-specific chromosome domain.