Genomic organization of the complex α-gliadin gene loci in wheat

Abstract

To better understand the molecular evolution of the large alpha-gliadin gene family, a half-million bacterial artificial chromosome (BAC) library clones from tetraploid durum wheat, Triticum turgidum ssp. durum (2n = 4x = 28, genome AB), were screened for large genomic segments carrying the alpha-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 alpha-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 alpha-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 alpha-gliadin probe. These results indicate that alpha-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.

Wheat Evolution, Domestication, and Improvement

“…Gu et al (2004) indicated that the B genome diverged before the separation of the A and D genomes. There has been considerable controversy over the donor of the S-genome progenitor, but it was correctly identifi ed as an ancestor of Ae.…”

“…However, the various wheat genomes and the rice genome appear to have similar genetic composition with a good macrolevel syntenic relationship (Gale and Devos 1998;Sorrells et al, 2003;Tang et al, 2008). Flavell et al (1974) and Gu et al (2004) established that over 80% of the hexaploid wheat genome comprised noncoded highly repeated DNA sequences and highly active and nonactive retrotransposons. The intergenic regions (Bennetzen 2000;Feuillet and Keller 2002;Wicker et al, 2003) and genic regions (Gu et al, 2004) of many species are mainly composed of retrotransposons, and the vast numbers of these retrotransposons are correlated with genome size (Kidwell et al, 2002).…”

“…Flavell et al (1974) and Gu et al (2004) established that over 80% of the hexaploid wheat genome comprised noncoded highly repeated DNA sequences and highly active and nonactive retrotransposons. The intergenic regions (Bennetzen 2000;Feuillet and Keller 2002;Wicker et al, 2003) and genic regions (Gu et al, 2004) of many species are mainly composed of retrotransposons, and the vast numbers of these retrotransposons are correlated with genome size (Kidwell et al, 2002). Most of the retroelements in the three wheat genomes are not colinear, which suggests that their present location was the result of genome divergence after the individual A, B, and D genome parental species were combined (Gu et al, 2004).…”

“…The intergenic regions (Bennetzen 2000;Feuillet and Keller 2002;Wicker et al, 2003) and genic regions (Gu et al, 2004) of many species are mainly composed of retrotransposons, and the vast numbers of these retrotransposons are correlated with genome size (Kidwell et al, 2002). Most of the retroelements in the three wheat genomes are not colinear, which suggests that their present location was the result of genome divergence after the individual A, B, and D genome parental species were combined (Gu et al, 2004). When analyzing the glutenin genes of wheat, Gu et al (2004) found that more genes from the glutenin region of the A genome contained retrotransposons than occurred in orthologous regions of either the B or D genome.…”

“…Most of the retroelements in the three wheat genomes are not colinear, which suggests that their present location was the result of genome divergence after the individual A, B, and D genome parental species were combined (Gu et al, 2004). When analyzing the glutenin genes of wheat, Gu et al (2004) found that more genes from the glutenin region of the A genome contained retrotransposons than occurred in orthologous regions of either the B or D genome.…”