Molecular characterization of Ph1 as a major chromosome pairing locus in polyploid wheat.

The foundation of western civilization owes much to the high fertility of bread wheat, which results from the stability of its polyploid genome. Despite possessing multiple sets of related chromosomes, hexaploid (bread) and tetraploid (pasta) wheat both behave as diploids at meiosis. Correct pairing of homologous chromosomes is controlled by the Ph1 locus. In wheat hybrids, Ph1 prevents pairing between related chromosomes. Lack of Ph1 activity in diploid relatives of wheat suggests that Ph1 arose on polyploidization. Absence of phenotypic variation, apart from dosage effects, and the failure of ethylmethane sulphonate treatment to yield mutants, indicates that Ph1 has a complex structure. Here we have localized Ph1 to a 2.5-megabase interstitial region of wheat chromosome 5B containing a structure consisting of a segment of subtelomeric heterochromatin that inserted into a cluster of cdc2-related genes after polyploidization. The correlation of the presence of this structure with Ph1 activity in related species, and the involvement of heterochromatin with Ph1 (ref. 6) and cdc2 genes with meiosis, makes the structure a good candidate for the Ph1 locus.

Effective chromosome pairing requires chromatin remodeling at the onset of meiosis.

During meiosis, homologous chromosomes (homologues) recognize each other and then intimately associate. Studies exploiting species with large chromosomes reveal that chromatin is remodeled at the onset of meiosis before this intimate association. However, little is known about the effect the remodeling has on pairing. We show here in wheat that chromatin remodeling of homologues can only occur if they are identical or nearly identical. Moreover, a failure to undergo remodeling results in reduced pairing between the homologues. Thus, chromatin remodeling at the onset of meiosis enables the chromosomes to become competent to pair and recombine efficiently.

Genomic regions influencing gene expression of the HMW glutenins in wheat.

Bread wheat (Triticum aestivum L.) produces glutenin storage proteins in the endosperm. The HMW glutenins confer distinct viscoelastic properties to bread dough. The genetics of HMW glutenin proteins have been extensively studied, and information has accumulated about individual subunits, chromosomal locations and DNA sequences, but little is known about the regulators of the HMW glutenins. This investigation addressed the question of glutenin regulators. Expression of the glutenins was analyzed using QRT-PCR in ditelosomic (dt) Chinese Spring (CS) lines. Primers were designed for each of 4 CS glutenin genes and a control, non-storage protein endosperm-specific gene Agp-L (ADP-glucose pyrophosphorylase). Each line represents CS wheat, lacking one chromosome arm. The effect of a missing arm could feasibly cause an increase, decrease or no change in expression. For each HMW glutenin, results indicated there were, on average, 8 chromosome arms with an up-regulatory effect and only one instance of a down-regulatory effect. There were significant correlations between orthologous and paralogous HMW glutenins for effects of chromosome groups B and D. Some or all the glutenin alleles shared regulatory loci on chromosome arms 2BS, 7BS, 4DS, 5DS and 6DS, and Agp-L shared regulatory loci with glutenins on arms 7AS, 7BS, 2DS, 3DS, 4DS and 5DS. These results suggest a few chromosome arms contain putative regulatory genes affecting the expression of conserved cis elements of 4 HMW glutenin and Agp-L genes in CS. Regulation by common genes implies the regulators have diverged little from the common wheat ancestor, and furthermore, some regulation may be shared by endosperm-specific-genes. Significant common regulators have practical implications.