High-molecular-weight glutenin subunits (HMW-GS) are important determinants of wheat dough quality as they confer visco-elastic properties to the dough required for mixing and baking performance. With this important role, the HMW-GS alleles are key markers in breeding programs. In this work, we present the use of a PCR marker initially designed to discriminate Glu1 Bx7 and Glu1 Bx17 HMW-GS. It was discovered that this marker also differentiated two alleles, originally both scored as Glu1 Bx7, present in the wheat lines CD87 and Katepwa respectively, by a size polymorphism of 18 bp. The marker was scored across a segregating doubled-haploid (DH) population (CD87 x Katepwa) containing 156 individual lines and grown at two sites. Within this population, the marker differentiated lines showing the over-expression of the Glu1 Bx7 subunit (indicated by the larger PCR fragment), derived from the CD87 parent, relative to lines showing the normal expression of the Glu1 Bx7 subunit, derived from the Katepwa parent. DNA sequence analysis showed that the observed size polymorphism was due to an 18 bp insertion/deletion event at the C-terminal end of the central repetitive domain of the Glu1 Bx 7 coding sequence, which resulted in an extra copy of the hexapeptide sequence QPGQGQ in the deduced amino-acid sequence of Bx7 from CD87. When the DH population was analysed using this novel Bx7 PCR marker, SDS PAGE and RP HPLC, there was perfect correlation between the Bx7 PCR marker results and the expression level of Bx7. This differentiation of the population was confirmed by both SDS-PAGE and RP-HPLC. The functional significance of this marker was assessed by measuring key dough properties of the 156 DH lines. A strong association was shown between lines with an over expression of Bx7 and high dough strength. Furthermore, the data demonstrated that there was an additional impact of Glu-D1 alleles on dough properties, with lines containing both over-expressed Bx7 and Glu-D1 5+10 having the highest levels of dough strength. However, there was no statistically significant epistatic interaction between Glu-B1 and Glu-D1 loci.
Genetic maps were compiled from the analysis of 160–180 doubled haploid
lines derived from 3 crosses: Cranbrook Halberd, CD87 Katepwa, and Sunco
Tasman. The parental wheat lines covered a wide range of the germplasm used in
Australian wheat breeding. The linkage maps were constructed with RFLP, AFLP,
microsatellite markers, known genes, and proteins. The numbers of markers
placed on each map were 902 for Cranbrook Halberd, 505 for CD87 Katepwa, and
355 for Sunco Tasman. Most of the expected linkage groups could be determined,
but 10–20% of markers could not be assigned to a specific linkage
group. Homologous chromosomes could be aligned between the populations
described here and linkage groups reported in the literature, based around the
RFLP, protein, and microsatellite markers. For most chromosomes, colinearity
of markers was found for the maps reported here and those recorded on
published physical maps of wheat. AFLP markers proved to be effective in
filling gaps in the maps. In addition, it was found that many AFLP markers
defined specific genetic loci in wheat across all 3 populations.
The quality of the maps and the density of markers differs for each
population. Some chromosomes, particularly D genome chromosomes, are poorly
covered. There was also evidence of segregation distortion in some regions,
and the distribution of recombination events was uneven, with substantial
numbers of doubled haploid lines in each population displaying one or more
parental chromosomes. These features will affect the reliability of the maps
in localising loci controlling some traits, particularly complex quantitative
traits and traits of low heritability.
The parents used to develop the mapping populations were selected based on
their quality characteristics and the maps provide a basis for the analysis of
the genetic control of components of processing quality. However, the parents
also differ in resistance to several important diseases, in a range of
physiological traits, and in tolerance to some abiotic stresses.
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