Genetic dissection of grain yield in bread wheat. II. QTL-by-environment interaction.

The grain yield of wheat is influenced by genotype, environment and genotype-by-environment interaction. A mapping population consisting of 182 doubled haploid progeny derived from a cross between the southern Australian varieties 'Trident' and 'Molineux', was used to characterise the interaction of previously mapped grain yield quantitative trait locus (QTL) with specific environmental covariables. Environments (17) used for grain yield assessment were characterised for latitude, rainfall, various temperature-based variables and stripe rust infection severity. The number of days in the growing season in which the maximum temperature exceeded 30 degrees C was identified as the variable with the largest effect on site mean grain yield. However, the greatest QTL-by-environmental covariable interactions were observed with the severity of stripe rust infection. The rust resistance allele at the Lr37/Sr38/Yr17 locus had the greatest positive effect on grain yield when an environment experienced a combination of high-stripe rust infection and cool days. The grain yield QTL, QGyld.agt-4D, showed a very similar QTL-by-environment covariable interaction pattern to the Lr37/Sr38/Yr17 locus, suggesting a possible role in rust resistance or tolerance. Another putative grain yield per se QTL, QGyld.agt-1B, displayed interactions with the quantity of winter and spring rainfall, the number of days in which the maximum temperature exceeded 30 degrees C, and the number of days with a minimum temperature below 10 degrees C. However, no cross-over interaction effect was observed for this locus, and the 'Molineux' allele remained associated with higher grain yield in response to all environmental covariables. The results presented here confirm that QGyld.agt-1B may be a prime candidate for marker-assisted selection for improved grain yield and wide adaptation in wheat. The benefit of analysing the interaction of QTL and environmental covariables, such as employed here, is discussed.

Genetic dissection of grain yield in bread wheat. I. QTL analysis.

Grain yield forms one of the key economic drivers behind a successful wheat (Triticum aestivum L.) cropping enterprise and is consequently a major target for wheat breeding programmes. However, due to its complex nature, little is known regarding the genetic control of grain yield. A doubled-haploid population, comprising 182 individuals, produced from a cross between two cultivars 'Trident' and 'Molineux', was used to construct a linkage map based largely on microsatellite molecular makers. 'Trident' represents a lineage of wheat varieties from southern Australia that has achieved consistently high relative grain yield across a range of environments. In comparison, 'Molineux' would be rated as a variety with low to moderate grain yield. The doubled-haploid population was grown from 2002 to 2005 in replicated field experiments at a range of environments across the southern Australian wheat belt. In total, grain yield data were recorded for the population at 18 site-year combinations. Grain yield components were also measured at three of these environments. Many loci previously found to be involved in the control of plant height, rust resistance and ear-emergence were found to influence grain yield and grain yield components in this population. An additional nine QTL, apparently unrelated to these traits, were also associated with grain yield. A QTL associated with grain yield on chromosome 1B, with no significant relationship with plant height, ear-emergence or rust resistance, was detected (LOD > or =2) at eight of the 18 environments. The mean yield, across 18 environments, of individuals carrying the 'Molineux' allele at the 1B locus was 4.8% higher than the mean grain yield of those lines carrying the 'Trident' allele at this locus. Another QTL identified on chromosome 4D was also associated with overall gain yield at six of the 18 environments. Of the nine grain yield QTL not shown to be associated with plant height, phenology or rust resistance, two were located near QTL associated with grain yield components. A third QTL, associated with grain yield components at each of the environments used for testing, was located on chromosome 7D. However, this QTL was not associated with grain yield at any of the environments. The implications of these findings on marker-assisted selection for grain yield are discussed.

Identification of genetic loci associated with ear-emergence in bread wheat.

A doubled haploid population constructed from a cross between the South Australian wheat cultivars 'Trident' and 'Molineux' was grown under winter field conditions, under field conditions over summer and under artificial light both with and without vernalisation. The duration from planting to ear-emergence was recorded and QTL associated with heading date were detected using a previously constructed genetic linkage map. Associations were shown with chromosomal regions syntenous to previously identified photoperiod (Ppd-B1) and vernalisation (Vrn-A1) sensitive loci. Additional QTL associated with time to heading were also identified on chromosomes 1A, 2A, 2B, 6D, 7A and 7B. Comparisons between the genetic associations observed under the different growing conditions allowed the majority of these loci to be classified as having either photoperiod-sensitive, vernalisation-sensitive or earliness per se actions. The identification of a photoperiod-sensitive QTL on chromosome 1A provides evidence for a wheat gene possibly homoeologous to Ppd-H2 previously identified on chromosome 1H of barley. The occurrence of a putative major gene for photoperiod sensitivity observed on chromosome 7A is presented. The combined additive effects at these loci accounted for more than half the phenotypic variance in the duration from planting to ear-emergence in this population. The possible role of these loci on the adaptation of wheat in Australia is discussed.

Mapping of a novel QTL for resistance to cereal cyst nematode in wheat.

Cereal cyst nematode (CCN; Heterodera avenae Woll.) is a root pathogen of cereals that can cause severe yield losses in intolerant wheat cultivars. Loci for resistance to CCN, measured by a seedling bioassay, were identified by creating a genetic map based on a Trident/Molineux doubled haploid population of 182 lines. A novel locus accounting for up to 14% of the resistance to CCN was mapped to chromosome 1B of Molineux by association with microsatellite marker loci Xwmc719 and Xgwm140. This locus acts additively with the previously identified CCN resistance loci identified on chromosomes 6B (Cre8) and 2A (Cre5 on the VPM1 segment) in this population to explain 44% of the genetic variance for this major wheat pathogen.

The genetic control of milling yield, dough rheology and baking quality of wheat.

Improving the end-use quality of wheat is a key target for many breeding programmes. With the exception of the relationship between glutenin alleles and some dough rheological characters, knowledge concerning the genetic control of wheat quality traits is somewhat limited. A doubled haploid population produced from a cross between two Australian cultivars 'Trident' and 'Molineux' has been used to construct a linkage map based largely on microsatellite molecular makers. 'Molineux' is superior to 'Trident' for a number of milling, dough rheology and baking quality characteristics, although by international standards 'Trident' would still be regarded as possessing moderately good end-use quality. This population was therefore deemed useful for investigation of wheat end-use quality. A number of significant QTL identified for dough rheological traits mapped to HMW and LMW glutenin loci on chromosomes 1A and 1B. However, QTL associated with dough strength and loaf volume were also identified on chromosome 2A and a significant QTL associated with loaf volume and crumb quality was identified on chromosome 3A. A QTL for flour protein content and milling yield was identified on chromosome 6A and a QTL associated with flour colour reported previously on chromosome 7B was confirmed in this population. The detection of loci affecting dough strength, loaf volume and flour protein content may provide fresh opportunities for the application of marker-assisted selection to improve bread-making quality.

Sequence tagged microsatellites for the Xgwm533 locus provide new diagnostic markers to select for the presence of stem rust resistance gene Sr2 in bread wheat ( Triticum aestivum L.).

The stem rust resistance gene Sr2 has provided durable broad-spectrum, adult-plant resistance to the fungal pathogen Puccinia graminis Pers. f. sp. tritici throughout wheat-growing regions of the world for more than 50 years. The ability to select for Sr2 in wheat breeding programs was recently improved by the identification of a tightly linked microsatellite marker gwm533. This marker typically amplifies a 120-bp polymerase chain reaction fragment from wheat lines carrying Sr2. In instances where the 120-bp fragment is not associated with the presence of Sr2, DNA sequence analysis has shown that a second allele was amplified, differing in the structure of the microsatellite repeat. To discriminate this allelic homoplasy (alleles identical in size, but not identical by descent), sequence-tagged microsatellites (STM) markers were developed for the Xgwm533 locus. These markers were shown to be diagnostic for the presence of Sr2 in a wide range of germplasm, representative of all major wheat varieties historically grown in Australia. The STMs will be particularly useful for marker-assisted selection in Southern Australian breeding programs, where the use of the marker gwm533 is often precluded by the presence of the non- Sr2-associated 120-bp allele in the pedigree of current breeding germplasm. The STMs also revealed a high incidence of previously undetected allelic homoplasy at the Xgwm533 locus and may have broader utility in genetic research and breeding, as this locus is also reported to be strongly associated with a major gene conferring resistance to Fusarium head blight.

Differences in delta9 desaturase activity between Jersey- and Limousin-sired cattle.

An experiment examined delta9 desaturase activity and FA composition in subcutaneous adipose tissue in two differing breeds of cattle. Jersey-sired cattle had significantly higher rates of desaturase activity than Limousin-sired cattle (1.55 vs. 0.75 nmol/mg protein/min). This difference was also demonstrated by a lower concentration of individual (e.g., 18:0) and total saturated FA (38.3 vs. 45.1 wt%), and a higher concentration of individual (e.g., 16:1) and total monounsaturated FA (58.2 vs. 52.7 wt%) in the Jersey animals. Other indices of desaturation calculated from the FA composition showed this same difference. The slip point of adipose tissue of Jersey cattle (36.8 degrees C) was significantly lower than that of Limousin cattle (39.2 degrees C), but Jersey adipose tissue had a greater content of beta-carotene. The positive relationship between adipose tissue beta-carotene and desaturation opposes the negative relationship between dietary beta-carotene and desaturation determined elsewhere. These results, however, lead to the hypothesis that some cattle have a reduced capacity to metabolize beta-carotene to various forms of vitamin A, a compound that can reduce delta9 desaturase enzyme activity. In addition, the higher level of intramuscular fat in Jersey cattle (6.97 vs. 3.82%) is possibly related to a lack of inhibition of the adipocyte differentiation genes by vitamin A.