Wheat (Triticum spp.) is one of the founder crops that likely drove the Neolithic transition to sedentary agrarian societies in the Fertile Crescent more than 10,000 years ago. Identifying genetic modifications underlying wheat’s domestication requires knowledge about the genome of its allo-tetraploid progenitor, wild emmer (T. turgidum ssp. dicoccoides). We report a 10.1-gigabase assembly of the 14 chromosomes of wild tetraploid wheat, as well as analyses of gene content, genome architecture, and genetic diversity. With this fully assembled polyploid wheat genome, we identified the causal mutations in Brittle Rachis 1 (TtBtr1) genes controlling shattering, a key domestication trait. A study of genomic diversity among wild and domesticated accessions revealed genomic regions bearing the signature of selection under domestication. This reference assembly will serve as a resource for accelerating the genome-assisted improvement of modern wheat varieties.

No abstract

H igh temperature stress is an important yield limiting factor in both spring and winter wheat (Triticum aestivum L.). At the present rates of greenhouse gas emissions and population growth, it is expected that mean surface air temperatures will increase in the range of 1.8 to 5.8°C by the end of this century (Intergovernmental Panel on Climate Change, 2007). It is predicted that future climates will not only be associated with an increase in mean temperatures (Easterling et al., 1997) but also with an increase in the frequency of episodes of high temperatures . In addition, climate models foresee that there will be a relatively greater increase in nighttime temperatures as compared to daytime temperatures. Over the past century global daily minimum temperatures increased more than twice compared to increases in daily maximum temperatures (Easterling et al., 1997). Recent studies have shown that historical yields of rice (Oryza sativa L.; Peng et al., 2004) and wheat (Lobell et al., 2005) were strongly correlated with minimum (nighttime) temperatures, rather than daytime maximum temperatures. Decreasing rice yields in the Philippines were related to increasing nighttime temperatures (Peng et al., 2004), and increasing wheat yields in Mexico were related to decreasing nighttime temperatures (Lobell et al., 2005). ABSTRACTClimate models predict greater increases in nighttime temperature in the future. The impacts of high nighttime temperature on wheat (Triticum aestivum L.) are not well understood. Objectives of this research were to quantify the impact of high nighttime temperatures during reproductive development on phenology, physiological, vegetative, and yield traits of wheat. Two spring wheat cultivars (Pavon-76 and Seri-82) were grown at optimum temperatures (day/night, 24/14°C; 16/8 h light/dark photoperiod) from sowing to booting. Thereafter, plants were exposed to four different nighttime temperatures (14, 17, 20, 23°C) until maturity. The daytime temperature was 24°C across all treatments. There were signifi cant infl uences of high nighttime temperatures on physiological, growth, and yield traits, but no cultivar or cultivar by temperature interactions were observed. High nighttime temperatures (>14°C) decreased photosynthesis after 14 d of stress. Grain yields linearly decreased with increasing nighttime temperatures, leading to lower harvest indices at 20 and 23°C. High nighttime temperature (≥20°C) decreased spikelet fertility, grains per spike, and grain size. Compared to the control (14°C), grain fi lling duration was decreased by 3 and 7 d at night temperatures of 20 and 23°C, respectively. High nighttime temperature increased the expression of chloroplast protein synthesis elongation factor in both cultivars suggesting possible involvement of this protein in plant response to stress.

Wheat (Triticum aestivum L.) cultivars must possess suitable enduse quality for release and consumer acceptability. However, breeding for quality traits is often considered a secondary target relative to yield largely because of amount of seed needed and expense. Without testing and selection, many undesirable materials are advanced, expending additional resources. Here, we develop and validate whole-genome prediction models for end-use quality phenotypes in the CIMMYT bread wheat breeding program. Model accuracy was tested using forward prediction on breeding lines (n = 5520) tested in unbalanced yield trials from 2009 to 2015 at Ciudad Obregon, Sonora, Mexico. Quality parameters included test weight, 1000-kernel weight, hardness, grain and flour protein, flour yield, sodium dodecyl sulfate sedimentation, Mixograph and Alveograph performance, and loaf volume. In general, prediction accuracy substantially increased over time as more data was available to train the model. Reflecting practical implementation of genomic selection (GS) in the breeding program, forward prediction accuracies (r) for quality parameters were assessed in 2015 and ranged from 0.32 (grain hardness) to 0.62 (mixing time). Increased selection intensity was possible with GS since more entries can be genotyped than phenotyped and expected genetic gain was 1.4 to 2.7 times higher across all traits than phenotypic selection. Given the limitations in measuring many lines for quality, we conclude that GS is a powerful tool to facilitate early generation selection for end-use quality in wheat, leaving larger populations for selection on yield during advanced testing and leading to better gain for both quality and yield in bread wheat breeding programs.

No abstract