44 research outputs found
A haplotype map of allohexaploid wheat reveals distinct patterns of selection on homoeologous genomes
BACKGROUND: Bread wheat is an allopolyploid species with a large, highly repetitive genome. To investigate the impact of selection on variants distributed among homoeologous wheat genomes and to build a foundation for understanding genotype-phenotype relationships, we performed population-scale re-sequencing of a diverse panel of wheat lines. RESULTS: A sample of 62 diverse lines was re-sequenced using the whole exome capture and genotyping-by-sequencing approaches. We describe the allele frequency, functional significance, and chromosomal distribution of 1.57 million single nucleotide polymorphisms and 161,719 small indels. Our results suggest that duplicated homoeologous genes are under purifying selection. We find contrasting patterns of variation and inter-variant associations among wheat genomes; this, in addition to demographic factors, could be explained by differences in the effect of directional selection on duplicated homoeologs. Only a small fraction of the homoeologous regions harboring selected variants overlapped among the wheat genomes in any given wheat line. These selected regions are enriched for loci associated with agronomic traits detected in genome-wide association studies. CONCLUSIONS: Evidence suggests that directional selection in allopolyploids rarely acted on multiple parallel advantageous mutations across homoeologous regions, likely indicating that a fitness benefit could be obtained by a mutation at any one of the homoeologs. Additional advantageous variants in other homoelogs probably either contributed little benefit, or were unavailable in populations subjected to directional selection. We hypothesize that allopolyploidy may have increased the likelihood of beneficial allele recovery by broadening the set of possible selection targets
Effective curricula for at-risk students in vocational education:a study of teachersâ practice
Abstract Background This study focused upon a curriculum for at-risk students in vocational education aimed at enhancing studentsâ motivation and engagement for learning. The study explored teachersâ view on effective curricular characteristics and teachersâ strategies to create positive learning experiences for students. Methods Qualitative research has been conducted regarding four curricula for at-risk students; data were collected and examined by means of 16 focus group interviews with teachers and analysis of relevant documents. Results Teachers pointed out the central position of the student, resulting in individual trajectories. Teachers used peer group dynamics, job orientation and sports as tool for student development. Teachers regarded themselves as coaches, as experts in sport activities and as group managers. For positive learning experiences two key curricular characteristics have been identified: (1) equality in the relationship between student and teacher, (2) a positive fit between the curriculum and the students
Crystal plasticity prediction of Lankford coefficient using the MULTISITE model: influence of the critical resolved shear stresses
The MULTISITE model [1] is based on polycrystalline plasticity and the underlying hypotheses of the model are (i) that the deformation of each grain is significantly influenced by the interaction with a limited number of adjacent grains, and (ii) that local strains deviate from their macroscopic average according to specific ârelaxation modesâ. The LAMEL model [2] is reformulated into the more general elastic-viscoplastic MULTISITE model permitting various relaxation modes. This model has been validated for cubic materials but hexagonal close-packed (HCP) crystals usually demonstrate larger anisotropy than cubic crystals. The model was used to simulate uniaxial tensile tests performed on rolled sheets made of Ti-6Al-4V. The Lankford coefficients (r) calculated in various directions in the plane of the sheet were analysed. In this study, different grain interaction hypotheses were tested. Besides, it appeared that the value of the critical resolved shear stresses (CRSS) of the different slip system families of the HCP metal had significant effects on the results. Their influence as well as the influence of the strain rate sensitivity parameter was examined
âEveryone has their own qualitiesâ : tracking and academic self-appraisal in Flemish secondary education
Track position has an impact on studentsâ perceptions of educational
success. These perceptions matter as they relate to educational and professional
aspirations and choices. In this ethnographic study, based on ethnographic observations
and semi-structured interviews in three secondary schools in Flanders
(northern part of Belgium), we want to increase our understanding of the combination
of within- and between track comparisons for studentsâ perceptions of their
educational success. The hierarchical nature of the Flemish educational system is
reflected in studentsâ comparison groups to evaluate their school success. In academic
tracks, most students considered their educational career automatically as
successful, regardless of their actual grades. Conversely, students in vocational
tracks developed more contradictory feelings with respect to their personal school
success and felt the need to distinguish themselves from their classmates. Thus,
regardless the existence of different school cultures across tracks, students searched
for strategies to define themselves as successful in school