Crop yield reduction as a consequence of increasingly severe climatic events threatens global food security. Genetic loci that ensure productivity in challenging environments exist within the germplasm of crops, their wild relatives and species that are adapted to extreme environments. Selective breeding for the combination of beneficial loci in germplasm has improved yields in diverse environments throughout the history of agriculture. An effective new paradigm is the targeted identification of specific genetic determinants of stress adaptation that have evolved in nature and their precise introgression into elite varieties. These loci are often associated with distinct regulation or function, duplication and/or neofunctionalization of genes that maintain plant homeostasis.

Genetic mechanisms of abiotic stress tolerance that translate to crop yield stability.

The central goal of human genetics is to understand the inherited basis of human variation in phenotypes, elucidating human physiology, evolution and disease. Rare mutations have been found underlying two thousand mendelian diseases; more recently, it has become possible to assess systematically the contribution of common SNPs to complex disease. The known role of copy-number alterations in sporadic genomic disorders, combined with emerging information about inherited copy-number variation, indicate the importance of systematically assessing copy-number variants (CNVs), including common copy-number polymorphisms (CNPs), in disease. Here we discuss evidence that CNVs affect phenotypes, directions for basic knowledge to support clinical study of CNVs, the challenge of genotyping CNPs in clinical cohorts, the use of SNPs as markers for CNPs and statistical challenges in testing CNVs for association with disease. Critical needs are high-resolution maps of common CNPs and techniques that accurately determine the allelic state of affected individuals.

Copy number variation in association studies of human disease

The timing of flowering during the year is an important adaptive character affecting reproductive success in plants and is critical to crop yield. Flowering time has been extensively manipulated in crops such as wheat (Triticum aestivum L.) during domestication, and this enables them to grow productively in a wide range of environments. Several major genes controlling flowering time have been identified in wheat with mutant alleles having sequence changes such as insertions, deletions or point mutations. We investigated genetic variants in commercial varieties of wheat that regulate flowering by altering photoperiod response (Ppd-B1 alleles) or vernalization requirement (Vrn-A1 alleles) and for which no candidate mutation was found within the gene sequence. Genetic and genomic approaches showed that in both cases alleles conferring altered flowering time had an increased copy number of the gene and altered gene expression. Alleles with an increased copy number of Ppd-B1 confer an early flowering day neutral phenotype and have arisen independently at least twice. Plants with an increased copy number of Vrn-A1 have an increased requirement for vernalization so that longer periods of cold are required to potentiate flowering. The results suggest that copy number variation (CNV) plays a significant role in wheat adaptation.

http://digital.csic.es/bitstream/10261/59223/1/PlosOne_7_e33234.pdf

Copy number variation affecting the Photoperiod-B1 and Vernalization-A1 genes is associated with altered flowering time in wheat (Triticum aestivum)

Crop Breeding Chips and Genotyping Platforms: Progress, Challenges, and Perspectives

Human selection during crop domestication has resulted in remarkable transformations of plant phenotypes, providing a window into the genetic basis of morphological evolution. Recent progress in our understanding of the genetic architecture of novel plant traits has emerged from combining advanced molecular technologies with improved experimental designs, including nested association mapping, genome-wide association studies, population genetic screens for signatures of selection, and candidate gene approaches. These studies reveal a diversity of underlying causative mutations affecting phenotypes important in plant domestication and crop improvement, including coding sequence substitutions, presence/absence and copy number variation, transposon activation leading to novel gene structures and expression patterns, diversification following gene duplication, and polyploidy leading to altered combinatorial capabilities. The genomic regions unknowingly targeted by human selection include both structural and regulatory genes, often with results that propagate through the transcriptome as well as to other levels in the biosynthetic and morphogenetic networks.

A Bountiful Harvest: Genomic Insights into Crop Domestication Phenotypes

Frost Resistance-1 (FR-1) and FR-2 are two loci affecting freezing tolerance and winter hardiness of the temperate-climate cereals. FR-1 is hypothesized to be due to the pleiotropic effects of VRN-1. FR-2 spans a cluster of C-Repeat Binding Factor (CBF) genes. These loci are genetically and functionally linked. Recent studies indicate CBF transcripts are downregulated by the VRN-1 encoded MADS-box protein or a factor in the VRN-1 pathway. Here, we report that barley genotypes ‘Dicktoo’ and ‘Nure’ carrying a vrn-H1 winter allele at VRN-H1 harbor increased copy numbers of CBF coding sequences relative to Vrn-H1 spring allele genotypes ‘Morex’ and ‘Tremois’. Sequencing bacteriophage lambda genomic clones from these four genotypes alongside DNA blot hybridizations indicate approximately half of the eleven CBF orthologs at FR-H2 are duplicated in individual genomes. One of these duplications discriminates vrn-H1 genotypes from Vrn-H1 genotypes. The vrn-H1 winter allele genotypes harbor tandem segmental duplications through the CBF2A–CBF4B genomic region and maintain two distinct CBF2 paralogs, while the Vrn-H1 spring allele genotypes harbor single copies of CBF2 and CBF4. An additional CBF gene, CBF13, is a pseudogene interrupted by multiple non-sense codons in ‘Tremois’ whereas CBF13 is a complete uninterrupted coding sequence in ‘Dicktoo’ and ‘Nure’. DNA blot hybridization with wheat DNAs reveals greater copy numbers of CBF14 also occurs in winter wheats than in spring wheats. These data indicate that variation in CBF gene copy numbers is widespread in the Triticeae and suggest selection for winter hardiness co-selects winter alleles at both VRN-1 and FR-2.

Andrea K. Knox

Papers

CBF gene copy number variation at Frost Resistance-2 is associated with levels of freezing tolerance in temperate-climate cereals