Marker assisted selection: a strategy for wheat improvement Bhakti Rana1, Preeti Rana 2, Manoj K. Yadav1, Sundeep Kumar1

Introduction
Wheat is most widely grown crop in the world, best adapted to temperate region and is a staple food of about 35% of the world population. Wheat is a major source of energy, protein and dietary fiber in human nutrition since decades. Wheat is a major crop contributing importantly to the nutrient supply of the global population. Since the beginning of agriculture, ten thousand years ago, the importance of varietals improvement is well known. In the ancient time, selection and introduction were commonly used method, since knowledge about use of hybridization, mutation and polyploidy were not in practice. Selections were made on the basis of physical appearance and to fulfill the requirements. Till 15th century, most of the varieties were developed either through primary selection or introduction. Breeders had the advantages of variability until 1970s and due to intensive crossing programmes; green revolution took place during 1967-1968. The development and promotion of modern, high yielding varieties was the most important factor contributing to the enormous success of green revolution. During two generations leading up to the turn of the century the global population grew by 90 percent whilst food production expanded by 115 percent. The global food security is quite fragile, particularly when looking towards the middle of the century because of projected needs for human, animal and industrial uses. Global wheat production is expected to increase from nearly 600 million tons of present production level to around 760 million tons in 2020 with limited expansion of sown area. But now, due to non-existence of variability for the yield trait, it is not possible to develop a new variety either by selection or introduction. Selection based on knowing the location of the genes of interest gives the breeder a significant advantage, particularly for quantitative traits, where classical selection is done on the phenotype as a whole rather than on the underlying genetic determinants. Identification of significant QTL marker associations forms the baseline for MAS of quantitative traits. Furthermore, other breeding methods like hybridization, which reshuffle existing variability in the population and tools like polyploidy and mutation bring challenges for allele in the available traits and are not possible to introduce a new trait from unrelated species. Molecular markers have been introduced over last two decades, which has revolutionized the entire scenario of biological sciences. DNA based molecular markers have acted as versatile tools and have found there position in various fields like taxonomy, physiology, embryology, genetic engineering etc. PCR brought about a new class of DNA profiling markers that facilitated the development of marker based gene tags, map based cloning of agronomically important genes, variability studies, phylogenetic analysis, synteny mapping, marker assisted selection of various genotypes. Molecular markers are identifiable DNA sequences found at specific locations on the chromosomes and transmitted by the standard laws of inheritance from one generation to next and considered as landmarks in the chromosome maps that can be useful to monitor the transfer of specific chromosome segments known to carry useful agronomic traits. Molecular markers have also provided an excellent opportunity to develop saturated genetic maps and to integrate genetic, cytological and molecular maps. Molecular markers are being used to tag specific chromosome segments bearing the desired gene(s) to be transferred into the breeding lines.
Traditionally, breeders have relied on visible traits to select improved varieties however; MAS rely on identifying marker DNA sequences that are inherited alongside a desired trait during the first few generations. Thereafter, plants that carry the traits can be picked out quickly by looking for the marker sequences, allowing multiple rounds of breeding to be run in quick succession (Kumar et al. 2007). Molecular markers make selection possible for breeders to combine desirable alleles at a greater number of loci and at earlier generations than is possible with conventional breeding methodologies. Molecular markers can circumvent more cumbersome, established pedigree breeding strategies and even generate plant genotypes unachievable by conventional methods (Young 1999). Molecular markers are required in a broad spectrum of gene screening approaches, ranging from gene-mapping with traditional ‘forward-genetics’ approaches through QTL identification studies to genotyping and haplotyping studies. Molecular markers are also considered as useful tools for pyramiding of different resistance genes and developing multi-line cultivars targeting for durable resistance to the disease (Xia et al. 2005).
Conventionally, plant breeding depends upon morphological/phenotypic markers for the identification of agronomic traits. With the development of methodologies for the analysis of plant gene structure and function, molecular markers have been utilized for identification of traits to locate the gene(s) for a trait of interest on a plant chromosome and are widely used to study the organization of plant genomes and for the construction of genetic linkage maps. Molecular markers are independent from environmental variables and can be scored at any stage in the life cycle of a plant. Over the last several years, there has thus been marked increase in the application of molecular markers in the breeding programmes of various crop plants. Molecular markers not only facilitate the development of new varieties by reducing the time required for the detection of specific traits in progeny plants, but also fasten the identification of desired genes and their corresponding molecular markers, thus accelerating efficient breeding of resistance traits into wheat cultivars by marker assisted selection (MAS).