Molecular Approaches to Crop Improvement

MOLECULAR MARKERS

The progress in plant breeding is easy and quick for qualitative characters controlled by major genes with easily identifiable effect on the phenotype. But there are a number of plant characters which are difficult to be observed and where final phenotype does not give a reliable composition of the underlying genetic constitution. An individual may be carrying genes for resistance to say an insect but shall be known after it is exposed to insect attack. Similarly a male sterile plant cannot be identified before flowering stage and grain quality may not be known until before maturity of the crop. The identificatio of desirable genotypes for the quantitative characters like yield, quality, earliness, adaptation etc. is still more complicated. In all such cases the breeder attempts to select the desirable plants on the basis of some other easily observable attribute as a marker for the genes associated with the character of interest. Plant breeders have the convention of using many morphological characters as 'proxy' for the genes affecting complex characters. But the available number of such markers is too limited to serve as an index of each and every segment of DNA of the entire genome which may be controlling especially the quantitative characters. The available mutants can be assembled through breeding into the stocks under investigation but it requires additional efforts only for the purpose of finding association of genes with markers. The polymorphism at the DNA level provides a unique type of markers to serve as an index of genetic worth of the entire genome. An exact location of genes especially the loci governing quantitative traits (QTL) is basic and of paramount importance for map based cloning of these genes. Such markers can also be used to determine the nature of genetic variation in wild and cultivated types. But the most important use of these markers lies in the indirect manipulation of desirable genes in the form of marker-assisted selection (MAS). The following three classes of markers are available for use in plant breeding:

1. Morphological markers

2. Protein markers

3. DNA markers