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Markers have been used over the years for the classification of plants. Markers are any trait of an organism that can be identified with confidence and relative easy, and can be followed in a mapping population on another hand markers be defined as heritable entities associated with the economically important trait under the control of polygenes. Morphological markers can be detected with naked eye (naked eye polymorphism) or as difference in physical or chemical properties of the macromolecules. In other words, there are two types of genetic markers viz. morphological markers or naked eye polymorphism and non-morphological markers or molecular markers.
Application of molecular markers, have now been increasingly adopted to address the problems in Citrus taxonomy. Compared to morphological data, molecular tools provide abundant information, highly efficient and are insensitive to environmental factors. Molecular markers has provided an ideal means for identifying genotypes, estimation of relatedness between different accessions and following inheritance of economically important characters. In Citrus, a wide variety of DNA based markers has been used in order to study their genetic variation as well as phylogenic and taxonomic relationship among different genera. RAPD markers provide a fast and easy approach for taxonomic classification and cultivar typing of Citrus fruits. SSR have proven to be the marker of choice in Citrus breeding research, because of their variability, ease of use, accessibility of detection and reproducibility. ISSR, SRAP, CAPSSNP, AFLP are also used to study the genetic diversity of Citrus throughout the world. In addition, cpDNA is especially useful in phylogenetic analyses due to its evolutionary conservatism, relative abundance in plant tissue, small size and pre dominant uniparental inheritance.
Key words: Citrus, Genomics, Molecular characterization, Microsatellites, Molecular markers, AFLP, ISSR, RAPD, SSR, Polymorphism, Genetic diversity