Comparison of the current of ex situ conservation programmes of crop plants in developing countries, the United States, and the IARCs (After Cohen et. Al. 1991; Science 253; p. 868)

Often conditioned by national policies and by number of accessions with adequate material for distribution (see Table 52.1 and 52.2). t Limitations imposed by lack of resources, coordinating mechanisms, and availability of appropriate sites. t No one genetic resources programme has local access to a complete range of sites for regeneration. §Limited by availability of personnel and conflicting responsibilities. II Except in very few cases.

Gene banks and core collections. Since it has been considered necessary to study and conserve all available useful genetic diversity of crop plants, the accessions in gennplasm collections at gene banks are ever increasing. Thus, the national gene banks and those at IARCs have entered an era of increased activity and responsibility, particularly for their own indigenous   crop gennplasm. Under the Convention of Biological Diversity (CBD) also, individual nations have been assigned the responsibility to conserve and use their own biodiversity. These responsibilities have become difficult to be discharged with limited resources, particularly due to unrestrained growth of accessions in different collections. Consequently, the issue of handling, maintenance and sustainable use of crop genetic resources has been widely discussed in the recent past. It is often argued that we should devote our energy, effort and resources to maximise the useful genetic diversity in a limited gennplasm collection and ensure its use in plant breeding, while still retaining in reserve those accessions which are apparently not of so much of a direct immediate use. In view of this, Frankel (1984) proposed the" concept of 'core collections' as a solution. A core collection would represent the genetic diversity of a crop species and its relatives with minimum repetitiveness and still representing the entire range of biodiversity that is available. The remaining accessions will not be discarded but would be managed as a 'resource collection'.

While accepting and implementing the concept of 'core collections', a study of the nature and structure of total genetic diversity becomes important. The methods used for a study of genetic diversity may involve clustering based on similarity, and branching based on dissimilarity. While 'clustering methods' are based on phenetic analysis of data on accessions, the 'branching methods' are based on passport data (cladistic analysis) combined with knowledge about natural selection, domestication, distribution and utilization influencing the structure of the gene pool (actual accessions may not be needed). Genetic markers used in these diversity studies include morphological markers, storage proteins, allozymes and a number of molecular markers (RFLPs, RAPDs, SSRs, AFLPs, etc.). With the availability of molecular markers, it has also become necessary and possible to locate new useful genes  n a representative, yet severely limited sample, Le. the 'core collection'. Thus the use of molecular markers has given an added impetus to the concept of 'core collections', which may generally represent 10% of the total collections and may still effectively retain 70% of the alleles available in the whole collection.