Esterase isozyme fingerprinting of the cassava germplasm collection held at CIAT

Abstract

Esterase isozymes were chosen for fingerprinting CIAT`s cassava germplasm colleciton, because of (1) their technical repeatability; (2) high polymorphism (i.e., many alleles per locus); and (3) high numner of bands detectable in cassava clones (i.e., at least a few loci are expressed). A total of 4304 accessinos (about 86 of the total collection) has been analysed, and a total of 22 different bands were found. All bands have been coded and the existence of each band recorded in a computerized system. A total of 2146 different banding patterns were found among the 4304 accessions. A large numner (1407) of patterns were represented by only one clone, while the rest had 2 to 39 clones with the same banding patterns. The former group, 1407 clones, represents unique genotypes, because different banding patterns imply genetic differences. Those clones in the latter group may represent the possibility of their being duplicates. The presence of duplicates in a germplasm collection has serious implications for germplasm conservation, as well as for use in breeding programs. A systematic procedure for duplicate identification in the collection, through the combined use of morphological and esterase isozyme descriptor, has been developed and a progressive elimination of duplicates will be carried out. A tentative list of a cassava core collection, consisting of 630 accessions, has been developed. The SAS "FASTCLUS" procedure was used to select 51 accessions, representing the range of variation of isozyme patterns, for inclusion in the core collection. Thus, data of fingerprinting has been successfully used for identification of possible duplicates and development of a core collection. Data on esterase isozyme were also used to stydy the distribution of genetic diversity in a subsample of the collection.