Constituting a core collection of Saccharum spontaneum L. and comparison of three stratified random sampling procedures

In this investigation, sampling procedures for constituting a core collection of accessions of Saccharum spontaneum germplasm collection maintained and evaluated at Sugarcane Breeding Institute, Coimbatore, India are examined. Data on a set of 21 qualitative and 10 quantitative descriptors were used to evaluate the diversity of individual characters. The overall diversity of the collection was also evaluated by pooling the Shannon Diversity Index (SDI) of the individual traits. In order to delineate a suitable core collection, stratified random sampling was attempted. For this purpose, the accessions were grouped using three different methods, namely, plant-type (habit) based grouping, multivariate cluster analysis and a grouping algorithm on the basis of an information measure. The relative efficiency of these grouping procedures was evaluated by partitioning the total diversity into between and within group diversity. The quantum of diversity (i.e. mean pooled diversity index or pooled SDI) and its sampling variance for core subsets of various sampling fractions drawn from the collection using stratified random sampling was evaluated through simulation. In general, the estimated diversity of core subsets among various methods of sampling did not differ much from the overall diversity of the base collection. Nevertheless, the sample standard deviation of the pooled SDI differed among the sample sizes and methods of allocation. Hence the optimal sample size was determined based on least sample standard deviation of the pooled SDI. In general, core subsets selected through stratified random sampling had lesser sample standard deviation of the pooled diversity index as compared to simple random sampling. Among the methods of stratification, the one based on the information measure was found to be better than plant-type based grouping or multivariate cluster analysis. A core size of 10% (about 60 accessions) drawn through stratified random sampling from the diversity groups constituted using the information measure was found to be optimum.