In spite of its shortcoming, analysis of PCR-derived rDNA libraries is being employed increasingly to investigate microbial diversity within many ecosystems. In the present investigation, the effects of the number of PCR cycles (10 vs 25 cycles) on the inferred structure of a 16S rDNA library have been examined. Seventy-five 25-cycle sequences were retrieved and analysed in comparison with 284 10-cycle sequences already described in a previous study. The 359 clones obtained were classified into 94 molecular species (at least 98% sequence similarity). At the level of large phylogenetic groups, the two cloned rDNA libraries were not different. A mathematical model was developed in order to estimate the number of molecular species expected if further sequencing was performed. Coverage-based computing, projections and statistical analysis demonstrated that the structures of the two PCR-derived rDNA libraries were different and that the 25-cycle rDNA library displayed reduced diversity. It is suggested that the number of PCR cycles used for amplification of 16S rDNA genes for phylogenetic diversity studies must therefore be kept as small as possible.


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