PCR amplification of intergenic spacers in the ribosomal DNA of Drosophila melanogaster reveals high levels of turnover in length and copy-number of spacers in geographically separated populations

A recently described PCR-based method for the analysis of intergenic spacer (IGS) length variation in the ribosomal (r) DNA of Drosophila melanogaster was used to analyse the distribution of IGS length variants in the rDNA of a number of recently collected D. melanogaster populations. One group of populations, from Europe and North Africa, was shown to have low intrapopulation IGS length variation following maintenance of massed populations in the laboratory for an extended period. However, a greater degree of IGS profile variability was detected at a number of levels in the majority of laboratory-maintained isofemale lines from two of these populations plus a second group of populations which were collected more recently from the eastern coast of Australia; all of which were immediately divided into isofemale lines following collection. Interestingly, PCR analysis of pooled DNA extracts from 30 individuals of either sex showed almost identical PCR profiles from each of the Australian populations. These preliminary results are discussed with regard to the possible combinations of forces (natural selection, neutral drift and genomic molecular drive) on the patterns of IGS length variation.