The 'rare allele phenomenon' in a ribosomal spacer

We describe the increased frequency of a particular length variant of the i nternal transcribed spacer 1 (ITS-1) of the ribosomal DNA in a hybrid zone of the land snail Albinaria hippolyti. The phenomenon that normally rare al leles or other markers can increase in frequency in the centre of hybrid zo nes is not new. Under the term 'hybrizyme' or 'rare allele' phenomenon it h as been recorded in many organisms and different genetic markers. However, this is the first time that it has been found in a multicopy locus. On the one hand, the pattern fits well with the view that purifying selection in h ybrid populations works on many loci across the genome and should thus have its effect on many independent molecular markers. On the other hand, the r esults are puzzling, given that the multiple copies of rDNA are not expecte d to respond in unison. We suggest two possible explanations for these conf licting observations.