A 7-year cytogenetic study on the leaf litter frog Craugastor fitzingeri from Costa Rica and Panama revealed the existence of highly mobile nucleolus organizing regions (NORs) in their genomes. Silver (Ag)-staining of the active NORs demonstrated an exceptional interindividual pattern of NOR distribution at the telomeres of the chromosomes. All individuals examined showed a different and specific NOR location in their karyotypes. Furthermore, intraindividual variation in the NOR sites was found. This observation suggested the existence of mobile NORs in C. fitzingeri. Confirmation of this phenomenon was possible by systematic FISH analysis using an 18S + 28S rDNA probe. The extremely variable number and position of the NORs in C. fitzingeri is best explained by highly mobile NORs that move freely between the telomeres of the chromosomes. These transpositions must occur preferentially in premeiotic, meiotic, or postmeiotic stages, but also at a lower incidence in the somatic tissues of the animals. It is hypothesized that transposable (mobile) elements are closely linked to the NORs or are inserted into the major 18S + 28S rDNA spacers of C. fitzingeri. When such transposable elements spread by transpositions, they can carry with them complete or partial NORs. The present study provides detailed information on various differential chromosome banding techniques, in situ hybridization experiments, chromosomal hypermethylation patterns, determination of the genome size, and analyses of restriction fragment length polymorphisms of the DNA.
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