Fish mapping of ribosomal DNA and telomeric repeats in modelspecies of Platyhelmithes and Acanthocephala

Molecular cytogenetics is little exploited in metazoan parasites despite considerable relevance of the karyotype structure and chromosomal location of specific DNA sequences to phylogenetic and systematic evaluations. We applied fluorescence in situ hybridization (FISH) to determine the type of telomeric DNA and localize rDNA genes in chromosomes of several acantocephalan, monogenean and cestode species. In Paradiplozoon homoion-complex (Monogenea, 2n = 12), the chromosome pair no. 5 carried a nucleolar organizer region (NOR) composed of a cluster of rDNA genes as confirmed by both the classical banding approaches and FISH with 18S rDNA probe. In two acanthocephalan sibling species Pomphorhynchus laevis and P. tereticollis (Acanthocephala, both 2n = 6 + XX in female and X0 in male), two NORs were found, located each in different sites of the first two autosomal pairs. Furthermore, we hybridized the vertebrate telomeric probe, (TTAGGG)n, with chromosomes of above-mentioned helminths and cestodes Caryophyllaeus laticeps(2n = 20), Caryophyllaeides fennica (2n = 20), and Nippotaenia mogurndae (2n = 28). The probe hybridized with chromosome ends in all Platyhelminthes (Monogenea, Cestoda) but not in Acanthocephala. No hybridization signals in P. laevis and P. tereticollis were also obtained with the arthropod (TTAGG)n or nematode(TTAGGC)n telomeric probes. Our results suggest that whereas the telomeres in Platyhelminthes are composed of the TTAGGG repeates, this ancestral motif of telomeres in Bilateria was lost in Acanthocephala and replaced with unknown telomeric sequences.