FISH with the 18S rDNA probe (red) on chromosomes of Acanthocephalus anguillae males from HN and OL rivers. A Mitotic metaphase (HN). B, C Mitotic metaphases (OL). D Interphase nucleus. E Pachytene (HN). F Zygotene and G pachytene bivalents showing clusters of interstitial 18S rDNA signals associated with DAPI-positive heterochromatic blocks (OL). H Diplotene (HN). I Diplotene/diakinesis with already separated bivalents No. 2 and J early anaphase with the largest bivalents almost separated (OL). K Metaphase I (HN). L Metaphase II (OL). Note the larger signals of the 18S rDNA probe on a homologous chromosome of pair No. 1 in the OL population, indicated by arrows. Asterisks indicate the X chromosome. Chromosomes were counterstained with DAPI. Bar = 10 μm.