New cytogenetic data on and , parasites of evolutionary interest.

Caryophyllideans are intestinal parasites of freshwater fishes, occupying a basal position among the ‘true’ tapeworms. We performed detailed cytogenetic analyses of the well-known caryophyllidean species . For comparison, we also examined for the first time the chromosomes of , a specific parasite of loaches in China. Both species showed a diploid chromosome number of 2 = 20, = 10. Chromomycin A (CMA)/diamidino-2-phenylindole (DAPI) staining performed for the first time in the class Cestoda revealed CMA/DAPI bands in the pericentromeric regions of the short arms of chromosome pair no. 7 in the karyotype of . Fluorescence hybridization with the 18S rDNA probe confirmed the presence of a single cluster of major rDNA near the centromere on a pair of small chromosomes in both species. These findings support the hypothesis that the ancestral state in the family Caryophyllaeidae is a single interstitial cluster of major rDNA genes and thus one nucleolar organizer region per haploid genome. Our results, which we presented together with literature data plotted on a phylogenetic tree, show stability of caryophyllidean karyotypes at the genus level, but showed differences between genera without a clear phylogenetic signal. The data allowed us to at least formulate a hypothesis about the ancestral haploid chromosome number of = 10 for the family Caryophyllaeidae and possibly for the sister family Capingentidae. In addition, we compared two populations of from water bodies with different levels of polychlorinated biphenyl contamination, showing a slightly increased incidence of chromosomal abnormalities at the contaminated site.