Chromosome analysis of Phyllodistomum folium (Trematoda, Gorgoderidae) infecting three European populations of zebra mussels.

The mitotic chromosome sets of the larval stages of Phyllodistomum folium infecting three European populations of Dreissena polymorpha were studied using conventional Giemsa staining and karyometrical analysis. The karyotype, described herein for the first time, consisted of nine chromosome pairs (2 n=18). The chromosomes were comparatively small and measured from 1.19 micro m to 5.22 micro m. The first and second longest chromosome pairs were, respectively, approximately 20% and 18% of the mean total chromosome set length and were clearly differentiated from the other seven pairs, which gradually decreased in size. Most chromosomes were biarmed with medially or submedially located centromeres; only pairs 1 and 4 contained subterminally located centromeres. No significant differences in the relative length values of corresponding chromosomes were observed between the three populations studied. The main interpopulation differences were detected in the centromeric index values of some corresponding chromosome pairs. These data, when analysed in conjunction with those of other Gorgoderidae, indicate that the karyotype of P. folium and other species in this family do not show any clear affinities with "typical" plagiorchiate chromosome sets.