Molecular and karyological identification and morphological description of cystocercous cercariae of Phyllodistomum umblae and Phyllodistomum folium (Digenea, Gorgoderidae) developing in European sphaeriid bivalves

- Cystocercous cercariae of P. umblae was found in sphaeriid bivalves in Norway.
- Morphology of cercariae of P. umblae and P. folium is closely related.
- Identity of cercariae was proved by molecular and karyological analysis.
- Two congeneric species differ in diploid chromosome number.
- Life-cycle stages need to be described based on both morphological and genetic data.

Two cystocercous gorgoderid cercariae of the genus Phyllodistomum Braun, 1899, Phyllodistomum umblae (Fabricius, 1780) and Phyllodistomum folium (Olfers, 1816), developing in sphaeriid bivalves from Norway and Lithuania, were studied and compared. Our previous molecular studies revealed very close phylogenetic relation of these two species and proved that cystocercous cercariae of P. folium develop in sporocysts in the gills of sphaeriid bivalves. In the present study morphological descriptions are given for the two cercariae, together with karyological and molecular characteristics. Comparative karyological and sequence analysis using ITS2 and 28S rDNA revealed the conspecificity of cercariae emitted from Pisidium hibernicum and Sphaerium corneum with adult P. umblae from Thymallus thymallus and Coregonus albula. The cercariae of P. folium and P. umblae are very similar morphologically with main differences in the structure of the tail. The two species clearly differ in karyotype structure. The diploid set of P. folium is composed of 18 chromosomes, but 16 chromosomes with one pair of large metacentrics were found in mitotic cells of P. umblae. Interspecific karyotypic difference presumably arose from Robertsonian fusion of two uni-armed chromosomes.

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