Molecular relationships between closely related species of Bothriocephalus (Cestoda:Platyhelminthes).

DNA/DNA hybridization has been carried out to establish the evolutionary relationships among several host-specific bothriocephalid tapeworms species (Cestoda: Platyhelminthes). Comparative anatomy is not informative for deciphering the relationships among these sibling morphological taxa, and clearcut genetic differences have previously identified seven biological species which are each highly specific to a single host. We show that two species (Bothriocephalus gregarius and B. renaudii) infesting the same host (turbot: Psetta maxima) in two different geographic areas are not sister-taxa. Moreover, a strong decrease in the amount of DNA change is observed in one species of tapeworm, documenting a marked heterogeneity of rates of nucleotidic substitution among these very closely related organisms. Based on these observations, different hypotheses are developed for understanding the evolutionary history of this assemblage of parasites, suggesting that host-switching has played an important role in the recent past.