Systematic relationships of hymenolepidid cestodes of rodents and shrews inferred from sequences of 28S ribosomal RNA

This study attempts to elucidate systematic relationships of hymenolepidid cestodes of rodents (18 species), shrews (13 species) and bats (one species) using sequences of partial 28S ribosomal RNA, with special reference to the genus Rodentolepis. The main finding is the presence of four multispecies clades of hymenolepidid cestodes showing pronounced morphological variation and frequent colonizations between unrelated hosts. Neither the hymenolepidid cestodes of shrews nor rodents were monophyletic. Also, the genus Rodentolepis sensu Vaucher in Czaplinski & Vaucher (1994, Keys to the Cestode Parasites of Vertebrates. Commonwealth Agricultural Bureaux International, Cambridge) is clearly non-monophyletic. Although rostellar morphology is obviously a key feature on specific and generic levels, on higher systematic levels it seems to be a rather poor indicator of phylogenetic affinity in hymenolepidid cestodes. The presence of clades with more than one rostellar type (armed rostellum present, rudimentary unarmed rostellum present and rostellum absent) also conflicts with the proposed subfamilial and tribal classifications of hymenolepidid cestodes. The overall evidence suggests that the recent trend of splitting hymenolepidid cestodes into multiple genera will produce a more stable and practical classification than the earlier practice of favouring a few, morphologically variable genera. New classifications of hymenolepidid cestodes should, however, consider both morphological and molecular evidence.